Sexual development of dairy bulls in the Mexican tropics

Sexual development and pubertal traits were studied in Holstein Frisian (Ho) and Brown Swiss (BS) bulls born and maintained under tropical conditions. Characteristics evaluated every 2 weeks, from 27 to 63 weeks of age, included live weight, scrotal circumference, testicular diameter, semen quality and sexual behavior. 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% progressive motility. Testicular growth was linear in Ho bulls and quadratic in BS bulls. There was no breed difference in age at puberty (Ho, 333 +/- 15.8 days; BS, 311 +/- 10.5 days). However, at puberty, live weight and scrotal circumference tended to be greater in Ho (276 +/- 16.9 kg and 28.4 +/- 1 cm, respectively) than in BS bulls (233 +/- 11.3 kg and 25.9 +/- 0.7 cm, respectively), and testicular diameter was larger for Ho (5.5 +/- 0.24 cm) than for BS bulls (4.8 +/- 0.16 cm). Pooled data for all bulls for semen characteristics at puberty were: volume, 6.3 +/- 0.6 ml; progressive motility, 26.8 +/- 4.4%; sperm concentration, 58.5 +/- 13.9 x 10(6) spermatozoa/ml, and 351.5 +/- 91.2 x 10(6) spermatozoa/ejaculate. These values improved until at least 18 weeks after puberty. Eighty-five percent of bulls mounted heifers by 206 days of age, but only a few bulls had mounts with ejaculation during the study. It was concluded that reproductive development was similar between Ho and BS bulls, but slower than that reported for dairy bulls in temperate areas. Variation in some characteristics, such as scrotal circumference, was observed among bulls within each breed group, which might be of benefit for genetic selection.