Fixed-time insemination in peripuberal, lightweight replacement beef heifers after estrus synchronization with PGF2alpha and GnRH

Estrus synchronization contributes to optimizing the use of time, labor, and financial resources by shortening the calving season, in addition to increasing the uniformity of the calf crop. We determined whether acceptable pregnancy rates could be achieved after synchronization of ovulation and fixed-time artificial insemination (AI) in peripuberal replacement beef heifers using gonadotropin-releasing hormone (GnRH) and PGF2alpha. Crossbred heifers from two herds (MH, n = 239; SS, n = 330) were wintered at a single location. After a prebreeding examination revealed that 55 heifers had a reproductive tract score (RTS) of I (infantile reproductive tracts), they were culled and the remaining heifers were assigned randomly to one of three treatment groups: administration of 25 mg PGF2alpha i.m. on Days - 12 and 0 followed by estrus detection and insemination between 10 and 14 h after an observed estrus (Control; n = 173); administration of 100 μg GnRH i.m. on Day -6, followed by 25 mg PGF2alpha i.m. on Day 0, then fixed-time AI and administration of 100 μg GnRH i.m. on Day +2 (GPG; n = 172); and, treatment as for group GPG in addition to administration of 100 μg GnRH i.m. on Day -12 (GGPG; n = 169). Bulls were introduced 10 days after AI for 60 days to breed heifers which did not conceive after AI (clean-up bulls). On Days -12, -6, and 0 transrectal ultrasonography was used to monitor ovarian structures in a subset of heifers (30 per treatment). At 30-35 days after AI, ultrasound was used to determine the presence of a viable fetus. Presence of a fetus and stage of pregnancy were determined via palpation per rectum 61-63 days after the conclusion of the breeding season. Heifers in the MH herd (309 ± 1.9 kg) were heavier (P < 0.001) than those in the SS herd (283 ± 1.7 kg) at initiation of the breeding season. Synchronized pregnancy rates were greater (P < 0.05) in GGPG (25.4%) and GPG (22.1%) than Control (12.7%) heifers. Pregnancy rates were 9, 21, 32, or 31% for heifers with RTS of 2, 3, 4, or 5, respectively. The average diameter of 22 follicles induced to ovulate in heifers treated with GnRH (GPG and GGPG treatments) was 14.2 ± 0.8 mm (range = 10.0-23.6 mm). In conclusion, a fixed-time ovulation synchronization program using GnRH and PGF2alpha improved pregnancy rates in peripuberal, lightweight replacement beef heifers.