Responses to selenium in a range beef herd

S e l e n i u m has long been known to be toxic at high levels, but it was less than thirty years ago that it was shown to be an essential element also in livestock diets. In California, selenium deficiencies were first recognized in the northern part of the state. Early research showed that these deficiencies could be corrected by intramuscular injection of a selenium-vitamin E compound or by provision of additional selenium in the diet. A broader survey of the state was undertaken in 1981-82, using blood levels of resident cattle to indicate selenium status. This survey identified deficient areas in a number of Central Valley counties. Blood selenium concentrations above 0.08 parts per million (ppm) are generally regarded as adequate, and less than 0.04 ppm as deficient. Several herds along the east side of Fresno and Tulare counties had readings well below the 0.04 ppm level. Clinical symptoms of deficiency (white muscle disease, retained placentas), however, had not been observed in cattle grazing native range in foothill areas of that part of the state. One of the herds sampled was on the Johnson ranch, in the lower foothills on the Tulare-Fresno county line. This is a well-managed herd with relatively high conception rates and weaning weights. In most aspects, it is typical of other cowcalf ranches in that area. Because of the owners' interest in further improving economic returns from their cattle, we began a study in July 1984 to determine whether providing additional selenium would elevate blood content and improve productive performance. The method chosen was to place two pellets, or boluses, into the animal's reticulum. Each bolus weighed about 1 ounce and contained 10 percent selenium and 90 percent iron. Rumen action causes the boluses to rub together and release small amounts of selenium over an extended period. The production characteristics measured were cow fertility and calf weaning weights. The breeds of cows used in the trial were Hereford, Angus, and HerefordAngus cross.