Our study was aimed to investigate the effects of dietary metabolizable energy (ME) and crude protein (CP) levels on the digestive physiology in the jejunal fluid of Muscovy ducks to provide digestive parameters for in vitro digestion. There were 6 ME levels (11.0, 11.5, 12.0, 12.5, 13.0, and 13.5 MJ/kg; Exp. 1) and six CP levels (140, 155, 170, 185, 200, and 215 g/kg; Exp. 2) and each treatment included 6 replicates with 3 ducks each replicate. In Exp. 3, the comparison of energy utilization was investigated between in vivo and in vitro using the digestion parameters obtained from Exp. 1 and 2. As dietary ME was increased, the chymotrypsin activity was increased linearly (P < 0.05), and the concentrations of Ca2+, K+, and Mg2+ were increased quadratically (P < 0.05) in the jejunal fluid. As dietary CP was increased, amylase activity was increased linearly (P < 0.05), whereas trypsin and chymotrypsin activities and Ca2+ concentration were increased quadratically (P < 0.05). The pH values were decreased quadratically with the increased dietary ME or CP levels (P < 0.05). The optimal digestion parameters for energy feedstuffs with 307.26 U/mL amylase, 54.68 U/mL trypsin, 24.90 U/mL chymotrypsin, 104.39 mmol/L Na+, 51.25 mmol/L Cl-, and pH 7.79; for protein feedstuffs with 381.88 U/mL amylase, 72.84 U/mL trypsin, 11.98 U/mL chymotrypsin, 93.53 mmol/L Na+, 46.25 mmol/L Cl-, and pH 7.80, respectively. Using the optimal digestion parameters for in vitro digestion, energy utilization in vitro reflected the degree of the apparent energy utilization of corn, sorghum, and barley as well as true energy utilization of soybean meal, rapeseed meal, and cottonseed meal in vivo and the variation of digestion was lower in vitro than in vivo.
Keywords: crude protein; digestive enzyme; electrolyte ion concentration; energy utilization; metabolizable energy.
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