Effects of GnRH administration on pregnancy rate and litter size in ewes subjected to a fixed time insemination program during the breeding season.

The objective of this study was to compare the effects of GnRH administration at sponge insertion, insemination, and day 7 post-insemination on pregnancy rate and litter size in ewes subjected to a fixed time insemination program during the breeding season. Estrus was synchronized in 2740 adult ewes at four experimental sites. An intravaginal sponge impregnated with flurogestone acetate was left in the vagina for 12 days followed by an injection of 330 IU of eCG at sponge removal. Each ewe was in seminated twice at 52 h and 64 h after sponge removal. In the Insertion group, the Insemination group, and the Post-insemination group, ewes were given an intramuscular administration of 25 μg of the GnRH agonist triptorelin at sponge insertion, insemination, and day 7 post-insemination, respectively. No GnRH administration was given in the Control group. Pregnancy rates and litter size were calculated after all deliveries were completed. The results showed that the pregnancy rates in the Insertion group were significantly higher compared to those in the Control group, the Insemination group, and the Post-insemination group. The litter size in the Insertion group and the Insemination group were not significantly different and were significantly higher compared to those in the Control group and the Post-insemination group. In conclusion, the fixed time artificial insemination program, in combination with GnRH administration at sponge insertion, is a simple and potentially effective strategy for increasing fertility in estrus-synchronized ewes during the breeding season.