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Molybdenum in Horse DietsBy Kentucky Equine Research Staff · December 17, 2015

The nutritionists at Kentucky Equine Research (KER) are occasionally asked about little-known aspects of equine feeding. Here’s an example of such a query from a veterinarian:

I have a customer that did bloodwork on her horse, and the results showed high levels of molybdenum. Could you provide me with information on this mineral and if this level would be toxic to horses?

Molybdenum is commonly found in low concentrations in most dietary feedstuffs. An average concentration value for this mineral in forage is 1-100 ppm (mg/kg), according to Nutrient Requirements of Horses, published by the National Research Council. However, high levels of molybdenum can be found in plants grown on soils naturally contaminated with molybdenum due to mining, smelting, or industrial operations, or soil types containing peat, muck, or shale.

“Molybdenum is an essential element, though horses can easily meet their daily requirement under normal feeding management, even with low forage intakes,” said Catherine Whitehouse, M.S., an equine nutritionist with KER. Accordingly, equine feed manufacturers do not include molybdenum in premix that is added to commercial feeds. Nutrient Requirements of Horses does not contain a dietary requirement for molybdenum.

Molybdenum toxicity is linked closely with its interactions with sulfur and copper, though these interactions differ among species, as ruminants are much more sensitive to high molybdenum intakes than horses. A study in horses found that molybdenum at 20 mg/kg of the diet did not affect the absorption or utilization of copper. This intake level is much higher than the levels reported to affect ruminants. The horse is ranked as having one of the highest tolerances for molybdenum among both ruminants and nonruminants, said Whitehouse.

“I would recommend analyzing the horse’s forage if there is concern it may contain high levels of molybdenum, especially if the horse exhibited clinical signs of molybdenum toxicity,” she continued.

Removing the horse from molybdenum-rich feedstuffs is recommended, and this would include any and all nutritional supplements. “Overuse of nutritional supplements has been indicated in other cases of nutrient overload, and the possibility exists in this case as well. A thorough accounting of the horse’s diet, especially with a forage analysis in hand, can rule out a nutritional cause of molybdenum excess.”

Why did the horse owner and veterinarian test blood concentrations of molybdenum? Was it a comprehensive blood panel and molybdenum flagged as high based on the normal ranges for that laboratory? Molybdenum is primarily excreted in the urine, so testing urine for molybdenum concentration may be a more useful tool than the blood test to determine exposure to high levels of the mineral, Whitehouse recommended.