Cattle grazing green grass typically consume sufficient beta-carotene to meet their vitamin A requirements. As a result, vitamin A deficiency is rarely an issue during summer grazing. However, winter feeding programs can lead to vitamin A deficiency or insufficiency. Deficiencies are often first observed in weak or sick newborn calves, as most vitamin A is transferred through colostrum.
Vitamin A is stored in the liver and utilized when diets lack adequate vitamin A or its precursor, beta-carotene. Cattle can rely on these liver reserves until they are depleted. When cattle are fed diets devoid of vitamin A or its precursors, liver stores of retinol (vitamin A) can be depleted in approximately three months.
Vitamin A deficiency may result in several clinical signs, including reduced vision or night blindness, decreased conception rates, retained placentas, abortions or stillbirths, weak calves, reduced growth rates, and a dull hair coat.
Current vitamin A requirements are as follows: 2,200 IU/kg of dry matter for feedlot cattle; 2,800 IU/kg for pregnant beef cows and heifers; and 3,900 IU/kg for lactating cows and breeding bulls (Nutrient Requirements of Beef Cattle, 8th Revised Edition, 2016). These values equate to approximately 998, 1,270, and 1,769 IU per pound of dry matter, respectively.
For example, when feeding 4 ounces of a vitamin-mineral supplement to a lactating cow consuming 40 pounds of dry matter, providing 100% of the recommended vitamin A requirement would require approximately 283,040 IU of vitamin A per pound of supplement.
Most commercial mineral supplements contain higher concentrations of vitamin A to account for storage degradation and to help replenish animals that may already be deficient.
Recent research by Speer et al. (2024) evaluated vitamin A supplementation in confined beef cows and their calves. The study found that higher supplementation levels (93,000 IU per day for 165 days) were required to fully replenish liver retinol stores. Cows grazing green summer forage likely meet or exceed this intake naturally. The liver serves as a storage reservoir during periods when dietary vitamin A is limited.
Once liver stores are depleted, replenishment through dietary intake takes time. Speer et al. also reported that after 81 days of supplementation at three times the recommended vitamin A requirement, liver retinol concentrations returned to adequate levels. Injectable vitamin A is an option for rapid intervention; however, consistent dietary supplementation is still necessary to maintain adequate liver reserves.
Source : ndsu.edu