Vitamin A

Properties and Metabolism

Vitamin A alcohol (retinol) is a nearly colorless, fat-soluble, long-chain, unsaturated compound with five double bonds. Since it contains double bonds, vitamin A can exist in different isomeric forms. Vitamin A and the precursors, carotenoids, are rapidly destroyed by oxygen, heat, light and acids. Presence of moisture and trace minerals reduces vitamin A activity in feeds (Olson, 1984). The combined potency of a feed, represented by its vitamin A and carotene content, is its vitamin A value. Vitamin A occurs in three forms: retinol, retinal, and retinoic acid. The basic structure of vitamin A is all-trans retinol (Illus. 2-1), which gives rise to several isomers, the most important of which are all-trans retinal and all-trans retinoic acid. Vitamin A is one of the least stable vitamins and is highly labile to light and oxidation. Chemically, retinol consists of a substituted cyclohexene ring with an aliphatic side-chain marked by a series of conjugated double bonds. This unique structure allows retinol and its derivatives to function as both a visual pigment and a regulator of cellular growth and differentiation (Olson, 1991).

Vitamin A (retinol) does not occur per se in plants, but its precursors the carotenes occur in several forms. Carotenes are orange-yellow pigments that occur in green leaves and to a lesser extent in corn grain. Four of these compounds, alpha-carotene, beta-carotene, gamma-carotene and cryptoxanthin (the main carotenoid of corn), are of particular importance because of their provitamin A activity. The vitamin A activity of beta-carotene is substantially greater than that of other carotenoids. As an example, both alpha-carotene and cryptoxanthin are about one-half the conversion rate of beta-carotene to vitamin A (Tanumihardjo and Howe, 2005). However, research has consistently shown that pure vitamin A has twice the potency of beta-carotene on a weight-to-weight basis, indicating a maximum conversion efficiency of 50%. Carotenoids are commonly referred to as “provitamin A” because they can be enzymatically transformed to vitamin A by most species. The primary site of carotene conversion to retinol is the intestinal mucosa. Beta-carotene is cleaved by the enzyme: beta-carotene 15, 15′ dioxygenase (Bauernfeind, 1981), yielding one molecule of retinaldehyde, which is enzymatically reduced to retinol. Normal processes of fat digestion and absorption and adequate dietary fat content are required for the absorption and subsequent conversion of beta-carotene to retinol. In dairy cows supplemental fat elevates plasma levels of beta-carotene (Weiss et al., 1994). Lycopene is an important carotenoid for its antioxidant function but does not possess the beta-ionone ring structure (required for vitamin A activity) and, therefore, is not a precursor of vitamin A. In humans, beta-carotene and lycopene are the predominant carotenoids in tissue (Ribaya-Mercado et al., 1995). Beta-carotene is the most potent vitamin A precursor in plants. The efficiency of carotene transformation varies with species. Some species, such as cats, are unable to convert carotenoids to vitamin A (Bauernfeind, 1981), because they lack the enzyme β-carotene-15, 15-dioxygenase. Compared to goats, cattle have a low enzyme activity for conversion of β-carotene to retinol (Mora et al., 2000). Lower enzyme levels of duodenal and jejunal 15, 15-dioxygenase in cattle compared with goats may explain the greater pigmentation of adipose tissue in cattle.In dairy cattle, the accepted standard of conversion is that 1 mg of all-transbeta-carotene will provide 400 IU of vitamin A activity, for a 12% conversion efficiency (Bauernfeind, 1981, NRC, 1989). The Jersey and Guernsey breeds absorb a greater proportion of carotene intact from the intestine, leading to the orange-yellow color of their body and milk fat, but they have the capacity to convert carotene to retinol in the liver, lung and other tissues (McGinnis, 1988). Cattle reportedly have the capacity to convert beta-carotene to retinol in the ovary (Sklan, 1983; Schweigert et al., 1988), where beta-carotene may act as a local supply of retinol.Vitamin A activity (IU) is defined in terms of the all-trans isomer of retinol. One mg of all-trans retinol equates to 3,333 IU of vitamin A activity (Machlin, 1991). The 13-cisisomer has a relative biological activity of 50% for chicks (Ullrey, 1972). The carotene content of most feedstuffs is quite variable and subject to large losses under less than ideal harvest and storage conditions.

Vitamin A and the precursor carotenoids are rapidly destroyed by oxygen, heat, light and acids. Presence of moisture and trace minerals accelerates destruction of vitamin A activity in feeds (Olson, 1991). Common forms of vitamin A for use in commercial feeds are the acetate, propionate and palmitate esters of retinol. These are normally produced as hardened or cross-linked gelatin beadlets to improve stability. Vitamin A acetate is the most common source used in dry feeds for livestock. Propionate and palmitate esters are generally used in liquid feeds.A number of factors influence the digestibility of carotene and vitamin A in ruminants. Working with lambs, Donoghue et al. (1983) reported that dietary levels of vitamin A, ranging from mildly deficient to toxic levels, affect digestion and uptake. Percentage absorption from supplemental levels of 0, 100 and 12,000 µg retinol (0, 333 and 40,000 IU) per kg diet were 91%, 58% and 14%, respectively. Enteric disease states, such asCryptosporidium infection, reduce vitamin A absorption in calves (Holland et al., 1992). Wing (1969) reported that the apparent digestibility of carotene in various forages fed to dairy cattle averaged about 78%.

Variables that influence carotene digestibility include month of forage harvest; type of forage (hay, silage, greenchop or pasture); plant species; plant dry matter content; and harvest and storage conditions. In general, carotene digestibility was above average during warmer months and below average during winter. Preruminant calves (Bierer et al., 1995) readily absorb carotenoid compounds, especially beta-carotene, which contributes to liver vitamin A stores (Hoppe et al., 1996). Conversion of beta-carotene to retinol by animals is inversely related to vitamin A status (McDowell, 1992). Conversion of carotene to retinol decreases with increasing intake of carotene or vitamin A and with increasing vitamin A concentrations in liver (van Vliet et al., 1996). There are notable species differences in the absorption and transport of carotenoids by plasma lipoprotein fractions (Yang et al., 1992). The physiological significance of these species differences is not yet understood.

A. Rumen Metabolism of Vitamin A

Several studies indicate that appreciable amounts of vitamin A are degraded in the rumen. Studies with various diets have reported pre-intestinal vitamin A disappearance values ranging from 40% to 70% (Ullrey, 1972). The amount of concentrate in a diet is one factor associated with ruminal destruction. Rode et al. (1990) compared microbial degradation of vitamin A (retinyl acetate) in steers fed concentrate, hay or straw diets. Estimated effective rumen degradation of biologically active vitamin A was 67% for cattle fed high-concentrate diets, compared to 16% and 19% for animals fed hay and straw diets, respectively. Weiss et al. (1995) reported 72% in vitro rumen degradation of retinyl esters with a 50% concentrate ration and 16 to 20% degradation when diets of greater than 75% forage were fed. On the other hand, beta-carotene degradation was 23% in sheep, in vivo, across a wide range of dietary starch levels. Weiss (1998) interpreted these data to mean that in ruminants fed rations with 50% or more concentrate, retinyl esters would be only 50% as available on a weight basis compared to beta-carotene. However, preformed vitamin A (e.g., retinyl acetate) has approximately eight times the vitamin A activity per mg as compared to beta-carotene in cattle (3,000 IU/mg vs. 400 IU/mg), and this must taken into account in making any comparison. Interestingly, Florida workers (Aréchiga et al., 1998), reported that milk production was significantly increased in each of three separate experiments in which cows were fed 400 mg of beta-carotene per head daily, despite being supplemented with 200,000 to 250,000 IU of vitamin A per day. The two different rations averaged 28.8% and 28% forage on a dry matter basis, although fibrous byproduct feeds made up a significant portion of the rations. Ruminal destruction of vitamin A may have been a factor in this study. The results suggest that beta-carotene supplementation of dairy cows may be warranted under certain dietary or climatic conditions. 

B. Intestinal Absorption, Transport and Storage

The absorption and transport of vitamin A has been reviewed in more detail elsewhere (Solomons, 2006; Ross and Harrison, 2007). The primary site of vitamin A and carotene absorption is the proximal jejunum (Frye et al., 1991; Olson, 1991). Normal pancreatic, liver and bilary function and adequate fat intake are required for absorption of vitamin A and its precursors. Dietary retinyl esters are hydrolyzed to retinol by pancreatic esterase in the small intestine. They are absorbed as the free alcohol in association with lipid micelles and then re-esterified to form retinyl palmitate in the mucosa. The retinyl esters are transported via the lymphatic system, mainly in association with chylomicrons, to the liver where they are hydrolyzed to retinol and re-esterified for storage in parenchymal cells. Hydrolysis of the stored retinyl esters liberates retinol that is combined with retinol-binding protein for secretion into the bloodstream. Small amounts of retinyl palmitate are absorbed directly into portal blood and are adsorbed to plasma lipoproteins. In vitamin A toxicity, plasma retinyl palmitate concentrations are elevated while retinol concentrations remain normal (Herdt and Stowe, 1991). Retinol is released from the hepatocyte complexed with retinol binding protein (RBP) and transported in this form to the tissues. The retinol-RBP complex is itself associated with transthyretin, a thyroid hormone carrier. Therefore, there is a metabolic interdependence between vitamin A and thyroid function. Iodine deficiency impairs vitamin A metabolism, and vitamin A deficiency impairs thyroid function (Olson, 1991). Goitrogenic feedstuffs in the diet may increase the vitamin A requirement or potentiate cattle responses to supplemental beta-carotene.

