RESUMEN
This study was conducted to optimize the cryopreservation of epididymal bison sperm harvested in the field. In the first experiment, epididymal bison sperm were treated with or without seminal plasma (n = 6) and cooled to 5 °C over 2 hours. In a separate experiment, glycerol was added at different times and sperm was held at 5 °C for different periods of time before cryopreservation (n = 11). In addition, epididymal sperm frozen with and without seminal plasma (n = 6) and after 4, 24, and 48 hours (n = 5) of equilibration at 5 °C, were evaluated for their in vitro fertilizing ability. Post-thaw motility of bison epididymal sperm was similar when cryopreserved with or without seminal plasma or when glycerol was added at either 0, 4, 24, or 48 hours before freezing (P > 0.05). However, sperm incubated at 5 °C for 24 hours before freezing exhibited higher percentages of motile sperm (44% vs. 35% for 4 hours or 48 hours, P < 0.05). Fertilization rates of bison oocytes were not different for any treatments. Chilling the whole epididymis for 24 or 48 hours resulted in complete loss of sperm viability. In conclusion, bison epididymal sperm can be chilled outside of the epididymis for at least 48 hours before cryopreservation without compromising post-thaw sperm motility providing flexibility for technicians performing field collections.