Corn Grain Processing and Digestion

Grains are fed to livestock primarily to supply energy, and the major energy source in cereal grains is starch. For maximum starch digestion, corn and sorghum grain must be processed. For non-ruminants, starch from finely ground grain is fully digested, but for ruminants fed concentrate diets, finely ground grain can cause metabolic diseases. Hence, steam rolling or flaking and fermentation (high moisture storage) rather than fine grinding are used for grains fed to ruminants to increase the extent of starch digestion. Such processing methods increase starch digestion both in the rumen (of dietary starch) and postruminally (of starch reaching the small intestine). The lower the density of corn flakes, 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 particles spend less time in the rumen for digestion in lactating cows than in feedlot cattle. This shortened ruminal residence time can explain why ruminal and total tract starch digestibility is lower for lactating cows than for finishing cattle. Contrary to popular belief, digestion of starch reaching the small intestine does not decrease as abomasal supply of starch increases. However, neither dry rolled or whole corn is digested well post-ruminally. Due to reduced loss of methane and heat, available energy supply for the animal is greater for starch digested in the small intestine than for starch fermented in either the rumen or large intestine. Different hybrid characteristics are desired for different processing methods. For whole and dry rolled corn, very fine grinding of grain with a floury endosperm, a thin or loose pericarp, and a low amylose:amylopectin ratio all will help to maximize starch digestion. For fermented corn grain with adequate moisture content and for adequately processed steam flaked corn, starch digestion usually exceeds 97%. Therefore, the remaining differences among various corn samples in digestibility (1 to 3%) must be attributed to differences in digestibility of 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, corn grain richer in starch (and thereby lower in NDF and protein) will provide more digestible energy.