DIFFERENCES IN EARLY PATTERNS OF GONADOTROPIN-SECRETION BETWEEN EARLYAND LATE MATURING BULLS, AND CHANGES IN SEMEN CHARACTERISTICS AT PUBERTY

In prepubertal bull calves there is an early transient rise in gonadot rophin secretion between 10 and 20 wk of age, and it has been suggeste d that this plays a role in the attainment of sexual maturation. To te st this, we looked for differences in the gonadotrophin secretory patt ern from birth to puberty between early and late maturing bulls. We al so characterized the changes in semen morphology that occur about the time of puberty. Blood samples were collected (n=28) every wk from 2 t o 20 wk of age and then every 2 wk until 50 wk of age. Semen was colle cted by electroejaculation at approximately 4-wk intervals from 36 to 49 wk of age. Puberty was defined as the first age at which an ejacula te contained 50 million spermatozoa with a minimum of 10% motility. Bu lls were divided into early (n=14) and late (n=14) maturing groups bas ed on the age at puberty (41.9+/-0.3 and 48.3+/-0.7 wk of age, respect ively). There was a transient increase in serum concentrations of LH a nd FSH between 2 and 24 wk of age; LH concentrations were greater in e arly maturing bulls than in late maturing bulls at 12, 13, IS, 17 and 48 wk of age (P<0.05). Serum concentrations of testosterone and FSH di d not differ between groups (P>0.05). As the bulls matured there was a n increase in the percentage of normal and live sperm cells, cell moti lity and the number of cells per ejaculate (P<0.05), and a decrease in the percentage of proximal droplets and knobbed acrosomes(P<0.05). We concluded that, during the early rise in LH secretion, early maturing bulls had higher circulating LH concentrations than late maturing bul ls. During the weeks preceding and following puberty there was an incr ease in the quality of semen collected by electroejaculation.