Resynchronization of previously timed-inseminated beef heifers with progestins

Abstract The objective was to determine the efficacy of a previously used CIDR or melengestrol acetate (MGA; 0.5 mg/head/day) for resynchronization of estrus in beef heifers not pregnant to timed-AI (TAI). In three experiments and a field trial, heifers were reinseminated 6–12 h after first detection of estrus. Pregnancy diagnosis was done from approximately 25–43 days after either TAI or reinsemination. In Experiment 1, 79 heifers received a once-used CIDR from 13 to 20 days after TAI and 80 heifers were untreated controls. For these two groups, there were 34 and 35 heifers, respectively, not pregnant to TAI; median ± S.E. intervals from TAI to onset of estrus were 22 ± 0.2 days versus 20 ± 0.6 days ( P P  = 0.1); conception rates were 62.5% versus 76.7% ( P P  = 0.07), for CIDR and untreated (control) groups, respectively. In Experiment 2, heifers ( n  = 651) were TAI (Day 0) and 13 days later randomly assigned to one of seven groups ( n  = 93 per group) to receive a once-used CIDR (three groups; Days 13–20), MGA (three groups; Days 13–19), or no treatment (control group). Groups given a CIDR or MGA also received: no further treatment (CIDR or MGA alone); 1.5 mg estradiol-17β (E-17β) and 50 mg progesterone (P4) in 2 mL canola oil on Day 13; or E-17β and P4 on Day 13 and 0.5 mg E-17β on Day 21 (24 h after CIDR removal or 48 h after the last feeding of MGA). Pregnancy rate to TAI was lowest ( P P P P P P P P P  = 0.1) and pregnancy (38.6% versus 52.2%, P