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Molybdenum Toxicity and Hypocuprosis in Ruminants: A Review¹

Colorado State University, Fort Collins 80523

Abstract

Elevated Mo intakes depress Cu availability and may produce a physiological Cu deficiency in ruminants. Total sulfur or sulfate in the ration generally potentiates the effect of Mo. The ratio of Cu to Mo in feed is important regardless of the absolute amount of each. For this reason, and because of the importance of the S content of the diet, it is impossible to define safe dietary limits for Cu and Mo.

Physiological Cu deficiencies are produced by four classes of feeds: (1) high Mo, generally above 100 ppm, (2) low Cu:Mo ratio, 2:1 or less, (3) Cu deficiency, below 5 ppm, and (4) high protein, 20 to 30% protein in fresh forage. The latter situation probably results from higher levels of sulfide produced from sulfur amino acids during rumen fermentation. Copper sulfide is largely unabsorbed. Future instances of hypocuprosis in the U.S. are most likely to occur from marginal copper deficiencies in forages.

¹ Invited paper sponsored by the Regulatory Agencies Committee and presented August 18, 1976 at the 68th Annual meeting of the American Society of Animal Science, Texas A&M University, College Station.