Ovarian characteristics, serum concentrations of progesterone and estradiol, and fertility in lactating dairy cows in response to equine chorionic gonadotropin

We hypothesized that administration of eCG during the proestrous maturation phase of the preovulatory ovarian follicle would increase follicle size and serum estradiol concentrations. Our objectives were to evaluate the effects of eCG administration on preovulatory ovarian follicle size and growth rate, serum concentrations of estradiol and progesterone, estrual activity, posttreatment luteal activity, and pregnancy per AI. Lactating Holstein cows milked thrice daily were enrolled in a Presynch-Ovsynch timed AI program before the first AI. Cows (N = 128) in a single herd were given two doses of prostaglandin F2α (PGF2α) 14 days apart (Presynch), with the second dose 11 days before the onset of an Ovsynch protocol (treatment with GnRH 7 days before and 56 hours after PGF2α, with AI 16 to 18 hours after the second GnRH treatment). Cows were assigned randomly to receive either saline or 400 IU eCG concurrent with PGF2α treatment of the Ovsynch protocol (Day 0). Serum concentrations of progesterone and estradiol were assessed to determine if eCG would increase estrual activity, improve ovulatory response to GnRH, and enhance postovulatory luteal function. Compared with controls, treatment with eCG did not increase diameter or growth rate of the largest follicle during 48 hours after eCG, but tended (P = 0.09) to increase growth rate of the second largest follicle. Serum estradiol concentrations and estrual activity were not altered by eCG. Serum progesterone concentrations did not differ between treatments on Days 0, 2, 4, 9, or 16 after eCG treatment. Number of CL per cow on Days 9 and 16 after administration of eCG did not differ between treatments, but total luteal tissue volume tended (P = 0.06) to be greater on Day 16 for eCG-treated cows compared with controls. Pregnancy per AI at first service was similar between eCG (35.3%) and control cows (39.0%). We concluded that eCG treatment administered 3 days before insemination at the dose of 400 IU failed to increase follicle growth size and growth rate 48 hours after treatment, and did not enhance pregnancy outcomes in dairy cattle programmed for AI at first service.