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The objectives were to evaluate the effects of equine chorionic gonadotropin (eCG) supplementation (with or without eCG) and type of ovulatory stimulus (GnRH or ECP) on ovarian follicular dynamics, luteal function, and pregnancies per AI (P/AI) in Holstein cows receiving timed artificial insemination (TAI). On Day 0, 742 cows in a total of 782 breedings, received 2mg of estradiol benzoate (EB) and one intravaginal progesterone (P4) insert (CIDR). On Day 8, the CIDR was removed, and all cows were given PGF2α and assigned to one of four treatments in a 2×2 factorial arrangement: (1) CG: GnRH 48h later; (2) CE: ECP; (3) EG: eCG+GnRH 48h later; (4) EE: eCG+ECP. There were significant interactions for eCG×ovulatory stimulus and eCG×BCS. Cows in the CG group were less likely (28.9% vs. 33.8%; P<0.05) to become pregnant compared with those in the EG group (odds ratio [OR]=0.28). There were no …