Effect of dietary protein content on growth, uniformity and mortality of two commercial broiler strains

Two experiments were conducted to determine the response in performance, including uniformity and mortality, of two broiler strains to dietary protein content. In Experiment 1, 480 Cobb 500 and 480 Ross 788 day-old sexed broiler chickens were housed in cages to 21 d with 10 chickens per cage, and in Experiment 2, lasting 42 d, 1680 sexed broiler chickens of each of these two strains were placed in 48 floor pens with 70 chickens per pen. Males and females were reared separately in both experiments. Six levels of dietary protein were fed for 2 1 d in both experiments, the composition changing for the period 22 to 42 d in Experiment 2. Body weight of each bird was measured at weekly intervals up to 21 d in Experiment 1, and of 20 randomly selected birds from each pen on days 1, 21 and 42 in the second experiment. Broilers in the latter trial were group-weighed by pen at weekly intervals. Mortality was monitored daily. The highest body weight gains and feed conversion efficiencies (FCE, g gain/ kg food) were recorded in Cobb, with a correspondingly higher food intake for the starter feed in both experiments. In the finisher period Ross birds consumed significantly more than Cobb broilers (9 g/d) but in this case there was no difference in growth rate between the two strains, resulting in a significantly poorer FCE for Ross (487 vs. 522 g gain/kg feed). The pattern of food intake in the finisher period also differed between the two strains: the Cobb birds increased their food intake as the dietary protein content was decreased, but food intake decreased with protein content in the Ross. Uniformity was greatest in both strains when they were fed the highest protein feeds in both experiments, the variation in live weight increasing as the protein content decreased. There was no nutritional effect on mortality, although mean overall mortality was twice as high in Cobb broilers.