The hindgut (structure of the digestive tract after the small intestine) in all three organisms contains a diverse population of fermenting microbes. These microbes release gases, as well as fatty acids and lactic acid as waste products of fermentation. Of the three organisms, the most hindgut fermentation occurs in horses.

If a horse feeds on too much starchy grain, a lot of undigested carbohydrate will pass from the foregut into the hindgut. The same digestion problem happens when a horse feeds on spring grasses in the pasture. Grass plants produce many carbohydrate-rich leaves during the spring, but in the summer they produce more stem than leaf. Microbes that ferment the excess starch produce an increased level of lactic acid, which lowers the pH of the hindgut. Digestion in the hindgut stops and the horse often has to be treated for impaction. Why do you suppose excess starch halts digestion?