The nutritional value of forage changes throughout the growing season and changes in crude protein and digestibility are often discussed. However, other nutrients also change in forage throughout the growing season such as vitamin A. Vitamin A is a dietary essential nutrient, meaning that the animal cannot synthesize it, important for vision, immune function, and reproduction. The estimated vitamin A requirement for gestating cows, lactating cows, and growing calves is 1,270, 1,770, and 1,000 IU/lb dry feed or 27, 38, and 21 IU/lb body weight. Thus, a 1300-lb spring calving cow would need to consume 50,000 IU from March to October and 35,000 from October to March.
The vitamin A content of forages is not equal across species or locations. Figure 1 illustrates the vitamin A content of forages in Ohio and North Carolina. Fescue pasture easily meets the nutritional requirements of gestating and lactating cows, but alfalfa, fescue, and orchardgrass hay may be marginal to deficient. Additionally, growing native prairie grasses exceed the Vitamin A requirement of beef cows, but dormant forages are deficient (Figure 2). Thus, vitamin A supplementation is probably not necessary during the grazing season to meet the current nutritional requirements, but dry conditions can significantly decrease vitamin A in grazed forage.
Beef cows can store vitamin A in their liver for 4 to 6 months and so may be able to go through the fall with minimal supplementation. We have little information on the amount of vitamin A necessary to build up liver stores and, thus, summer mineral supplements often contain enough vitamin A to meet the current requirements. There is little chance of toxicity problems with over feeding vitamin A.
Vitamin A needs to be supplemented from late fall through early spring until cows are grazing green pastures. A vitamin and mineral supplement with a target intake of 4 oz/head/day should contain 150,000 to 200,000 IU/lb of vitamin A.
Changing vitamin A content of forages throughout the year, the species of forage, and the storage method and time is important to consider when evaluating vitamin and mineral supplements. Work with your veterinarian or nutritionist to make sure the supplementation program is adequate, but not overly costly.