A number of factors influence digestibility of carotene and vitamin A. Working with lambs, Donoghue et al. (1983) reported that dietary levels of vitamin A ranging from mildly deficient to toxic levels affect digestion and uptake. Percentage transfer from the digestive tract from supplemental dietary levels of 0, 100 and 12,000 µg retinol per kg were 91%, 58% and 14%, respectively. Wing (1969) reported that the apparent digestibility of carotene in various forages fed to dairy cattle averaged about 78%. In most mammals the product ultimately absorbed from the intestinal tract as a result of feeding carotenoids is mainly vitamin A itself. There is considerable species specificity regarding the ability to absorb dietary carotenoids. In some species, such as the rat, pig, goat, sheep, rabbit, buffalo and dog, almost all the carotene is cleaved in the intestine. In humans, some breeds of cattle, horses and carp, significant amounts of carotene can be absorbed. Absorbed carotene can be stored in the liver and fatty tissues. Hence, these latter animals have yellow body and milk fat, whereas animals that do not absorb carotene have white fat. In the case of cattle, there is a strong breed difference in absorption of carotene. The Holstein is an efficient converter, having white adipose tissue and milk fat. The Guernsey and Jersey breeds, however, readily absorb carotene, resulting in yellow adipose fat and also butterfat. A number of factors affect absorption of carotenoids. Cis-trans isomerism of the carotenoids is important in determining their absorbability, with the trans forms being more efficiently absorbed (Stahl et al., 1995). Among carotenoids, there is a differential uptake and clearance of specific carotenoids (Bierer et al., 1995; Johnson et al., 1997). Dietary fat is important (Roels et al., 1958; Fichter and Mitchell, 1977). Dietary antioxidants (e.g., vitamin E) also appear to have an important effect on the utilization and perhaps absorption of carotenoids. It is uncertain whether the antioxidants contribute directly to the efficient absorption or whether they protect both carotene and vitamin A from oxidative breakdown. Protein deficiency reduces absorption of carotene from the intestine. Once the retinoids are transferred into the cell they are quickly bound by specific binding proteins in the cell cytosol. The intracellular retinoid-binding proteins bind retinol, retinal and retinoic acid for purposes of protection against decomposition, solubilize them in aqueous medium, render them nontoxic and transport them within cells to their site of action. These binding proteins also function by presenting the retinoids to the appropriate enzymes for metabolism (Wolf, 1991). Some of the principal forms of intracellular (cytoplasmic) retinoid-binding proteins are cellular retinol-binding proteins (CRBP, I and II), cellular retinoic acid-binding proteins (CRABP, I and II), cellular retinaldehyde binding protein (CRALBP) and six nuclear retinoic acid receptors (RAR and RXR, with alpha, beta and gamma forms). There are two classes of nuclear receptors with all-trans retinoic acid it is the ligand for RAR; for 9-cis-retinoic acid it is the ligand for RXR (Kasner et al., 1994; Kliewer et al., 1994). Several studies have shown that liver can store enough vitamin A to protect animals from deficiency for long periods of low vitamin A/carotene intake. Hepatic retinol concentrations above 20 µg/g of fresh tissue provide adequate reserves to maintain serum retinol within normal limits (Herdt and Stowe, 1991). This large storage capacity must be considered in studies of vitamin A requirements to ensure that intakes that appear adequate for a given function are not being supplemented by reserves stored prior to the period of observation. Measurement of the liver store of vitamin A at slaughter or by biopsy (Illus. 2-2) is a useful technique in studies of vitamin A status and requirements. Plasma retinol concentration is not a reliable measure of liver stores, because of its tight homeostatic regulation.

Besides being partially converted to retinol, beta-carotene and other carotenoids such as lutein are absorbed in lipid micelles and transferred to the bloodstream, where they are transported primarily in association with lipoproteins (Yang et al., 1992). Sheep, goats and cattle differ in the proportion of carotenoids associated with the VLDL, LDL and HDL lipoprotein fractions of plasma (Yang et al., 1992). Compared to sheep and goats, cattle have ten-fold higher concentrations of beta-carotene in the liver, 7 versus 0.7 µg/g of tissue (Yang et al., 1992). Retinol is readily transferred to the egg in birds, but the transfer of retinol across the placenta is marginal and mammals are born with very low liver stores of vitamin A. In the pig, uterine RBP has been identified in the uterus, with the function of delivering retinol to the fetus (Clawitter et al., 1990). The main excretory pathway for vitamin A is by elimination as glucuronide conjugates in the bile prior to fecal excretion. Since ruminant animals are born with very low reserves of vitamin A, it is critical that calves receive adequate amounts of colostrum, which contains high levels of vitamin A activity, within a few hours after birth. Colostrum deprivation during the first 24 hours of life impairs overall absorption of vitamins A, D and E through seven days of age (Blum et al., 1997). Adequate fat intake is required for fat-soluble vitamin absorption in calves (Rajaraman et al., 1997) and is a consideration in the formulation of milk replacers. Low vitamin A intake by the cow during pregnancy increases the likelihood of vitamin A deficiency in the calf. This is because the cow’s body reserves will be low and thus, colostrum will have a subnormal vitamin A content (Miller et al., 1969). Deficiencies of dietary protein, phosphorus, zinc and iodine during gestation can also impair vitamin A metabolism in the cow and reduce colostral vitamin A supply to the calf. Addition of 200 g/day of supplemental fat to the diet of dry dairy cows increased plasma beta-carotene (as well as vitamin E) concentrations in the peripartum period (Weiss et al., 1994). During the transition from nonlactating to the lactating state in the dairy cow, changes in beta-carotene and vitamins A and E status occur. The regulatory role of the liver in maintaining retinol concentrations in plasma appear to be compromised in cows with fatty liver (Rosendo et al., 2010). In normal cows, the lower liver beta-carotene and greater plasma retinol found at calving suggest that stored retinol in liver first forces available beta-carotene stores to be converted into retinol, thus decreasing liver beta-carotene but not retinol. However, for cows with fatty liver, less beta-carotene was mobilized at calving. Cows that develop fatty liver during transition have a higher risk for retained placenta, metritis and mastitis, conditions all associated with lower retinol, alpha-tocopherol or beta-carotene status (LeBlanc et al., 2002; 2004; Bobe et al., 2004).


Vitamin A is necessary for support of growth, health, and life of major animal species. In the absence of vitamin A, animals will cease to grow and eventually die. Knowledge of the metabolic function of vitamin A in biochemical terms is still incomplete. Vitamin A deficiency causes at least four different and probably physiologically distinct lesions:

  1. loss of vision due to a failure of rhodopsin formation in the retina;
  2. defects in bone growth and maturation;
  3. reproductive failure (i.e., failure of spermatogenesis in the male, and fetal resorption in the female);
  4. defects in growth and differentiation of epithelial tissues, frequently resulting in keratinization.

Keratinization of epithelial tissues due to vitamin A deficiency results in loss of protective functions in the alimentary, genital, reproductive, respiratory and urinary tracts, increasing the susceptibility to infection. Abortions, increased prevalence of retained fetal membranes, and increased calf morbidity and mortality are indicators of vitamin A deficiency in gestating cows. Thus, diarrhea and pneumonia are typical secondary effects of vitamin A deficiency. The skin and haircoat are also affected. Vitamin A is required for normal immune function through its role in the control of cell differentiation and gene expression. Neutrophil function is impaired by vitamin A deficiency (Twining et al., 1997). Neutrophils and alveolar macrophages from young calves respond to increasing concentrations of vitamin A, in vitro (Eicher et al., 1994). Vitamin A also affects the integrity of blood vessels as evidenced by edema observed in the legs and brisket of deficient cattle.

More is known about the role of vitamin A in vision than any of its other functions. Retinol is utilized in the aldehyde form (trans form to 11-cis-retinal) in the retina of the eye as the prosthetic group in rhodopsin for dim light vision (rods) and as the prosthetic group in iodopsin for bright light and color vision (cones). Retinoic acid, a form of vitamin A present in the body, has been found to support growth and tissue differentiation but not vision or reproduction (Scott et al., 1982). Vitamin A-deficient rats fed retinoic acid were healthy in every respect, with normal estrus and conception, but failed to give birth and resorbed their fetuses. When retinol was given even at a late stage in pregnancy, fetuses were saved. Male rats on retinoic acid were healthy but produced no sperm, and without vitamin A both sexes were blind (Anonymous, 1977).

Vitamin A and its derivatives, the retinoids, have a profound influence on organ development, cell proliferation, and cell differentiation. Their deficiency originates or predisposes a number of disabilities (McDowell, 2000; Estean-Pretel et al., 2010). Even with a mild vitamin A deficiency in pregnant rats, there was reduction of nephrons in the fetuses resulting in kidney malfunction (Marín et al., 2005; Bhat and Manolescu, 2008). Vitamin A deficiency in rats induced anatomic and metabolic changes comparable to those associated with neurodegenerative disorders; slowing of cerebral growth was correlated with retinol level (Rahab et al., 2009). During embryogenesis, retinoic acid has been shown to influence processes governing the patterning of neural tissue and cranio-facial, eye and olfactory system development; retinoic acid continues to influence the development, regeneration, and well-being of neurons (Asson-Batres et al., 2009). Retinoic acid also regulates the differentiation of epithelial, connective and hematopoietic tissues (Safonova et al., 1994). The nature of the growth and differentiation response elicited by retinoic acid depends upon cell type. Retinoic acid can be an inhibitor of many cell types with a potential to reduce adipose tissues in meat producing animals (Suryawan and Hu, 1997).

Vitamin A functions in the embryo begin soon after conception and continue throughout the life of all vertebrates (Ross et al., 2000). Except for its role in vision, the function of vitamin A at the cellular level is not yet clear. It is known that retinoic acid acts to regulate cell differentiation. Research findings have indicated that vitamin A increased RNA synthesis by polymerase II in rat testes and the induced change in transcription was due in part to altered chromatin structure (Porter et al., 1986). A hormonal effect is suggested for retinoic acid, with its actions mediated by specific nuclear receptor proteins that control gene expression by binding to specific DNA sequences in regulatory regions of target genes (Franceschi, 1992). The biological affect of retinoic acid is through nuclear retinoic receptors (RAR) and retinoid “x” receptors (RXR). This allows binding to target DNA sequences of responsive genes to activate or repress gene transcription and thus regulate a myriad of biological processes (Ross, 2003).

At the time of birth, ruminants have very low liver reserves of vitamin A, which is reflected by the low concentration of vitamin A in blood (Swanson et al., 2000; Zanker et al., 2000). To prevent vitamin A deficiency the newborn needs to ingest high amounts of vitamin A immediately after birth, which is supplied by colostrum. Because it is clear that nuclear receptors play a vital role in the expression of many important enzymes, it is likely that feeding of vitamin A would have major effects on metabolism, health status, and growth performance of young ruminants. Krüger et al., (2005) fed high levels of vitamin A to cows. The result was that calves fed the colostrum from supplemented cows were now more able to metabolize and eliminate foreign substances from external and internal sources. This further emphasizes the importance of vitamin A-rich colostrum and has great significance for other species that depend on colostrum for the health of their offspring (e.g., humans).

