The use of estradiol and/or GnRH in a two-dose PGF protocol for breeding management of beef heifers

Abstract

The objective was to determine reproductive performance following AI in beef heifers given estradiol to synchronize ovarian follicular wave emergence and estradiol or GnRH to synchronize ovulation in a two-dose PGF-based protocol. In Experiment 1, 561 cycling (confirmed by ultrasonography), Angus heifers received 500 μg cloprostenol , i.m. (PGF) twice, 14 days apart (days 0 and 14) and were equally allocated to four groups in a 2×2 factorial design. On Day 7, heifers received either 2 mg estradiol benzoate (EB) and 50 mg progesterone (P), i.m. in oil (EBP group) or no treatment (NT group). Half the heifers in each group received 1 mg EB, i.m. in oil on Day 15 (24 h after the second PGF treatment) with TAI 28 h later (52 h after PGF), and the other half received 100 μg GnRH, i.m. on Day 17 (72 h after PGF) concurrent with TAI. All heifers were observed for estrus twice daily from days 13 to 17; those detected in estrus more than 16 h before scheduled TAI were inseminated 4–16 h later and considered nonpregnant to TAI. Overall pregnancy rate (approximately 35 days after AI) was higher in heifers that received EBP than those that did not (61.6% versus 48.2%, respectively; P <0.002); but was lower in heifers that received EB after PGF than those that received GnRH (50.0% versus 59.8%; P <0.02). Although estrus was detected prior to TAI in 77 of 279 heifers (27.6%) treated with EBP (presumably due to induced luteolysis), they were inseminated and 53.2% became pregnant. Overall pregnancy rates were 51.4, 68.3, 45.0, and 55.0% in the NT/GnRH, EBP/GnRH, NT/EB, and EBP/EB groups, respectively ( P <0.05). In Experiment 2, 401 cycling, Angus heifers were used. The design was identical to Experiment 1, except that 1.5 mg estradiol-17β (E-17β) plus 50 mg progesterone (E-17βP) and 1 mg E-17β were used in lieu of EBP and EB, respectively. All heifers receiving E-17β 24 h after the second injection of PGF (NT/E-17β and E-17βP/E-17β) were TAI 28 h later without estrus detection, i.e. 52 h after PGF. Heifers in the other two groups received 100 μg GnRH, i.m. 72 h after PGF and were concurrently TAI; heifers in these two groups that were detected in estrus prior to this time were inseminated 4–12 h later and considered nonpregnant to TAI. Estrus rate during the first 72 h after the second PGF treatment was higher ( P <0.05) in the E-17βP/GnRH group (45.0%; n =100) than in the NT/GnRH group (16.0%; n =100), but conception rate following estrus detection and AI was not different (mean, 57.4%; P =0.50). Overall pregnancy rate was not significantly different among groups (mean, 46.9%; P =0.32). In summary, the use of EB or E-17β to synchronize follicular wave emergence and estradiol or GnRH to synchronize ovulation in a two-dose, PGF-based protocol resulted in acceptable fertility to TAI. However, when 2 mg EB was used to synchronize follicular wave emergence, early estrus occurred in approximately 28% of heifers, necessitating additional estrus detection. A combination of estrus detection and timed-AI in a two-dose PGF protocol resulted in highly acceptable pregnancy rates.