Growing cattle in the United States consume up to 6 kg of
starch daily, mainly from corn or sorghum grain. Total tract apparent digestibility of
starch usually ranges from 90 to 100% of
starch intake. Ruminal
starch digestion ranges from 75 to 80% of
starch intake and is not greatly affected by intake over a range of 1 to 5 kg of
starch/d.
Starch apparently digested in the small intestine decreases from 80 to 34% as
starch entering the small intestine increases from 0.2 to 2 kg/d.
Starch apparently digested in the large intestine ranges from 44 to 46% of
starch entering the large intestine. Approximately 70% of
starch digested in the small intestine appears as
glucose in the bloodstream. Within the range of
starch intakes that do not cause rumen upsets, increasing
starch (and energy) intake increases the amount of
starch digested in the rumen, increases the supply of
starch to the small intestine, increases
starch digested in small intestine (albeit at reduced efficiency), and increases
starch digested in the large intestine, such that total tract digestibility remains relatively constant. With increased
starch intake, most of the
starch is still digested in the rumen, but increasing amounts of
starch escape ruminal and intestinal digestion, and disappear distal to the ileocecal junction. Again, within the range of
starch intakes that do not cause rumen upsets, as
starch intake increases, hepatic gluconeogenesis increases,
glucose entry increases, and
glucose irreversible loss increases, with a significant portion lost as CO2. The ability to increase use of dietary
starch to support greater
weight gains or improved marbling could come from increasing
starch digestion in a healthy rumen or in the small intestine, but we conclude that the main limit to use of dietary
starch to support live
weight gain is digestion and absorption from the small intestine. Increased oxidation of
glucose at greater
starch intakes may alter energetic efficiency by sparing other oxidizable substrates, like
amino acids.