Effect of thawing rate and post-thaw temperature on motility and acrosomal-maintenance in bovine semen frozen in plastic straws.

One ejaculate from each of 13 Holstein bulls was diluted in egg yolk-tris-gly~erol, packaged in .25 ml Continental straws and frozen in liquid nitrogen vapor. Two thawing rates, 35 C for 1 min and 5 C for 3 min, were compared after immediate post-thaw exposure to either 1 C, 20 C or 37 C for either 1 or 3 minutes. Straws were plunged into water baths to achieve these thaw and post-thaw treatments. Following treatment, samples were incubated at 37 C and evaluated at 0 (immediately posttreatment), and again at 4 and 8 hr for percent intact acrosomes and percent motility. Time of exposure (1 vs 3 min) to the post-thaw temperature did not affect percent intact acrosomes. Considering mean percent intact acrosomes (mean of 0-, 4and 8-hr incubation), the 35 C thaw resulted in significantly greater (P~.01) percent intact acrosomes after exposure to the 20 C and 37 C post-thaw treatments (47.1 and 48.8, respectively) than did the 5 C thaw (30.6 and 30.0, respectively). There was no difference in the mean percent intact acrosomes between the 5 C and 35 C thaws (37.6 vs 39.3, respectively) when exposed to the 1 C postthaw treatment. For the 5 C thaw, there was a higher (P<.01) incidence of acrosomal damage when the semen was exposed immediately after thawing to 20 C or 37 C as compared to 1 C. A thaw rate x post-thaw treatment interaction was significant (P<.01) for percent intact acrosomes and percent motility. Results for motility were similar to those obtained for acrosomal maintenance. (