Puberty in beef bulls: hormone concentrations, growth, testicular development, sperm production and sexual aggressiveness in bulls of different breeds.

Postweaning growth and pubertal traits were studied in Hereford, Angus, Red Poll, Brown Swiss, Hereford-Angus crossbred (HXA) and Angus-Hereford crossbred (AX H) bulls from 7 through 13 months of age. Pubertal factors characterized included body weight, testicular size, hormone concentrations, sexual aggressiveness and sperm production. Puberty was defined as the age at which a bull first produced an ejaculate containing at least 50 x 106 spermatozoa with a minimum of 10% motility. Average pubertal age in days was 264 -+ 9 for Brown Swiss, 283 -+ 9 for Red Poll, 295 -+ 9 for Angus, 296 + 9 for AX H, 300 + 8 for HX A and 326 + 9 for Hereford bulls. Brown Swiss bulls were heaviest (295 kg), while Red Poll and Hereford bulls were lightest (258 and 261 kg) in body weight at puberty. Concentrations of serum luteinizing hormone (LH) and testosterone (T), averaged across all bulls, increased linearly (P<.01)from 7 through 13 months of age. Breed differences in LH concentration were not observed from 7 months of age through puberty. Breed groups with high average T concentrations between 7 and 13 months of age reached puberty earlier than breed groups with low T concentrations. Bulls exhibited first sexual interest approximately 3 weeks before reaching puberty, and attained mating ability approximately 6 weeks after reaching puberty. Brown Swiss bulls were