Luteal function, estrous response, and pregnancy rate after treatment with norgestomet and various dosage of estradiol valerate in suckled cows

Two experiments were conducted to determine whether dosage of estradiol valerate (EV) or day of estrous cycle when treatment was administered affected response to Syncro-Mate-B (SMB). Suckled beef cows received a 6-mg Norgestomet (NOR) ear implant (in situ 9 d) and a 3-mg NOR i.m. injection. In Exp. 1, 74 cows received NOR treatments concurrently with either 5 or 6 mg of EV administered i.m. on either d 1, 3, or 5 of an estrous cycle (estrus = d 1). In Exp. 2, 169 cows received NOR treatments concurrently with either 5, 7, or 9 mg of EV administered i.m. on either d 3 or 9 of an estrous cycle. In Exp. 1, 67, 50, and 92%, respectively, of cows receiving 5 mg of EV on d 1, 3, or 5 had functional corpora lutea (CL) at implant removal, whereas 42, 75, and 77%, respectively, of cows receiving 6 mg of EV had functional CL at implant removal (dosage P > .10; day P < .05). Synchronized estrous responses were 50, 33, and 23%, respectively, when cows received 5 mg of EV on d 1, 3, or 5, and 67, 33, and 38%, respectively, when cows received 6 mg of EV (dosage P > .10; day P < .05). First-service pregnancy rate was decreased (P < .05) in cows treated on d 5 (33%) compared with cows treated on d 1 (63%), but dosage of EV had no effect on pregnancy rate. In Exp. 2, 52, 39, and 46%, respectively, of cows treated on d 3 with 5, 7, or 9 mg of EV had functional CL at implant removal, whereas only one cow treated on d 9 had a functional CL at implant removal (dosage P > .10; day P < .01). Synchronized estrous responses were 62, 68, and 75% for cows treated on d 3, and 96, 87, and 96% for cows treated on d 9 with 5, 7, or 9 mg of EV, respectively (dosage P > .10; day P < .01). First-service pregnancy rates were 79, 44, and 62% in cows treated with 5, 7, or 9 mg of EV, respectively (P < .05), whereas day of treatment had no effect on pregnancy rate. Dosages of EV as high as 9 mg did not differentially affect luteal response to treatment or synchronized estrous response. First service pregnancy rate was decreased in cows receiving higher dosages of EV. Luteal response, synchronization of estrus, and first-service pregnancy rate were affected by day of an estrous cycle when treatment was administered.