Extent and Partition of Cereal Grain Starch Digestion in Ruminants

Abstract

Corn and sorghum starch represents 80% of all concentrate starch fed to ruminants in the U.S. Considerable additional corn and sorghum starch is fed in silage. Barley, flaked corn, steam flaked sorghum and probably wheat and oat starches are about 94% fermented in the rumen. Ground corn is 74% fermented in the rumen and this value is 1) affected by corn source, 2) greater for sheep than cattle, and 3) similar at 20 and 80% rations but less on 40% and least on 60% corn rations. Untreated milo starch is 42% fermented in the rumen. The bovine small intestine digested up to 7.7 g corn starch/kg¾/day. The bovine small and large intestine has received up to 14.2 g sorghum starch/kg¾/day with total tract digestibility at 97%. Total tract digestibility of starch in these experiments averaged 99% although significant amounts of corn and sorghum starch fed at high levels and coarse particle sizes to cattle do escape digestion. Starch fermentation in the rumen has an energetic efficiency of 75 to 80%, i.e., a loss of 20 to 25%. Corn starch escaping fermentation may contribute up to 8.6 g glucose/kg¾ to the ruminant. Bacterial protein production is reduced between 8.2 and 23.3 g/100 g starch escaping ruminal fermentation.