Inducing luteolysis prior to atresia of the first follicular wave by altering the interval to prostaglandin F2ɑ administration after long-term progestin presynchronization

A series of experiments was designed to evaluate treatment schedules for control of the estrous cycle in which luteolysis is induced prior to atresia of the first follicular wave following a long-term progestin presynchronization treatment in beef heifers. The overarching hypothesis was that the proportion of heifers undergoing luteolysis and expressing estrus following PG administration would be affected by the duration of treatment with an intravaginal progesterone-releasing insert (CIDR; 1.38 g progesterone) and/or by the interval from CIDR removal to prostaglandin F2ɑ (PG; 500 μg cloprostenol) administration. In each experiment, heifers were blocked by reproductive tract score (RTS) and body weight (BW) and were assigned randomly within block to treatment. In Experiment 1, heifers (n = 91) were assigned to one of four treatments in a 2 x 2 factorial design: presynchronization via CIDR treatment for either 14 d or 18 d with administration of PG either 8 d or 9 d following CIDR removal. In Experiment 2, heifers (n = 63) were assigned to receive administration of PG either 9 d or 10 d following a 14 d CIDR presynchronization. In Experiment 3, heifers (n = 83) were assigned to receive administration of PG either 9 d or 16 d following a 14 d CIDR presynchronization. In Experiment 1, there was a tendency for a greater proportion of heifers to express estrus by 96 h after PG administration when PG administration occurred 9 d versus 8 d after CIDR removal (P = 0.07). Additionally, serum estradiol (E2) concentrations were greater at the time of PG administration when PG was administered 9 d versus 8 d after CIDR removal following an 18 d CIDR treatment (P = 0.006). In Experiment 2, pregnancy rate to AI (P/AI) was greater among heifers receiving PG administration 9 d versus 10 d after CIDR removal (P = 0.03). There was a tendency for largest follicle diameter (LFD) to be greater among non-estrous heifers receiving PG administration 10 d versus 9 d after CIDR removal when measured at timed AI (P < 0.08). In Experiment 3, there was a tendency for serum progesterone (P4) concentrations at the time of PG administration to be greater among heifers receiving PG administration 16 d versus 9 d after CIDR removal (P = 0.07). These results provide a preliminary evaluation of the feasibility of inducing luteolysis prior to atresia of the first follicular wave following long-term progestin presynchronization.