Induction of accessory corpus luteum in cows by gonadotropin-releasing hormone administrated after insemination

Sufficiency of progesterone limits conception rate in dairy cows. This study evaluates the effect of gonadotropin-releasing hormone on induction of accessory corpus luteum, progesterone concentrations and pregnancy in dairy cows. The hormone (0.05 mg) was administrated intramuscularly to cows with one corpus luteum on day 5-7 (Group E1, n = 57) or 11-13 (Group E2, n = 40) after insemination; control groups C1 (n = 54) and C2 (n = 41) were not treated. Detection of pregnancy by ultrasound examination and measurement of serum progesterone concentration was performed on day 25-39 and around day 90 after insemination. The occurrences of two corpora lutea in groups E1 and E2 versus C1 and C2 were 43.9% and 33.5% vs. 3.7% and 9.8% (P < 0.001). Progesterone concentrations were 5.9 +/- 2.92 ng/ml and 7.8 +/- 2.35 ng/ml in untreated and treated cows (P = 0.016), respectively. Conception rates in groups E1 and E2 were 100% and 86.7% in cows bearing two corpora lutea, and only 16.7% and 38.9% in cows bearing one corpus luteum (P < 0.01); similarly as in groups E1 and E2 compared to groups C1 and C2 (49.1% and 50% vs. 50% and 31.7%). Pregnancy rates around day 90 in groups E1 and E2 compared to groups C1 and C2 were 45.6% and 45% vs. 42.6% and 26.8%, respectively. Administration of gonadotropin-releasing hormone on day 5-7 or 11-13 after insemination was efficient to induce accessory corpus luteum and increase the concentration of serum progesterone but had no effect on the conception rate.