Zwitterionic buffers preserve ram semen quality more efficiently than TRIS during storage at 15 °C

This study was designed to evaluate the effect of various buffers on the storage of ram semen at 15 °C. Second ejaculates from six adult males were collected using an artificial vagina and diluted in either MOPS, TRIS, TES, HEPES, citrate, or phosphate-based extenders. Semen samples were stored at 15 °C and the sperm motility and viability (membrane integrity) variables assessed after 0, 24 and 48 h intervals. Significantly higher progressive sperm motility rates were recorded at 0 h of storage, and higher motile and progressive sperm motility at 24 and 48 h, when zwitterionic-based extenders (MOPS, TES and HEPES) were used, compared to citrate, TRIS, and phosphate-based extenders—with the last group showing a drastic reduction in sperm motility during storage. The zwitterionic groups were also superior to the other treatments in terms of sperm velocity (straight line velocity, VSL; curvilinear velocity, VCL; average path velocity, VAP) at 0 h of storage, although at 24 and 48 h the differences were minimal in the CITRATE group—regarding all velocity variables, and in the TRIS group, regarding the VCL parameter. Sperm diluted in the TRIS-based extender showed a marked increase in the proportion of circular sperm trajectories (lower sperm linearity, LIN, and straightness, STR), and a decrease in the VAP. The reduction in the vigour of the sperm in the TRIS extender (measured by VCL) was less pronounced than in the other groups. At the same time, VSL was reduced, as more sperm moved in circles, and the amplitude of lateral head displacement (ALH) was also dramatically increased. The CITRATE diluent recorded intermediate results—between that of TRIS and the other treatment groups, regarding the variables related to the quality of sperm movement at 0 h storage. However, following CITRATE dilution, semen underwent a clear improvement after a period of 24 and 48 h, so that differences with the zwitterionic groups were attenuated or disappeared. Similarly, the CITRATE group obtained similar or higher viable sperm values, compared to zwitterionic buffers during storage. The TRIS, and particularly the PHOSPHATE diluents, recorded poorer sperm viability after 24 and 48 h of storage. It was concluded that zwitterion-based buffers may be an acceptable alternative for inclusion in the composition of diluents for chilled ram semen storage. On the other hand, TRIS may be seen as having caused drastic modifications of certain sperm kinematic parameters during storage at 15 °C.