Vitamin A is rarely a concern in range cattle nutritional programs because it is readily synthesized from carotene that is common in green growing plants. However, in drought situations where plants become dead or dormant, the carotene content becomes practically devoid and may lead to a deficiency of the precursor to vitamin A. Carotene is very low in mature, weathered forages, grains and many crop residues. Carotene will be lost in stored hay crops over extended periods of time. Therefore if hay that was stored throughout all of last fall and winter is to be fed in the upcoming winter, the vitamin A content will be considerably less than when that forage was originally harvested. In addition some scientists have suggested that high nitrate forages common in drought years can exaggerate vitamin A deficiencies. Deficiencies of Vitamin A usually show up first as weak, blind or stillborn calves. Other signs are scours, respiratory problems, poor gains and poor reproduction.
Fortunately, the liver of cattle is capable of storing vitamin A for long periods and frequent supplementation is not necessary. A singular injection of one million International Units (IU) of vitamin A provides sufficient vitamin for 2 to 4 months in growing and breeding cattle. A word of caution: Vitamin A and A,D, and E injections have been found to on very rare occasions cause a severe reaction to the vaccine. Please consult your veterinarian about the use of these products.
Because the daily requirements of beef cows range from 30,000 to 50,000 IU, depending on size, stage of production, and level of milk production, supplements can be fortified with vitamin A to supply the minimum daily requirement. Depending on the quantity of range supplement being provided, vitamin A can be added to supplements at the rate of 5000 to 10,000 IU per pound of feed. Read more about vitamin and mineral needs for grazing cattle in the Oklahoma Cooperative Extension circular E-861: "Vitamin and Mineral Nutrition of Grazing Cattle".
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist