Duodenal supply of glutamate and casein both improve intestinal starch digestion in cattle but by apparently different mechanisms

Greater postruminal flows of protein increase small intestinal starch digestion in cattle. Our objective was to determine if small intestinal starch digestion is increased by duodenal supplementation of AA. We fed 5 duodenally and ileally cannulated steers a low-starch soybean hull-based diet in 5 x 5 Latin square designs and provided continuous duodenal infusion of raw cornstarch in combination with AA or casein and measured small intestinal starch digestion. In Exp. 1 treatments were continuous duodenal infusion of 1) no supplement (control), 2) casein (400 g/d), 3) crystalline AA similar in amount and AA composition to the casein (CASAA), 4) crystalline nonessential AA similar to those provided by casein, or 5) crystalline essential AA similar to those provided by casein. In Exp. 2 treatments were continuous duodenal infusion of 1) no supplement (control), 2) casein (400 g/d), 3) Glu (133 g/d), 4) Phe and Trp plus Met (30.4, 6.5, and 17.5 g/d, respectively; PTM), or 5) a combination of Glu and PTM. Duodenal infusion of casein increased (P <= 0.05) small intestinal starch digestion. When CASAA was infused, small intestinal starch digestion was similar (P = 0.30) to casein infusion. Infusion of only nonessential AA tended to increase (P = 0.14) small intestinal starch digestion relative to the control, but infusion of essential AA alone did not affect (P = 0.84) small intestinal starch digestion. In addition, infusion of casein or CASAA increased ileal flows of ethanol-soluble starch (small-chain alpha-glycosides), but nonessential AA alone were not different than the control. Duodenal infusion of Glu increased (P = 0.05) small intestinal starch digestion, whereas PTM did not. Neither Glu nor PTM increased ileal flow of ethanol-soluble starch, but Glu and PTM provided together tended (P = 0.07) to increase ileal flows of small chain alpha-glycosides. Our data suggest that Glu alone can increase small intestinal starch digestion in cattle similar to casein, but increases in small intestinal starch digestion in response to Glu are not associated with an increase in ileal flows of small chain alpha-glycosides.