Evaluation of the effects of crude protein and lysine on the growth performance of two commercial strains of broilers using meta-analysis

A meta-analysis was conducted to evaluate the effects of crude protein (CP) and Lys on feed intake (FI; g/broiler chicken per day), average daily gain (ADG; g/broiler per day), and feed efficiency (FE) of broilers of Ross and Cobb strains during the first 21 d of age. A series of criteria was set to search the literature and a database was extracted and used to generate 6 different data sets: (1) male and female broilers of Ross and Cobb strains (n=169); (2) Ross and Cobb male broilers (n=124); (3) Ross males and females (n=80); (4) Cobb males and females (n=89); (5) Ross males (n=64); and (6) Cobb males (n=60). Each of these 6 data sets was analyzed separately for the effects of CP and Lys on FI, ADG, and FE. All models were able to demonstrate a linear effect of Lys (P≤0.08) except in 1 case, while a quadratic effect for all models was highly significant (P≤0.001). However, the results for CP level were not as consistent as for Lys. The FE response was more affected by CP level than were FI and ADG. The models developed showed that the interaction between CP and Lys was an important factor affecting broiler growth performance (P≤0.06 except for 2 cases). Results showed that the optimal value of Lys was increased by an increase in CP level, though its proportion to CP (% of CP) decreased. The Lys levels required to maximize FI, ADG, and FE in a diet containing 170 g CP for the first group (male and female broilers of Ross and Cobb strains) were 11.3, 12.2, and 12.8, while these values for a diet containing 245 g CP increased to 13.4, 14.2, and 14.5 g/kg of diet, respectively. By comparing results from the different data sets, it was concluded that the Lys requirement for males is greater than for females. Optimization of the models for all data groups showed that Lys required for maximum ADG was greater than for FI and was greatest for maximum FE. The results of this study illustrate that dietary CP level, desired response (FI, ADG, and FE), sex, and strain are important factors affecting the Lys requirement of broilers.