Effects of male body weight, strain and dietary protein content on fertility and musculo-skeletal disease in naturally mated broiler breeder males

1. The effects of male body weight at mating and different rates of gain on overall fertility were studied in two strains of broiler breeder males fed on a diet containing 160 or 120 g crude protein/kg. Sexual activity, musculo-skeletal disease and culling were also assessed. 2. There were no differences in early fertility between males weighing 3.0 or 3.5 kg at the start of the mating period, or between males gaining 1.0 or 1.5 kg body weight from mating to 60 weeks of age. 3. There were no differences in fertility between males fed on the two diets differing in crude protein content. 4. One strain of male had lower fertility than the other and this was associated with a greater weight of breast muscle, lower frequency of observed copulations and a higher number of incompleted matings. 5. Sexual activity declined and the number of spermatozoa trapped on the perivitelline membrane increased with age. 6. There were no differences among the treatments in the prevalence of musculo-skeletal lesions (destructive cartilage loss). As a proportion destructive cartilage loss occurred in 0.67 of the males at 66 weeks of age. Degeneration of the antitrochanter was present in 3 of 50 females examined and a cartilage flap in the medial condyle of the tibiotarsus occurred in 15 of these birds. 7. Beak lesions were the most important cause of male culling which averaged 0.24 as a proportion of the males at the start of the experiment.