Abstract: 【Objective】 The objective of the present study was to determine the effect of exogenous protease supplementation on the in vitro dry matter digestibility (DMD) and enzymatic hydrolysate gross energy (EHGE) using a computer-controlled simulated digestion system (CCSDS) and obtain the optimum protease supplementation into a corn-soybean meal-based diet for broilers. It would provide a reference for the establishment of in vitro method for efficacy evaluation of exogenous enzyme.【Method】 A single factor completely randomized design with 8 treatments was adopted and each treatment contained 5 replicates with 1 digestion tube per replicate was adopted in the present study. The control diets for starter and finisher phrases (24.29% and 21.72% of crude protein, respectively, dry matter basis) of broilers were formulated to meet or exceed the nutrient requirements for broilers, and the test diets were supplemented with 0, 15 000, 75 000, and 150 000 PROT·kg^-1 exogenous protease (90 000 PROT·g^-1 ) into the control diets for starter and finisher phrases of broilers, respectively. The DMD, in vitro gross energy digestibility (GED) and EHGE of the diets were determined using a CCSDS (SDS-II). The correlation between DMD or EHGE and protease supplementation (PS) of diets were also analyzed. 【Result】 The results showed as follow: 1) The in vitro total tract DMD, GED and EHGE of diets increased as the level of exogenous protease supplementation in the control diet for starter and finisher phrases of broilers increased (linear, P＜0.01). 2) The in vitro gastric DMD and GED of diets decreased as the level of exogenous protease supplementation in the control diet for starter phrases of broilers increased (linear, P＜0.01). However, the in vitro gastric DMD and GED of diets were not affected by the exogenous protease supplementation in the finisher control diet (P＞0.05). 3) A linear regression analysis, taking into account protease supplementation (g·kg^-1), explained the DMD (%) of starter and finisher diets for DMD = 2.56 PS - 0.82 PS × PS + 73.90 (R² = 0.82, RSD = 0.42, P＜0.001) and DMD = 3.56 PS – 1.46 PS × PS + 75.20 (R² = 0.70, RSD = 0.60, P＜0.001), respectively. A linear regression analysis, taking into account protease supplementation (g·kg^-1), explained the EHGE (MJ·kg^-1) of starter and finisher diets for EHGE = 0.75 PS - 0.30 PS × PS + 15.02 (R² = 0.89, RSD = 0.07, P＜0.001) and EHGE = 0.61 PS - 0.30 PS × PS + 15.67 (R²= 0.79, RSD = 0.06, P＜0.001), respectively. There was greatest EHGE when protease supplementation was  112 500 PROT·kg^-1 in a starter diet and 91 800 PROT·kg^-1 in a finisher diet, respectively. 【Conclusion】 In conclusion, the in vitro gastrointestinal DMD, GED and EHGE were increased by the protease supplementation into a corn-soybean diets for broilers. However, the effect of protease supplementation on nutrition digestibility in different intestinal segments was not simple consistent with a dose-response pattern. The 112 500 PROT·kg^-1 of protease supplementation in a starter diet and 91 800 PROT·kg^-1 of protease supplementation in a finisher diet for broilers may improve more effectively in the present study.

Key words: exogenous protease, dry matter digestibility, enzymatic hydrolysate gross energy, computer-controlled simulated digestion system, broiler