By Applied Animal Science
A new research article in Applied Animal Science details how beef cows and calves maintained in confinement and fed brown stored forages combined with grain or grain byproducts may not be getting sufficient amounts of vitamin A.
Champaign, IL, September 30, 2024–Unlike cows maintained on pasture or fresh green forages, cows in confinement systems cannot get enough vitamin A from their diets without supplementation. The National Academies of Sciences, Engineering, and Medicine (NASEM) recommends 31,000 IU/day of vitamin A, but a new study published in Applied Animal Science finds that beef cows being managed on nongreen stored forages and concentrates for long periods of time have inadequate amounts of retinol–an indicator of vitamin A status–in their livers.
David K. Beede, PhD, editor in chief of the journal, noted, “This research demonstrates the effects of supplementation of vitamin A to beef cows from mid-gestation through early lactation and subsequent liver retinol concentrations of their calves. The study highlights that current recommendations are likely not adequate compared with typical reference ranges. Greater supplementation of vitamin A is needed when diets consist mainly of concentrates and brown forages fed in a confinement system.”
As lead author Mary Drewnoski, PhD (Department of Animal Science, University of Nebraska, Lincoln, NE, USA) explained, “Liver retinol concentration was selected as the indicator of cow and calf vitamin A status because it is a more sensitive measure of status compared with blood plasma retinol concentrations. Approximately 90% of total-body vitamin A is stored in the liver and will increase as dietary vitamin A intake increases–but not in a linear fashion.”
Drewnoski continued, “The objective of this study was to determine the effects of 3 different supplemental vitamin A amounts on liver retinol concentrations of gestating beef cows and their calves in a production system where cows were fed stored forages and concentrates year-round. Our hypothesis was that current NASEM recommendations for supplemental vitamin A would not be sufficient to meet the vitamin A needs of confined gestating cows and their calves as indicated by liver retinol concentrations.”
Accordingly, cows were divided into three groups; these groups received either the NASEM-recommended vitamin A supplementation of 31,000 IU/day (1X) or three (3X) or five (5X) times that amount. The cows’ mean initial liver retinol concentration was 186 μg/g of dry matter (DM), and that concentration was maintained between 172 and 192 μg/g of DM throughout the study, well below the target liver retinol concentration of 300 to 700 μg/g of DM, despite receiving the recommended supplementation in addition to the 14,433 IU/day of vitamin A that was provided by their diets.
A newborn calf is at greater risk of vitamin A deficiency than the cow, especially if the cow experiences low vitamin A during pregnancy. According to Drewnoski, “Liver retinol concentrations of 100 to 350 μg/g of DM are considered adequate for calves at 32 days of age,” but the researchers observed values of only 51 μg/g of DM in calves of dams fed the NASEM-recommended supplemental amount. However, calves from cows fed the 3X and 5X supplemental amounts of vitamin A had retinol concentrations of 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,” added Drewnoski.
Drewnoski concluded by noting, “Overall, our results demonstrate that for cows fed primarily brown forages and concentrates long term, supplementing gestating cows with the current NASEM recommendation for vitamin A will not result in their calves’ liver vitamin A concentrations being within the adequate reference range. It is important to note that both cow liver concentrations and dietary vitamin A intake will affect calf vitamin A status, because 60% and 40% of vitamin A contributions to colostrum come from diet and cow liver stores, respectively. Thus, while feeding 93,000 IU/day resulted in a continued increase in cow liver stores, feeding less than 93,000 IU/day to cows who are of adequate status may not result in sufficient vitamin A concentrations in colostrum for the calf, given that dietary vitamin A contributes slightly more to colostrum than cow liver stores.”
The article appears in the October issue of Applied Animal Science.