Effect of time of eCG administration on follicular response and reproductive performance of FGA-treated Ossimi ewes

Abstract

Equine chorionic gonadotrophin (eCG) is commonly used to improve litter size and for synchronized lambing in ewes. The efficiency, however, may be influenced by the time of application. The aim of the present study was to test the hypothesis that the time of eCG treatment induces a change in follicular development and ovulation rate in progestagen—PGF 2α treated ewes. Eighteen Ossimi ewes were treated with 40 mg FGA sponges for 8 days. Two days prior to sponge removal ewes (G1) were injected with 500 IU eCG ( n = 6), the second group (G2) received eCG (500 IU) on the day of sponge removal ( n = 6), while the third group (G3) was not treated with eCG and acted as the control ( n = 6). PGF 2α was injected to all the ewes on the day of sponge removal. Follicular development was observed ultrasonographically twice daily from sponge withdrawal. Administration of eCG in the G1 group caused a rapid reduction in the number of small follicles and an early appearance of the medium and large follicles ( P < 0.05). Estrus and ovulation occurred in 100% of the ewes in G1 and in 83.3% of the ewes in the G2 and G3 groups. The interval from treatment to estrus and ovulation was shorter ( P < 0.05) in the G1 (32 ± 5.6 and 72 ± 0.0 h, respectively) than the G2 (69 ± 9.9 and 108 ± 11.1 h, respectively) and the control groups (80 ± 13.8 and 124 ± 15.4 h, respectively). Administration of eCG 2 days before sponge withdrawal synchronized the time of ovulation and all ewes in the G1 group ovulated at the same time − the time of ovulation was spread over a period of 48 h in G2 and 72 h in G3, respectively. The ovulation rate was higher ( P < 0.05) in the eCG-treated ewes than the controls (2.33 ± 0.2; 2.33 ± 0.5 and 1.0 ± 0.2 for the G1, G2 and G3 groups, respectively). Characteristics of ovulatory follicles, including the interval from follicle emergence to ovulation, follicular growth rate and maximum size before ovulation did not differ between the treatment groups. The administration of eCG increased the number of fetuses and the lambs born, compared to control group ( P < 0.05). Results demonstrate that eCG administration to FGA-treated Ossimi ewes in the subtropics stimulate follicular development and increase prolificacy. The administration of eCG before sponge removal resulted in a shorter interval to estrus and ovulation linked to earlier development of large follicles and could be beneficial in the use of fixed-time AI.