Effect of cooling of equine spermatozoa before freezing on post-thaw motility: preliminary results

The ability to ship cooled stallion semen to a facility that specializes in cryopreservation of spermatozoa would permit stallions to remain at home while their semen is cryopreserved at facilities having the equipment and expertise to freeze the semen properly. To accomplish this goal, methods must be developed to freeze cooled shipped semen. Three experiments were conducted to determine the most appropriate spermatozoal extender, package, time of centrifugation, spermatozoal concentration and length of time after collection that spermatozoa can be cooled before cryopreservation. In the first experiment, spermatozoa were centrifuged to remove seminal plasma, resuspended in either a skim milk extender, a skim milk-egg yolk-sugar extender or a skim milk-egg yolk-salt extender, cooled to 5 degrees C and frozen in 0.5- or 2.5-mL straws either 2.5 or 24 h after cooling. Samples frozen 2.5 h after cooling had higher percentages of progressively motile (PM) spermatozoa (27%) than samples frozen 24 h after cooling (10%; P < 0.05). Samples frozen 2.5 h after cooling in skim milk extenders containing egg yolk had higher percentages of PM spermatozoa (average 32%) than did spermatozoa frozen in extender containing skim milk alone (average 16%; P < 0.05). The percentages of PM spermatozoa frozen in 0.5- or 2.5-mL straws were similar (21 and 28%, respectively; P > 0.05). In the second experiment, spermatozoa were centrifuged to remove seminal plasma either before (25 degrees C) or after cooling (5 degrees C), and spermatozoa were frozen after being cooled to 5 degrees C for 2, 6, or 12 h. The percentages of PM spermatozoa were higher (P < 0.05) for spermatozoa centrifuged before cooling (30%) than for spermatozoa centrifuged after cooling (19%). Spermatozoa centrifuged at 25 degrees C then cooled for 12 h to 5 degrees C had higher (P < 0.05) post-thaw progressive motility (23%) compared to spermatozoa cooled for 12 h and centrifuged at 5 degrees C (13%). In the third experiment, spermatozoa were centrifuged for seminal plasma removal, resuspended at spermatozoal concentrations of 50, 250 or 500 x 10(6)/mL, cooled to 5 degrees C for 12 h and then frozen. Samples with spermatozoa packaged at 50 or 250 x 10(6)/mL had higher (P < 0.05) percentages of PM spermatozoa (25 and 23%) after freezing than did samples packaged at 500 x 10(6) spermatozoa/mL (17%). We recommend that semen be centrifuged at 25 degrees C to remove seminal plasma, suspended to 250 x 10(6) spermatozoa/ml and held at 5 degrees C for 12 h prior to freezing.