Effect of bovine follicular fluid administration during or after norgestomet treatment on the fate of the proestrus dominant follicle in heifers

The objective was to examine the influence of follicle stimulating hormone (FSH) on the maintenance and ovulation of the proestrus dominant follicle (DF) in cattle. This was investigated by subjecting a proestrus DF, maintained by a single norgestomet (N) implant, to bovine follicular fluid (BFF) injections during N treatment and immediately after its removal. Earlier, we demonstrated that with an insertion of a single N implant the proestrus DF could be maintained for 9 days without affecting its ovulatory capacity. Eighteen cycling Holstein heifers were treated with a N implant at proestrus. The day of implant insertion was designated day 1 of the implant period and the implant was retained for 9 days. Heifers (n = 6 per group) were randomly allocated to receive saline for 4 days from day 5 to day 8 of the implant period and for 4 days from the day of implant removal to day 3 after removal (CONTROL) or BFF from day 5 to day 8 of implant period (BFF-DURING) or BFF from the day of implant removal to day 3 after implant removal (BFF-AFTER). Injections (10 ml) were given i.v. twice daily and the ovaries monitored by ultrasonography daily, throughout and after the implant period. All CONTROL heifers maintained the DF during treatment and ovulated following implant withdrawal. In all BFF-DURING heifers, the BFF injections caused regression of the DF and its disappearance. In three of the BFF-AFTER heifers, BFF injections caused regression of the DF. In the remaining three BFF-AFTER heifers, the DF ovulated. Mean plasma FSH concentrations did not differ (P > 0.05) between the CONTROL and BFF-DURING heifers. However, the mean plasma FSH concentrations were lower in BFF-AFTER heifers compared with CONTROLS (P < 0.05). Mean plasma LH concentrations did not differ among treatment groups (P > 0.05). In summary, BFF treatment caused atresia of the proestrus DF when maintained by N and this was not associated with suppression of circulating FSH. Administration of BFF after implant removal resulted in an equal chance of ovulation or regression of DF. Regression was associated with suppression of FSH and LH.