According to the recent Nutrient Requirements for Horses (2007), molybdenum intakes at 1 to 3 mg/kg of diet interfere with copper utilization in ruminants, but much higher levels of molybdenum are tolerated by the horse. Molybdenum at 20 mg/kg of the ration did not interfere with copper absorption. Therefore, the likelihood of a molybdenum problem in the horse is minimal. There is considerable difference among animals in the tolerance of high levels of molybdenum. Cattle have the least tolerance, followed by sheep. Horses have failed to show any signs of toxicity in pastures that severely affect cattle. One study, however, showed that the presence of 5 to 25 ppm molybdenum in forages caused some disturbance in copper utilization in horses. Tolerance of non-ruminant animals to molybdenum is much higher than that of ruminant species. Because of this high tolerance, little research has been conducted to determine whether excess molybdenum might cause problems with non-ruminants under certain conditions. Tolerance to high levels of molybdenum varies with the age of the animal, quantity and form of ingested molybdenum, inorganic sulfate content in the diet, and copper status. Molybdenum tolerance also varies with intake of methionine, cystine, and protein, which are capable of being oxidized to sulfate in the body. The level of sulfate in the diet can alter the absorption, retention, and excretion of absorbed molybdenum.

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