Seasonality affects post-thaw plasma membrane intactness and sperm velocities in spermatozoa from Thai AI swamp buffaloes (Bubalus bubalis)

Altogether 218 frozen semen AI doses, prepared between 1980 and 1989 and also between 2003 and 2005 from 18 AI Thai swamp buffalo sires, were examined to determine whether seasonality affects post-thaw viability, as plasma membrane integrity (PMI, using SYBR-14/PI), plasma membrane stability (PMS, using Annexin-V/PI), or motility (Mot, using CASA). A thermoresistance test (38 °C for 60 min) was used to further analyze sperm survivability in vitro. All variables were compared over 3 seasons of the year (rainy: July–October; winter: November–February; and summer: March–June), with distinct ambient temperature and humidity. PMI (% of alive spermatozoa) was higher in winter (54.6%, P < 0.001) than in the rainy (43.5%) or summer (46.7%) seasons. Outcomes of PMS (Annexin-V/PI assay) confirmed those of PMI, the highest PMS in spermatozoa processed in winter (55.7%, P < 0.001). Spermatozoa depicting linear Mot post-thaw ranged from 48.2% to 48.8% across seasons (ns), proportions that decreased during incubation (33.5–37.9%), albeit without seasonal differences. The mean percentages of straight linear velocity (VSL), average path velocity (VAP), or curvilinear velocity (VCL) were higher (P < 0.05–0.001) in the rainy season than in winter or summer, while average lateral head displacement (ALH) was higher (P < 0.05) in summer, differences maintained after incubation. In conclusion, post-thaw PMS and PMI, assessed by flow cytometry, were significantly better in sperm samples processed during winter than in samples processed during the other seasons of the year, a seasonal difference not picked up by CASA, probably due to the larger number of spermatozoa assessed.