FSH-stimulated follicle development in ewes in high and low body condition and chronically treated with gonadotrophin-releasing hormone agonist.

Ovarian follicle development in response to FSH infusion was investigated in Scottish Blackface ewes with high and low body condition scores in which endogenous gonadotrophin secretion and follicle development to > or = 2.5 mm diameter was suppressed using subcutaneous implants containing a GnRH agonist. In two experiments conducted during the normal breeding season, groups of 20 (Expt 1) and 15 (Expt 2) ewes were fed to achieve body condition scores > or = 2.75 (high; H) or < or = 1.75 (low; L). In both experiments GnRH agonist implants were inserted four weeks before FSH was infused for 72 h at 7 micrograms h-1 to group H animals or at 5 micrograms h-1 to group L animals; the infusion rates were designed to ensure similar circulating FSH concentrations in animals of both groups. In Expt 2, additional subcutaneous implants containing oestradiol were inserted 21 days after insertion of GnRH agonist implants and 7 days before the FSH infusion began. In both experiments, FSH infusion was associated with an increase in circulatory concentrations of LH (P < 0.01) and FSH (P < 0.001), but there was no difference with body condition in mean circulating gonadotrophin concentrations, the numbers of ovarian follicles > or = 2.5 mm diameter, the proportion of these follicles that were oestrogenic or the mean rate of oestradiol secretion in vitro. It is concluded that differences in body condition of ewes do not affect the responsiveness of the ovary to FSH, in the presence or absence of oestradiol, as measured by the number, size and steroidogenic capacity of ovarian follicles present following FSH infusion.