Effect of presynchronization prior to Ovsynch on ovulatory response to first GnRH, ovulatory follicle diameter and pregnancy per AI in multiparous Holstein cows during summer in Iran

The aim was to evaluate the effect of presynchronization with GnRH and PGF2α prior to Ovsynch on ovulatory response to first GnRH, diameter of largest follicle at TAI and pregnancy per AI (P/AI) in multiparous dairy cows during summer. Cows (n=1069) were randomly assigned to one of three timed-AI (TAI) protocols. The TAI protocols were: 1) Ovsynch (O; n=425), GnRH- 7d-PGF2α-56h-GnRH-16h-TAI), 2) double-Ovsynch (DO; n=302), GnRH-7d-PGF2α-3d-GnRH and Ovsynch was initiated 7 days later, and 3) G7G-Ovsynch (G7G; n=342), PGF2α-2d-GnRH and Ovsynch was initiated 7 days later. Ovarian examinations were performed by transrectal ultrasonography during Ovsynch to determine ovulatory response to first GnRH and diameter of largest follicle at TAI. Presynchronization increased ovulatory response after first GnRH of Ovsynch (P=0.001), which was greater in DO (74.0%) and G7G (76.0%) groups compared to O group (50.0%). Means (±SEM) diameter (mm) of largest follicle at TAI was smaller in cows presynchronized before Ovsynch (DO and G7G, overall 15.7±0.3) compared to that in cows subjected to a standard Ovsynch without presynchronization (18.5±0.42). P/AI at 32 d after Al was greater (P=0.001) in G7G (32.7%) and DO (31.1%) groups compared to Ovsynch (19.7%) group. Presynchronization prior to Ovsynch also affected P/AI at 60 and 150 d after AI (P<0.05). In conclusion, DO and G7G protocols resulted in greater ovulatory response to first GnRH, smaller ovulatory follicles and greater P/AI compared to a standard Ovsynch protocol. Therefore, TAI protocols that include a presynchronization with GnRH and PGF2α prior to Ovsynch should be used in multiparous cows during summer to achieve acceptable reproductive performance.