Toxicity of ammonium molybdate added to drinking water of calves.

Molybdenum, as ammonium molybdate, was added to the drinking water of 5-wk-old calves to establish the minimum toxic concentration. A basal diet with 13 ppm copper and .29% sulfur was fed ad libitum for 21 days. The concentration of copper in liver was reduced with 50 ppm added molybdenum in water but not with 1 or 10 ppm. However, copper in plasma was elevated with 50 ppm added molybdenum in water while changes in ceruloplasmin concentration were nonsignificant. The calculated percent copper as ceruloplasmin copper in plasma decreased from 61% to 43% with all additions of molybdenum. Apparently uptake of plasma copper by tissues was reduced by molybdenum decreasing the bioavailability of copper. These data indicate the difficulty of detecting molybdenum-induced hypocuprosis from plasma copper and ceruloplasmin without data on tissue copper. With dietary levels of 13 ppm copper and .29% sulfur, the minimum toxic concentration of molybdenum in drinking water for calves is between 10 and 50 ppm, and the critical copper-to-molybdenum ratio is less than .5. Molybdenum in water may be less toxic to calves than molybdenum in fresh forages.