Molybdenum Toxicity in cattle: an underestimated problem

Molybdenum toxicity is a controversial subject. However, much of the contention is due to inappropriate diagnosis. This paper shows the flaw in using plasma copper levels alone to diagnose a molybdenum toxic condition, based on samples from >7,500 dairy cattle. Evidence that thiomolybdates are absorbed into the ruminant body and have toxic effects (molybdenum toxicity) on critical copper metallo-enzymes is detailed. The consequences are seen as impaired fertility and production. Prevention of molybdenum toxicity is achieved by reducing the iron intake of the animal and by supplementation with copper that is sacrificial in the rumen and not primarily supplied for absorption. A correct diagnosis will allow appropriate supplementation to be undertaken and the risk of toxicity from inappropriate supplementation reduced.