Summary. Plasma FSH concentration was significantly higher in Romanov than Ile-de-France ewe lambs at 5, 6 and 7 weeks of age (P < 0·001, P < 0·02 and P < 0·02, respectively) and at 5, 6 and 7 weeks of age (P < 0·001, P < 0·01 and P < 0·05, respectively) compared to Finn lambs. FSH concentrations were similar and unaffected by time in Ile-de-France and Finn lambs. Ovariectomy at 5 weeks of age produced similar increases in FSH concentrations in Romanov and Ile-de-France ewe lambs, but at 3 months of age the increase in FSH concentrations after ovariectomy was significantly steeper (P < 0·02) in Romanov than Ile-de-France lambs. Sensitivity to oestradiol feedback was related to the age of the lambs. At 5 weeks of age, oestradiol (30 μg in oil per lamb) produced a significant decrease (P < 0·001) in FSH concentrations in Romanov and Ile-de-France lambs, demonstrating that negative feedback can be triggered by oestradiol at this age. Positive feedback after an oestradiol challenge was identified in lambs of both breeds at 6 weeks of age. Sensitivity to the negative feedback of follicular fluid compounds was also established at 5–6 weeks and did not differ between breeds. At 9–10 weeks of age, while there was no breed effect of an oestradiol challenge on FSH concentrations, suppression of FSH concentrations by follicular fluid was shorter in Finn than in the other lambs.
As all the feedback mechanisms are functional at 5–6 weeks of age, it is likely that the between breed differences of FSH profile during infancy are linked to differences in gonadal development.