| This experiments aimed to study the influence of production performance, slaughter performance, nutrient apparent metabolic rate, digestive enzyme activity and serum biochemical indexes on Cobb broiler by quadratic regression rotation design of different combinations of metabolizable energy(ME) and crude protein(CP) levels, in order to determine the best combination of ME and CP levels of different stages of broiler, and provide a theoretical basis to the development and implementation of fine broiler nutrition system as well of large-scale and efficient farming.This experiment used two-factor quadratic regression orthogonal rotation combination design method which set ME and CP levels as the two trials factors combining with growth performance and carcass performance to form regression equation, obtain the optimal parameters by fitting the target equation optimal parameters, and analyze the impact of each index the optimal value in every test. The experiment was divided into two stages: The first stage was 0-21 d, and a total of 480 one-day-old Cobb broilers of similar weight and health status were divided into 12 groups(4 replicates per group and 10 broilers per replicate) in random and male and female in half. The second stage was 22-42 d and broilers were groups on the same basis as that of the first group. The diet was corn-soybean meal, and Groups 1-8 were experiment groups while groups 9-12 were center groups(zero level) that were fed at the same nutrient level, in order to maintain the stability of the quadratic regression orthogonal rotation of the rotary design. The ME and CP levels of center groups were respectively as follow: 1-21d: 12.4 MJ/kg, 21%; 22-42d: 12.8 MJ/kg, 20%, which were referenced the standard of NRC(1994), and the ME and CP levels of other experiment groups were set according to quadratic regression rotation design parameter table. The results showed that:(1) The Cobb broiler for 1-21d: The results showed that a certain level of ME and CP could improve average daily gain(ADG), feed /gain(F/G), breast muscle percentage, thigh muscle percentage and reduce abdominal fat percentage of broilers aged 1 to 21 days(P<0.05), significantly improve the dietary fat, crude protein apparent metabolic rate(P<0.05), improve intestinal protease activity(P<0.05), increase serum levels of glucose(GLU) and albumin(ALB)(P<0.05), decrease triglyceride(TG) and uric acid(UC) content(P<0.05). The response graphs and regression analysis showed that when ME level was 10.05~12.40MJ/kg, the ADG was increased with the increases of CP, the effect become weak gradually with the CP levels increase further. When ME and CP was 11.80 MJ/kg and 22.85%, the ADFI was maximum, when the ME level was 12.15MJ/kg with CP level of 20.65%, the F/G was minimum of 1.65, when the ME and CP levels were at the time of 12.05~12.15MJ/kg and 20.85%~21.25%, the breast muscle percentage and thigh muscle percentage were higher, while the abdominal fat percentage was lower.(2) 22-42d: The results showed that the increase the level of ME and CP could improve ADG and F/G of Cobb broilers aged 22 to 42 days(P<0.05), increase breast muscle percentage and thigh muscle percentage(P<0.05), improve meat quality and decrease abdominal fat percentage(P<0.05). Significantly improve the dietary fat, crude protein apparent metabolic rate(P<0.05), improve intestinal lipase and protease activity(P<0.05), increased serum levels of GLU, triglycerides(TG)(P<0.05) and UC(P<0.05). The nonlinear regression analysis and response graphs showed that ADG was on a higher level when the CP level was around 20% and energy level was between 10.80 ~12.80MJ/kg, but the ADG was decreased with the increased of ME from 14.21MJ/kg to 14.80MJ/kg; The ADFI was the biggest when the energy and crude protein level were 12.70 MJ/kg and 19.78% respectively and the lowest F/G was 1.95 when the ME and CP level were 12.65 MJ/kg and 20.16% respectively. When ME and CP levels were 12.68MJ/kg and 20.15%, the breast muscle percentage was the best theoretically, similarly, the thigh muscle percentage was the best when ME and CP were 12.68MJ/kg and 20.15%, and abdominal fat percentage was lower when ME and CP were 12.70MJ/kg and 19.55%.(3) Overall, a certain level of ME and CP could significantly improve the production performance and slaughter performance, improve nutrient apparent metabolic rate, increase digestive enzyme activities and various biochemical indexs play balancing roles in the serum of Cobb broiler. Therefore, under these experimental conditions, the ME and CP levels of different stages for Cobb broiler diets were respectively recommended as: 1-21d: ME: 12.05~12.35 MJ/kg, CP: 20.85%~21.25%; 22-42d: ME:12.68~12.95MJ/kg, CP: 19.78%~20.15%. |
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