Luteal response and follicular dynamics induced with equine chorionic gonadotropin (eCG) administration after insemination in sheep

• We evaluated the ovarian response induced by the administration of eCG after insemination in the preimplantation period.
• 400 IU of eCG on Day 10 after insemination, but not on Day 5, increased luteal size and serum progesterone concentrations.
• The eCG treatment slightly affected follicular dynamics.

The objective of this study was to evaluate the ovarian response to equine chorionic gonadotropin (eCG) administrated during the luteal phase after insemination in sheep. Thirty cyclic multiparous ewes received a fixed-time intrauterine insemination (Day 0) and were assigned to three experimental groups that received 400 IU of eCG on Day 5 (n = 10) or Day 10 (n = 10), while another group remained as control group without eCG (n = 10). Corpus luteum area and follicular diameter were daily measured by transrectal ultrasonography and serum progesterone concentrations were daily determined by radioimmunoassay during 30 days after insemination in pregnant ewes or until the following ovulation in non pregnant ewes. The administration of eCG on Day 10, but not on Day 5, increased corpus luteum size and serum progesterone concentrations (P < 0.05). Follicular dynamics were influenced by both eCG treatments on Day 5 and Day 10, promoting the growth of the largest follicle present during treatment or increasing the length of the wave that emerged after treatment (P < 0.05). In conclusion, the administration of 400 IU of eCG 10 days after insemination in sheep increased corpus luteum size and progesterone concentrations, as well as promoted follicular development. This information could be useful to develop new pharmacological strategies to improve the establishment of pregnancy in sheep.