Guinea Grass Toxicity

I have a gelding that can’t eat guinea grass because it causes his hair to fall out in patches.
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I have a 12-year-old gelding that can’t eat guinea grass (Panicum maximum), not even for three days, because it causes the hair around his face, neck, and belly to fall out in patches. As soon as he stops eating it, his hair grows back within a week or two; by four weeks you wouldn’t even know he had lost his hair. He is otherwise healthy, and it doesn’t seem to hurt him in any other way. My other horses have no problem with it. To avoid this, I keep him in a separate property that doesn’t have much of that type of grass. Do you have any ideas as to what might be happening? I live in Puerto Rico, and this is a very common type of grass for horses to eat.

AI don’t have much firsthand experience with guinea grass, nor can I find much information on its toxicity. There are reports of photosensitization occurring with some Panicum grasses, especially Panicum coloratum (kleingrass). The lesions will occur on the white or non-pigmented skin areas, and will look like severe sunburn. The toxin in the grass is one of the diosgenins, and causes biliary (pertaining to the bile, bile ducts, or gallbladder) obstruction and liver disease. The skin photosensitivity is secondary to the biliary obstruction. Your veterinarian could do some liver tests to determine if this is a skin problem secondary to liver disease.

From the duration of the symptoms described, I am not convinced that the horse’s skin problems are due to the toxin in the guinea grass; it could be more of an allergic response in the horse. The rapid recovery suggests this.

If it is an allergic response to something in the grass, or due to the toxins causing photosensitivity, there would appear to be little choice but to keep the horse from eating the guinea grass

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Written by:

Anthony P. Knight, BVSc, MS, Dipl. ACVIM, is a professor of large animal medicine in the College of Veterinary Medicine and Biomedical Sciences at Colorado State University. He received his veterinary degree from the University of Nairobi, Kenya, in 1968. After completing a master’s degree at Colorado State University, he joined the faculty in 1974. His current professional interests include livestock heath, foreign animal diseases, emergency management, and plant toxicology. He has written two books on poisonous plants of animals in North America, and maintains a poisonous plants website for use by anyone wanting poisonous plant information.

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