Vet Advice

A balanced mineral intake is essential for optimal performance. Mineral requirements in cattle vary depending on body type, age, pregnancy, weight gain and milk production. As well, the mineral content of feed varies widely from year to year and the unreliability of guessing what the mineral content might be lies at the root of most mineral-related problems. For instance, the calcium (Ca) and phosphorus (P) content of alfalfa hay average around 1.33 per cent and 0.25 per cent respectively, with Ca variance ranging from 0.31 per cent to 2.5 per cent and that of P, 0.09 to 0.49 per cent.

Time can be another factor. Depending on natural mineral stores, it might be as short as a week or so out to several months before signs appear on mineral-deficient diets. Stores can be prematurely depleted by drought conditions during the grazing season, or fluctuate unexpectedly when a variety of lower-quality feedstuffs are introduced during the winter feeding period. Nutritionists across Western Canada caution cattle producers that forage quality in 2010-11 is generally poor and in many areas, weathered hay little better than straw sits in feed yards.

The practice of analyzing feed and balancing rations not only makes good business sense it is the primary tool of investigating problems associated with mineral imbalances, especially those of Ca and P. Technologies associated with feed testing include wet chemistry and NIRS (near infrared reflectance spectrophotometry). NIRS, the cheapest and quicker of the two, is suitable for crude protein, acid and neutral fibre analysis, but may not suitable for mineral analysis when troubleshooting nutritional problems.

Mineral imbalances in brood cows result in reduced performance, lowered resistance to disease and reproductive disorders. Imbalance between the macro minerals Ca and P can cause disease during late pregnancy around the time of parturition. Animals with Ca/P levels badly askew end up as downer cows or exhibit the paralysis typically associated with milk fever in dairy cows at calving -- a condition reported more and more frequently in beef breeds.

Macro minerals are required in relatively large amounts everyday and include salt, calcium, phosphorous, potassium (K) and magnesium (Mg). Their inclusion in rations is measured in grams per day or as a percentage of the diet.

Forages are usually good sources of calcium; cereals, minimally so. Good-quality legume forages contain high levels of calcium, while grasses contain only moderate amounts. Calcium is an essential "building block" for bones, teeth, enzymes, hormones and muscle. Calcium availability and absorption is influenced by a number of factors. Low vitamin D levels and high levels of dietary phosphorus will reduce calcium absorption. Excess magnesium reduces calcium absorption while a deficiency of magnesium decreases calcium mobilization into the blood, which may result in milk fever symptoms in freshening or lactating cows. Calcium supplementation is easily accomplished by the addition of calcium carbonate (feed grade limestone) to a high grain ration or a 2:1 or 3:1 mineral supplement mix included in the ration or fed free choice. Limestone contains about 36 per cent calcium and is relatively inexpensive. Mineral supplements for cattle are identified by the ratio of calcium to phosphorus. Most mineral supplements also contain trace minerals and vitamins. Range minerals often contain salt.

Grains are considered a good source of phosphorus. Forages are marginal suppliers. Pasture- and forage-based diets are generally deficient in phosphorus.

Phosphorus deficiency can result in low conception rates, reduced feed intake, poor feed efficiency, lower growth rate, reduced milk production, reproductive failures and skeletal abnormalities. The most critical need for phosphorus is the last trimester of pregnancy and immediately prior to breeding season. A common symptom of phosphorus deficiency is the failure to return to heat after calving. Marginally deficient cows frequently go onto spring pastures and miss being bred early. Poor body condition exaggerates the period of anestrus. Since the body pool of phosphorus is low, phosphorus deficiencies are quickly expressed physiologically. Vitamin D deficiency or excess levels of dietary calcium contribute to reduction of P absorption. This important mineral can be provided free choice under range conditions or supplemented as required in complete rations.

Calcium and phosphorus are normally present in the body at a 2:1 ratio. They are absorbed in proportions present in the diet. Rations with ratios less than 1.5:1 have been shown to be detrimental to production, while at the other end of the scale rations in the range of 1.5:1 to 7:1 have proven satisfactory. Levels exceeding 8:1 can result in depressed performance.

High levels of calcium and phosphorus may decrease availability of dietary magnesium. Excessive levels of potassium can impair calcium and magnesium absorption. Cases of "downer cows" have been identified in cows receiving cereal forage or cereal silage diet high in potassium. Supplementing the diet with two to four ounces of limestone per day usually prevents these symptoms from occurring and will help correct an imbalance.

Though free-choice feeding of minerals is probably the easiest and most common practice of supplying minerals, wide variation of intake does exist. Factors affecting intake include: palatability of the mineral preparation, water quality and hardness, mineral content of the feeds, types of feeds, physical location of the mineral and individual animal preferences. Mixing salt with the cattle mineral supplement will generally encourage consumption and tends to prevent excessive intakes.

Optimal health and performance hangs in the "balance." Involve your veterinarian in decisions that need to be made regarding mineral supplementation between now and next grazing season.

-- Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen at gren@fbcpublishing.com or WCABP at info@wcabp.com . Reprinted from Canadian Cattlemen, February 2011, page 31.