Efficacy of PGF2α to synchronize estrus in water buffalo cows (Bubalus bubalis) is dependent upon plasma progesterone concentration, corpus luteum size and ovarian follicular status before treatment

This study was conducted to identify factors affecting PGF2α efficacy to synchronize estrus in water buffalo cows. After detection of a corpus luteum (CL) by rectal palpation, cows were treated (im) with dinoprost (12.5, 25 or 50 mg) or d(+) cloprostenol (75, 150 or 300 μg) in a total of 66 treatments. Blood samples were collected 0, 24 and 48 h after treatment and ultrasound examinations and observations for estrus were performed daily to the day of ovulation or to 6 days after treatment. No PGF2α dose–response pattern was observed and overall rates of luteal regression (progesterone <1.0 ng/ml at 48 h), estrus, no detected behavioral estrus with ovulation occurring, and ovulation were 71.2, 36.4, 19.7 and 54.5%, respectively. To analyze plasma progesterone concentrations and ovarian dynamics, cows were divided in three groups according to their response to treatment. Cows that failed to have ovulations from a follicle after treatment (Group A, n=30) had (P<0.05) a lower plasma progesterone concentration (2.98 ng/ml) and smaller CL area (CLA; 187.3 mm2) before treatment as compared with cows that had an ovulation from a follicle (4.43 ng/ml and 223.7 mm2, respectively; Groups B and C, n=36). In cows that failed to ovulate, plasma progesterone concentration decreased in the first 24 h, but did not decline further and was >1.0 ng/ml 48 h after treatment. Moreover, no significant change in CLA after treatment was detected, indicating that treatment induced only partial luteolysis. In cows that ovulated, plasma progesterone concentration and CLA decreased continuously from treatment to ovulation (consistent with complete luteolysis). Threshold values of 2.8 ng/ml for plasma progesterone concentration and 189 mm2 for CLA were identified as the best predictors of ovulation before treatment (83.3 and 80.6% sensitivity and 58.6 and 65.5% specificity, respectively, with positive and negative predictive values around 71%). When the origin of the ovulatory follicle was investigated, the interval from treatment to ovulation was shorter (91.9 versus 113.3 h; P<0.05), and the ovulatory follicle had a slower growth rate (1.02 versus 1.55 mm per day; P<0.005), a lesser increase in diameter from treatment to ovulation (4.7 versus 8.0 mm; P<0.001), and a greater maximum diameter (13.2 versus 12.1 mm; P<0.05) in cows that ovulated from the largest follicle present in the ovary before treatment (Group B, n=27) compared with cows that ovulated from the second largest follicle present in the ovary before treatment (Group C, n=9). In summary, the efficacy of PGF2α for causing luteolysis and synchronizing estrus and ovulation in buffalo cows was dependent upon plasma progesterone concentration, CL size and ovarian follicular status before treatment.