Use of Ovsynch in dairy herds--differences between primiparous and multiparous cows.

Ovsynch protocols are used to increase service rate and decrease days open and cullings for infertility. Recent reports have indicated better results after Ovsynch in primiparous than in older cows. However, this was not observed in all investigations on the subject. The objective of the study was to evaluate differences between primiparous and multiparous cows after synchronization of ovulation with an Ovsynch protocol in six trials. A total of 1584 cows (583 primiparous and 1001 multiparous cows, respectively) on three dairy farms were synchronized with an Ovsynch protocol consisting of a GnRH-analogue at Days 0 and 9, and a prostaglandin F(2alpha) analogue on Day 7. AI was carried out in all cows 16-20 h after the last treatment. Cows were categorized into primiparous and multiparous cows for analysis. Conception rate (CR) to timed AI, to further AI, overall conception rate and proportion of cows pregnant by 200 days in milk were compared between the age groups. Finally, two logistic regression models were calculated with conception to first service and conception by 200 DIM as the outcome variables. Independent variables were trial (categorical) and age group (primiparous versus multiparous). Conception rates to TAI were higher in primiparous than in older cows (37.9% versus 31.6%, P=0.015). Likewise pregnancy rates by 200 DIM were higher in primiparous cows (81.8% versus 75.4%, P=0.003). However, the extent of the difference varied between trials. Results indicate that Ovsynch protocols are more effective in primiparous than in older cows.