Effect of high progesterone concentrations during the early luteal phase on the length of the ovulatory cycle of goats

The effect of exogenous progesterone exposure early in the oestrous cycle on the duration of the interovulatory interval was studied in dairy goats. A controlled intravaginal drug release (CIDR-G) device was inserted for 5 days starting at day 0 (D0 group, n = 6) or day 3 (D3 group, n = 5) postovulation. A third group was composed of untreated control goats (control group, n = 7). Daily transrectal ultrasound was carried out during the interovulatory interval to assess the ovarian dynamics. Oestrous behaviour was checked twice a day and serum progesterone levels were assayed in daily jugular blood samples. Treated goats showed two different responses. In three D0 goats and one D3 goat, progesterone concentrations fell immediately after CIDR withdrawal and this was followed by oestrus and ovulation between days 8 and 11 (short cycles). In the other three D0 goats and in four D3 goats the treatment significantly reduced the interovulatory interval (18 .3 ± 0.3 and 18.5 ± 0.3 days, respectively) (shortened cycles) compared with the control group (20.0 ± 0.2 days; P< 0.05), but the intervals with progesterone concentrations over 1 ng/ml were not different (15.7 ± 0.3, 15.8 ± 0.7 and 16.0 ± 0.5 days for D0, D3 and control goats, respectively). In all D0 goats with a short cycle response, the ovulatory follicle arose from the first follicular wave but in the D3 goat with a short cycle it arose from the second follicular wave. These results showed that premature progesterone exposure early in the ovulatory cycle of the goat affected its length inducing short or shortened cycles. The effect of progesterone could either affect luteotropic support of the corpus luteum (CL) and/or stimulate a premature release of the luteolysin. © 2001 Elsevier Science B.V. All rights reserved.