Are Grass Clippings Toxic to Horses?

Q. A friend swears that I put my horses at serious risk of "grass tetanus" by mowing my pasture with a rotary finish mower. He says that all of the short pieces produced by small, high-speed equipment expose so much of the grass juices that horses can get very sick. Is this true?

Peter, via e-mail

A. It's usually not recommended that horses be fed grass clippings, which are basically the type produced by a rotary mower. The small particle size enhances the chance of rapid fermentation in the horse's digestive system, which could potentially lead to colic or laminitis if he rapidly eats large amounts, especially if he is not used to being out on pasture. Also, if the clippings are left in large clumps, as so often happens when the grass is long when cut, and if heat/humidity conditions are just right, the clippings could ferment and mold, which is also potentially detrimental if the horse eats them.

However, the main concern with grass clippings is that they might also contain pieces of common ornamental plants that are highly toxic, such as oleander or Japanese yew. Plus, they are usually bagged in plastic before being dumped in a large pile, a situation that encourages the mold and rapid consumption that are major concerns.

In mowing your pastures, you're merely cutting the grass they are accustomed to being on and leaving it "in situ" (in the original place). If you keep your pasture mowed regularly so the clippings are dispersed and not accumulating in large clumps, I don't think there should be tremendous concern as long as your horses are used to being on the pasture you are mowing and aren't prone to laminitis.

About the Author

Sarah Ralston, VMD, PhD, Dipl. ACVN

Sarah L. Ralston, VMD, PhD, Dipl. ACVN, is a professor in the Department of Animal Sciences at Rutgers' School of Environmental and Biological Sciences, specializing in equine nutrition. Her research has focused on the effects of diet on metabolism, behavior, and the development of orthopedic disease in young horses, and she has additional interests in nutritional modulation of stress, metabonomics (the study of metabolic responses to drugs, environmental changes, and diseases), and pasture management. Previous research highlights were the pioneering work she did in nutrition for geriatric horses and post-surgical colics while at Colorado State University in the 1980s and the discovery of the correlation of hyperinsulinemia with development of osteochondrosis in young Standardbreds.

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