Ruminal and Postruminal Starch Digestion by Cattle

Grains are fed to livestock primarily to supply energy, and most of the digestible energy in cereal grains comes from starch. To maximize starch digestion by livestock, corn and sorghum grain must be processed. For nonruminants, starch from finely ground grain is fully digested, but for ruminants fed high concentrate diets, finely ground grain often causes metabolic diseases. Hence, rather than finely grinding corn, processes including steam rolling, steam flaking and fermentation (high moisture storage) are used to increase extent of starch digestion from grains fed to ruminants. These processing methods usually increase starch digestion in the rumen, in the intestines (of starch reaching the small intestine), or at both locations. The lower the density (bushel weight) of flaked corn, the greater the digestibility of starch, particularly in the small intestine. For maximum ruminal starch digestion, a thinner flake is needed for lactating cows than for feedlot cattle because grain particles spend less time for digestion in the rumen of lactating cows than of feedlot cattle. This shorter ruminal retention time can explain why ruminal and total tract starch digestibility generally is lower for lactating cows than for finishing cattle. Ruminal escape of starch is greater with dry rolled and whole corn grain than with steam flaked and high moisture corn, but starch from dry rolled and whole corn grain is poorly digested in the small intestine. Averaged across processing methods, starch digestibility in the small intestine decreases as the quantity of starch entering the small intestine increased, but when grain processing methods are considered individually, disappearance of starch in the small intestine remains roughly proportional to starch entry rate. Due to reduced loss of energy as methane and heat, available energy supply for the ruminant is greater when starch is digested in the small intestine than when starch is fermented in either the rumen or large intestine. But if starch digestion in the small intestine is below about 70%, no energetic benefit from increasing ruminal output of starch will be achieved. Characteristics that make a grain or a hybrid ideal for livestock differ with processing method. For whole and dry rolled corn, the combination of very fine grinding of grain with a floury endosperm, a thin or loose pericarp, and a low amylose:amylopectin ratio will maximize starch digestion. For fermented corn grain with adequate moisture content as well as adequately processed steam flaked corn, starch digestion usually exceeds 97% so any remaining differences in digestibility among corn samples (1 to 3%) are due to components other than starch (NDF, protein). For maximum feed efficiency, energy digestibility must be maximized. For dry rolled or ground corn, incomplete starch digestibility is of primary concern, but with more extensively processed grain, altering the starch content (more starch and less NDF and protein) is the simplest way to increase its content of digestible energy.