Vitamin A has been called the vitamin most likely to be deficient in cattle diets. It has also been assumed vitamin A has a well-defined requirement, so supplementation is relatively common for cattle on diets that are deficient in vitamin A. But some of the things beef cattle nutritionists have long believed about vitamin A are being questioned.

“I would argue that the current recommendations should be changed,” stated Hannah Speer, now an associate professor at Montana State University’s Northern Ag Research Center at Havre, Mont. Speaking at the December 2023 Range Beef Cow Symposium in Loveland, Colo., Speer talked about research conducted during her graduate studies at the University of Nebraska, the evidence suggesting higher levels of vitamin A supplementation may be particularly important for cows in late gestation. But, it’s not so much for the benefit of the cows as for their calves.

Eat your carrots

Remember how Mom coaxed you to eat your carrots so you could see better? There is some truth to that. Carrots contain beta-carotene that your body converts to vitamin A, which is important to eye health and vision. Vitamin A is essential to a cowbrute for the same reason and others.

According to Speer, vitamin A is important to proper immune function and the health of epithelial cells that line the gastrointestinal and respiratory tracts. The vitamin A status of a cow during gestation also is important to reproduction, affecting embryonic and fetal development and placental growth.

“Deficiencies could result in fetal resorption, abortion or birth defects. It could result in complete failure to breed,” said Speer, noting that effects on young calf survival and subsequent health are probably even more significant.

“Calves are at risk of vitamin A deficiency because there is limited placental transfer, so calves need to consume colostrum that is high in vitamin A,” warned Speer, explaining that the amount of vitamin A needed by the cow for maintenance and reproduction is much lower than the amount required to also supply her calf’s need through colostrum.

The greener, the better

According to Speer, green forage is the best dietary source of beta-carotene, containing up to 14 times more of that vitamin A precursor than mature and harvested forages. Beta-carotene content is directly related to the green color of forages. Forages that have lost their “green,” like dormant range and pasture, crop residues and sun-bleached hay, are poorer sources of vitamin A.

“Green alfalfa hay is pretty good, but it provides half as much vitamin A as green, growing pasture. Silage can be a fair source, but it’s highly variable,” said Speer, noting that beta-carotene content of harvested forages generally declines further during storage.

Consequently, in a majority of cow-calf systems, winter is the period of time that cow diets are low in beta-carotene. It helps that cattle grazing green forage in the spring and summer will build up liver stores of vitamin A to draw on during winter. It’s sort of a limited emergency supply. However, given the variability of feedstuffs used in winter cow diets and the variability of beta-carotene availability from various feedstuffs, there often is need for supplementation.