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Functions of Vitamin A are discussed more completely in Vitamin Basics. This section deals more specifically with the roles of vitamin A in ruminant livestock.
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:
- loss of vision due to a failure of rhodopsin formation in the retina;
- defects in bone growth and maturation;
- reproductive failure (i.e., failure of spermatogenesis in the male, and fetal resorption in the female);
- 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. 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.
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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). 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 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.
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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.
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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 (Olson, 1991; Tompkins and Hussey, 1989). 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). 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 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).
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 (Tjoelker et al., 1990). Daniel et al. (1991a, b) reported that beta-carotene enhanced the bactericidal activity of blood and milk PMNs against Staph. 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). The recent 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).
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