291 - Synchronization and Seasonality Effects on Follicular Development, Estrus Expression, and Pregnancy in Brahman Females

Abstract: Control of follicular development is essential for estrous synchronization. This experiment determined differences in follicular development during synchronization in Brahman females. Cows (n=219) and heifers (n=90) in Fall (n=170) and Spring (n=139) were synchronized with a CIDR insert and administration of GnRH or PG treatment on d0 (1/2GnRH: n=106, GnRH: n=103, PG: n=100), and CIDR removal and PG administration on d6 followed by estrus detection and AI for 96h. Blood samples were collected on d0, d6, and d8 and/or AI to determine circulating progesterone (P4) concentrations. Transrectal ovarian ultrasonography occurred daily. Cows were grouped by CL presence (d0CL, d6CL) and P4 concentrations (d0P4, d6P4, d8P4, Low< 1 ng/mL, High≥1 ng/mL) on d0 and d6. The GLIMMIX procedure of SAS was used to analyze follicular wave initiation, wave emergence day, estrus expression, and pregnancy success. There were significant interactions of treatment by d0CL (P=0.0237), and season by d0P4 (P=0.0118) on wave initiation. Cows receiving 1/2GnRH with a CL on d0 (97±2%) and Fall cows having High d0P4 (96±3%) had the greatest probability of wave initiation. Age significantly impacted wave initiation (P=0.0213; heifers: 93±4%, cows: 79±5%). Interactions of treatment by season (P=0.0405) and treatment by d0CL (P=0.0379) significantly impacted wave emergence timing. Season (P< 0.0001; Fall: 23±5%, Spring: 65±6%), and the interactions of treatment by d0P4 (P=0.0291) and new wave by d0P4 (P=0.0357) influenced estrus expression. Cows receiving 1/2GnRH with High d0P4 and cows with High d0P4 that initiated a new wave had the greatest probability of estrus expression. Pregnancy rate differed by estrus expression (P=0.0055; no estrus=25±6%, estrus=54±8%) and semen type (P=0.0105; conventional=52±9%, sexed=26±6%), and tended to differ by d8P4 (P=0.0570; Low=30±7%, High=48±8%). These results suggest that differences in follicular development and behavioral estrus in synchronized Brahman females may be due to seasonality and endogenous progesterone production.