Administration of PGF-2α and GnRH is a widely used method for ovulation synchronization. The objective of this study was to assess whether regular cyclic activity prior to synchronization treatment increases conception rate. The experiment was performed on 326 multiparous Italian Friesian dairy cows synchronized with GnRH and PGF-2α 80 days at least, after calving. The treatment begins with an injection of GnRH, given at any stage of the estrous cycle. Seven days later, an injection of PGF-α is given followed two days later by a second injection of GnRH. Cows were inseminated 8 to 24 h after the second GnRH injection without regard to estrus behavior. Cyclic activity of animals before hormonal treatment was assessed by daily measurement of milk progesterone. Moreover progesterone concentrations were used to discriminate animals in two groups: HHL (H= high progesterone levels at the first GnRH injection; H= high progesterone levels at the PGF2α injection; L= low progesterone levels at insemination) and NNN (HLL, HHH or LHL). This experiment demonstrates that synchronisation treatment is more efficacious on cows showing regular cyclic activity (conception rate of 49.4% vs 19.4%; P<0.001). This difference between cyclic and non-cyclic cows is also observed in the HHL group (48% vs 31%, P<0.01) and in the NNN group (38% vs 7%, P<0.01). In conclusion, this study demonstrated that a regular cyclic activity improved synchronization treatment efficiency.
