Starch Digestion and Utilization in Ruminants

For most grain, except corn and sorghum, 90% or more of starch is normally fermented in the rumen. With corn, up to 30% or more could escape ruminal fermentation; most of the starch escaping fermentation would be digested in the small intestine or fermented in the large intestine. The capacity for digestion of raw starch in the small intestine is limited to 100 to 200 g/d in sheep. Gelled starch could be digested in quantities up to 200 to 300 g/d. The capacity would ultimately be limited by lack of enzymes involved in hydrolysis of short chain di- and oligosaccharides and also by capacity for absorption of glucose. Starch fermented in the cecum leads to an increase of N loss in the feces. In some instances, the high propionic acid proportion arising from fermentation of starch exceeds the capacity of the liver for metabolism. In growing lambs and goats, this creates problems of synthesis of branched-chain fatty acids in adipose tissue and, in dairy cows, problems of low milk yield and milk fat production. The low ruminal pH that often occurs when starchy grain is included in diets for ruminants can cause depression in fiber digestion. Some of these problems can be overcome by reducing extent of cereal processing and other methods that prevent low ruminal pH. It is concluded that due to limited capacity for starch digestion postruminally and high animal variability, deliberate attempts to increase postruminal digestion of starch are unlikely to be beneficial and are likely to create potential digestive problems.