Reproductive efficiency of three estrus synchronization schemes comprisingfixed-time insemination in dairy cows

The objective of this field trial was to compare the odds of pregnancy at 2 5 days (P25) and at 60 days (P60), of embryonic death (ED) between 25 and 6 0 days, and of non-pregnant cows returning to heat 20-25 days after artific ial insemination (AI). Three fixed-time insemination protocols involving co mbined administration of either double gonadotrophin-releasing hormone (GnR H) and prostaglandins (A) or GnRH, human chorionic gonadotrophin (hCG) and prostaglandins (B), or double treatment with prostaglandins (C) were used. Cows included in the trial were selected from five herds, comprising a tota l of 735 (herd size range 85-250, median 120) Holstein dairy cows. Animals used for the study were multiparous, 4-7 years of age, had expressed at lea st one overt postpartum heat, were between 60 and 120 days of lactation and had at least one palpable corpus luteum and no gross morphological anomali es of their reproductive tracts. They were randomly allocated to receive tr eatment A (n = 89), B (n = 93), or C (n = 65). Pregnancy of cows not observ ed in heat 18-24 days post AI was diagnosed by transrectal ultrasonography at 25-30 days and confirmed by rectal palpation at 60-75 days post AI. Thos e that were detected in heat in the normal period post AI were re-inseminat ed. No differences were detected among the three treatment groups concernin g the odds of P25 (P = 0.5), P60 (P = 0.2) and ED (P = 0.13). In contrast, non-pregnant cows that were administered treatment A were less likely to ex hibit estrous signs in the normal period post AI than those administered tr eatments B (P = 0.02) or C (P = 0.005).