The effect of dual-hemisphere breeding on stallion fertility.

The research looked into the effect of dual-hemisphere breeding on the fertility rates of Thoroughbred stallions. The study did not find any significant negative effects; rather, it documented higher fertility outcomes when stallions were bred in the Southern Hemisphere due to smaller book sizes and better mare reproductive quality.

Research Objective and Methodology

• The study aimed to evaluate the fertility outcomes of Thoroughbred stallions subjected to breeding in both the Northern and Southern Hemispheres (dual-hemisphere breeding or DH).
• The researchers also differentiated between stallions experiencing dual-hemisphere breeding for the first time (novice/NOV) and those that were already experienced (EXP).
• Book size (number of mares a stallion is expected to breed with), total number of covers (actual breeding events), the distribution of mare type (maiden, foaling, and barren), cycles per mare, and mare age were some of the additional factors considered in the study.

Research Findings

• The research discovered that fertility values including the seasonal pregnancy rate, pregnancy rate per cycle, and first-cycle pregnancy rate were generally higher in the Southern Hemisphere (SH) than the Northern Hemisphere (NH).
• In contrast, the book size, total number of covers, and cycles per mare were higher in the NH, highlighting a negative correlation with first-cycle pregnancy rate and pregnancy rate per cycle for novice stallions.
• Fertility outcomes between the first (O1) and second (O2) seasons within a year of dual-hemisphere breeding did not show a significant difference.
• A comparison between dual-hemisphere and single hemisphere breeding seasons did not reveal any significant difference in fertility rates. Same goes for the effect of consecutive dual-hemisphere breeding seasons on fertility rates.

Conclusions

• The study did not find any detrimental effects of dual-hemisphere breeding on stallion fertility. In fact, fertility was found to be higher when stallions were bred in the Southern Hemisphere compared to the Northern Hemisphere.
• The potential reasons for the higher fertility rates in the SH were attributed to smaller book sizes and better reproductive quality of mares.
• Thus, the data suggests that dual-hemisphere breeding could be a viable breeding approach under the right conditions, especially in the Southern Hemisphere.