Vitamin A: What You Need to Know
Not just for eye health, vitamin A helps with immunity, kidney and bone function.
Successfully incorporating mineral and vitamin supplementation into your nutrition program stems from understanding why cattle need those nutrients in the first place. Producers who have a better understanding of key nutrient functions, more specifically the biology behind bodily functions, will be better able to decide on appropriate supplements and timing, as well as the efficacy of the product that the nutrition consultant is pushing this year.
What is vitamin A?
According to Homer Sewell, University of Missouri (MU) Extension, vitamin A is considered a required nutrient for beef cattle because, unlike vitamins B and K, microbes in the rumen are unable to synthesize it. Animals are the only carriers of vitamin A. Plants, the natural source of vitamin A for cattle, contain carotenes, which the animal then converts into vitamin A through the wall of the small intestine.
Vitamin A is a fat-soluble vitamin and is stored in the liver.
Daniel Rivera, Mississippi State University (MSU) assistant research/Extension professor, says, “Carotenes are pigments synthesized by plants for use in the photosynthetic process. They are found in green and yellow plants (forages) and are readily destroyed by sunlight (cured hay). They are also found in minor amounts in yellow corn, but are in low amounts in other grains.”
Functions
Many immune system cells require vitamin A to function well, says Rivera. It plays a role in the epithelial (lining) tissue of the respiratory, digestive and reproductive tracts. Vitamin A assumes the role of a nonprotein-helper molecule or cofactor in kidney function and bone development, he adds.
Sewell explains that the eye uses vitamin A to adapt from light to dark.
How much is needed?
Because cattle convert carotenes to vitamin A and then store it in the liver, an animal’s needs are primarily dictated by its available liver stores. Requirements change based on the circumstances the cattle are experiencing. Rivera says, “Stresses such as heat stress, immune challenge, inflammation and parasites might interfere with the animal’s ability to convert carotene to vitamin A, thereby increasing the requirement.”
Like many minerals, vitamin A absorption can be inhibited by other minerals and chemical compounds. According to Rivera, nitrate can reduce absorption, and low phosphorus is thought to play a role in decreased absorption, as well.
Should it always be available?
Interestingly enough, “Once [vitamin A] stores have become depleted, the animal has difficulty in converting carotene to vitamin A,” says Rivera, who advises that cattle should be supplemented with vitamin A year-round. This will allow liver stores to build, and a reserve will be available for times of stress or when the forage quality decreases during drought or dormancy. He says that most commercial mineral supplements include vitamin A in the mix, and the actual supplementation cost of the vitamin is minimal.
Baby calves are born with limited stores of vitamin A, but Rivera says as long as the calf receives colostrum, an injection dose is not needed.
Vitamin A deficiency
A deficiency occurs when cattle have mobilized all reserves of one or many nutrients. Vitamin A deficiency, in particular, causes night blindness. Sewell explains that the easiest way to check for night blindness is to place an object in the pathway of an animal at twilight and notice if it stumbles over it.
Other conditions generally linked to vitamin A deficiency are loss of appetite, rough hair coat, greater susceptibility to infection and disease, diarrhea, decreased performance and reproductive problems, says Rivera. He cautions that these are similar to many other nutrient deficiencies, making it difficult to assess if the problem is associated entirely with vitamin A.
Remember, carotenes — the sources of vitamin A — are found in green and yellow growing plants. “Cattle that have been on drought-stressed pastures or high-grain diets for an extended period of time are more susceptible to vitamin A deficiency via a carotene deficiency,” says Rivera.
He says cows can take up to 180 days to deplete their liver stores, and young cattle can deplete theirs in half the time. On the flip side, it can take nearly 40 days to correct a deficiency problem using supplementation only. That’s a long time to suffer the effects of a deficiency, so the fastest, safest way to address the problem is through an injection.
A producer Rivera once worked with quit feeding a complete mineral mix that included vitamins A, D and E for about two years. He noted a decreased calving rate and weaned calves having a greater susceptibility to bovine respiratory disease (BRD). However, once he got back on the complete mineral mix, the problem was resolved.
“Since many minerals and vitamins are cofactors in many biological processes it was difficult to ascertain if there was one specific deficiency,” Rivera says.
