Effects of presynchronization and eCG on pregnancy rates to GnRH-based, fixed-time artificial insemination in beef heifers

Three experiments were conducted to determine the effects of presynchronization and treatment with equine chorionic gonadotropin (eCG) on corpus luteum (CL) and ovarian follicular development, plasma progesterone concentrations, and pregnancy rates in beef heifers subjected to a gonadotropin releasing hormone (GnRH)-based, fixed-timed AI (TAI) protocol. All heifers were given GnRH on day 0, prostaglandin F2α (PGF) on day 7, and a second GnRH on day 9 concurrent with TAI (54 h after PGF). In exp. 1( N = 148), presynchronization with PGF (days -22 and -11) decreased the percentage of heifers with non-luteal plasma progesterone concentrations on day 0 (5.4 vs 29.7%) and day 7 (0 vs 11.6%; P<0.05), but not on day 9 (74.3 vs. 66.2%; P>0.20), and reduced the number of heifers in estrus and bred before TAI (P<0.05). Although presynchronization reduced preovulatory follicle diameter (12.9±0.3 vs. 14.9±0.3 mm; mean± SEM; P<0.01), it did not affect TAI pregnancy rates (36.5 vs. 29.7%; P>0.20). In exp. 2, heifers (N = 128) were presynchronized with melengestrol acetate (MGA) (days -27 to -12), and received a controlled internal drug release (CIDR) on day 0; on day 7, half were given 300 IU of eCG at CIDR removal. Treatment with eCG tended to increase preovulatory follicle diameter in heifers that did not ovulate to GnRH on day 0 (P = 0.06), but did not affect the percentage of heifers with non-luteal plasma progesterone concentrations on day 9 (57.8 vs. 57.8%) or TAI pregnancy rates (48.4 vs. 53.1%; P>0.20). Experiment 3 was a 2 × 2 factorial arrangement of presynchronization (PGF concurrent with a CIDR on day -7) and eCG treatments (on day 7) applied to heifers in three herds (A, N = 150, B, N = 260 and C, N = 40). All heifers had a once-used CIDR from days 0 to 7. Presynchronization increased the percentage of heifers (Herd A) with low-luteal plasma progesterone concentrations on day 0 (70.7 vs. 22.7%) and day 7 (90.7 vs. 53.3%; P<0.01), but did not affect the percentage of heifers with non-luteal concentrations of progesterone on day 9 (97.3 vs. 93.3%; P>0.20). Combined for all herds, presynchronization reduced the prevalence of a CL on day 0 (23.5 vs. 73.7%; P<0.01), and increased the prevalence of follicles ≥10 mm on day 7 (96.8 vs. 86.7%; P<0.01); however, TAI pregnancy rates (195/ 439 = 44.4%) were not improved by either presynchronization or eCG treatment (P>0.20). © 2010 Agricultural Institute of Canada.