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