Changes in the P I T U I T a R Y - G O N a D a L Axis Associated with Puberty in Holstein Bulls 1

The temporal pattern of the endocrine changes associated with puberty were studied using 52 bulls born in October or April. Blood samples were taken weekly and at 30-min intervals for 5 h every 4 wk. Bulls were castrated at one of six 4-wk intervals between 12 and 32 wk and blood samples were taken. Season of birth affected concentrations of testosterone (greater for spring-born) in intact bulls, but not luteinizing hormone (LID or follicle stimulating hormone (FSH). The concentration of FSH increased about 30% between 4 and 32 wk, without evidence of pulsatile discharge. Basal concentration of LH was low and pulsatile discharges were infrequent at 4 or 8 wk. At 12, 16 and 20 wk, however, basal LH concentration was elevated and LH discharges were at <2-h intervals. Testosterone concentration did not rise until 18 to 20 wk, but then continued to rise; Lt! discharge was suppressed concomitantly. Bulls castrated at 16 or 20 wk had higher concentrations of LH in their blood both before and shortly after castration than older bulls, but by 21 d after castration values for bulls of all ages were similar. It was concluded that elimination of an unidentified suppressive factor allows frequent discharges of LH between