by Mary Drewnoski & Marissa Eekhoff, University of Nebraska

Vitamin A is important for immune function, reproduction, vision and growth. In cow-calf production, the young calf is particularly vulnerable to vitamin A deficiency, which can impair immune function and increase susceptibility to scours and respiratory disease.

Fresh, green forage is an excellent source of vitamin A; however, dormant, weathered or long-stored forage may contain very little. That means even in a normal year, spring-calving cows may not consume enough vitamin A during late gestation before green grass is available. This year, drought may increase that concern for many herds because cows may have spent more time consuming brown forage and are entering winter with lower vitamin A stores.

Calves are born with very little vitamin A and depend on colostrum to build their stores. Vitamin A in colostrum comes from both the cow’s liver reserves (where she stores vitamin A) and what she is consuming during late gestation. However, one of our recent studies illustrates that liver stores alone are not enough.

The take-home message is that good vitamin A stores in the cow do not guarantee adequate vitamin A for her calf.

Cows entered the study after grazing green pasture and had very high liver vitamin A stores. They were then fed alfalfa hay and corn silage with relatively little supplemental vitamin A. At approximately one month after calving, the cows still had adequate liver vitamin A stores, but their calves did not. The take-home message is that good vitamin A stores in the cow do not guarantee adequate vitamin A for her calf.

Her vitamin A intake during late gestation matters, too. It has been shown that the late-gestation diet contributes around 60% of the vitamin A in the colostrum, with the liver only contributing 40%.

How much vitamin A is enough?

Current recommendations would suggest providing approximately 31,000 IU of supplemental vitamin A per day to a 1,200-pound (lb.) gestating cow. However, recent Nebraska research suggests that may not be enough when cows are consuming stored or dormant forages.

In our study, cows had been fed stored feeds for a year or more and were receiving approximately 31,000 IU of supplemental vitamin A per day before the study began. Despite that supplementation, their average liver vitamin A stores were below the range considered adequate. Cows that continued to receive 31,000 IU per day remained below the adequate range, while cows receiving 93,000 IU per day reached adequate liver vitamin A concentrations after 81 days. Calves from cows receiving 31,000 IU per day also had inadequate liver vitamin A concentrations at about 1 month of age, while the average concentrations of calves from cows receiving 93,000 IU per day were adequate.

These cows represented a relatively high-risk situation: They had been fed stored feeds for a year or more and began the study with low liver vitamin A stores. Thus, 93,000 IU per day should not necessarily be interpreted as the minimum needed by every cow. Cows entering winter with greater stores or consuming hay that retains some green color will likely receive more vitamin A from their diet.

For most spring-calving cows consuming dormant or stored forage, targeting at least 75,000 IU of supplemental vitamin A per cow daily during late gestation is a reasonable approach. That may sound like a lot, but it is still below the 93,000 IU per day that was needed to bring cows with low vitamin A stores into the adequate range in our research. In herds that have experienced prolonged drought, have been consuming low-quality or weathered forage for an extended period, or are otherwise likely to have low vitamin A stores, targeting closer to 90,000 IU per day may be warranted. This number aligns well with the total vitamin A intake recommendations suggested by other countries.