The relationships between dietary crude protein, body weight, and fertility in naturally mated broiler breeder males

1. Naturally mated male broiler breeders were fed to achieve five levels of body weight gain on a high (160 g crude protein (CP)/kg) or low (110 g CP/kg) protein diet. Males were separately fed in 9 of the experimental treatments and fed with the females in the other. 2. There was an optimum body weight for maximum fertility which changed with age. The best fertility was shown by males weighing 3.2 kg at 26 weeks rising to 4.0 kg at 60 weeks of age. 3. The low protein diet was associated with higher fertility, particularly during the latter part of the breeding period (49-60 weeks). 4. Head width reached a plateau after 26 weeks of age and was 1 mm greater in males weighing more than 5 kg compared with males weighing 3 to 4 kg at 60 weeks of age.