A study was conducted to compare the performance of a modern broiler breed (Ross; RS) with those of two 1970’s heritage broiler breeds (HB1 and HB2) fed drug free recommended protein (RP; 22% CP) and low protein (LP; 19% CP) diets from 1 to 63 d of age. Six hundred mixed sex d old chicks from each of the three breeds were divided into ten groups of equal weight and randomly placed in 30 floor pens. Five replicates pens of each breed were randomly assigned to the RP and LP starter (1-16 d of age) and grower (16-30 d of age) diets, respectively. The LP grower diet was used to feed all birds from 31-63 d of age. The BW gain (BWG) and feed intake (FI) of the RS birds were higher (P < 0.05) than those of the two heritage breeds at 16, 30, 35, 49 and 63 d of age. Among the heritage breeds, the BWG and FI of HB1 birds were higher (P < 0.05) than those of HB2 birds at all the measured periods. For the entire experimental period, the BWG and FI of RS birds were higher than those of HB1 and HB2 birds by 35.6 and 53.4%, and 21.7 and 37.5%, respectively. The feed conversion ratio (FCR) was also better (P < 0.05) for RS birds than for HB1 and HB2 birds. The HB1 and HB2 birds were, however, similar (P > 0.05) in FCR. The BWG and FCR of all the three breeds were depressed (P < 0.05) when they were fed LP starter and grower diets up to 30 d of age, but not (P > 0.05) when they were fed a common finisher diet from 30 to 63 d of age. Regarding FI, a significant depression (P < 0.05) of a LP diet on the same was only observed for RS birds from 1 to 35 d of age. For the entire experimental period, the LP starter and grower diets only negatively affected BWG of the RS birds (3992 vs 3771 g/bird). The results show that the modern RS broilers compared with the 1970’s heritage breeds have superior performance, but are more sensitive to dietary protein level.

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