Reproductive efficiency of three estrus synchronization schemes comprising fixed-time insemination in dairy cows
The objective of this field trial was to compare the odds of pregnancy at 25 days (P25) and at 60 days (P60), of embryonic death (ED) between 25 and 60 days, and of non-pregnant cows returning to heat 20-25 days after artificial insemination (AI). Three fixed-time insemination protocols involving combined administration of either double gonadotrophin-releasing hormone (GnRH) 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 total 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 least one overt postpartum heat, were between 60 and 120 days of lactation and had at least one palpable corpus luteum and no gross morphological anomalies of their reproductive tracts. They were randomly allocated to receive treatment A (n = 89), B (n = 93), or C (n = 65). Pregnancy of cows not observed 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. Those that were detected in heat in the normal period post AI were re-inseminated. No differences were detected among the three treatment groups concerning 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 exhibit estrous signs in the normal period post AI than those administered treatments B (P = 0.02) or C (P = 0.005).