Effect of GnRH injection at day 6 and 12 after insemination on fertility of Holstein dairy cows during the warm season

The objective of this study was to determine the effects of two different time of GnRH injection after artificial insemination (A.I) (d 6 and d 12) on reproductive performances of Holstein cows during the warm season. Cows randomly assigned to treatments 1) No GnRH injection (control, n = 300),2) GnRH injection at day 6 after A.I (G6, n=300) and 3) GnRH injection at day 12 after A.I (G12, n=300). Ultrasonography was used to detect pregnancy and late embryonic/fetal mortality between Days 32 and 60 after AI. Data were analyzed using Proc GLM of SAS. Results showed treatment affected the percentage of cows diagnosed pregnant at 32±1 d and 60±3 d after resynchronized AI. A greater percentage of G6 (25.0%) cows were pregnant at 32 d after resynchronized AI than control cows (19.3 %) and G12 cows (20.0 %). The risk of pregnancy loss between d 32 and 60 of gestation was affected by treatments and decreased in cows received GnRH injection after AI Compared to control cows. Percentage of cows that showed estrus at 19-24 d after AI was highest in G6 cows and lowest in control cows compared to other treatments. In summary, administration of a GnRH on Day 6 after AI improved reproductive performance in dairy cows.