OVARIAN FOLLICULAR WAVES IN DAIRY-COWS AFTER ADMINISTRATION OF GONADOTROPIN-RELEASING-HORMONE AT ESTRUS

A crossover design experiment was conducted to examine the effects of GnRH on hormonal concentrations and changes in ovarian structures of l actating dairy cows during the estrous cycle. Cows were assigned rando mly to receive either 100 mug of GnRH or saline 12 h after estrus (d 0 ) was first observed. Ultrasonographic examinations of the ovaries per formed daily throughout the estrous cycle detected either three (n = 7 ) or four (n = 6) follicular waves. Transient increases in FSH general ly preceding the follicular wave containing the dominant (preovulatory ) follicle, and the emergence of the dominant follicle before the subs equent estrus occurred later in GnRH-treated than in saline-treated co ws with either three or four follicular waves. Day of emergence of dom inant follicles within follicular waves was preceded 83% of the time b y a transient increase in FSH and was followed consistently by increas ed serum estradiol-17beta, which generally reached peak concentrations concomitant with the peak diameter of the dominant follicle. Although clearly defined pulses were detected in 19 out of 26 sampling periods on d 7 and 14, number of FSH pulses and percentage of cows with pulse s of FSH were similar in GnRH-treated and saline-treated cows with thr ee or four follicular waves. In conclusion, treatment of cows with GnR H 12 h after the onset of estrus delayed the appearance of the next ov ulatory follicle in cows with either three or four follicular waves. R egardless of treatment, emergence of dominant follicles was preceded i n most cases by a transient increase in FSH and followed consistently by increased estradiol-17beta.