A. Beta-Carotene and Reproduction in Dairy Cattle

Since 1978, several studies have suggested that beta-carotene has a function independent of vitamin A in bovine reproduction (Lotthammer, 1979; Sklan, 1983; Rakes et al., 1985; Ascarelli et al., 1985; Wang et al., 1987; Schweigert et al., 1988a, b; Graves-Hoagland et al., 1988, 1989; Bonsembiante et al., 1986; Aréchiga et al., 1998; de Ondarza and Engstrom, 2009). Cows fed supplemental beta-carotene have exhibited a reduced interval to first estrus, increased conception rates and reduced frequency of follicular cysts compared to animals receiving only vitamin A. Aréchiga et al. (1998) reported an improvement in pregnancy rate, but only under conditions of heat stress and only when beta-carotene had been fed for 90 days or more. In this study, high levels of dietary vitamin A (200,000 to 250,000 IU/day) were fed to all cows, and beta-carotene elicited a consistent increase in milk production across all three trials. Cystic ovarian degeneration is correlated with reduction of plasma beta-carotene (Lopez-Diaz and Bosu, 1992). A Quebec study (Block and Farmer, 1987) reported a weak inverse correlation between plasma carotene concentration and reproductive performance.However other studies have found no significant effect (Folman et al., 1979; Wang et al., 1982,1988; Bindas et al., 1984; Marcek et al., 1985; Greenberg et al., 1986; Akordor et al., 1986) or adverse effects (Folman et al., 1987) of beta-carotene supplementation on fertility of cattle. In some of these studies, there have been trends for improved reproduction; in others, there were no effects at all. The corpus luteum and follicular fluid of the cow have a high concentration of beta-carotene (Chew et al., 1984). It has been suggested that beta-carotene has a specific effect on reproduction in addition to its role as a precursor of vitamin A. Both the corpus luteum and the follicle possess 15,15′-dioxygenase activity and convert beta-carotene to retinol within the granulosa tissue (Sklan, 1983; Schweigert et al., 1988a, b; Rapoport et al., 1998). Uptake and conversion of beta-carotene to retinol has been demonstrated in human and mouse fibroblasts, rabbit corneal epithelia and rat liver cells maintained in cell culture (Wei et al., 1998). Graves-Hoagland et al. (1988, 1989) reported a positive relationship between postpartum progesterone production and plasma concentrations of beta-carotene in dairy cows. In a survey of commercial farms in Quebec, Block and Farmer (1987) reported a modest positive correlation (0.23) between reproductive performance and both plasma retinol and plasma beta-carotene. A Swedish survey reported no relationship between blood beta-carotene or vitamin A and reproduction (Jukola et al., 1996). Other surveys have reported seasonal variability in plasma beta-carotene content, with significantly lower levels observed during winter months and higher levels during spring and early summer, especially when green forages were fed. Jackson et al. (1981) reported that cows with low levels of beta-carotene during winter months exhibited irregular cycles of plasma reproductive hormones. Results of several studies are summarized in Table 2-1. In five trials with 168 Holstein cows and 20 heifers (Tharnish and Larson, 1992) found no benefit of feeding very high levels of vitamin A (1 to 2 million IU, versus 100,000 IU, per day) on plasma progesterone concentration or on measures of reproductive efficiency.

One potential interaction initially explored by Lotthammer et al. (1976) was that of beta-carotene and thyroid function. The diets fed in these initial studies contained goitrogenic Brassica feedstuffs such as kale, forage rape and turnips. Beta-carotene affected serum thyroxine (T-4) levels in these studies. Serum beta-carotene and T-4 were inversely related. Beta-carotene has been reported to increase the conversion of T-4 to T-3 in dairy cows (Pethes et al., 1985). This may have, in turn, mediated some of the reproductive responses observed in response to beta-carotene supplementation in these studies. An interdependency between vitamin A and carotene status and thyroid function has been demonstrated in the rat (Coya et al., 1997; Mutaka et al., 1998) and the chicken (Bhat and Cama, 1978). Seasonal variance in plasma carotenes (Block and Farmer, 1987; Cetinkaya and Ozcan, 1991) may also be related in part to seasonal changes in thyroid function in cattle. Interrelationships of vitamin nutrition, stress load, endocrine and immune function are still emerging (Miller et al., 1993; Gross and Seigel, 1997) and should prove to be a productive area of research in the future.

B. Milk Production in Dairy Cattle, Vitamin A, Beta-Carotene, and Mammary Gland Immune Function

A sufficiently long dry period before parturition is well known to be a prerequisite for mammary gland regeneration and high milk yields during the ensuing lactation. During the dry period, the mammary gland epithelium involutes and then regenerates (Capuco et al., 2003). Because milk production is dependent on the number of mammary secretory cells and on their synthetic and secretory capacities, the regulation of the mammary epithelial cell population during the dry period has important consequences for milk production and lactation persistency (Capuco et al., 2003). Milk production is dependent on vitamin A. In vitro studies show important interactions among vitamin A, lactoferrin, and insulin-like growth factor (IGF) binding proteins (Muri et al., 2005; Puvogel et al., 2005; Schottsedt et al., 2005). As a consequence vitamin A is important for mammary gland epithelial cell proliferation and apoptosis during the dry period and potential milk yield can be affected (Puvogel et al., 2005). Feeding vitamin A to calves influenced concentrations of vitamin A, hemoglobin, and triglycerides and tended to affect the IGF binding proteins (Muri et al., 2005). Circulating levels of vitamin A (retinol) and lactoferrin are low in calves at birth. Bovine colostrum contains relatively high amounts of vitamin A and lactoferrin and both substances are intestinally absorbed by neonatal calves. Ingestion of colostrum in the neonatal calf is followed by changes in the development and functions of the gastrointestinal tract, causing metabolic and endocrine changes (Blum and Baumrucker, 2002) and influencing immune systems of the gastrointestinal tract (David et al., 2003; Norman et al., 2003). Vitamin A and lactoferrin supplementation influenced growth of the ileum and colon in neonatal calves. Interactions were observed between vitamin A and lactoferrin on epithelial cell maturation, villus growth, and size of follicles in intestinal immune tissues (Peyer’s patches) (Schottstedt, 2005). The finding that supplemental beta-carotene increased milk production in three experiments in Florida (Aréchiga et al., 1998), when fed to high producing cows on low forage rations with high levels of supplemental vitamin A, suggests that beta-carotene may complement preformed vitamin A in the diet under certain conditions. In the latter study, rations were based on corn silage as the primary forage and cows were intensively managed with three times per day milking and use of bovine somatotropin (Aréchiga et al., 1998). Disease resistance is a key function of vitamin A, which is required for the development and function of immune cells, the maintenance of mucous membranes and epithelial linings of the respiratory, digestive, urinary and reproductive tracts, and normal functioning of the adrenal gland and thyroid glands. An animal’s ability to resist infectious disease depends on a responsive immune system, and vitamin A deficiency reduces the immune response. In many experiments with laboratory and domestic animals, the effects of both clinical and subclinical deficiencies of vitamin A on the production of antibodies and on the resistance of different tissues to microbial infection or parasitic infestation have been clearly demonstrated (Tompkins and Hussey, 1989; Olson, 1991). Supplemental vitamin A reportedly improves the health of animals infected with roundworm, of hens infected with the genus Capillaria and of rats with hookworms (Herrick, 1972). Vitamin A has been used as an adjunct in treating ringworm (Trichophyton verrucosum) infestations in cattle. Cryptosporidia infection reduces vitamin A absorption (Holland et al., 1992). Vitamin A and beta-carotene have important roles in disease resistance, including bovine mastitis (Bendich, 1993; Chew, 1993; NRC, 2001; LeBlanc et al., 2004). Rezamand et al. (2007) reports, that dairy cows with greater tissue energy stores prepartum and reduced plasma proteins, beta carotene, and alpha-tocopherol had a greater risk for developing a new intramammary infection during the periparturient period. In the last week prepartum, a 100 ng/ml increase in serum retinol was associated with a 60% decrease in the risk of early lactation clinical mastitis (LeBlanc et al., 2004). There are reports of improved mammary health in dairy cows supplemented with beta-carotene and vitamin A during the dry period (Dahlquist and Chew, 1985) and lactation (Chew and Johnston, 1985) (Table 2-1). Dairy cows supplemented with 53,000 IU of vitamin A per head daily plus 300 mg of beta-carotene starting 30 days prepartum had significantly lower milk somatic cell counts during lactation than unsupplemented animals (Chew, 1984). Additionally, cows fed 173,000 IU of vitamin A showed a reduction in somatic cell count compared to controls, but not as large a response as the cows fed both vitamin A and beta-carotene (Chew, 1984). Addition of beta-carotene improved calf neutrophil function in vitro (Eicher et al., 1994).

In addition to a reproductive effect, carotenoids have been shown to have other biological actions independent of vitamin A (Chew, 1995; de Ondarza and Engstrom 2009; Koutsos, 2003). Recent animal studies indicate that certain carotenoids with antioxidant capacities, but without vitamin A activity, can enhance many aspects of immune functions, can act directly as antimutagens and anticarcinogens, can protect against radiation damage, and can block the damaging effects of photosensitizers. Also, carotenoids can directly affect gene expression and this mechanism may enable carotenoids to modulate the interaction between B-cells and T-cells, thus regulating humoral and cell-mediated immunity (Koutsos, 2003).

In animal models, beta-carotene and canthaxanthin have protected against UV-induced skin cancer as well as some chemically induced tumors. In some of these models, an enhancement of tumor immunity has been suggested as a possible mechanism of action of these carotenoids (Bendich, 1989). Beta-carotene can function as a chain-breaking antioxidant, it deactivates reactive chemical species such as singlet oxygen, triplet photochemical sensitizers and free radicals which would otherwise induce potentially harmful processes (e.g., lipid peroxidation; McDowell, 2004). Lymphocyte function, phagocytosis, and in vitro intracellular killing by blood neutrophils against Staphylococcus aureus were enhanced with increasing dietary alpha-tocopherol, retinol, and beta-carotene in dairy cows (Erskine et al., 1997).

Polymorphonuclear neutrophils (PMNs) are the major line of defense against bacteria in the mammary gland. Beta-carotene supplementation appears to exert a stabilizing effect on PMNs and lymphocyte function during the early dry period (Tjoelkeret al., 1990). Daniel et al. (1991a, b) reported that beta-carotene enhanced the bactericidal activity of blood and milk PMNs against S. aureus but did not affect phagocytosis. Vitamin A either had no effect or suppressed bactericidal activity and phagocytosis. Control of free radicals is important for bactericidal activity but not for phagocytosis. The antioxidant role of vitamin A is limited; it does not quench or remove free radicals. Beta-carotene, on the other hand, does have significant antioxidant properties and effectively quenches singlet oxygen free radicals (Di Mascio et al., 1991; Zamora et al., 1991), which may explain its effects on immune cell function. In this role, beta-carotene may complement the antioxidant activity of vitamin E.

