Horse News & Views - June 2006

Ministry of Agriculture, Food and Rural Affairs

Ontario Association of Equine Practitioners

Feed tags specify the species of animals for which the feed is intended. Diets made for cattle, sheep and other ruminants should never be fed to horses. They often contain urea, other non protein nitrogen sources and, on occasion, monensin. The non protein nitrogen (NPN) in itself is not usually harmful to horses, but monensin and other ionaphores are highly toxic to horses. The LD50 (lethal dose for 50% of horses) for monensin is 1-2 mg/kg or 1-2 ppm. Cattle feed may contain 250-375 ppm.

Microbial organisms present in the rumen of ruminants utilize the nitrogen in urea or other non protein nitrogen-containing substances to synthesize protein, provided the animal consumes sufficient grain. The protein produced by these organisms passes from the rumen to the stomach and small intestine, where it is digested and absorbed.

In contrast, the nitrogen in NPN sources, when fed to horses, is absorbed from the small intestines and excreted in the urine. Very little NPN ever reaches the cecum and colon for microbial utilization and protein synthesis. Rations containing NPN will have lower than expected protein concentrations when fed to horses, since the nitrogen is not available for microbial protein synthesis. On the reverse side, NPN does not cause harm to the horse unless fed at extremely high concentrations that are above those normally found in cattle and sheep diets. (Ref: Lewis, LD. Feeding and Care of the Horse, second edition, Lippincott Williams & Wilkins, 1995.)

Occasionally, horses will develop hives, commonly called protein bumps, on the skin. These can be an allergic reaction of the body to foreign proteins in the feed, but are more likely the result of skin contact with some chemical in the environment, e.g., insect repellents. Insect bites also cause these hives. Generally, the hives are short lived and cause no serious health problems.

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