The efficacy of different PGF2α treatments to promote luteolysis on D 7 or D 9 of the estrous cycle in nonlactating Nellore cows.
The objective was to evaluate the luteolytic competence of different PGF2α (PGF, Lutalyse®) treatments on d 7 and 9 of the estrous cycle. Nonlactating Nellore cows (n=270) were synchronized with the 7-d EB + CIDR program. Cows received Estrotect® patches at CIDR removal to determine estrus response. Presence of an ovulatory follicle and its disappearance were confirmed 48 and 72 h after CIDR removal, respectively. Cows detected in estrus within 48 h and with confirmed ovulation 72 h after CIDR removal remained in the study (n=225). Cows were assigned to treatments according to BW (389 ± 3.1) and BCS (2.7 ± 0.01, scale 1 – 5). One of four PGF treatments were administered either 7 or 9 d after confirmed ovulation ( a single 12.5, 25 or 50 mg dose or two 25 mg doses 8 h apart), in a 2 x 4 factorial. Presence of a corpus luteum was determined by ultrasound and progesterone (P4) analyses (P4 ≥ 1 ng/ml) on either d 7 (h 0) or d 9 (h 0) of the estrous cycle. Blood samples were collected at h 24, 48 and 72 after PGF treatment to assess the incidence of luteal regression (defined as concentrations of P4 < 1 ng/ml at 48 and 72 h after PGF). Serum P4 concentrations were quantified using a chemiluminescent immunoassay. Cows received a new Estrotect®patch at PGF and were observed for estrus twice daily for 5 d. The incidence of luteal regression was greater (P < 0.05) in cows receiving either a single 50 mg (89.3%; 50/56), or two 25 mg doses of PGF (89.5%; 51/57) compared to cows receiving 12.5 mg (67.9%; 38/56) or one 25 mg PGF dose (66.1%; 37/56). Moreover, estrus response was greater (P < 0.05) for cows receiving the single 50 mg or two 25 mg doses of PGF (80.4 and 78.9%, respectively) compared to cows receiving 12.5 mg (55.4%); cows that received a single 25 mg dose were intermediate (66.1%) and did not differ from other treatments. Neither day of estrous cycle (7 or 9) nor its interaction with PGF treatment influenced luteal regression and estrus response. In conclusion, luteal regression and estrus response were greater in nonlactating cycling Nellore cows treated with 50 mg of PGF either in a single or split dose injection on d 7 or d 9 of a synchronized estrous cycle.
Keywords: beef cows, luteolysis, PGF