Effect of extruding the cereal and/or the legume protein supplement of a compound feed on in vitro ruminal nutrient digestion and nitrogen metabolism

Summary An experiment was performed to evaluate the effect of extruding the cereal and/or the protein supplement of a compound feed for intensively reared calves on ruminal N metabolism using an in vitro culture system. A conventional compound feed was produced without extruding [treatment non-extruded (NE)], with the cereal blend extruded (CE), with the protein blend extruded (PE) and with both cereal and protein blends extruded (CPE). Four experimental diets, consisted of 0.90 of each experimental compound feed and 0.10 barley straw were assessed using dual-flow continuous-culture fermenters. 15N infusion as ammonia sulphate was used to label the microbial population. Average NH3 concentration in fermenter effluents ranged from 270 mg/l with diet NE to 69 mg/l for diet CPE (p < 0.05) and volatile fatty acid concentrations ranged from 161 mm in diet PE to 130 mm in diet CPE (p < 0.05). Diets PE and CPE showed a lower true organic matter degradability (49.5% and 48.2%) than NE and CE (52.8% and 52.2%). Non-ammonia nitrogen flow in effluents was highest on diet CPE, intermediate on diets CE and PE and lowest on diet NE (p < 0.01), reflecting the differences in dietary N flow and dietary protein degradability (71.2%, 63.7%, 61.2% and 50.0%, respectively, for NE, CE, PE, CPE; p < 0.001). In contrast, microbial protein synthesis efficiency was lower for treatments including the cereal blend extruded, although the resulted differences were only significant (p < 0.001) for CPE diet. © 2007 The Authors. Journal compilation 2007 Blackwell Publishing Ltd.