Effect of Season on Ovulatory Response and Reproductive Performance in Noncyclic Lactating Dairy Cows Synchronized with Ovsynch.

The aim of this study was to compare the ovulatory response and reproductive performance after Ovsynch protocol in noncyclic dairy cows during the warm (n = 43) and cool seasons (n = 70). Noncyclic cows (n=113) received Ovsynch protocol; GnRH1 (d 0); PGF2α (d 7); GnRH2 (d 9); FTAI (d 10). Ultrasonographic examinations were performed to determine the preovulatory follicle size (d 0, d 10) and ovulatory response to the GnRH1 (d 7) and GnRH2 (d 17). Follicle size at the onset of Ovsynch was not different in cool season (20.8 ± 0.9 mm) compared to that in warm season (19.1 ± 1.1 mm, P > 0.05). The percentage of small-sized follicles at the onset of Ovsynch was higher (P < 0.05) in warm season (37.2%) than cool season (18.6%). However, follicle size at FTAI was similar (P > 0.05) between warm (15.3 ± 0.3 mm) and cool (15.8 ± 0.3) seasons. Ovulatory response to the GnRH1 and the GnRH2 of Ovsynch were not different during the warm (90.7%, 83.7%) and cool seasons (81.4%, 87.1%), respectively (P > 0.05). Pregnancy rate was also similar (P > 0.05) during the warm (34.9%) and cool seasons (35.7%). Cows that had large-sized follicles at the onset of Ovsynch had a tendency (P = 0.08) for lower pregnancy rate in warm season (12.5%) compared to cool season (52.6%). It was concluded that season did not affect the ovulatory response to the first GnRH and pregnancy rate in noncyclic dairy cows that were synchronized with Ovsynch. [ABSTRACT FROM AUTHOR]