Reproductive Efficiency in Stallions with Reduced Fertility: Results from a Frozen Semen Breeding Program

The aim of this study was to evaluate the reproductive efficiency of a stallion with reduced fertility using frozen semen within a controlled artificial insemination program. Frozen semen from a single batch collected from a genetically valuable Oldenburg stallion was used to inseminate seven sport mares. Post-thaw semen quality was assessed using a computer-assisted sperm analysis (CASA) system. Estrous cycles were closely monitored by transrectal ultrasonography, with examinations intensified around ovulation. Ovulation occurred spontaneously or was induced with human chorionic gonadotropin (hCG) when appropriate. A single deep intrauterine insemination was performed per cycle, with deposition of 6–10 straws into the uterine horn ipsilateral to ovulation. Post-insemination monitoring focused on uterine response and detection of inflammatory or ovarian abnormalities. Post-thaw semen analysis revealed severely compromised quality, characterized by low progressive motility (1.33%), reduced viability (~35%), and a high incidence of secondary morphological abnormalities (35–40%). Despite these limitations, four out of seven mares conceived, resulting in an overall pregnancy rate of 57.1%. Successful pregnancies were associated with optimal uterine receptivity and rapid resolution of post-breeding inflammatory responses. These findings demonstrate that acceptable reproductive outcomes can be achieved using low-quality frozen semen when insemination timing, technique, and uterine management are carefully optimized. The controlled use of subfertile but genetically valuable stallions may therefore be justified within assisted reproduction programs.