Effect of preovulatory concentrations of estradiol and initiation of standing estrus on uterine pH in beef cows

Previous studies have indicated that initiation of standing estrus within 24 h of fixed-time AI influenced pregnancy rates. Furthermore, uterine environment at time of insemination can influence sperm transport. We hypothesized that preovulatory concentrations of estradiol would influence uterine pH at time of insemination. The objective of this study was to determine the influence of elevated preovulatory concentrations of estradiol on uterine pH following a fixed-time AI protocol. Cows were synchronized with the CO-Synch (n = 57) protocol, and 29 cows were treated with an injection of estradiol cypionate (ECP; 1 mg) 36 h before the second injection of GnRH. Cows that exhibited standing estrus or were treated with ECP had increased (P < 0.05) concentrations of estradiol compared to cows not in estrus and not administered ECP, respectively. There was an ECP by standing estrus interaction on uterine pH (P = 0.01). Control cows that exhibited estrus had a reduced uterine pH (6.72 ± 0.10; P = 0.05) compared to control cows not exhibiting estrus (7.0 ± 0.06). Cows treated with ECP and detected in standing estrus had a greater uterine pH (7.0 ± 0.07) compared to control cows in estrus (P = 0.02) and ECP cows not in estrus (6.81 ± 0.09; P = 0.06). The interval between the initiation of standing estrus and when pH was determined also influenced uterine pH. Cows that initiated standing estrus within 4 h of pH determination had a lower uterine pH (6.74 ± 0.12) compared to cows that initiated estrus 4–8 h (7.09 ± 0.08; P = 0.07) or 8–12 h (7.10 ± 0.15; P = 0.03) after pH determination. In summary, elevated concentrations of estradiol influenced standing estrus but only influenced uterine pH when pH was determined within 4 h of the initiation of standing estrus.