The objective of this study was to compare the performance of a modern commercial broiler strain (Ross 308; RS) with that of two 1970s broiler strains (broiler strains 1 and 2) fed a drugfree low-protein or recommended-protein diet. All chicks were vaccinated on d 1 against coccidiosis using the edible immunocox gel vaccine (Vetech Laboratories Inc., Guelph, Ontario, Canada). Six hundred straight-run chicks of each strain were divided into 10 groups of equal weight and randomly placed in 30 floor pens. Five pens of each strain were randomly assigned to the recommended-protein or low-protein dietary regimen. The low-protein grower diet was used as the finishing diet (d 30 to 63). The BW gain (BWG) and FCR of the RS birds were better (P < 0.05) than those of broiler strain 1 birds by 35.6 and 12.2% and those of broiler strain 2 birds by 53.4 and 12.3%, respectively. The BWG and FCR up to 30 d of age were depressed (P < 0.05) regardless of the strain when the low-protein diets were fed to birds. The low-protein diets, however, depressed (P < 0.05) the overall (d 63) BWG of only the RS birds (3,992 vs. 3,771 g/bird). Therefore, the 1970s broiler strains compared with the RS strain had slower growth and performance, but were less sensitive to a decrease in dietary protein. The faster growth rate of the modern RS broiler strain compared with the 1970s strains was due to genetic improvement of the RS strain.

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