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estrus, estradiol concentrations, uterine ph, pregnancy rates, gnrh injections


Research has demonstrated that cows that are in estrus within 24 h of fixed-time AI had elevated concentrations of estradiol and greater pregnancy rates compared to cows that are not in estrus. Our objective was to determine if estradiol and/or estrus had an effect on uterine pH during a fixed-time AI protocol. Non-lactating beef cows (n = 20) were treated with the CO-Synch protocol (100μg GnRH on d -9; 25 mg PG on d -2; and 100μg GnRH on d 0). Half (n = 10) the cows received an injection of estradiol cypionate (ECP; 1mg) 12 h following PG. Cows detected in standing estrus within 24 h of the second GnRH injection were considered to be in standing estrus. Cows treated with ECP had greater concentrations of estradiol compared to non-treated cows (8.3 ± 0.7 and 5.2 ± 0.7 PG/mL, respectively), and a treatment by time interaction influenced concentrations of estradiol. All cows had similar concentrations of estradiol at time of ECP, but ECP treated cows had elevated concentrations of estradiol following the second GnRH injection compared to control cows. Treatment, time, and treatment by estrus by time influenced uterine pH. All cows had a similar uterine pH 12 h after ECP, but control cows that did not exhibit estrus had a higher uterine pH compared to control cows that did exhibit estrus and ECP cows that exhibited estrus at time of the second GnRH injection, the time insemination would occur (pH 7.0 ± 0.1, 6.7 ± 0.1, 6.8 ± 0.1, respectively). ECP cows not exhibiting estrus were intermediate (6.8 ± 0.1). All cows had a similar uterine pH from 24 h after time of insemination through ovulation. In summary, ECP treatment elevated concentrations of estradiol and lowered uterine pH to a level similar to the uterine pH of control cows that exhibited estrus within 24 h of when insemination would occur.

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