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 < 0.001); estrus rates were 70.6% versus 85.7% ( P = 0.1); conception rates were 62.5% versus 76.7% ( P < 0.3); and pregnancy rates were 44.1% versus 65.7% ( 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 < 0.05) for the group given a CIDR plus E-17β and P4 on Day 13 and E-17β on Day 21. Variability in return to estrus was greater ( P < 0.001) in the control and MGA groups than in CIDR groups. Conception and pregnancy rates in heifers given a CIDR (65.1 and 61.4%) were higher ( P < 0.01) than those fed MGA (49.6 and 40.4%), but not different from controls (62.2 and 54.9%, respectively). In Experiment 3, 616 heifers received a once- or twice-used CIDR for 7 days, beginning 13 ± 1 days after TAI, with or without a concurrent injection of 150 mg of P4 (2 × 2 factorial design). Pregnancy rate to TAI was 47.2%. In heifers that returned to estrus, there was no significant difference between a once- or twice-used CIDR for rates of estrus (68.8%, P < 0.3), conception (65.9%, P < 0.6) and pregnancy (45.3%, P < 0.8). Injecting progesterone at CIDR insertion increased the median interval from CIDR removal to onset of estrus ( P < 0.05) and reduced rates of estrus (63.8% versus 73.8%, P < 0.05), conception (60.5% versus 70.6%, P = 0.1) and pregnancy (38.6% versus 52.2%, P < 0.02). In a field trial, 983 heifers received a once-used CIDR for 7 days, beginning 13 ± 1 days after TAI. Pregnancy rate to TAI was 55.2%. The median (and mode) of the interval from CIDR removal to estrus was 2.5 days. Estrus, conception and pregnancy rates were 78.2, 70.3 and 55.0% (overall pregnancy rate to TAI and rebreeding, 78.7%). In summary, a once- or twice-used CIDR for 7 days, starting 13 ± 1 days after TAI resulted in the majority of nonpregnant heifers detected in estrus over a 4-day interval, with acceptable conception rates; however, injecting progesterone at CIDR insertion significantly reduced both estrus and pregnancy rates, and estradiol treatment after CIDR removal was associated with a decreased pregnancy rate to TAI. Fertility was higher in heifers resynchronized with a once-used CIDR than with MGA.