Increases in duodenal glutamic acid supply linearly increase small intestinal starch digestion but not nitrogen balance in cattle

Small intestinal starch digestion (SISD) in cattle is often limited; however, greater postruminal flow of high- quality protein (e.g., casein) can increase SISD, and Glu can mimic responses to casein for SISD. We evaluated effects of increasing Glu flows to the duodenum on SISD and N retention in cattle. Cattle received (DM basis) continuous duodenal infusion of raw cornstarch (1.5 +/- 0.08 kg/d) and 0, 30.9 +/- 0.6, 62.4 +/- 1.2, or 120.4 +/- 3.4 g/d Glu or 387.9 +/- 17.5 g/d casein. As expected, the positive control (i.e., casein) increased (P = 0.05) SISD. Interestingly, SISD linearly increased (P = 0.02) with increasing amounts of Glu. Starch flow to the ileum linearly decreased (P = 0.04) in response to greater postruminal Glu and tended to decrease (P = 0.07) with duodenal casein infusion. Ileal flow of ethanol- soluble starch was not affected by duodenal Glu (P = 0.16) or casein (P = 0.42). There was a tendency (P = 0.08) for a quadratic response to Glu for ileal glucose flow with greater flows for intermediate levels of Glu, but casein had no effect (P = 0.81) on glucose flows to the ileum. Greater postruminal supplies of Glu (linear, P = 0.05) and casein (P = 0.02) decreased fecal starch flow. Postruminal starch digestion was increased by both casein (P = 0.03) and Glu (linear, P = 0.05). Nitrogen intake from feed was not different among treatments (P = 0.23). By design, infusate N increased from 0 to 13 +/- 1.5 g/d with greater amounts of Glu, and casein provided 61 +/- 1.3 g N/d. Urinary N excretion was not affected (P = 0.30) by postruminal Glu flow, but urine N was increased by casein (P < 0.01). Glutamic acid did not affect N retention (P = 0.34), but casein increased N retention (P < 0.01). However, N retained as a percent of N intake (26.7 +/- 1.7%) was not different when cattle were provided Glu (P = 0.16) or casein (P = 0.38).