Season and temperature-humidity index affect sperm quality and in vivo fertility of beef bulls

Increased temperature and humidity can reduce sperm quality and fertility of bulls. The objective was to evaluate effects of season and temperature-humidity index (THI) on quality of sperm collected from Bos taurus, Bos indicus and crossbred beef bulls and its fertility in fixed-time artificial insemination (FTAI) programs, considering season and THI at semen collection and 30 d before collection. Semen was collected and processed in Botucatu, SP, Brazil, between 2020 and 2022; there were 120 bulls from three breeds (Angus, Brangus, and Nelore). Frozen-thawed semen was used for fixed-time artificial insemination on 63 farms in southern, southeastern, and central-western Brazil, with 26,800 pregnancy records (5129 from Angus semen; 1217 from Brangus; and 20,454 from Nelore). Season and THI affected (P < 0.05) sperm quality characteristics (fresh and frozen-thawed) and pregnancy rates. In summer, high THI negatively affected pre- and post-cryopreservation characteristics of Angus sperm, whereas sperm from Brangus and Nelore bulls were less affected; with high THI, sperm from Nelore bulls had the best quality, both before and after freezing. Pregnancy rates were highest with semen processed in winter (Angus and Nelore) or spring (Brangus). With semen processed in the summer or with the highest THI, Brangus and Nelore bulls had significantly higher pregnancy rates than Angus. In conclusion, Angus bulls were more sensitive to heat stress during the summer and high THI conditions, with significant reductions in fresh and post-thaw semen quality. Results were attributed to Brangus and Nelore sires being better adaptated to high THI, sustaining semen quality and fertility in all seasons under in a subtropical climate.