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Cow Nutrient Requirements

David P. Price is a consulting nutritionist specializing in feedlot and range cattle

Whenever drought is an issue, it is good to review cow nutrient requirements. While this may seem remedial, the consequences of an oversight can be devastating. As taught in Ruminant Nutrition 101, there are five classes of nutrients to be concerned with: energy, protein, vitamins, macro-minerals and trace or micro-minerals.

Of the vitamins, only vitamin A is of any real practical importance. Bacteria in a functioning rumen can synthesize all the B vitamins as well as vitamin C. In order to synthesize B12, the microorganisms must have a supply of cobalt, but the amount is so minute, most any mineral from a reputable manufacturer will be adequate. If due to prolonged drought no green forage has been available, theoretically vitamin E might be a concern. But other than the relationship with selenium (in Se-deficient areas), I have yet to see real-world clinical problems. Vitamin A is the concern. In normal years (when cows have had green grazing) approximately 90 to 120 days supply can be stored in the liver. Not so during dry years. To be safe, a dry cow should receive 25,000-30,000 IUs of vit. A daily. Old timers often injected vitamin A, but since vitamin A can be toxic in high levels, I much prefer to supply vitamin A in whatever supplement might be provided. A marginal deficiency of vitamin A will result in cows that fail to clean (retained placenta) at calving. More severe deficiencies can result in fetal resorption, as well as severe health issues in the cows.

Macrominerals include calcium (Ca) phosphorous (P), potassium (K), magnesium (Mg) and sodium (Na). Most minerals and supplements contain an excess of salt, so Na is seldom a problem. Most range forage is adequate to high in K, so K seldom needs to be supplemented. Likewise Ca is adequate in most range forage, is often contained in ground water, and most supplements contain excess levels; so Ca is seldom deficient. While it is not a bad idea to supplement Mg, it is normally only an issue in lactating cows, and even then clinical problems normally only occur on irrigated pasture. Only P is a common real-world problem and often requires supplementation. (Having said that, I must point out that for several decades the National Research Council recommendations for P were excessive.

Microminerals include copper, zinc, manganese, cobalt, iodine and in specific areas selenium. Iron is often included in range minerals, but in the vast majority of instances is not required. The inter-relationships between trace minerals is extremely complex and would require entire columns to describe. It is always good to supplement microminerals, but with the exception of copper, on most native pasture, seldom do true clinical deficiencies occur. But once again I must add the caveat that if summer grazing was on dry, dormant grass — the importance of trace minerals is significantly increased (since dormant grass will contain lower levels).

Of all the nutrients, protein is the most critical. Critical in that daily supplementation is required. Unlike all the other nutrients, protein cannot be stored in the body to be mobilized during periods of deficiency; as can vitamin A, minerals and energy (as fat). Having said that, I must emphasize that statement is true only from a production point of view. It is not true physiologically. From a physiological point of view, there is a protein store. It is called the fetus. If the cow receives inadequate protein, the fetus will either not develop fully, or in severe deficiencies will be rebsorbed. Most commonly, protein deficiencies manifest themselves in what is known as the “weak calf syndrome.” Cows inadequately supplemented with protein will have smaller, weaker calves. In the case of even mild dystocia, these calves will be much less likely to survive and otherwise will have lowered immunities. They are therefore much more susceptible to scours, respiratory infections or death from physical factors such as rough terrain or hypothermia.

Cows in good condition can withstand moderate energy deprivation (up to a point) by mobilizing fat stores. However, once the fat reserves are gone, the fetus will be affected.

If energy is the only nutrient lacking (in mild deficiency) the cow can lose weight and give birth to a normal calf — if adequately nourished just before calving. Likewise she must receive compensatory energy after calving, or she will not cycle and breed adequately. We can supply this energy as a supplement, but it is far more efficient and economical to target calving dates when the growing season begins and the grass is green. That is, green grass contains three to five times as much energy as vernalized (winter grass).

Bottom line: If a ranch has winter grazing, the main focus of supplementation is protein, vitamin A, phosphorus and trace minerals. If drought has caused grass to go dormant during what should have been the growing season, this supplementation becomes much more critical.

Beyond that, protein deficiency may have already adversely affected fetal growth. (It is important to realize this process cannot be compensated for. Beginning proper protein supplementation after the fact can prevent total resorption — but the calf will still be smaller and weaker at birth.)

If drought has caused the cow to be thinner than normal going into the winter, then energy supplementation will be required. This is the most expensive and difficult of all supplementation.

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