The time of eCG administration in progesterone injection-based estrus synchronization protocol could affect the time of estrus expression in ewes during non-breeding season

The objectives of this study were to investigate the suitable time of eCG administration following progesterone injections, and to propose the suitable time for AI in progesterone injection-based estrus synchronization protocol in ewes during non-breeding season. Healthy Shal ewes (n = 140) were used in this study. All ewes were exposed to rams for 10 h, after two months of isolation from males. Six days after ram exposure, all ewes were given progesterone (37.5 mg; SC; Day 0 of experiment) in association with GnRH analogue (7.5 µg Alarelin acetate; IM). Subsequently, the second (25 mg; SC) and the third (12.5 mg; SC) doses of progesterone were given, 3 days apart. Concurrent with the third dose of progesterone, all ewes received prostaglandin F2α analogue (250 µg Cloprostenol; IM). On Day 6, ewes were divided into four groups considering their age, BCS and parity. Ewes in groups 1, 2 and 3 received eCG (400 IU, IM), concurrent, 24 h, and 48 h following the last progesterone injection, respectively. Ewes in control group did not receive any further treatment. Estrus detection was carried out from the last progesterone injection for 6 days. Ewes that exhibited standing estrus were drafted and inseminated laparoscopically within 24 h after standing estrus. Data were analyzed using GLM and GENMOD procedures in SAS. The incidence of estrus was similar in all groups (P > 0.05). The time to estrus and AI after the last progesterone injection was longer in control than in other treatment groups (P < 0.05). Lambing rate in group 2 (66.7%) was higher than in control group (40%; P < 0.05). There was no significant difference in prolificacy among groups (P > 0.05). Fecundity in group 3 (83.3%) was higher than in control (43.3%; P < 0.05). The percentage of ewes that exhibited estrus within 24 h occurs earlier in eCG treated groups than in control (P < 0.05). In conclusion, although the time of eCG administration after the last progesterone injection could result in almost similar reproductive performance in ewe, it will shift the time of estrus expression and AI, affecting the proper time for AI.