EFFECT OF TIME OF ARTIFICIAL-INSEMINATION ON PREGNANCY RATES, CALVINGRATES, PREGNANCY LOSS, AND GENDER RATIO AFTER SYNCHRONIZATION OF OVULATION IN LACTATING DAIRY-COWS

In order to assess the optimal time of artificial insemination (AI) in relation to ovulation, lactating dairy cows (n = 732) from herds with rolling herd averages of 9980 to 11,800 kg from three milkings per da y were randomly assigned to five groups by stage of lactation and pari ty. Ovulation was synchronized by administration of GnRH followed 7 d later with PGF(2 alpha) followed 2 d later with a second treatment wit h GnRH. Cows were inseminated at 0, 8, 16, 24, or 32 h after the secon d injection of GnRH (ovulation occurs between 24 and 32;h after GnRH). Pregnancy diagnoses were performed by ultrasound at 25 to 35 d post-A l. Pregnancy rates per Al were similar for the groups inseminated at 0 , 8, 16, and 24 h and lower for the group inseminated at 32 h. A signi ficant quadratic effect of treatment suggests that the middle time per iods (8, 16, and 24 h) may produce the greatest pregnancy rate per AI. However, the group inseminated at 0 h had lowest pregnancy loss, and the group inseminated at 32 h tended to have the greatest pregnancy lo ss compared with that of the other groups. The calving rate was simila r between the groups inseminated at 0, 8, 16, and 24 h and lower in th e group inseminated at 32 h. The time of AI also appeared to affect ge nder of calf: cows bred at 0 and 32 h having a higher percentage of fe male offspring. In conclusion, there appears to be substantial flexibi lity in the time of AI after the second injection of GnRH, and lower r eproductive rates were observed only when Al was after the time of ovu lation.