The role of beta-carotene in mammary disease resistance is not completely clear. While the above studies, conducted primarily in the northwestern United States, have found specific effects of beta-carotene on immune cell function and beneficial effects of beta-carotene supplementation on udder health in dairy cows, others have reported no effect of beta-carotene on the incidence of mastitis or somatic cell count (Oldham et al., 1991). Batra et al. (1992) reported that mastitic cows had lower plasma concentrations of vitamin A and beta-carotene than healthy cows, while Johnston and Chew (1984) reported a positive association between plasma beta-carotene postpartum and somatic cell count. Some studies aimed at assessing reproductive effects reported that beta-carotene supplementation reduced somatic cell count (Rakes et al., 1985) and reduced treatments required for clinical mastitis (Wang et al., 1988).


Extensive research has been conducted to determine the vitamin A requirements of various species. Requirements have been published in the United States by the Committee on Animal Nutrition of the National Academy of Science-National Research Council (NRC). Vitamin A requirements can be expressed as IU per kg or lb of body weight, per head per day, or per kg or lb of diet. In agreement with general practice, the requirements are normally expressed on a dietary rather than body weight basis. The National Research Council (NRC) subcommittees have established vitamin A requirements for the ruminant species of beef cattle (NRC, 2000) dairy cattle (NRC, 2001), sheep (NRC, 1985), and goats (NRC, 1981). Typical requirements are illustrated in Table 2-2. A newer version of the NRC (2007b) is designed for small ruminants to include not only sheep and goats, but also cervids and new world camelids.

Minimum requirements have been determined by various methods, including amounts required to prevent night blindness, amounts required for storage and reproduction, and maintenance of normal pressure in the cerebrospinal fluid. The minimum vitamin A requirement for normal growth may be lower than the amount required for higher growth rates, resistance to various diseases, and normal bone development. It was suggested that calves born with low vitamin A liver stores should receive a minimum of 16,500 IU per 100 kg (7,500 IU per 100 lb) body weight, and that three to five times this level are necessary for adequate vitamin A liver stores in calves during the critical first few months of life. The NRC (2001) for dairy cattle has changed how vitamin requirements are listed from NRC (1989). For lactating cows examples are given for both a Holstein and Jersey at a specific weight, body condition score, age and milk composition. The different vitamin A requirements relate to milk production and range from 2,123 to 3,685 IU per kg (965-1,766 IU per lb) feed. Quantity of milk is an important consideration as milk is an important source of vitamin A, higher producing cows requiring more dietary vitamin A. Vitamin A content of milk from different species is illustrated in Table 2-3. Vitamin A requirements for dry pregnant cows are based on days pregnant and range from 5,576 to 8,244 IU per kg (2,535 to 3,747 IU per lb) feed. A growing pregnant heifer would have a vitamin A requirement of 6,486 to 7,075 IU per kg (2,948 to 3,216 per lb) feed.

Feedlot beef cattle (NRC, 2000) require 2,200 IU per kg (1,000 IU per lb) feed; pregnant heifers and cows require 2,800 IU per kg (1,273 IU per lb), and lactating cows and bulls require 3,900 IU per kg (1,773 IU per lb) feed. Growing lambs (NRC, 1985) require 1,567 IU per kg (712 IU per lb) of vitamin A while lactating and gestating ewes require between 2,667 or 3,305 IU per kg (1,212 or 1,502 IU per lb) of feed, respectively. Both growing kids and goats in late pregnancy require approximately 1530 IU per kg (695 IU per lb) feed of vitamin A (NRC, 1981).The NRC (2007b) nutrient requirements for small ruminants expresses requirement for vitamin A, not in concentration of feed, but as retinol equivalents (RE, 1 RE equals 1µg of all-trans retinol) per kg of body weight. The vitamin A requirements for small ruminants are 100 RE per kg (45.5 RE per lb) of body weight for growth, 45.5 RE per kg (20.7 RE per lb) body weight for late gestation and 53.5 RE per kg (24.3 RE per lb) body weight for lactation.

Feedlot beef cattle (NRC, 2000) require 2,200 IU per kg (1,000 IU per lb) feed; pregnant heifers and cows require 2,800 IU per kg (1,273 IU per lb), and lactating cows and bulls require 3,900 IU per kg (1,773 IU per lb) feed. Growing lambs (NRC, 1985) require 1,567 IU per kg (712 IU per lb) of vitamin A while lactating and gestating ewes require between 2,667 or 3,305 IU per kg (1,212 or 1,502 IU per lb) of feed, respectively. Both growing kids and goats in late pregnancy require approximately 1530 IU per kg (695 IU per lb) feed of vitamin A (NRC, 1981).The NRC (2007b) nutrient requirements for small ruminants expresses requirement for vitamin A, not in concentration of feed, but as retinol equivalents (RE, 1 RE equals 1µg of all-trans retinol) per kg of body weight. The vitamin A requirements for small ruminants are 100 RE per kg (45.5 RE per lb) of body weight for growth, 45.5 RE per kg (20.7 RE per lb) body weight for late gestation and 53.5 RE per kg (24.3 RE per lb) body weight for lactation.

Table 2-4: Factors Influencing Vitamin A Requirements

  • Type and level of production (growth, pregnancy, location).
  • Genetic differences (species, breed, strain).
  • Carryover effect of stored vitamin A (principally in the liver).
  • Conversion efficiency of carotenes to vitamin A.
  • Variations in level, type and isomerization of carotenoid vitamin A precursors in feedstuffs.
  • Presence of adequate bile in vivo.
  • Destruction of vitamin A in feeds through oxidation, long length of storage, high temperatures of pelleting, catalytic effects of trace minerals and peroxidizing effects of rancid fats.
  • Presence of disease and/or parasites.
  • Environmental stress and temperature.
  • Adequacy of dietary fat, protein, zinc, phosphorus and antioxidants (including vitamin E, vitamin C and selenium).
  • Pelleting and subsequent storage of feed.

Adapted from McDowell (2000)

Practical factors influencing vitamin A requirements are listed in Table 2-4. Minimum requirements have been determined by various bioassay methods, including prevention of night blindness, maintenance of liver stores, maintenance of reproduction and maintenance of normal pressure in the cerebrospinal fluid (CSF). The minimum vitamin A requirement for normal growth may be lower than the requirements for higher rates of gain, resistance to various diseases, normal bone development and nervous system function in ruminants (Weichenthal et al., 1963). Vitamin A deficiency is often seen in heavily parasitized animals that supposedly were receiving an adequate amount of the vitamin (McDowell, 2004). It is believed that requirements for immunity are higher than for growth or reproduction. In humans, vitamin A stores were positively associated with several measures of innate immune activity across a broad range of stress, suggesting that vitamin A enhances protection against diverse pathogens even at concentrations above those needed to maintain normal vision (Ahmed et al., 2009).

The vitamin A requirement of calves was increased by as much as seven-fold, depending on the criteria used to determine it (Lewis and Wilson, 1945). The vitamin A requirement values (IU per 100 kg body weight) were 2,640 (1,200 IU per 100 lbs) for adequate weight gains, 5,280 (2,400 IU per 100 lbs) for increased weight gains and 17,600 (8,000 IU per 100 lbs) for optimum weight gains and vitamin A liver stores 2,640, 5,280 and 17,600 IU per kg (1,200, 2,400 and 8,000 IU per 100 lbs body weight, respectively). The authors suggested that calves born with low vitamin A liver stores or calves deprived of adequate colostrum should receive a minimum of 16,500 IU per 100 kg (7,500 IU per 100 lbs) of body weight. Three to five times this level may be necessary for adequate vitamin A liver stores in calves during the critical first few months of life. Many commercial calf milk replacers contain 44,000 IU vitamin A per kg (20,000 IU per lb) of powder.

Calf vitamin A requirements and fortification rates have been the subject of studies. Franklin et al. (1998) supplemented milk with 0, 15,000 or 30,000 IU per day of vitamin A as retinyl acetate and studied growth and immune system parameters through six weeks of age. Vitamin A administered by this route did not significantly elevate plasma retinol concentrations or affect growth performance or immune cell parameters, but it did lower plasma vitamin E concentrations. Given that normal milk contains approximately 10,000 IU of vitamin A activity per kg (4,545 IU per lb) solids, and that calves were fed 4.54 kg (10 IU per lb) of milk per day (12% solids), the total vitamin A intakes of calves in the study would have been 5,448, 20,448 and 35,448 IU per day. Hammell et al. (1998) reported greater weight gains in bull calves fed no supplemental vitamin A than in calves fed either 34,000 or 68,000 IU vitamin A per day, while weight gain of calves fed 1,700 IU vitamin A per day was not statistically different from either the controls or the higher vitamin A levels. In contrast to these findings, Eicher et al. (1994) reported that feeding milk replacer fortified with 87,000 IU vitamin A per kg (39,463 IU/lb) solids (39,474 IU/calf/day) did not depress plasma vitamin E concentrations and improved fecal scores of calves through 45 days of age, compared to feeding 7,000 IU per kg milk replacer solids. Vitamin E was fed at either 11.2 or 57 IU per kg (5.1 or 25.9 IU per lb) solids. There was a positive interaction between vitamins A and E on bactericidal activity of neutrophils. The route of administration of vitamin A in these studies, added directly to milk versus manufactured into milk replacer, may be a factor in the contrasting outcomes. Swanson et al. (1999) reported that current NRC vitamin A requirements for dairy calves may be too low to maintain liver stores during the first weeks of life. Calves were fed milk replacers containing 1,900, 3,800, 7,600, 15,200 or 44,000 IU vitamin A per kg solids (863, 1,727, 3,454, 6,908 or 20,000 IU/lb solids). Feeding rates of milk replacer (12% solids) were 10% of body weight during week 1 and 12% of body weight during weeks 2 to 4 of the experiment. Liver biopsies were performed at 4, 9, 15, 21 and 28 days of age and showed that hepatic vitamin A (retinol) stores were only maintained by the 7,600 IU level of vitamin A per kg (3,456 IU per lb) and continued to increase through the highest level of 44,000 IU per kg (20,000 IU per lb) of vitamin A supplementation. The current NRC (2001) vitamin A requirement for calves has been increased to 9,000 IU per kg (4091 IU per lb) of dry matter for milk replacer and 4,000 IU per kg (1,818 IU per lb) of dry matter for starter and grower diets. In the Swanson et al.(1999) study, the vitamin A concentrations of both liver and serum were low, despite feeding of adequate colostrum at birth. Only at the highest level of supplementation (44,000 IU per kg or 20,000 per lb) did liver and serum vitamin A reach levels generally accepted as being adequate (Figure 2-1).

