There has not been much wheat pasture this winter after a slow start last fall and a long winter drought in most of the Southwest. Many pastures that usually have stockers grazing all winter were either not stocked or had the calves pulled already. Moisture from recent snow will give the wheat a boost, and many of these fields could be used for spring-calving cows to graze. Wheat growing this time of year is high in protein and highly digestible and will easily meet the requirements of a beef cow in early lactation.
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A problem with wheat and other small grain pastures is the mineral content in comparison to the needs of the lactating cow. The table shows the average mineral content from five experiments across Oklahoma and Arkansas.
There is considerable variation in the mineral composition of small grain forages, depending on management, growth conditions and soil mineral content. This data shows that magnesium, copper and zinc are deficient in wheat forage; calcium and phosphorus are adequate; and potassium is excessive.
The high potassium content of wheat forage is problematic because it interferes with magnesium absorption in the gastrointestinal tract. Grass tetany in mature cows is caused by low blood magnesium levels that can result from either low magnesium intake or poor absorption. It is commonly a problem in nursing mature cows grazing small grain pastures in the spring due to magnesium excretion in the milk and reduced resorption of magnesium from the bone in mature cows compared to younger cows or growing calves.
Grass tetany is a condition associated with inadequate magnesium in the diet. Magnesium is a mineral that is necessary in nerve function and therefore muscle contraction. Cattle with grass tetany become excitable, develop muscle tremors and have difficulty breathing. In the worst case, death is possible.
The most common method of preventing grass tetany is to supplement the herd with magnesium beginning at least one month prior to spring grazing. Mineral supplements that contain 10-12 percent magnesium as magnesium oxide, called high-mag minerals in layman’s terms, are commonly used.
At 3 to 4 ounces intake, such minerals will provide 40-50 percent of a cow’s daily magnesium requirement. Mineral supplement intake is often reduced when higher rates of magnesium are added due to the unpalatability of magnesium oxide. Mineral mixtures will not effectively offset mineral deficiencies if desired amounts are not consumed, so intake must be monitored. — Paul Beck, Oklahoma State University Extension beef cattle specialist
