In the past we have reviewed the necessity of various nutrients in cattle diets. In this issue and the one to come we'll take a more in-depth look at the need for vitamins in the nutritional programs for cattle. It seems that in our day-to-day living we are constantly bombarded with the need for one type of nutritional supplement or another in our diets. Much of this marketing focuses on the various vitamin supplements that are available.

Like people, cattle require vitamins in their diets. It is very common to find vitamins added to mineral supplements as well as feeds of various types. Generally we find fairly high levels of vitamins added simply because they are fairly inexpensive and can therefore be used as �tag-dressing.� But the question becomes: �which vitamins do cattle really need and at what levels.� Hopefully the following will shed some light on these questions.

Vitamin needs of beef cattle can be confined largely to A, D and E. This is because bacteria in the rumen of cattle are considered to have the ability to synthesize vitamin K and the B vitamins in sufficient quantities to meet the animal's requirement. This varies depending on the type of diet the cattle are on, whether it is forage based or grain based. Cattle on grain based rations often have a higher requirements for K and B vitamins.

Vitamin A

Vitamin A needs special attention in beef cattle rations. This vitamin is found only in animals. Plants, however, are the natural source of vitamin A activity for animals. Green and yellow plants contain carotene, a pigment which animals convert to vitamin A. The wall of the small intestine is the principal site for conversion of carotene to vitamin A.

In recent years, many cases of vitamin A deficiency have been reported for beef cattle receiving rations considered to be adequate or high in vitamin A. This increased incidence of vitamin A deficiency in beef cattle has been attributed to:

*Greater use of milo and barley to replace yellow corn and the emphasis on high-grain rations with less roughage, all of which lower the carotene level of rations.

*Large destruction of carotene in the components of cattle rations because of longer storage and heat treatments for drying and processing feeds.

*Use of younger calves by feedlots. Young animals have lower body stores of vitamin A and are fed longer than older animals.

*Stress of feeder cattle from hauling, handling, disease and parasites.

*Larger daily gains as a result of nutritional and genetic improvement. Vitamin A requirements are greater per unit of body weight.

*Higher nitrate content of forages, especially when grown under adverse weather conditions, with high nitrogen fertilizer applications.

*Higher nitrite and nitrate levels in livestock water.

Function of Vitamin A in the Animal's Body

Although some of the metabolic functions of vitamin A are not yet known, a main role is the maintenance of epithelial tissues such as those found in skin and lining of respiratory, digestive and reproductive tracts. It also functions in visual purple, a compound in the eye needed for sight when an animal adapts from light to dark. Vitamin A is also essential for proper kidney function and normal development of bones, teeth and nerve tissue.

Signs of Vitamin A Deficiency

One of the first easily detected signs of vitamin A deficiency in cattle is night blindness. An easy way to check for this condition is to place an obstacle in the pathway of cattle and notice if they stumble over it at twilight. Other early signs are loss of appetite, rough hair coat, dull eyes, slowed gains and reduced feed efficiency. Diarrhea and pneumonia may be the first indicators, especially in young animals. Later developments include excessive watering of the eyes, staggering gait, lameness or stiffness in knee and hock joints, and swelling of the legs and brisket (and sometimes in the abdominal region). Feedlot cattle with advanced vitamin A deficiency often pant excessively at high temperatures and go into convulsions when excited.

Signs of vitamin A deficiency in breeding herds include lowered fertility and calving percentage. Cows abort, deliver dead or weak calves, and are difficult to settle.

Liver Storage of Vitamin A

Cattle have from 70 to 90 percent of their total vitamin A stores in the liver. The remainder is deposited in fat and other organs. Carotene that escapes conversion to vitamin A is also stored predominantly in the liver. It is, however, distributed more evenly in the body than vitamin A, since it is more prevalent in fat. Yellow fat and yellow milk are due to the inability to convert all the carotene in the diet to vitamin A.

Body deposits of vitamin A are low at birth and young animals have smaller reserves than older animals that have consumed diets high in vitamin A activity. Young animals fed vitamin A-deficient rations usually show deficiency symptoms sooner than older animals

Factors that Affect Vitamin A Requirements

1)      Breed. Some dairy breeds are more efficient than others at converting carotene to vitamin A. The yellow milk and carcass fat of Guernseys and Jerseys indicate their poor utilization of carotene for vitamin A. Some studies indicated Holsteins were twice as efficient as Guernseys in changing carotene to vitamin A. There does not appear to be any difference among beef breeds in ability to convert carotene to vitamin A.

2)      Carotene. Beta-carotene makes up a larger percent of the total carotene in some plants than in others. Other carotenes yield less vitamin A activity than beta-carotene.

