A study was conducted to evaluate the effects of dietary protein level and protein digestibility on the growth performance and carcass characteristics of broilers from 1 to 35 d of age. Three hundred and twenty broiler chickens were fed four different ideal protein-balanced, isocaloric diets in a 2 x 2 factorial design experiment with two levels of protein (high protein (HiPro; 20 and 18%) and low protein (LoPro; 18 and 16%) for day 1 to 14 and day 15 to 35, respectively and two levels of crude protein digestibility (85% digestibility (HiDig) and 80% digestibility (LoDig). The HiDig diets were formulated using soybean meal and fishmeal, whereas the LoDig diets used wheat distillers dried grains with solubles and meat and bone meal as the primary protein sources. The standardized ileal digestibility (SID) values for lysine for wheat distillers dried grains with solubles (56.5%) and meat and bone meal (72.0%) were measured before the experiment to improve the accuracy of the diet formulations.

During the starter phase, the interaction was significant for ADG; birds fed the LoPro-LoDig diet grew slower than birds fed the other three diets (P < 0.05). During the grower phase, the interaction was significant for ADFI; birds fed the LoPro-LoDig diet had the lowest ADFI compared with those fed the other three diets. The interaction between protein level and digestibility was significant for the SID of most of the AA and was significantly higher for birds fed the HiPro-HiDig diet compared with those fed the other three diets. Total breast meat yield was significantly higher in birds fed the HiPro diets than in those fed the LoPro diets, whereas birds fed the HiDig diets had significantly more abdominal fat than those fed the LoDig diets. The results suggest that low-protein diets can support growth performance equal to high-protein diets when highly digestible ingredients such as soybean meal and fish meal are used. However, maximum breast meat yield requires a high-protein diet and is not affected by ingredient digestibility. (Note-This research emphasizes the importance of using highly digestible protein feedstuffs when attempting to lower protein levels in practical broiler diets).