In grow-finish lambs, May et al. (1987) used cerebrospinal fluid (CSF) pressure and plasma and liver retinol concentrations to assess minimum vitamin A requirements. Lambs were first depleted of liver vitamin A stores and then for 16 weeks fed varying levels of vitamin A (2, 4, 8, 16, 32 or 64 µg per kg (.91, 1.8, 3.64, 7.28, 14.5 or 29.0 µg per lb) liveweight per day). This equates to 6.7, 13.3, 26.6, 53.3, 106.6 or 213.1 IU per kg (3.0, 6.0, 12, 24.2, 48.4 or 96.7 IU per lb) liveweight. Under these experimental conditions, CSF pressure was maintained at normal levels by actual vitamin A intakes of 14 µg/kg (6.4 µg per lb) live weight or greater. Plasma concentrations of 20 µg retinol/dl were adequate to maintain normal CSF pressure. The minimum vitamin A intake required to prevent elevated CSF pressure in this study was 14.1 µg per kg liveweight (6.4 µg per lb).

A. Factors Affecting Vitamin A Requirement

As discussed earlier in this section, different animal species convert beta-carotene to vitamin A with various degrees of efficiency. The conversion rate of the rat has been used as the standard value, with 1 mg of beta-carotene equal to 1,667 IU of vitamin A, for 50% conversion efficiency. Of the species studied, only poultry are equal to the rat in vitamin A conversion efficiency. For ruminants, 1 mg of beta-carotene in the diet is equivalent to approximately 400 IU of vitamin A (retinol) for beef cattle (NRC, 1996), dairy cattle (NRC, 2001) and goats (NRC, 1981), and 400 to 700 IU for sheep (NRC, 1985) (Table 2-5). There has been variation in experimental results (Bauernfeind, 1981).

Factors that influence the rate at which carotenoids are converted to vitamin A include type of carotenoid, class and production level of animal, individual genetic differences in animals and level of carotene intake (NRC, 1996). Efficiency of conversion of beta-carotene to retinol decreases with increasing intake of either carotenes or vitamin A (van Vliet et al., 1996). For example, the conversion efficiency ratio varies from 4:1 (25%) to 16:1 (6.25%) in the dairy calf as intake of vitamin A equivalents increases from markedly deficient to excess levels (Bauernfeind, 1981; Dolge et al., 1956). Stress conditions, such as extremely hot weather, viral infections and impaired thyroid function, have also been suggested as causes for reduced carotene conversion to vitamin A. Vitamin A requirements are increased by stress conditions such as environmental stress and increased disease exposure. This may reflect increased activity of the adrenal gland, which is known to be dependent on vitamin A. In general the disease state results in decreased serum concentrations of vitamin A (Figure 2-2).

Dietary factors can affect vitamin A metabolism and increase requirements. These factors include (a) deficiencies of protein, vitamin E, zinc, iodine or phosphorus; (b) elevated nitrate levels in feeds or water; (c) the presence of mycotoxins or ethanol in feed; and (d) the feeding of high concentrate rations or the feeding of significant quantities of polyunsaturated fatty acids (Chhabra and Afora, 1987; Gallup et al., 1953; Frye et al., 1991; Miller et al., 1969; Harris, 1975; Bauernfeind, 1981). Conversely, very low-fat diets impair absorption of vitamins A, D, E and K. There is considerable research and some controversy on the relationship of nitrates to vitamin A nutrition. In a review of this subject by Rumsey (1975), it was concluded that although nitrates can be shown to have an adverse effect on vitamin A in vitro, this effect does not appear to be significant under most feeding conditions. Bauernfeind (1981) cites data showing that nitrite, the initial reduction product of nitrate, destroys vitamin A activity in stored forages, especially at a pH below 5, which is typical of silages.


A. Carotenoids and Vitamin A

The vitamin A requirements of ruminants can potentially be met by carotenes in feedstuffs. A number of carotenoid compounds with provitamin A activity occur in plants. Beta-carotene is the most biologically active of these; the potency of the other carotenoids range from 0% to 57% of beta-carotene (Bauernfeind, 1981). Carotenes are chemically unstable, and as a result, many stored feedstuffs are deficient in provitamin A activity. All green parts of growing plants are rich in carotene and therefore have a high vitamin A value. In fact, the degree of green color in roughage is a good index of its carotene content. Although the yellow color of carotenoids is masked by chlorophyll, all green parts of growing plants are rich in carotene and thus have a high vitamin A value. Good pasture always provides a liberal supply, and type of pasture plant-whether grass or legume-appears to be of minor importance. At maturity, however, leaves contain much more than stems, and thus legume hay is richer in vitamin A content than grass hay (Maynard et al., 1979). With all hays and other forage, vitamin A value decreases after the bloom stage. Plants at maturity can have 50% or less of the maximum carotenoid value of immature plants.

Patel el al. (1966) conducted a study of carotenes in alfalfa harvested by different methods. Initial losses were greatest in alfalfa wilted for two days after cutting, and least in alfalfa that was dried under a shed to protect it from sunlight. Once harvested, the loss of carotene through the first 100 days of storage ranged from 44% to 74%. The alfalfa with the highest initial carotene value (the smallest loss of activity during harvest) had the largest loss of carotene in storage. The highest carotene content at 100 days after harvest was for alfalfa wilted for one day and then dried and stored under a protective shed. Under typical harvest and storage conditions in North America, carotene losses in forage crops would be significant.

Both carotene and vitamin A are destroyed by oxidation. This process is accelerated at high temperatures, but heat without oxygen has a minor effect. Butter exposed in thin layers in air at 50°C loses all its vitamin A potency in 6 hours, but in the absence of air there is little destruction at 120°C over the same period. Cod liver oil in a tightly corked bottle has shown activity after 31 years, but it may lose all its potency in a few weeks when incorporated in a feed mixture stored under usual conditions (Maynard et al., 1979).

Much of carotene content is destroyed by oxidation in the process of field curing. Russell (1929) found that there may be a loss of more than 80% of the carotene in alfalfa during the first 24 hours of the curing process. This loss occurs chiefly during daylight hours, partly because of photochemical activation of the destructive process. Much of carotene content is destroyed not only by oxidation, but also by plant enzymes during the process of field curing. In alfalfa leaves, sunlight-sensitized destruction is 7% to 8% of the total pigment present, while enzymatic destruction amounts to 27% to 28% (Bauernfeind, 1981). Enzymatic destruction requires oxygen, is greatest at high temperatures and ceases after complete dehydration.

Hay crops cut in the bloom stage or earlier, and are cured without exposure to rain or too much sun, retain a significant portion of their carotene content, while those cut in the seed stage and exposed to rain and sun for extended periods lose most of their carotene activity. Green hay curing in the swath may lose one-half of its vitamin A activity in one day of exposure to sunlight. Thus, hay usually has only a small proportion of the carotene content of fresh grass. Under similar harvest and curing conditions, alfalfa and other legume hay can have less carotene content than a good grade of grass hay (Maynard et al., 1979).

The carotene content of dried or sun-cured forages decreases in storage with the rate of destruction depending on factors such as temperature, exposure to air and sunlight, and length of storage. In relation to light, beta-carotene has been shown to protect against photo oxidation as it absorbs a major portion of light below 500 mm and thereby reduced reactions with photosensitizers (Airado-Rodiguez et al., 2011). Under average conditions, carotene content of hay can be expected to decrease by about 6% to 7% per month. In artificial curing of hay with a “hay drier,” there is only a slight loss of carotene because of the rapidity of the process and protection against exposure to oxygen, with the final product having 2 to 10 times the value of field-cured hay. Severe heating of hay in the mow or stack reduces vitamin content, and there is a gradual loss in storage, so old hay is poorer than new. All-trans beta-carotene is the predominant isomer in feeds, thermal processing can substantially increase the proportions of cis isomers (9-c and 13-c); beta-carotene is the more bioavailable isomer (Deming et al., 2002). It would seem likely that the isomers of beta-carotene would increase in the fermentation process of silage, resulting in a lower bioavailable beta-carotene.

Aside from yellow corn and its byproducts, practically all concentrates used in livestock feed are devoid or nearly devoid of vitamin A activity. In addition, yellow corn contains a high proportion of non-beta-carotenoids (i.e., cryptoxanthin, lutein, and zeacarotene) that have much less or no provitamin A value than beta-carotene, although they are absorbed and deposited in body tissues (Yang et al., 1992).

The provitamin A activity of yellow corn grain is only one-eighth that of good roughage. There is evidence that yellow corn loses carotene rapidly during storage. For example, a hybrid corn high in carotene lost 50% of its carotene content during eight months storage at 77°F (25°C) and 75% in three years at 77°F. Carotene loss was reduced if corn grain was stored at 45°F (7°C) (Quackenbush, 1963).

Bioavailability of natural beta-carotene is less than chemically synthesized forms (Hussein and El-Tohamy, 1990; White et al., 1993). Using ferrets, all-trans beta-carotene was less bioavailable from carrot juice than from beta-carotene beadlet-fortified beverages (White et al., 1993). In calves, a small enhancing effect of mild heat treatment of carrots on serum and tissue accumulation of carotenoids has been reported (Poor et al., 1993).

A marked discrepancy exists between the carotene content of corn-silage and the vitamin A status of ruminants fed corn silage. On average, corn silage-carotenes were found to be only two-thirds as effective as beta-carotene in maintaining liver vitamin A levels in rats (Miller et al., 1969; Rumsey, 1975). Martin et al. (1971) reported five-fold less carotene in corn silage harvested in October and November than in corn silage harvested in September. The more mature corn silage was not able to sustain liver vitamin A stores in beef steers, particularly if the silage was finely chopped. Diets high in corn silage harvested after a killing frost would be marginal in both vitamins A and E. After a killing frost, four Florida grasses showed dramatically reduced beta-carotene and alpha-tocopheral concentrations (Arizmendi-Maldonado et al., 2003). Milleret al. (1969) reported that ethanol, an occasional byproduct of corn-silage fermentation, may reduce liver vitamin A. Fungal mycotoxins occur in corn silage as well and may increase the vitamin A requirement.

Wing (1969) reported carotene digestibility in plants to be greater during the warmer months. Variations were found in the digestibility of carotenes in plants according to year, species of plant, dry matter content, and form of forage; carotene digestibility was somewhat lower in silages than in pastures or hay. Average published values of carotene content can serve only as approximate guides in feeding practice because of many factors affecting actual potency of individual samples as fed (NRC, 1982).

