Size and number of corpora lutea and serum progesterone concentrations when administering two doses of eCG in an estrous synchronization treatment regimen for dairy cattle

•Administration of eCG at the time follicle emergence is associated with co-dominant follicle growth in cattle.•In 8 days synchronization protocol in cattle, double eCG on Days 2 and 6 could increase the number of ovulations.•Such protocol could enhance CL diameter and progesterone production. There was investigation of whether there were ovulations from co-dominant follicles following eCG administration. In all experiments, there was GnRH injection and CIDR insertion on day 0 (D0), CIDR withdrawal on D8, and cloprostenol administration on D8 (Exp. I and II) or D7 and D8 (Exp. III). Females in the control group were not administered any further treatment. Females in other group(s) were treated with eCG (500 IU) on Day 2 in Exp. I, Day 2 (eCG-2) or 8 (eCG-8) in Exp. II and Day 2 (eCG-2) or Days 2 and 6 (eCG-2-6) in Exp. III. Ovaries were examined using ultrasonography. In Experiments I and II, females had follicle emergence on Day 2. At the time of CIDR removal, more eCG-treated heifers (8/9; Exp. I) and cows (5/6; eCG-2; Exp. II) had co-dominant follicles compared to those in the control group (P