147. Impact of Estradiol and Progesterone Concentrations at CIDR Removal and 48 Hours Later on Bos Indicus Cattle Estrus Expression and Pregnancy Success.

Abstract Estradiol concentrations and estrus expression have been reported to significantly impact sperm transport, embryo survival, and pregnancy success. This study evaluated whether estradiol and/or progesterone concentrations at CIDR removal (d0) or 48 h later (d2) were different in the spring versus fall among Brahman females (n = 683) that did or did not exhibit estrus and did or did not become pregnant following a CIDR-based synchronization protocol. Data were collected across seven breeding seasons (Fall 2020 to Fall 2023). Females were synchronized using either the PG 6-day CIDR protocol or a modified version of this protocol (same protocol in spring and fall each year). Estrus was monitored from CIDR removal to d3 using Estrotect patches and visual observation. Sera samples were collected on d0 and 2 and analyzed for estradiol and progesterone via radioimmunoassay. Follicle size on d0 was determined by transrectal ultrasonography. Animals detected in estrus were inseminated using the am/pm rule; animals not expressing estrus were inseminated on d4 concurrent with an injection of GnRH. Differences in estradiol and progesterone concentrations between seasons, estrus status, and pregnancy status were analyzed using the MIXED procedure in SAS (v9.4), with age and year as covariates. Spring breeding had increased estrus expression (P < 0.01; 63±2% vs. 41±3%) and pregnancy success (P < 0.01; 49±3% vs. 38±3%) compared to fall breeding. Animals exhibiting estrus had increased concentrations of estradiol on d0 and 2 (P < 0.01), but estradiol did not differ between pregnancy status on either day (P > 0.67). There was an estrus by pregnancy interaction on d0 (P = 0.01) with animals that exhibited estrus and became pregnant having increased estradiol compared to all other groups. Estradiol concentrations were greater in spring versus fall (P < 0.01) on both days. Progesterone was greater (P < 0.01) in spring versus fall but did not differ (P > 0.08) between estrus expression or pregnancy status on either day. Follicle size on d0 was greater in spring versus fall (P = 0.02) and among animals exhibiting estrus (P < 0.01) but did not differ (P = 0.40) between pregnancy status. Among animals that exhibited estrus, interval from CIDR removal to estrus did not differ between seasons (P = 0.51) or pregnancy status (P = 0.71). In summary, Brahman cows synchronized for spring breeding had increased follicle size, estradiol concentrations, estrus expression, and pregnancy success compared to those bred in the fall. Estradiol on d0 and 2 significantly influenced estrus expression and pregnancy success, regardless of the season. Progesterone did not affect estrus expression or pregnancy success. Further understanding of the role and regulation of estradiol concentrations around the time of AI could help optimize pregnancy success in Bos indicus cattle.