Naturally occurring sources of vitamin A (retinyl esters) include fish oils, liver, milk fat, egg yolk and liver, although these are not typically major contributors to ruminant diets. Animal by-product proteins may contribute some vitamin A activity, primarily to dairy cattle diets. Their contribution would depend on losses of vitamin A during rendering.

Vitamin A value can be decreased by exposure to light (Whited et al., 2002). Light exposure can detrimentally affect the nutritional value and flavor quality of fluid milk products. Previous reports have detailed light-induced chemical reactions that result in vitamin A degradation and light-oxidized flavor defects. The presence of milk fat appears to protect against vitamin A degradation in fluid products, but adversely affects the flavor quality of milk after exposure to light (Whited et al., 2002). Vitamin A loss was directly influenced by the length and intensity of light exposure and inversely influenced by the fat content of the milk. To illustrate, after 16 hours, vitamin A content was reduced by 29% in reduced fat milk and by 49% in nonfat milk.

B. Commercial Sources of Vitamin A

Before the era of commercial chemical synthesis, marine fish oils were the principal source of vitamin A for human and animal diets. Alfalfa leaf meal was also used as a vitamin A source in livestock diets. Commercial chemical synthesis of vitamin A was introduced in 1949, and the synthetic form has become the major commercial source of vitamin A.

The major source of supplemental vitamin A used in ruminant diets is all-trans-retinyl acetate and all-trans retinyl palmitate. The acetate, propionate and palmitate esters are all produced by major vitamin A manufacturers. The propionate and palmitate esters are used in liquid feeds and human foods. Vitamin A is esterified in these forms for stability. Additionally, the retinyl esters are incorporated into hardened or cross-linked gelatin beadlets for even greater protection against oxidative destruction in premixes and finished feeds. Antioxidants are often included in the beadlet formulation to further increase the stability of the vitamin A. Beadlet technology provides both physical and chemical protection against factors normally present in premixes and feeds that are destructive to vitamin A. Beadlets are normally formulated to produce a flowable product that will mix easily with minimal dust. The vitamin A acetate products most frequently used in ruminant feeds contains 500,000 to 1,000,000 IU or United States Pharmacopeia Units (USP) per gram of product. (Note that IU and USP units are equal in value and one unit equals the activity of 0.3 µg of all-trans retinol or 0.344 µg of all-transretinyl acetate.) Beadlet technology is also used to produce combination products containing, for example, 500,000 IU vitamin A and 100,000 IU vitamin D3 per gram, or 1,000,000 IU vitamin A and 200,000 IU vitamin D3 per gram of product. In similar fashion, stabilized, dispersible, flowable liquid vitamin concentrates (DLC) are produced by major vitamin manufacturers for use in liquid feeds for livestock. Major vitamin A manufacturers also produce water-dispersible dry vitamin A products for use in milk replacers.


Vitamin A is required for normal visual function, maintenance of healthy epithelial tissues and mucous membranes, normal bone development and functional immunity in animals. Vitamin A deficiency signs observed in ruminants vary, but most relate to degenerative changes in these tissues. Numerous studies have demonstrated increased frequency and severity of infection in vitamin A-deficient animals. Low vitamin A status reduces antibody production and impairs cell-mediated immune response against pathogens (Davis and Sell, 1983). Vitamin A is required for production of leukocytes and other cells of the immune system. Clinical signs of vitamin A deficiency may be specific or nonspecific. General signs observed include loss of appetite, loss of weight, unthrifty appearance, thick nasal discharge and reduced fertility. The normal epithelia of the body are progressively replaced by stratified, keratinized tissue. This effect has been noted in the respiratory, alimentary, reproductive and genitourinary tracts as well as in the eye. Keratinization reduces the effectiveness of the epithelial tissues as a barrier to the entrance of infectious organisms. Thus, respiratory and upper respiratory diseases tend to be more severe in animals with vitamin A deficiency.

A. Vitamin A Deficiency in Cattle

In cattle, signs of vitamin A deficiency include reduced feed intake and growth rate; rough hair coat; edema of the joints and brisket; diarrhea; lacrimation; xerophthalmia; night blindness; blindness from corneal opacity; convulsive seizures; abnormal bone growth; low conception rates; abortion; stillbirths; weak, blind or stillborn calves; abnormal sperm and reduced libido in bulls; and increased susceptibility to respiratory and other infections (Illus. 2-3, 2-4, 2-5, 2-6 and 2-7) (NRC, 1996, McDowell, 2000). Cattle with marginal vitamin A status may be more susceptible to pinkeye or other diseases affecting the mucous membranes. Animals in advanced stages of deficiency may exhibit a staggering gait, convulsive seizures (“fainting” in feedlot cattle) and papilledema of the eye, resulting from elevated cerebrospinal fluid pressure (Illus. 2-8).


Illustration 2-3: Vitamin A Deficiency in Cattle. Unthrify Appearance

Note emaciation and evidence of diarrhea in calf. Animal also shows excessive lacrimation and nasal discharges characteristic of deficiency.

Courtesy of G. Patterson and Pfizer, Inc.

Illustration 2-4: Vitamin A Deficiency in Cattle. Blindness

Calf in Philippines (south of Manila) showing vitamin A deficiency characterized by copious lacrimation and blindness; six-month-old animal had been fed reconstituted milk powder and poor quality bleeched hay (practically devoid of carotene).

Courtesy of J.K. Loosli, University of Florida

Illustration 2-5: Vitamin A Deficiency in Cattle. Incoordination and Weakness

Vitamin A-deficient calf, showing incoordination and weakness.

Illustration 2-6: Vitamin A Deficiency in Cattle. Impaired Reproduction

Illustration 2-7: Vitamin A Deficiency in Beef Cattle

Edema of brisket. USDA.

Illustration 2-8: Vitamin A Deficiency in Cattle

Vitamin A is critical for successful reproduction. Vitamin A deficiency lowers reproductive efficiency in both males and females. Reduced libido and sterility in bulls with degeneration of seminiferous tubules has been reported (Larkin and Yates, 1964). Spermatozoa decrease in number and motility, and numbers of abnormal sperm increase markedly. In cows, key indications of deficiency are reduced conception rate (Table 2-6), shortened pregnancies, increased incidence of abortions, high incidence of retained placenta and birth of dead, weak, uncoordinated or blind calves. Blindness in newborn calves is caused by malformation and closure of the optic foramen, constricting the optic nerve (Miller, 1979). If born alive, calves have trouble gaining their balance and lack the instinct to nurse. Vitamin A-deficient newborn calves may show a very severe, often fatal diarrhea. In young calves, signs of vitamin A deficiency also include watery eyes, nasal discharge, muscular incoordination, staggering gait and convulsive seizures (McDowell, 2000). Elevated cerebrospinal fluid (CSF) pressure is the earliest change specific to vitamin A deficiency in the calf and is a precursor to most of the severe neurologic symptoms described.

The classic sign of vitamin A deficiency in ruminants is night blindness, due to the loss of activity of the rod cells in the retina, which are active in dim light. As vitamin A deficiency develops, the adaptation to dim light and darkness is reduced, eventually resulting in night blindness. This condition is readily detected when animals encounter obstacles in dim light. Night blindness and blindness may be the first noticeable sign of vitamin A deficiency in rapidly growing cattle fed high-concentrate rations. In severe vitamin A deficiency, characteristic changes occur in the eye, including excessive lacrimation (tearing), keratitis, softening and clouding of the cornea, and development of xerophthalmia, characterized by drying of the conjunctiva. In cattle, copious lacrimation (rather than xerophthalmia) is the most prominent clinical sign of vitamin A deficiency (Maynardet al., 1979). The degenerative changes in the eye in vitamin A deficiency are shown in Illus. 2-8. Blindness can result from either epithelial degeneration or secondary to eye infections caused by the deficiency. In finishing cattle, generalized edema can occur, with signs of lameness in the hock and knee joints and swelling in the brisket area (NRC, 1996). Booth et al.(1987) reported feedlot cattle with low serum vitamin A concentration, apparent blindness, fixed dilated pupils, severe ataxia and poor weight gains. Feedlot cattle with mild vitamin A deficiency exhibit reduced feed intake and weight gains. Reduced feed intake may result in deficiencies of other nutrients, particularly when the diet is marginal in those nutrients. Because vitamin A is involved in normal bone development, the long bones of deficient animals are altered in shape during growth. Teeth are also affected. Failure of the spine and other bones to develop normally causes increased pressure on and degeneration of the nerves. For example, blindness in calves results from constriction of the optic nerve caused by a narrowing of the bone canal through which it passes, the optic foraman (Maynard et al., 1979). Bone abnormalities in the spine and pelvis may be responsible for muscular incoordination and other neurologic symptoms exhibited by vitamin A-deficient cattle. Vitamin A is required for normal development, maintenance and function of the immune system. As part of the major role of maintaining a healthy epithelium, vitamin A and beta-carotene have been shown to have an important role in reducing the incidence and severity of mastitis in dairy cows (Chew, 1987). Feeding rations with low vitamin A activity has been shown to increase the incidence and severity of bovine mastitis (Chew, 1987, 1993). Adequate intake of vitamin A and beta-carotene is necessary for protection against mastitis. Vitamin A helps maintain epithelial integrity and normal immune cell function, while the antioxidant activity of beta-carotene increases the bactericidal activity of blood and milk polymorphonuclear neutrophils (PMNs) against S. aureus (Daniel et al., 1991a, b). In addition, some studies have shown that beta-carotene has a favorable effect on fertility (Table 2-1) of heifers and lactating cows (Lotthammer, 1979; Bonsembianteet al., 1986; Aréchiga et al., 1998), while others (Oldham et al., 1991; Akordor et al., 1986) have shown no effect of beta-carotene on reproductive performance or incidence mastitis. Additional studies are needed to clarify the physiologic role of beta-carotene as differences in results may relate to variations in body stores of beta-carotene and (or) vitamin A in experimental animals.

