Drought-Affected Forages may be Deficient in Vitamin A

NDSU Extension

Drought brings many nutritional and animal health-related challenges. Cow performance issues during the grazing season may result from reduced forage quality and quantity. It is possible that cows will enter the fall and winter in lower than desired body condition score and with other potential nutrient deficiencies that may not be noticeable. Vitamin A deficiency is one potential challenge that may arise when feeding drought-affected forages, according to North Dakota State University (NDSU) Extension specialists.

Vitamins are classified as water soluble (B vitamins, vitamin C) or fat soluble (vitamins A, D, E and K). Vitamins play critical roles in animal growth, immune function, nervous system function, reproduction, bone development and nutrient utilization. Under normal conditions, water soluble vitamins are synthesized in the body and do not need to be provided in the diet. However, with the exception of vitamin D, beef cattle are not able to synthesize fat soluble vitamins.

"Vitamin A (retinol) is the vitamin most likely to be deficient in beef cow diets this winter," says Janna Block, NDSU Extension livestock systems specialist at the Hettinger Research Extension Center. "Vitamin A is not present in plant material, but its precursors, also known as carotenoids such as beta-carotene, are present in high amounts in leafy green forage. Beta-carotene is converted to vitamin A mainly in the cow's small intestine."

A study published by researchers at the Ohio State University evaluated vitamin A equivalents of feedstuffs commonly fed to beef cattle in a five-state area. They reported that fresh pasture contained approximately five times more vitamin A equivalents than corn silage and 10 times more than hay. Corn, corn co-products and silage are other feedstuffs that typically contain moderate to good levels of carotenoids.

"During a grazing season with normal precipitation and adequate quantities of lush, green forage, daily intake of beta-carotene may be three to five times the animal's requirement," says Block. "When this occurs, excess vitamin A can be stored in the liver."

She adds that while cattle may store two to four months of vitamin A in the liver, storage is highly variable and cannot be assessed accurately without a liver biopsy sample.

"During drought, beta-carotene in growing forage is inadequate to meet requirements, limiting liver stores," says Block. "In addition, feed supplies after a drought year largely consist of harvested low quality forages which contain little to no beta-carotene. Even if forage was green at harvest, exposure to sunlight, heat and moisture reduce vitamin A activity during storage. Pregnant cows consuming low quality forages that have consumed minimal quantities of green grass this year are at high risk of vitamin A deficiency and should be supplemented."

Zac Carlson, NDSU Extension beef cattle specialist, cautions that high levels of nitrate in rations also can contribute to vitamin A deficiency through interference in the conversion of beta-carotene to vitamin A and reduced absorption in the animal.

Pregnant cows require approximately 1,300 International Units (IU) per pound of dry feed, and lactating cows and bulls require approximately 1,800 IU. For example, a dry cow weighing 1,300 pounds would require around 42,250 IU of vitamin A per day, assuming dry matter intake of 2.5% of body weight. A lactating cow weighing 1,300 pounds would require 59,000 IU of vitamin A per day.

"Vitamin A can be provided through dry or liquid supplements, mixed feeds or mineral supplements," says Carlson. "Vitamin A concentrates also are available and can be added to home-mixed supplements or rations. Commercial supplements should be evaluated to determine whether or not products supply adequate vitamin A."

Block provides the following example for calculating vitamin A provided in a mineral supplement. If a mineral supplement contains 200,000 IU per pound and target intake is 2 ounces of mineral per head per day, cows would be consuming approximately 25,000 IU of vitamin A per head per day (200,000 ÷ 16 oz/lb = 12,500 IU/oz; 12,500 IU/oz × 2 oz = 25,000 IU). This level of consumption would not meet the requirements for dry or lactating 1,300-pound cows, she notes.

Block also notes that vitamin A precursors are unstable and susceptible to oxidation when stored for long periods of time, particularly when included in an inorganic mineral mixture. To reduce vitamin A losses, she advises against storing mineral products for more than a few months.

"Injections of vitamin A also may be used," says Block. "However, it may be necessary to repeat injections every three to four weeks for deficient cows."

If dietary vitamin A is not adequate, a critical time for injection in pregnant cows is at least two months prior to calving to build up stores in the cow and ensure that adequate levels of vitamins are present in the colostrum. Calves have minimal vitamin reserves at birth and are highly dependent on an adequate supply of vitamins from the dam through colostrum and milk.

"Consequences of vitamin A deficiency include poor feed conversion and gain, reduced growth, low conception rates, abortion, stillbirths and weak calves," says Gerald Stokka, NDSU Extension veterinarian and livestock stewardship specialist. "Common symptoms of vitamin A deficiency include reduced feed intake, rough hair coat, night blindness, diarrhea and swelling of the legs and brisket."

Although required at relatively small amounts, vitamins are a critical component of beef cow diets, says Stokka. Forages and other low quality feeds that are deficient in vitamin A may also be deficient in other nutrients such as protein, energy and minerals. Forage analysis is recommended, and farmers and ranchers should consult with their veterinarians if they are interested in a liver biopsy to assess vitamin and mineral status of their cow herds.

Considerations for choosing the best supplement product include the production stage and weight of cattle, forage quality and expected intake of feeds. Contact your local Extension agent, nutritionist or veterinarian for more information about how vitamin A deficiency may affect your herd, and how you can prevent issues this winter.