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Vitamin A deficiency presents a special risk with the young calf. Because little of the vitamin is transferred from the dam to the fetus during gestation, practically all calves must be considered vitamin A deficient at birth. Researchers have reported that in fetal calves, liver stores, which are the most sensitive measure of vitamin A status, are about 1 percent of the levels found in adult cattle.
Colostrum generally provides the newborn calf with a rich initial source of vitamin A and the carotenoids that can serve as vitamin A precursors. However, concentrations decrease logarithmically as the mammary secretions change to normal milk. In addition, researchers such as Tomlinson et al. (1976) have reported significant differences in the vitamin A and carotenoid content of colostrum itself. Low levels are especially likely when cows receive inadequate vitamin A supplementation.
Once the preruminant calf has developed initial tissue stores of vitamin A from colostrum, it is important to ensure that adequate intake continues to maintain and increase vitamin A stores. Swanson et al. (1999) noted that the goal should extend beyond avoidance of clinical deficiency signs to promote optimum growth and performance--and provide stores that will allow important body functions to continue at times when the animal cannot meet its vitamin A requirements from the diet. Indeed, Herdt and Stowe (1991) noted that recently weaned calves may be the livestock at greatest risk of vitamin A deficiency in the United States, because they have so high a risk of infectious diseases, which increase vitamin A requirements. Without proper management and dietary vitamin A fortification beforehand, an infection in these animals could severely diminish body stores of vitamin A and compromise health and performance.
As a result, there have been questions about the current National Research Council (NRC) recommendation for minimum vitamin A fortification levels* in milk replacer. Assuming the calf consumes approximately 1.25 lbs (0.56 kg) of milk replacer solids per day, the NRC recommended level is 1,727 IU per pound (3,800 IU/kg) of solids. By comparison, whole milk itself contains approximately 4,545 IU per pound (10,000 IU/kg) of solids.
The NRC recommendation is based on work by Rousseau et al. (1954), in which the calves started the trial at 12.5 weeks of age and were already eating calf starter, chopped alfalfa hay and alfalfa meal. In later work, the same researchers (Eaton et al. 1972) revised their minimum recommendation, setting it at 225 percent of the NRC recommendation. Even in this later work, the calves could not truly indicate preruminant needs, since they were almost nine weeks old at the start of the study and consuming calf starter and chopped alfalfa hay.
There are also questions about the NRC recommendation because environmental stress, disease and other factors can significantly increase vitamin A requirements. Thus, most manufacturers of milk replacers fortify their products with about 10,000 to 20,000 IU of vitamin A per pound (22,000 to 44,000 IU/kg) of milk solids. This is roughly five to 10 times the NRC minimum recommendation, and it dramatically increases vitamin A stores in the calf to avoid deficiency under field conditions.
Recently, Swanson et al. (1999) investigated five levels of vitamin A fortification in milk replacer. Fortification ranged from half the NRC requirement to 20,000 IU/lb of milk replacer solids (Table 1). Calves began receiving the milk replacer at four days of age and continued the treatments until 28 days old.
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At the start of the study, the mean liver stores were 19.5 µg of vitamin A per gram of liver tissue, just below the deficiency level (< 20 µg/g) that Eaton et al. (1970) established for dairy calves. By four weeks of age, liver stores had become even more deficient in the calves receiving the NRC minimum recommendation or less. Calves receiving 200 percent of the NRC's recommendation maintained liver stores at marginal levels. Only the calves receiving the two highest fortification levels of approximately 7,000 or 20,000 IU/lb had significant increases in hepatic vitamin A levels.
Although the vitamin A-deficient calves did not exhibit clinical signs, the researchers noted that this study was conducted in warm, dry weather with minimal stress conditions and little disease on the farm. Under less ideal conditions, even the calves receiving double the NRC minimum would need higher levels simply to maintain their marginal tissue stores, the researchers noted. Indeed, in similar work by Poor (1994), calves that received 142 percent of the NRC requirement for vitamin A showed deficiency signs within two weeks after the study began.
The need for the vitamin A tissue stores that typical milk-replacer fortification levels promote was further suggested in a study by Hoppe et al. (1996). In this work, calves received a milk replacer balanced for all nutrients except vitamin A from four to 28 days of age. Vitamin A was included at 80 percent of the NRC requirement, and the animals then received various levels of beta-carotene to fulfill their vitamin A requirement.
Beta-carotene provides the greatest provitamin A activity for cattle, but it is only converted to vitamin A in response to need. The beta-carotene supplementation in this study provided daily intake of 1,200 to 19,000 IU of equivalent vitamin A. The calves continued to convert beta-carotene to vitamin A as beta-carotene intake increased through the highest level of supplementation (Figure 1). The results, like the levels, were similar to those in the study by Swanson et al. (1999).
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These results also suggested that any concern that current levels of vitamin A fortification levels in preruminant calves are too high is unfounded. In addition, Swanson et al. (1999) noted that in the few studies where researchers have reported an interaction between vitamins A and E, vitamin A was fed at much higher levels than found in milk replacers. For example, Hammell et al. (1997) fed 68,000 IU per head daily, whereas calves receiving a commercial milk replacer containing 18,000 IU per lb would actually consume about a third that amount, or 22,500 IU per day. Swanson et al. (1999) suggested that the current NRC requirement for vitamin A in preruminant calves should be evaluated and adjusted upward simply to avoid clinical deficiency signs. Current fortification levels in commercial milk replacers appear fully appropriate to help ensure that preruminant calves develop the vitamin A stores required for optimum health and performance.
*Allowance = total level from all sources (fortification plus feedstuffs)
References:
- Eaton, H.D., et al., 1970. Association of plasma or liver vitamin A concentrations with the occurrence of parotid duct metaplasia or of ocular papilledema in Holstein male calves. J. Dairy Sci. 53:1,775.
- Eaton, H.D., et al., 1972. Reevaluation of the minimum vitamin A requirement of Holstein male calves based on elevated cerebrospinal fluid pressure. J. Dairy Sci. 55:232.
- Hammell, D.C., et al., 1997. Effects of supplementation with vitamin A and vitamin E on health and weight gain of neonatal calves. J. Dairy Sci. 80(Suppl.):188.
- Herdt, T.H., and H.D. Stowe. 1991 Fat-soluble vitamin nutrition for dairy cattle. Vet. Clin. North Am. Food Anim. Pract. 7:391.
- Hoppe, P., et al., 1996, Dietary beta-carotene elevates plasma steady-state and tissue concentrations of beta-carotene and enhances vitamin A balance in preruminant calves. J. Nutr. 126:202.
- NRC. 1989. Nutritional Requirements of Dairy Cattle. 6th Rev. Ed. National Academy Press. Washington, D.C.
- Poor, C. 1994. Evaluation of the preruminant calf as a model for the study of beta-carotene metabolism in the human. Ph.D. Dissertation, Univ. Illinois, Urbana.
- Rousseau, J.E., et al., 1954. Relative value of carotene from alfalfa and vitamin A from a dry carrier fed at minimum levels to Holstein calves. J. Dairy Sci. 37:889.
- Swanson, K.S., et al., 1999. Influence of dietary vitamin A levels on serum and liver vitamin A concentrations in the preruminant calf. Proc. Experimental Biology (FASEB), Washington, D.C.
- Tomlinson, J.J., et al., 1976. Mammary transfer of vitamin A alcohol and ester in lactating dairy cows. J. Dairy Sci. 59:607.
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