Hypovitaminosis A in Cattle

By: Dr. Matt Fruge Production Animal Consultation

During World War II, the Air Ministry of Great Britain had an astounding knack for identifying Nazi bombers as they made their approach at night. The Royal military attributed this to the fact that their soldiers were eating a massive amount of carrots which are rich in vitamin A.

Of course, this was a rumor created to hide the invention of the first radar systems. Although consuming an abundance of vitamin A won’t necessarily give someone enhanced night vision, the effects of too little vitamin A (hypovitaminosis A) in the diet can have devastating consequences for not only vision but the neurologic and immune systems as well. This is also true for cattle.

Hypovitaminosis A is generally known to cause blindness in cattle, but the clinical symptoms vary with the age of the individual. Calves consuming inadequate amounts of vitamin A tend to show signs of ill thrift, diarrhea and pneumonia. Vitamin A also plays an important role in the immune system, so cattle entering the feedlot will typically be more susceptible to respiratory disease. Typically, in adult animals, hypovitaminosis A presents as vision loss and progresses to neurologic deficits and death.

Vitamin A is found naturally in several forms, one of which is retinal. Retinal is an important component of rhodopsin which is a pigment used in the retina for sensing low light conditions. When the body does not have sufficient retinal it cannot generate this pigment. As retinal is depleted the retina begins to lose its ability to sense light. The early sign of this process is night blindness which may not be apparent in cattle that are not handled in low light conditions, but if noticed this condition is usually reversible. As retinal levels continue to wane, the condition worsens to total blindness. Hypovitaminosis A may also cause a narrowing of bony openings within the skull which may compress nerves that travel through them. If this occurs to the optic nerve the blindness that follows is not reversible. Vitamin A deficiency may also cause the thickening of the protective covering of the brain and spinal cord which, along with the narrowing of bony openings, can cause an increase in cerebrospinal fluid pressure (CSF). This increase in CSF pressure yields neurological damage that presents as seizures. Affected cattle may start convulsing when stimulated and death may occur during convulsions. Vitamin A also plays an important role in the immune system by helping to maintain protective mucous membranes of the digestive and respiratory tracts.

In pasture cattle, incidence of vitamin A deficiency typically occurs in winter or times of drought. Beta-carotenes are naturally occurring precursors to vitamin A which can be found in all green plants and corn. Cattle that are out on low quality forage such as corn stalks, drought condition pasture or weather damaged hay should be supplemented with vitamin A. Special care should be taken in cattle that are in late gestation or are lactating, as deficiencies in the cows may lead to deficiencies in the calves which tend to display more severe symptoms of deficiency.

The rate of hypovitaminosis A that occurs in the feedlot is variable and directly tied to the nutritional history of the cattle entering the yard. Cattle have a very large capacity of vitamin A storage in the liver depending on age. As animals grow and mature, the ability to store larger amounts of vitamin A increases. Light feeder steers may only have the capacity to store 40 to 80 days of required vitamin A, with heavier or yearling animals able to store amounts that would last 80 to 150 days before exhaustion. Thanks to this storage capacity, if the animal experienced any time of abundance of this vitamin, the requirements in the diet will be much lower. Due to the fact that vitamin A deficiency has such serious and irreversible consequences, this vitamin has historically been supplied in the diet in large quantities. There is some research suggesting that supplementing vitamin A at such high concentrations has negative effects on carcass quality grades, and other research indicates that supplementing vitamin A increases feed conversion. With this in mind, we as an industry will need to look more into the actual requirements of individual animals.

Supplementation of vitamin A can be achieved by injection or by the oral route. Oral supplements of vitamin A can be included in liquid protein supplement or as a concentrate to be added to a mixed ration. Corn and most sources of roughage are high in carotene content so corn based or high roughage diets should not require such high levels of vitamin A. Conversely, it is important to remember that cattle being fed a diet of milo or barley as a corn replacement especially when the diet is low on roughage will require additional vitamin A supplementation. Also, cattle that are maintained on a high vitamin A or carotene diet become less efficient at converting carotenes to vitamin A and maintain the liver storage of vitamin A less securely. These factors may lead to rapid depletion if there is a sudden change to a diet deficient in vitamin A or carotenes. Vitamin A can also be mixed with salt to be fed as a supplement to cattle out on pasture during times that lush grass is not available.

Oral supplementation is a great way of supplying maintenance vitamin A to the animal, but it is not an efficient way of building up liver stores as an animal must consume 3-5 times its maintenance requirement of vitamin A/carotene before it begins to store it in the liver. The other route of administration is by injection. Vitamin A injection is a more efficient means of building the reserves in the liver quickly. A single, massive dose of vitamin A may be injected at once without having to rely on the animal consuming the supplement.

With the current global shortage of vitamin A, we as an industry will have to continue to be mindful of our applica-

References

Sewell, H.B. “Vitamins for Beef Cattle.” The Cattle Site, University of Missouri Extension, 21 August 2005, www. thecattlesite.com/articles/904/vitamins-for-beef-cattle/.

Stalker, A. “Vitamin A: An Important Winter Nutrient.” UNL Beef, February 2015, beef.unl.edu/vitamin-A-important-winter-nutrient.

Dewell, G. “Vitamin A Deficiency in Beef Calves.” Iowa State University Veterinary Diagnostic and Production Animal Medicine, vetmed.iastate.edu/sites/default/files/vdpam/Extension/Vitamin-A-deficiency-in-Beef-Calves.pdf.

Pickworth, C.L., et al. “Restriction of vitamin A and D

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