Aspects of the sexual development of Brahman versus Angus bulls in Florida

Postweaning growth and reproductive traits were studied in 10 Brahman and 12 Angus bulls from 8 through 20 months of age. Brahman bulls reached puberty at 15.9+/-.4 months of age, weighed 432+/-16 kg, had a scrotal circumference (SC) of 33.4+/-1.2 cm, and plasma testosterone of 3.96+/-1.03 ng/ml. Breed differences in SC averaged over the entire study were not significant. However, the breedxday interaction (BxD) (P<.01) showed that, initially, the Brahman SC was smaller than the Angus SC; however, by the end of the study, the Brahman SC was larger than the Angus. When SC was adjusted for body weight, breed differences (P<.01) and BxD (P<.01) for SC/body weight (BW) reflected the later age and heavier weight at which the Brahman bull reached puberty. Plasma testosterone differed between breeds (Angus>Brahman, P<.01) and increased at a linear (P<.01) rate with age. There was no BxD in plasma testosterone. No breed differences in sperm concentration were observed. However, other semen traits were different (P<.01), i.e., rate of forward movement, sperm motility, total abnormalities and semen volume. A BxD (P<.01) was also evident for breed differences in these semen traits. Sexual development of the Brahman bull occurred at a later chronological age and in a nonparallel pattern to that of the Angus. Between animal variation in SC within the Brahmans and differences between this study and other reports suggest that differences in SC exist for various populations of Brahman bulls and should provide opportunities for progress in selection for this trait.