Effect of time interval between prostaglandin F2α and GnRH treatments on occurrence of short estrous cycles in cyclic dairy heifers and cows

Prostaglandin F2α (PGF2α) and GnRH treatments, when administered 24 h apart during early diestrus, cause short estrous cycles in some dairy cows and heifers [J. Taponen, M. Kulcsar, T. Katila, L. Katai, G. Huszenicza, H. Rodriguez-Martinez, Short estrous cycles and estrous signs after premature ovulations induced with cloprostenol and gonadotropin-releasing hormone in cyclic dairy cows, Theriogenology 2002; 58, 1291–1302]. We investigated the effect of a time interval between PGF2α and GnRH administration on the appearance of short cycles. Estrus was induced in heifers with dexcloprostenol. A second luteolysis was induced similarly on day 7 after ovulation, and either 0 (T0) or 24 h (T24) later an injection of GnRH (0.1 mg of gonadorelin) was administered. We monitored ovarian activity with progesterone analyses from blood plasma samples and with ultrasonography. Fourteen cases (12 in T0 and 2 in T24) were excluded due to either incomplete luteolysis (2 cases) or unresponsiveness to GnRH (10 in T0 and 2 in T24). Short estrous cycles (7 to 8 d) were detected in 11/11 and 8/17 heifers in groups T0 and T24, respectively, with a significant difference in the incidence of short cycles (P < 0.01). In Experiment 2, estrus was induced in cows on day 8 (D8, n = 18), 9 (D9, n = 5), or 10 (D10, n = 3) with cloprostenol and gonadorelin administered simultaneously. Daily milk samples were collected for progesterone analysis until subsequent estrus was detected and ovarian ultrasound examinations were performed. Eight cases had to be excluded due to unresponsiveness to GnRH, leaving 18 cases eligible for the study. Short estrous cycles (7–12 d) were detected in 14/18 cows. In conclusion, shortening the time interval between PGF2α and GnRH treatments increased the incidence of short estrous cycles and appeared to increase the proportion of females unresponsive to GnRH treatment.