Invited Review: Vitamin A in beef cow-calf systems—Biological relevance, revisiting of requirements, and unresolved questions

Our purpose was to synthesize literature rel-evant to vitamin A use in beef cow-calf systems, with em-phasis on how current beef cow vitamin A requirements were derived, and the limitations of the underlying evi-dence, and key knowledge gaps that constrain confident supplementation decisions. Information was sourced from peer-reviewed papers and our own unpublished data. Vitamin A is a crucial micronutrient, af-fecting health, reproduction, and growth. Neonatal beef calves are the most vulnerable to vitamin A deficiency in cow-calf systems, are born with limited retinol stores, and are dependent on colostral retinol. There is limited under-standing of how cow retinol liver stores and dietary vita-min A consumption influence calf status. Calves deficient in vitamin A are more likely to suffer respiratory disease or diarrhea, leading to reduced productivity. Current sup-plemental recommendations for vitamin A as provided by NASEM (2016) are limited by the small number of stud-ies, which date to the mid 1900s. These recommendations do not account for variation in the basal diet β-carotene concentration, which can vary drastically. Colostral retinol is crucial to prevent vitamin A deficiency in newborn calves; however, limited research has explored the vitamin A needs of the gestating cow to support a healthy, produc-tive calf. Many factors need to be considered when evalu-ating the vitamin A requirements of beef cows, including stage of gestation, liver vitamin A stores, and harvest and storage conditions of forages. Further research is needed to identify supplemental vitamin A concentrations that best support the gestating cow providing for the calf, to identify proper supplemental vitamin A needs when cows are consuming varying basal diets, and to understand the interactions between vitamin A and other vital nutrients.