Inducing ovulation with human chorionic gonadotrophin improves cumulative pregnancy rates of dairy cows during the warm season

Abstract

This study was designed to assess the effects of the GnRH agonist gonadorelin and human chorionic gonadotrophin, during a timed artificial insemination program, on the fertility of lactating dairy cows during the warm and cold seasons of the year. Cows were treated intramuscularly with GnRH-agonist (Day 0) and PGF2α (Day 7), followed by either GnRH-agonist (GPG treatment; 38 animals) or hCG (GPH treatment; 26 animals) on Day 9. All cows were fixed-time inseminated (TAI) 16–22 h after the end of treatment. In this study, replacing the Day 9 dose of GnRH with a dose of hCG may result in an increase in the cumulative pregnancy rate (46.6% vs. 25%, P ≤ 0.05) but not the pregnancy rate (26.6% vs. 18.8%, P > 0.05) in heat-stressed cows (THI > 70). However, in cold season, no effects were observed in the pregnancy rates or cumulative pregnancy rates of the examined cows. This finding indicates that hCG therapy in TAI programs may mitigate the negative effects of heat stress on dairy cows reproduction. This tendency can be explained also by some studies that have shown that hCG can significantly reduce the number of small luteal cells while increasing the number of large luteal cells as the corpus luteum gets bigger in diameter, area, and volume.