Molybdenum - Sheep
In Hebridean sheep for example
An experiment with Hebridean sheep clearly demonstrates the toxic effects of molybdenum. One of the main symptoms of what we call copper deficiency in Hebridean sheep is that the wool turns grey. In one experiment at the University of Leeds Hebridean sheep were deprived of copper and as a result their blood copper levels reached such low levels that one would expect to see severe symptoms of deficiency. They showed no symptoms whatsoever.
The reason for this is that the sheep were fed a low copper diet containing iron and sulphur but which had no molybdenum in it. The rumen bacteria utilise the iron and sulphur to form copper-iron-sulphur compounds that render the copper unavailable in the rumen. As soon as molybdenum was introduced the wool turned grey, but when molybdenum was then removed from the diet the wool quickly turned back to black. This is part of a growing body of evidence that molybdenum acts as a toxic factor deactivating vital copper enzymes in the body.