Preovulatory follicle diameter, growth rate and time of ovulation during induced oestrus using a CIDR® in trypanotolerant female Bos taurus N'Dama cattle.

The aim of this study was to assess the dose (300 to 600 IU) effects of equine Chorionic Gonadotropin (eCG) on the preovulatory follicle diameter, growth rate, and time of ovulation characterized by echography. The eCG was injected at the end (D0) of the 7-day treatment with a controlled internal device release (CIDR) and a PGF2a being injected 2 days before the removal of the CIDR (D-2). The 120 female N'Dama cattle were distributed into five experimental groups. The control group (n = 26) was treated with physiological saline at the removal of the CIDR®, while the animals in the four treated groups received, respectively, 300 IU (n = 25), 400 IU (n = 24), 500 IU (n = 22) and 600 IU (n = 23) of eCG (Folligon®). Follicle growth rate (FGR) was significantly (P < 0.05) higher in animals treated using eCG (1.0 ± 0.4mm/day) than in the control group (0.9 ± 0.4mm/day). The diameter of the preovulatory follicle was significantly higher (P < 0.05) in the animals treated with 300 IU (10.1 ± 1.4mm) than in untreated animals (9.3 ± 1.2mm). Follicle growth rate was significantly higher (P<0.05) inn the animals treated with 300 IU (10,1 ± 1,4 mm) than in the control group (0.9±0,4 mm/day). The average interval between the time of eCG injection and ovulation was similar in the non-treated (83.7 ± 14.4 h) and treated animals (79.7 ± 11.9). Treated animals sowed a significant increase in the percentga of ovulation (94,7 %) compared to 73,1 %) (P<0.01). Use of eCG combined contributed towards synchronising the time of ovulation between 72 to 96 hours, which would facilitate the use of systematic insemination.