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 hor mone (FSH) on the maintenance and ovulation of the proestrus dominant follicle (DF) in cattle, This was investigated by subjecting a proestr us DF, maintained by a single norgestomet (N) implant, to bovine folli cular fluid (BFF) injections during N treatment and immediately after its removal. Earlier, we demonstrated that with an insertion of a sing le N implant the proestrus DF could be maintained for 9 days without a ffecting its ovulatory capacity, Eighteen cycling Holstein heifers wer e treated with a N implant at proestrus. The day of implant insertion was designated day 1 of the implant period and the implant was retaine d for 9 days, Heifers (n = 6 per group) were randomly allocated to rec eive saline for 4 days from day 5 to day 8 of the implant period and f or 4 days from the day of implant removal to day 3 after removal (CONT ROL) 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-AF TER), Injections (10 ml) were given i.v. twice daily and the ovaries m onitored by ultrasonography daily, throughout and after the implant pe riod, All CONTROL heifers maintained the DF during treatment and ovula ted following implant withdrawal. In all BFF-DURING heifers, the BFF i njections 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 l ower in BFF-AFTER heifers compared with CONTROLS (P < 0.05). Mean plas ma LH concentrations did not differ among treatment groups (P > 0.05). In summary, BFF treatment caused atresia of the proestrus DF when mai ntained by N and this was not associated with suppression of circulati ng FSH. Administration of BFF after implant removal resulted in an equ al chance of ovulation or regression of DF. Regression was associated with suppression of FSH and LH.