3)      Depletion. Cattle depleted of vitamin A are less efficient in converting carotene to vitamin A. This supports the practice of administering pre-formed vitamin A in the diet or by injection instead of depending upon carotene in the feed to replenish cattle severely deficient of vitamin A.

4)      High-carotene feeds. Cattle maintained on high-carotene diets convert carotene less efficiently to vitamin A. This condition could accelerate the depletion of liver stores of vitamin A when cattle are abruptly changed to diets with less carotene.

5)      Thyroid depression. Hot weather or components in the diet may cause thyroid depression, which is thought to decrease conversion of carotene to vitamin A.

6)      Stresses. Hot weather, disease, parasites and other stresses are believed to interfere with the animal's ability to convert carotene to vitamin A and to depress the efficiency with which vitamin A can be used to meet needs. Also, these and other factors may increase the animal's requirements for vitamin A. Inflammation and damage of the intestinal wall by diarrhea or parasites undoubtedly interfere with the absorption of carotene and vitamin A and the conversion of carotene to vitamin A.

7)      Silages, haylage, pasture. Cattle consuming rations high in corn silage, sorghum or oat silage and grass-legume haylage have been found to deplete normal stores of vitamin A in the liver, even though these feeds contained medium to high levels of what was thought to be beta-carotene. Cattle full-fed grain on pasture have benefited from vitamin A supplementation in some trials.

8)      Vitamin E. This vitamin appears to increase the efficiency of vitamin A and carotene utilization by reducing their oxidation before and after absorption from the digestive tract.

9)      Nitrate. High levels of nitrate or nitrite nitrogen in the ration or water have been found to cause a vitamin A deficiency syndrome in cattle and hogs in some cases. These compounds may have this effect by causing a greater destruction of vitamin A and carotene in the digestive tract, decreasing their absorption and interfering with the conversion of carotene to vitamin A, depression of the thyroid gland and increasing the requirement for vitamin A. Cattle used carotene less effectively when grazing orchard grass fertilized with nitrogen than when grazing unfertilized pastures in an Illinois trial.

10)      Phosphorus. Low levels of phosphorus in the diets of range cattle appeared to lower conversion of carotene to vitamin A. Plasma-carotene levels tended to rise when phosphorus intake was inadequate.

11)      Losses. Carotene and vitamin A are easily oxidized and destroyed in feeds by weather damage, exposure to air in lengthy storage, heat, and contact with minerals. Stemmy hay is low in carotene because most of the carotene is in the leaves of the plant.

Requirements for Vitamin A

The amount of vitamin A to use in the cattle ration depends upon the level of carotene in the feed, liver stores of vitamin A, and length of feeding period, among other factors.

1)      Newly Received Feeder Cattle. It is advisable to supply incoming feeders or other cattle under extreme stress conditions with 500,000 to 1 million I.U. of vitamin A. This amount may be given by injection in the muscle or rumen, or by putting 50,000 I.U. of vitamin A per head daily in the feed or drinking water for two to three weeks. Toxicity from administration of high levels of vitamin A has not been a problem in beef cattle. However, repeated dosing of cattle at short intervals with high injections of vitamin A should be avoided.

2)      Feedlot Cattle. The vitamin A requirements of feedlot cattle can be met by feeding around 6 milligrams of carotene or 2,500 I.U. of vitamin A for each 100 pounds of body weight; or by supplying 1,000 to 1,500 I.U. of vitamin A per pound of feed. See Table 1 for more detailed guidelines to use in feeding vitamin A to cattle.

Vitamin A can be purchased in dry or liquid protein supplements or in mixed feeds. Vitamin A concentrates can be bought for addition to farm-mixed supplements or rations. Mix vitamin A with salt. Adding 500,000 I.U. of stabilized vitamin A per pound of salt should be adequate. The mixture must be kept dry to prevent destruction of the vitamin A. Mixing one-fourth pound of calcium stearate, a drying agent, in each 100 pounds of salt-vitamin A mixture will help maintain the vitamin A potency of the mixture. A fresh mixture should be supplied every 10 to 14 days. In a Missouri trial, a salt-vitamin A mixture containing 0.25 percent calcium stearate retained 73 percent of its vitamin A value after one week, and 52 percent after two weeks.

Intramuscular Injections of Vitamin A

Another method of getting vitamin A into cattle has been the use of injectable forms. Vitamin A injected in the muscle is used more efficiently to increase liver stores than that given in the feed. This method is often used to supply vitamin A to new feeder cattle. However, the length of time that injections provide protection is not well established and is normally not considered to be a long term solution.

Conclusions

As we see from this discussion, Vitamin A plays a very important role in the health and performance of beef cattle. It needs to be balanced properly to insure adequate but not excessive levels are available to the animal. In the next issue, we'll continue this examination of vitamins in cattle nutrition.