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My guess is it's the grass
Posted by Sally on 11/16/2016, 9:41 am, in reply to "Intermittent diarrhea "
We've been blessed with rain and fairly warm temps, but the nights are getting colder and the days shorter. That means the cool season grasses in your pasture are manufacturing and storing lots of sugars. Grasses store sugars at the lower part of the stem, so short/overgrazed grass can be surprisingly high in sugar this time of year. One type of sugar that cool season grasses produce is fructan. Important: Fructan cannot be digested in the small intestine like other simple sugars because horses (and all mammals) lack the digestive enzyme needed to do this. Therefore, fructans pass through the digestive system to the large bowel, where they are fermented by gut microbes. Fructans are actually a prebiotic (food) for many types of gut microbes, but a large dose of them can overwhelm the microbe population (similar to starch overload) and cause some digestive disturbance. You may want to try adding some yeast, such as Yea-sacc 1026 to see if that helps.
One other possibility is the emergence of encysted small strongyles from the gut lining, which is common as the weather cools off. Even though you have wormed him with ivermectin, that drug does not get the earlier stages of encysted larvae, so that may be something you want to look at if it's been over 3 weeks since the last dose of ivermectin. Talk to your vet about that, but a dose of moxidectin (Quest) might be worth trying.