Van Merris et al. (2004) confirmed the decrease in serum retinol during the peripartum period of dairy cows and noted that profound changes in vitamin A metabolism occurred during the acute-phase reaction of coliform mastitis in heifers. All-transretinoic acid was found to be the most abundant circulating acid isomer during mastitis, providing an indication for a possible key role of all-transretinoic acid in the modulation of the immune response. In the 1950s, it was discovered that cattle developed signs of vitamin A deficiency, originally referred to as X-disease (hyperkeratosis), from consuming feeds that contained a chlorinated naphthalene found in lubricating oil. The depressed vitamin A levels in blood plasma led investigators to conclude that the toxic substance interfered with the conversion of carotene to vitamin A (Maynard et al., 1979). Removal of naphthalenes from oils eliminated X-disease. Studies have shown that vitamin A-deficient cattle lack heat tolerance. Deficient cattle stand panting, and daily feed consumption is reduced (Perry, 1980). Vitamin A-supplemented cattle show improved hot weather tolerance and spend more time ruminating. Vitamin A deficiency signs can result indirectly from deficiencies of zinc, iodine, phosphorus, protein or vitamin E, because these nutrients are required for the normal utilization and metabolism of vitamin A. Zinc deficiency interferes with the synthesis in the liver of retinol binding protein (RBP) which carries vitamin A (retinol) in plasma. Thus, in zinc deficiency decreased liver RBP levels may cause low plasma vitamin A concentrations. Zinc-deficient goats have been observed to have low serum vitamin A despite adequate dietary vitamin A (Chhabra et al., 1980). In calves, serum vitamin A was significantly higher for animals supplemented with 50 mg per kg (22.7 mg per lb) of zinc (Chhabra and Arora, 1987). Cattle from tropical Northern Australia showed a 12% annual mortality in part because of a slow release of liver vitamin A (Guerin, 1981). Apparently, high calcium and low zinc concentrations in native forages contributed to this slow liver vitamin A release. Since tropical forages have often been shown to be low in zinc (McDowell et al., 1984), conditioned vitamin A deficiencies may be resulting even though liver vitamin A values indicate adequate concentrations of this vitamin. This points out the importance of balanced vitamin-mineral nutrition for grazing livestock. Deficiencies of phosphorus, iodine, protein or vitamin E reduce vitamin A utilization and may cause vitamin A deficiency signs. Retinyl phosphate is an intermediary in retinol metabolism. Transthyretin, a thyroid hormone binding protein, forms a complex with retinol binding protein in plasma (Olson, 1991). Therefore iodine status and thyroid activity can influence retinol transport and uptake. Protein deficiency can limit synthesis of RBP, and vitamin E enhances vitamin A stability and utilization.

B. Vitamin A Deficiency in Sheep

Clinical signs of vitamin A deficiency in sheep (Illus. 2-9 and 2-10) are similar to those of cattle. Night blindness is the common means of determining the deficiency, although severe vitamin A deficiency in feedlot lambs may progress to total blindness (NRC, 1985). Vitamin A deficiency results in keratinization of the respiratory, alimentary, reproductive, urinary and ocular epithelia. Keratinization of these tissues reduces their resistance to infection (Weber, 1983). Bruns and Webb (1990) found that vitamin A-deficient lambs had depressed humoral immune response to ovalbumin. Adrenal function is compromised by vitamin A deficiency (Webb et al., 1968). Additional clinical deficiency signs include growth retardation, bone malformation, degeneration of the reproductive organs and elevated pressure in cerebrospinal fluid. Deficiency interferes with normal development of bone, which may contribute to muscular incoordination and nervous signs. Also, deficiency of the vitamin can result in lambs born weak, malformed or dead (Illus. 2-10). Retained placenta also occurs in vitamin A-deficient ewes.

Illustration 2-9: Vitamin Deficiency in Sheep. Weakness and Swayed Back

T.J. Cunha and Washington State University

Illustration 2-10: Vitamin A Deficiency in Sheep. Reproductive Failure

Ewe fed low carotene ration during gestation. One lamb was born dead, and the other died six hours after birth.

CA AG. Experiment Station, NRC, 1975

Vitamin A deficiency has resulted in low semen quality in rams (Lindley et al., 1949). Vitamin A deficiency has detrimental effects on wool production and characteristics, including shortened wool fibers and decreases in fiber thickness, strength, and elongation (Faird and Ghanem, 1982). 

C. Vitamin A Deficiency in Goats

Vitamin A deficiency in goats may first appear as a nonspecific rough, dull hair coat. Goats deficient in vitamin A exhibit keratinization of the epithelium of the respiratory, alimentary, reproductive and urinary tracts, and of the eye (NRC, 1981). Signs include increased susceptibility to multiple infections; especially respiratory infection; poor bone development; birth of abnormal offspring; and visual impairment. Night-blindness is the classic deficiency sign. Experimentally produced signs of vitamin A deficiency in goats include loss of appetite, loss of weight, unthrifty appearance, night blindness and a thick nasal discharge (Schmidt, 1941). In India, limited work with goats suggests that vitamin A deficiency leads to development of urinary calculi (Majumdar and Gupta, 1960). Vitamin A deficiency impairs fertility, either temporarily or permanently. Metritis may be caused by damage to the integrity of the uterine mucosa (Guss, 1977). In adult goats, reduced fertility is a common clinical sign. Doe goats show poor conception rates and shortened or delayed estrus, and bucks exhibit reduced semen quality.

Fortification Considerations

A. Stability in Feed Storage and Processing

With the exception of high quality, green forages, the vitamin A activity of typical ruminant diets is unpredictable and often inadequate. Ruminant animals consuming poor quality forages or a large proportion of dietary concentrate require a source of supplemental vitamin A, preferably as a stabilized product such as a cross-linked gelatin beadlet. Vitamin or vitamin-mineral premixes should be used within three months of manufacture to ensure that intended levels of vitamin A are delivered to livestock. Vitamin A activity declines over time, especially in the presence of trace minerals. Up to 25% of the original vitamin A activity can be lost after three months of storage, and up to 50% or more after one year. Vitamin A loss in commercial feeds was evident even if the commercial feeds contained stabilized vitamin A supplements. Dash and Mitchell (1976) reported the vitamin A content of 1,293 commercial feeds over a three-year period. The loss of vitamin A was greater than 50% within one year. Poor storage conditions, with exposure to moisture and heat, accelerate losses of vitamin A and carotenes from feedstuffs. When the common supplemental forms of vitamin A, all-trans retinyl acetate and all-transretinyl palmitate, are stored properly, vitamin A activity is relatively stable with losses of about 1% per month. When these retinyl esters are stored in combination with minerals or other feedstuffs or are pelleted, storage losses increase to 5% to 9% per month (Coelho, 1991). Several factors influence the loss of vitamin A from feeds during storage. Stabilized vitamin A (e.g., cross-linked gelatin beadlet) is relatively stable in vitamin premixes, but the presence of choline and trace minerals in the premix significantly reduces the stability of vitamin A. The stability of vitamin A in feeds and premixes has been improved considerably by chemical stabilization through esterification, and by physical and chemical protection using spray-dried gelatin beadlet technology with antioxidants (Shields et al.1982). Nevertheless, vitamin A product forms should not be stored for prolonged periods prior to use. Vitamin A and carotene destruction also occurs during feed processing with steam and pressure. The effects of pelleting on vitamin A in feed are related to die thickness and diameter. These variables produce friction, heat and shear forces capable of fracturing or melting vitamin A beadlets and exposing the vitamin to destructive factors. In addition, steam application exposes feed to heat and moisture. Running fines back through the pellet mill exposes vitamin A to the same factors a second time. Repelleting and extrusion are more destructive than conventional pelleting. Up to 30% to 40% of the vitamin A present initially at mixing may be destroyed during processing, depending on product form, temperature, moisture, throughput and other factors (Shields et al., 1982). In feed manufacturing, liquids, such as fats or oils, are sometimes applied to feeds after processing (pelleting, extrusion or expansion). In some cases, vitamin A is incorporated into these liquids for application. Antioxidants are added for stability, especially if the unsaturated fatty acid content is high. One disadvantage of surface application is that the vitamin A remains largely exposed to light, oxygen and moisture and may be more subject to degradation during handling and storage. Due to the relatively poor stability of vitamin A, particularly when exposed to light, moisture, oxygen, trace minerals, heat or friction, the feed industry has readily accepted the dry, stabilized vitamin A product forms. The stability of vitamin A in feeds and premixes has been improved in recent years by the chemical stabilization of esterification, and the physical and chemical protection provided by cross-linked gelatin beadlet technology (Bauernfeind and DeRitter, 1972; Shieldset al., 1982). Practical considerations that affect vitamin A stability are listed in Table 2-7. In the gelatin beadlet technology, vitamin A ester (acetate or palmitate) is emulsified with gelatin to form a liquid suspension and spray-dried into discrete dry particles. The beadlets are then dried and cross-linked for added stability. As with any commercial vitamin product form, the cross-linked beadlet has been developed through research to be an optimum formulation with a combination of high bioavailability, physical and chemical stability, and flowability in an economical product (Bauernfeind and DeRitter, 1972).

Table 2-7: Practical Factors that Affect Vitamin A Stability

Factors detrimental to Stability

  • Prolonged storage of the vitamin products, either in premixes prior to mixing of in the final feed product.
  • Vitamin premixes containing trace minerals.
  • High environmental temperature and humidity.
  • Pelleting, blocking and extrusion.
  • Hot feed bins that sweat inside upon cooling.
  • Rancid fat in the feed.
  • Moisture leakage into feedbins or storage facilities.

Factors Promoting Stability:

  • Minimizing time between manufacture of the vitamin product and consumption by the animal.
  • Storage of vitamins in a cool, dark, dry area in closed containers.
  • Not mixing vitamins and trace minerals in the same premix until ready to mix the feed.
  • Controling the premix to maintain pH within a range of 4.5 to 6.5; avoiding extremely acidic or alkaline pH.
  • Use of good quality feed ingredients and vitamins.
  • Use of appropriate antioxidant systems in the vitamin A product form.
  • Proper maintenance of storage bins and other equipment.
  • Minimizing time between purchase and use.

Adapted from Hoffmann-La Roche (1989)

Different vitamin A supplementation products have used specific coating technology for preventing vitamin A from ruminal degradation. There is a need for minimizing ruminal destruction to increase the amount of vitamin A that reaches the duodenum. To protect vitamin A from preintestinal destruction, gelatin beadlets have been developed commercially that contain not only vitamin A but also carbohydrates and antioxidants to stabilize the vitamin A (Hoffmann-La Roche, 1994). Bioavailability of different supplemental vitamin A sources has been compared (Jurjanz et al., 2006; Alosilla et al., 2007; Preveraud and Geraert, 2010). Different methods, to evaluate vitamin A bioavailability were an in vitro procedure using a mobile nylon bag (Preveraud and Geraert, 2010), vitamin A obtained from a liver bioposy procedure (Alosilla et al., 2007), and vitamin A milk concentrations using an area under the curve procedure (Jurjanz et al., 2010).

