|Harmon David L|
Submitted to: British Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/1994
Publication Date: N/A
Citation: N/A Interpretive Summary: Cereal grains comprise 80 to 90% of dietary dry matter for finishing cattle in the U.S. Because these grains contain 60 to 70% starch, starch represents the major energy component in these diets. Starch is not fermented completely in the rumen and flow to the small intestine varies. Greater flows of starch to the small intestine in cattle results in reduced starch digestibility. We infused glucose, corn dextrin or cornstarch into the small intestine of Holstein steers to determine what limits small intestinal starch digestion. The amount of starch infused was slightly less than the amount of starch flowing to the small intestine in a feedlot steer. Two thirds of the infused starch was digested in the small intestine, and only half was absorbed as glucose. Based on carbohydrates escaping small intestinal digestion, both pancreatic alpha-amylase and intestinal carbohydrase activities limit starch digestion. Because glucose absorption and the enzymes required for starch digestion in the small intestine are limited, it will be difficult to improve starch digestion in cattle when increased amounts escape ruminal fermentation.
Technical Abstract: Three Holstein steers (423 kg) with an elevated carotid artery, portal, and mesenteric venous catheters, and abomasal and ileal cannulae were used to determine the comparative extent of small intestinal starch digestion. Steers were abomasally infused with either water (275 ml/h), glucose (66 g/h), corn dextrin (66 g/h) or corn starch (66 g/h) in an incomplete Latin square design, including the three steers and eight infusion periods. In the small intestine, 85% of abomasally infused glucose, 78% of infused dextrin, and 66% of infused starch disappeared. Net portal glucose appearance could account for 73% of glucose, 60% of dextrin, and 57% of starch disappearance from the small intestine. Of the infused dextrin flowing past the ileum (14 g/h), 0.3 g was glucose, 6.2 g was soluble alpha-glucoside, and 7.5 g was insoluble alpha-glucoside. Infused starch flowing past the ileum (22.2 g/h), 0.9 g/h was glucose, 5.3 g/h soluble alpha- glucoside, and 15.9 g/h insoluble alpha-glucoside. The average chain length of the soluble alpha-glucoside in ileal digesta was 2.07 and 2.36 for dextrin and starch infusions, respectively, indicating mostly di- and, to a lesser extent, tri-saccharide. We conclude that 1) starch granular structure only partially limits small intestinal corn starch disappearance, 2) alpha- amylase small intestinal carbohydrases and glucose transport across the small intestine are limited, and 3) when 66 g of raw starch is presented to the small intestine per hour, about one- half appears as glucose in the portal vasculature.