Abstract
The aim of this investigation was to optimize fixed-time insemination in goats by clustering ovulations in prostaglandin F(2α)-synchronized goats either with gonadotropin releasing hormone (GnRH) or human chorionic gonadotropin (hCG). The underlying intention was to reduce the incidence of short cycles by providing a more sustained stimulation of the corpus luteum by substituting the commonly used GnRH with longer-acting hCG. It was conjectured that this might render the corpus luteum less prone to premature regression. Sixty pluriparous does were administered 5 mg of the prostaglandin F(2α) preparation dinoprost (Dinolytic; Pharmacia and Upjohn, Erlangen, Germany) during the luteal phase of the estrous cycle. Twenty of these does were administered 0.004 mg of the GnRH analog buserelin (Receptal; Intervet, Unterschleissheim, Germany) 48 hours later; another 20 does received 500 IU hCG (Chorulon; Intervet, Unterschleissheim, Germany) instead. Sixteen hours later the does were inseminated with frozen-thawed semen. The remaining 20 does served as controls and were inseminated 16-18 h after the onset of detected estrus. All 60 treated goats displayed estrous symptoms, the time of onset being similar for all groups (42.6, 37.6, and 40.5 hours after treatment for GnRH-treated, hCG-treated, and control does, respectively). The duration of estrus in the GnRH-treated group was 10 h less than in the other groups (45.1 vs. 56.4 and 54.4 h, P < 0.05). The number of ovulations (assessed by ultrasound monitoring) did not differ among groups (2.4, 2.1, and 2.5, P > 0.05). Monitoring of serum progesterone revealed that the incidence of corpus luteum insufficiency was significantly higher in GnRH- and hCG-treated does than in the control group (40% and 35% vs. 5%, P < 0.05). The pregnancy rate was 50% in the GnRH and 35% in the hCG group as compared with 60% in the controls. Corresponding kidding rates were 40%, 35%, and 60% (P > 0.05). When disregarding does with corpus luteum insufficiency, pregnancy rates would have been 83%, 54%, and 63%, and kidding rates 67%, 54%, and 63%, respectively. The average number of kids born was 1.88, 1.71, and 1.83, respectively (P > 0.05). It may be concluded that fixed time insemination of cycling does treated with prostaglandin F(2α) during the luteal phase, followed by ovulation induction with GnRH or hCG, would be an effective management tool if it were possible to control the high incidence of corpus luteum insufficiency. The attempt to achieve this by substituting GnRH with hCG, was not met with success. Until a solution for the problem has been found, it is advisable to inseminate prostaglandin-synchronized does 16-18 hours after the onset of detected estrus.