B. Optimum Vitamin A Nutrition and Fortification Levels

Vitamin A supplementation of ruminant diets is warranted when:

  • feeding poor quality forage with little or no green color;
  • feeding diets composed of 40% concentrate or more;
  • feeding corn silage as the sole or primary forage (Jordan et al., 1963);
  • feeding calves colostrum or milk from cows with a low vitamin A status;
  • feeding a limited roughage or poor quality roughage to weaned calves, lambs or kids;
  • feeding purchased cattle of unknown background and in unthrifty condition where liver stores of vitamin A are likely to be low or sub-optimal (Perry, 1980).


The use of concentrate and byproduct feeds in place of forages is probably the largest single factor that has increased the need for supplemental vitamin A in ruminant diets. Inefficient utilization of carotene from corn grain and the destruction of carotene and vitamin A in the rumen are the main reasons for adding supplemental vitamin A to high-concentrate diets (Rumsey, 1975; Weiss, 1998). Mold contamination is associated with a 98% reduction in carotene concentration in corn (Adams et al., 1975).

Supplemental vitamin A can be delivered via: (1) dry feeds, premixes, blocks or liquid feed supplements; (2) free-choice mineral mixtures; (3) injection, using a commercial preparation; (4) drinking water using a water-dispersible product. The most convenient and often most cost-effective means of providing supplemental vitamin A to livestock is in balanced rations that provide uniform consumption of the vitamin on a continuous and daily basis. Grazing livestock are often supplemented with free-choice minerals, liquid feeds or blocks (solidified products) that are formulated to be intake-limiting, usually by adjusting salt content or pH or through physical effects. These products can be both convenient and effective, but feeding directions must be followed and intake of the product monitored to ensure correct levels of supplementation. Furthermore, these products must be fed in appropriate feeders, such as covered mineral feeders, that afford adequate protection from moisture and sunlight. Liquid feed tanks must be located properly and kept in good working order.

The greatest limitation of administering vitamin A with free-choice minerals is unknown consumption by individual animals and destruction of the vitamin with time (McDowell and Arthington, 2005).

Vitamin A is often included, along with vitamins D3 and E, in liquid feed supplements. Since the viscosity, pH and solids content of liquid feed supplements vary considerably, development of vitamin A product forms that blend uniformly and are stable in such an environment was a challenge to manufacturers. Products of choice are dispersible liquid concentrates, which include fat-soluble vitamins A, D3, and E in tested formulations of emulsifiers, antioxidants and carriers.

In recent years, some livestock producers have followed the practice of administering vitamin A by intramuscular injection. This route of administration of vitamin A has been used to correct, or more frequently to prevent, vitamin A deficiency when feed or water administration is either inconvenient or impossible. Feeder cattle with an unknown history often receive one million IU vitamin A by intramuscular injection during the receiving process in preparation for entry into the feedlot. Vitamin A status is important for cattle to produce a viable immune response to the vaccines that are often administered at this time. Adequate liver vitamin A stores are a necessity for feedlot cattle entering a high-concentrate feeding program.

Intramuscular fat (marbling) increases carcass value. Retinoic acid, by controlling cell differentiation, can influence fat deposition. Attempts have been made to increase marbling in steers by reducing the vitamin A status (Gorocica-Buenfil et al., 2007, a, b, c). In Angus-cross steers, feeding low vitamin A diets for 145 or 168 days increased marbling and carcass quality (Gorocica-Buenfil, 2007a, b). Restricting vitamin A intake for 131 days at the end of the finishing period was insufficient to affect the site of fat deposition in Holstein steers (Gorocica-Buenfil et al., 2007c). In a later experiment, Gorocica-Buenfil et al. (2008) fed Angus-based steers a low vitamin A diet for 216 days. In this experiment neither marbling score or animal health were affected. In contrast to the results in steers and in in vitro experiments using lamb preadipocytes, in growing lambs in vivo a large concentration of vitamin A seemed to provoke greater development of intramuscular fat (Arana et al., 2008). The reasons for these differences are unknown. The risk of feeding a low vitamin A diet to ruminants would be substantial in relation to health of the animals. The duration of vitamin A restriction required to improve intramuscular fat deposition remains unknown. It is likely that to affect the vitamin A status of the animal, liver vitamin A stores need to be depleted. Also, animals entering a feedlot often have unknown vitamin A status, those with low liver stores of vitamin A would be at great risk for a deficiency.

Increased levels of vitamin A are important under stress conditions. Situations under which increased vitamin A supplementation may be valuable include:

  • in calves, lambs or kids during weaning;
  • as nutritional support of treatment for bacterial or viral enteric disease, intestinal parasites, ringworm or other parasites;
  • for newly arrived feedlot cattle;
  • at calving in beef or dairy cows that have received marginal nutrition during lat Pregnancy;
  • for animals under heat stress.

Seasonal periods of heat stress reduce milk yields and fertility in lactating dairy cows (Hansen and Aréchiga, 1999). Vitamin A and other natural retinoids have been shown to improve reproduction in heat-stressed dairy cows and prevent the development of otherwise developmentally incompetent oocytes (Livingston et al., 2002; Lawrence et al., 2004). Supplementing postpartum lactating dairy cow diets with beta-carotene during the summer increased pregnancy rates in the fall (Aréchiga et al., 1998). Administering retinol to ewes during superovulation followed by natural service increased the ability of recovered embryos to develop in vitro (Eberhardt et al., 1999).

Administration of vitamin A at high levels in drinking water or by injection is often recommended to support any specific therapy in the treatment of animal disease. This is of particular value in animals whose liver vitamin A stores may have been depleted or in animals with intestinal infections or disorders that have impaired vitamin A absorption. Also, high levels of vitamin A may be beneficial in reducing the incidence of mastitis in dairy cows (Chew, 1987) or increasing milk yield (Oldham et al., 1991).

The level of vitamin A supplementation used should be based on both the expected optimum requirements of the animal and the potential for subclinical deficiency from interfering factors, such as low dry matter intake, variable forage quality, ruminal destruction of vitamin A, feed product composition and storage time. As with most nutrients, a borderline deficiency of vitamin A is far more likely than a severe, outright deficiency. A marginal vitamin deficiency reduces animal performance by small increments and is not easily detected (Miller, 1979).

Optimal vitamin A fortification of ruminant diets is shown in Table 2-8. Higher levels of vitamin A are sometimes used when stress levels or disease pressures are high, when high-concentrate rations are being fed or when feed storage or formulation favors increased loss of vitamin A activity. Beta-carotene supplementation of dairy cattle is recommended when little or no green forage is being consumed, although this area of research remains unclear. The most recent study in this area (Aréchiga et al., 1998) reported a significant milk production response to beta-carotene in cows fed rations in which a significant proportion of forage was replaced with fibrous byproduct feedstuffs, despite being supplemented with high levels of vitamin A (200,000 to 250,000 IU per day) (Figure 2-3). This study also reported some beneficial effect of beta-carotene on reproductive performance during heat stress if cows received beta-carotene for 90 days or more.

Vitamin Safety

In general, the possibility of vitamin A toxicity in ruminants is remote because it has a wide margin of safety. However, of all vitamins, vitamin A offers the greatest risk of accidental toxicity. Presumed upper safe levels are four to 10 times the nutritional requirements for monogastric animals, including birds and fish, and about 30 times the requirements for ruminants (NRC, 1987). The higher vitamin A tolerance of ruminants may be due in part to microbial degradation of vitamin A in the rumen (Rode et al., 1990; Weiss, 1998). Most of the harmful effects have been obtained by feeding over 100 times the daily requirements for an extended period of time. Thus, moderate excesses of vitamin A administered for short periods of time should not produce any harmful effects. Recommended upper safe levels for adult cattle are 66,000 IU per kg (30,000 IU per lb) and for goats and sheep 45,000 IU per kg (20,455 IU per lb). In a short-term study, steers were fed 2.56 million IU per head per day, with no gross evidence of toxicity (Hale et al., 1961). Tharnish and Larson (1992) fed dairy cows 1.0 to 2.0 million IU per day for periods of several months in five experiments. No outright toxicity symptoms or depression in performance were noted. The most characteristic signs of hypervitaminosis A are skeletal malformation, spontaneous fractures, and internal hemorrhage (NRC, 1987). Other signs include loss of appetite, reduced growth or weight loss, skin thickening (hyperkeratosis), increased blood-clotting time, reduced erythrocyte count (hematocrit), enteritis, congenital abnormalities and conjunctivitis. Degenerative atrophy, fatty infiltration and reduced function of the liver and kidneys are also typical. For ruminants fed toxic levels of the vitamin, the most common clinical signs were osteoporosis, reduced feed intake and decreased spinal fluid pressure (NRC, 1987). The dietary level at which damage occurs in cattle varies among affected tissues; skeletal changes were observed with 132,000 IU vitamin A per 100 kg (60,000 IU per 100 lbs) of body weight per day, or about 30 times the requirement (Hazzard et al., 1964). In contrast, weight gains were depressed only at 880,000 IU of vitamin A per kg (400,000 IU per lb) of body weight or greater (Hazzard et al., 1964).High excess intake of vitamin A impairs absorption and metabolism of the other fat-soluble vitamins. Therefore, excess vitamin A in diets containing marginal levels of vitamins D, E and K may impair animal performance by inducing a marginal deficiency of one or more of the other fat-soluble vitamins. In particular, caution should be observed in formulation of milk replacers for veal calves and for replacement calves in accelerated-growth schemes to avoid potential problems with excessive vitamin A intake (NRC, 2001). Animals receiving direct exposure to sunlight will be protected to varying degrees from an induced vitamin D deficiency. Vitamin K is normally synthesized by rumen flora, but its absorption could be impaired by excess vitamin A. Vitamin E would be of greatest concern under practical conditions. The efficiency of conversion of beta-carotene to vitamin A (retinol) declines progressively with increasing intakes of either carotenes or preformed vitamin A. This appears to be a natural homeostatic control mechanism that protects grazing livestock from any potential harmful effects of the carotenes abundant in high quality green forages (Miller, 1979). Similarly, animals exhibit a relatively rapid catabolism and excretion of excess vitamin A. The conversion of retinol to retinoic acid is irreversible in animal tissues and provides a metabolic outlet for retinol and retinaldehyde. Retinoic acid derivatives are formed by conjugation to glucuronic acid in the liver and excreted in the urine. In practice, toxicity is far more likely to be produced by excess vitamin A than by carotene. However, vitamin A toxicity is normally not a practical problem for ruminant livestock, except when unreasonably high levels are accidentally administered or fed for extended periods of time (NRC, 1987).

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