Extension

Research has shown that the young calf is the most at risk for vitamin A deficiency in cow-calf systems. Vitamin A plays several important roles in the body, one of which is immune function. Marginal deficiencies can impact calf health and potentially cow productivity. Calves are born with very low vitamin A stores, and their primary source of vitamin A is colostrum. Vitamin A concentrations in colostrum have been reported to be six to fourteen times greater than that of milk, so colostrum is critical for establishing vitamin A stores in the young calf. Thus, calves not getting enough vitamin A from colostrum are at increased risk for diarrhea and respiratory disease in their first one to two weeks of life.

 

Fresh green forage contains high amounts of beta carotene, a vitamin A precursor. It is used by the cow to synthesize the vitamin A needed to support a variety of biological functions. Excess vitamin A can be stored in the liver and used during times when dietary vitamin A intake is low. In contrast, stored forages contain significantly less beta carotene compared with fresh forages. Green hay contains 14 times less beta carotene than fresh green forage, whereas ensiled feeds such as corn silage have roughly twice the amount found in green hay.¹ Grains and grain byproducts are also poor sources of dietary vitamin A because of their low carotene concentrations.1 Thus, cows fed diets consisting primarily of stored forages and concentrates may be at risk for vitamin A deficiency. Low amounts of vitamin A in the cow's diet during late gestation may lead to a deficiency in the calf impacting its health. University of Nebraska researchers conducted a study to determine the effects of three different supplemental vitamin A amounts on liver retinol concentrations (a measure of vitamin A status) of gestating beef cows and their calves in a production system where cows were fed stored forages and concentrates year-round.²

 

This experiment used 54 multiparous Angus cross-bred beef cows (Body Condition Score, BCS = 6.0) with an initial body weight of 1275 lb in mid-gestation (average of 125 days from calving) managed in a confinement system for a year or more. The cows were stratified by BCS and time spent in the confinement system and assigned to 1 of 9 pens. The pens were then randomly assigned to receive 1 of 3 amounts of supplemental vitamin A: the current recommendation for gestating beef cows (31,000 IU/day; 1X)³, 3 times (93,000 IU/day; 3X), or 5 times the current recommendation (155,000 IU/day; 5X). The 1X treatment assumed a cow weight of 1,200 lb that consumed 2.0% of body weight in dry matter (DM) per day.

 

Prior to treatment initiation, all cows were receiving 31,000 IU/day of supplemental vitamin A. Basal dietary intakes of vitamin A were on average 21,000 IU/day. Vitamin A supplementation was provided from late gestation through early lactation as retinyl acetate (International Nutrition; Omaha, NE) and mixed in the diet daily via a micronutrient machine. The cows were limit fed once daily to meet nutrient requirements during gestation and lactation a diet consisting of wheat straw, corn silage, and wet distillers grain plus solubles. The proportion of corn silage and wet distillers grain plus solubles in the diet was increased during lactation to increase energy density of the diet during lactation. Liver biopsies were collected for retinol analysis on cows 24 days before treatment initiation, day 40 and day 81 of supplementation, and both cows and calves were sampled 32 days post-calving (day 165 of supplementation)

 

These researchers observed no differences (P = 0.86) in initial cow liver retinol concentration [mean 186 μg/g of dry matter (DM); Figure 1] treatments. All cows were receiving the 31,000 IU/day supplemental vitamin A level for a year or more before the study began. Thus, the data suggest the current supplemental vitamin A recommendation of 31,000 IU/day was not enough to result in adequate liver retinol concentrations (300 - 700 μg/g of DM)⁴.

 

 

Figure 1. Effect of amount of supplemental vitamin A [IX= 31,000 IU/day (current recommendation), 3X = 93,000 IU/day, and 5X = 155,000 IU/day] on cow liver retinal concentrations. Dashed line indicates the liver retinal concentration considered adequate for cows (300 μg/g DM). Different letters (a-c) indicate significant differences (P ≤ 0.05) among treatments within time point: a = 1X versus 3X, b = 3X versus 5X, c = 1X versus 5X. Adapted from Speer et al., 2024.

 

In this study, liver stores did not reach adequate concentrations on the 1X treatment, suggesting that 31,000 IU/d may not be an appropriate recommendation for cows being managed primarily on nongreen stored forage and concentrates for a long period of time. Liver retinol concentrations of 1X remained below adequate reference ranges throughout the study (≤189 μg/g of DM), whereas 3X and 5X were elevated into the adequate range by day 81 (334 and 412 μg/g of DM, respectively).

 

A newborn calf is at greater risk of vitamin A deficiency than the cow, especially if the cow experiences low vitamin A during pregnancy. Calf liver retinol concentration also differed among treatments (P = 0.01; Figure 2) as calves of cows on 1X had lesser (P < 0.05) liver concentrations than 3X and 5X calves, which did not differ (P = 0.12). Liver retinol concentrations considered adequate for calves at 32 days of age (100 - 350 μg/g of DM)4 were not observed in 1X calves (51 μg/g of DM) but were observed in calves from 3X and 5X cows (119 and 165 μg/g of DM, respectively). Despite cows on the 3X and 5X treatment reaching adequate liver retinol status by day 81, only 60% of the 3X calves and 80% of the 5X calves reached liver retinol concentrations greater than 100 μg/g of DM.

 

 

Figure 2. Effects of cow supplemental vitamin A level (1X = 31,000 IU/day; 3X = 93,000 IU/day; 5X = 155,000 IU/day ) on calf liver retinol concentration at 32 days of age. The dashed line indicates the liver retinol concentration considered adequate for calves at 32 days of age (100 μg/g of DM; Puls, 1994). The main effect of treatment was significant (P = 0.01). Bars lacking a common letter (a, b) differ (P ≤ 0.05). Adapted from Speer et al., 2024.

 

These authors concluded that these results demonstrate that “providing the amount of supplemental vitamin A recommended by NASEM to cows in a long-term drylot did not result in cow or calf liver retinol concentrations within the adequate reference ranges. Supplementing cows with 93,000 IU/day of vitamin A for 165 days brought liver retinol concentrations of cows and their calves up within adequate reference ranges. Cows being fed diets consisting mainly of brown forages and concentrates long term may need more supplemental vitamin A than currently recommended to ensure calves receive enough vitamin A from colostrum.”

 

¹ Pickworth, C. L., S. C. Loerch, R. E. Kopec, S. J. Schwartz, and F. L. Fluharty. 2012. Concentration of provitamin A carotenoids in common beef cattle feedstuffs. J. Anim. Sci. 90:1553–1561.

 

² Speer, H. F., K. H. Wilke, and M. E. Drewnoski. 2024. Effects of vitamin A supplementation on liver retinol concentrations of beef cows and their calves managed in confinement. Appl. Anim. Sci. 40:619–626. Available at: https://doi.org/10.15232/aas.2024-02564.

 

³ NASEM (National Academies of Sciences, Engineering, and Medicine). 1976. Nutrient Requirements of Beef Cattle. Natl. Acad. Sci., Washington, DC.

 

⁴ Puls, R. 1994. Vitamin Levels in Animal Health: Diagnostic Data and Bibliographies. Sherpa International, Clearbrook, British Columbia.