Progesterone (CIDR)-based timed AI protocols using GnRH, porcine LH or estradiol cypionate for dairy heifers: Ovarian and endocrine responses and pregnancy rates

The overall objective was to compare the efficacy of GnRH, porcine LH (pLH) and estradiol cypionate (ECP), in a modified Ovsynch/fixed-time AI (FTAI) protocol that included a controlled internal drug [progesterone] release (CIDR) device. In Experiment 1, heifers received a CIDR on Day −10, and PGF (25 mg) on Day −3. At CIDR insertion, heifers received 100 μg of GnRH (n = 6), 0.5 mg of ECP (n = 6), 5.0 mg of pLH (n = 6) or 2 mL of saline (n = 7); these treatments were repeated on Day −1, except for ECP, that was repeated on Day −2, concurrent with CIDR-removal. The 5.0 mg pLH was the least effective with a longer interval to ovulation than the other groups combined (102 versus 64 h; P < 0.05). Overall mean LH concentrations (1.6 ng/mL) and area under the curve (AUC) did not differ among treatments, but mean peak LH concentration was lower in heifers given 5 mg of pLH compared to all other groups (4.5 versus 10.3 ng/mL; P < 0.05). In Experiment 2, heifers on CIDR-based Ovsynch protocols were given 12.5 mg pLH (n = 6; pLH-low), 25.0 mg pLH (n = 6, pLH-high), or 100 μg GnRH (n = 5; control). Heifers in the pLH-high group had greater (P < 0.01) plasma LH concentrations (between 12 and 20 h) than GnRH-treated heifers, but the pLH treatments did not differ (P > 0.10). Area under the curve for LH (ng/32 h) was at least 50% greater (P < 0.01) in pLH-treated heifers compared to GnRH-treated heifers (mean, 41.3, 56.3 and 20.3 for pLH-low, pLH-high and GnRH, respectively). Ovulation occurred in 15 of 17 heifers. Progesterone concentrations were higher on Days 9 and 14 in heifers given 25 mg of pLH, suggesting enhanced CL function. In Experiment 3, 240 heifers were assigned to CIDR-based Ovsynch/FTAI protocols. The first and second hormonal treatments (with an intervening PGF treatment on Day −3) were GnRH/GnRH (100 μg), ECP/ECP (0.5 mg), pLH/pLH (12.5 mg) or GnRH/ECP, respectively; pregnancy rates were 58.7, 66.1, 45.9 and 48.3%, respectively (ECP/ECP > both pLH/pLH and GnRH/ECP; P ≤ 0.05). In conclusion, CIDR-based Ovsynch/FTAI protocols using either GnRH/GnRH or ECP/ECP yielded pregnancy rates about 20% points higher than previously reported for dairy heifers bred to Ovsynch/FTAI in the absence of a CIDR.