Induction of cyclicity in postpartum anestrous beef cows using progesterone, GnRH and estradiol cypionate (ECP)

The objective of the present study was to determine whether treatment of postpartum multiparous and primiparous anestrous beef cows with an intravaginal progesterone-releasing insert (CIDR) and PGF2α, with and without the addition of GnRH or estradiol cypionate (ECP) at the time of CIDR insertion, is effective in stimulating onset of estrous cycles. Postpartum lactating Angus primiparous (n = 47, 2 years of age, 495 ± 6 kg) and multiparous (n = 76, ≥3 years of age, 553 ± 9 kg) cows were assigned by calving date to four blocks spaced 21-day apart. Cows were assigned sequentially by calving date to four treatment groups: (1) PGF2α (n = 30), (2) CIDR–PGF2α (n = 30), (3) GnRH–CIDR–PGF2α (n = 33), and (4) ECP–CIDR–PGF2α (n = 27). Intravaginal CIDR inserts were in place from days −7 to 0. A single 100 μg injection of GnRH or 2 mg ECP were administered on day −7, and 25 mg PGF2α was administered on day 0. Day 0 averaged 38 ± 1 day postpartum. Blood samples were collected on days −19, −9, 0, 5, 9, 12, 16, 19, 23, 26, and 30 for determination of plasma progesterone concentrations. Pre-treatment luteal activity (progesterone ≥ 1 ng/ml) was detected in 19% of primiparous and 8% of multiparous cows. Progesterone concentrations on day 0 were greater (P < 0.001) in primiparous (3.2 ± 0.3 ng/ml) than multiparous (2.0 ± 0.2 ng/ml) cows. Following CIDR withdrawal, progesterone concentrations from days 5 to 30 were used to categorize response profiles as either: (1) treatment-induced onset of estrous cycles, (2) continued anestrus, or (3) spontaneous ovulation and subsequent formation of a CL. Incidence of treatment-induced onset of estrous cycles, which was defined as progesterone concentrations ≥1 ng/ml in three or more consecutive samples from days 9 to 19, was influenced by treatment and parity. Percentages of cows initiating estrous cycles were greater (P < 0.001) in the three CIDR-treated groups than in the PGF2α group (55 and 8%, respectively). Percentages of cows initiating estrous cycles in the CIDR–PGF2α, GnRH–CIDR–PGF2α, and ECP–CIDR–PGF2α groups were 55, 58, and 52%, respectively. Incidence of treatment-induced estrous cycles in the three CIDR-treated groups of cows was greater (P = 0.008) in primiparous (76%) than multiparous (43%) cows. Treatment of postpartum anestrous primiparous and multiparous beef cows with CIDR–PGF2α approximately 40-day postpartum provides an approach to increase the percentage of cows that have reinitiated estrous cycles by the start of the breeding season.