Resynchronization of previously timed-inseminated beef heifers with progestins

• Published in: Theriogenology 2006-02, Vol.65 (3), p.557-572
• Main Authors: Colazo, M.G., Kastelic, J.P., Mainar-Jaime, R.C., Gavaga, Q.A., Whittaker, P.R., Small, J.A., Martinez, M.F., Wilde, R.E., Veira, D.M., Mapletoft, R.J.
• Format: Article
• Language: English
• Subjects: Animals; artificial insemination; Beef cattle; Breeding - methods; Cattle - physiology; controlled internal drug release devices; Drug Implants; Estradiol; Estradiol - administration & dosage; Estradiol - analogs & derivatives; estrus detection; Estrus synchronization; Estrus Synchronization - methods; Female; Fertility - drug effects; Insemination, Artificial - veterinary; Pregnancy; Pregnancy Rate; Progesterone - administration & dosage; Progestins; Progestins - pharmacology; Prostaglandins F - administration & dosage; Random Allocation; Reproduction - drug effects; Resynchronization; Time Factors
• ISSN: 0093-691X; 1879-3231
• DOI: 10.1016/j.theriogenology.2005.06.001

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