Clinical signs in cattle grazing high molybdenum forage

Abstract

Revegetation and sustainable cattle grazing are major objectives in the reclamation of mine tailings at the Highland Valley Copper mine in British Columbia, Canada. A total of 84 cow-calf pairs grazed forage extremely high in molybdenum (Mo) for 11 weeks in the summer and fall for 3 consecutive years (1999-2001). The average stocking rate was 0.63 ha AUM-1. The animals' diet consisted primarily of alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) containing 100-400 ppm Mo. Both cows and calves showed adequate daily gains of 0.78 kg head-1 day-1 and 1.33 kg head-1 day-1, respectively. Uptake of Mo was demonstrated by elevated levels of Mo in rumen contents and feces. Clinical signs of Mo toxicity were observed in less than half of the cows and in only a few calves. Clinical signs included a stiff shuffling gait, watery diarrhea, and a rough hair coat. Lameness, the primary sign, was resolved in all animals by the end of each trial. Diarrhea was also resolved by the end of the trial and hair coats returned to normal by the following spring. The onset and severity of the affliction appeared to be related to prevailing moisture conditions, which may have affected Mo availability in forage. Some affected animals were treated with Cu injections to no avail. Liver biopsies and serum samples showed marginal to adequate copper (Cu) levels but potentially toxic levels of Mo. In the third year of the trial, Cu-containing boluses were employed but they did not prevent the onset of clinical signs.