Molybdenum (Mo) isn’t usually a topic of conversation around the tack room, but this mineral is an important part of your horse’s diet. It is component of biological enzymes involved in the metabolism of purines, which help make up some of the building blocks of deoxyribonucleic acid and ribonucleic acid (better known as DNA and RNA, respectively).

Requirements and Sources

It’s unclear exactly how much Mo a horse needs in his diet. Most horse feeds contain an estimated 0.3 – 8 milligrams per kilogram of dry matter. Concentrations in plants or pasture growing on shale, granite, or marine-origin soils, or soils which have been contaminated by industrial pollution, could be notably higher (the latter over 200 mg/kg dry matter) (Lewis 1996, NRC 2005). Sandy soils, however, are low in Mo.

While specific values were not reported, the National Research Council’s Nutrient Requirements of Horses (NRC) reports that research found grass forage generally had higher Mo concentrations than legumes.

Deficiency and Excess

Neither Mo deficiency nor toxicity has been reported in horses. In birds and other mammals, a Mo deficiency can result in reduced growth rates. 

Deficiency has not been a great concern in horses, but excess certainly is. Although horses seem to tolerate high intake of Mo, too much can interfere copper absorption. Mo is absorbed quickly from the small intestine and excess is readily excreted in the urine, and doesn’t seem to interfere with other minerals’ absorption (Lewis 1996).

The maximum tolerable Mo intake for cattle, sheep, and horses is suggested to be 5 milligrams per kilogram of dry matter intake (NRC 2005). Although researchers believe animals, including horses, can likely tolerate higher intakes, anything more than 5 mg Mo/kg of dry matter feed likely begins to inhibit copper absorption, leading to other problems (NRC 2005). In other livestock species, researchers have proposed a copper-to-molybdenum ratio of no less than 4:1 to ensure each mineral is absorbed properly (Paterson 2012). Whether this ratio is proper for horses remains to be determined. 

Take-Home Message

While the exact Mo requirement in the equine diet remains unknown, the mineral is still an important part of the building blocks of genetic material. The NRC (2005) suggests that Mo requirements are usually met by feeding a balanced, practical diet. More research is needed to determine the exact needs, but since the risk of deficiency or toxicosis is very small, the primary concern is maintaining overall mineral balance.


Lewis, L.D. 1996. Minerals for horses. In: Feeding and care of the horse, 2nd edition. Lippincott Williams & Wilkins, Media, PA. pp. 19-41.

National Research Council. 2007. Nutrient Requirements of Horses, 6th Edition. National Academies Press, Washington, DC.

National Research Council. 2005. Mineral Tolerance of Animals, 2nd Edition. National Academies Press, Washington, DC.

Paterson, J. 2012. How did we discover that trace minerals were necessary for livestock?, accessed online at: