Effects of linear cooling rate on motion characteristics of stallion spermatozoa

Three experiments were designed to analyze the effects of cooling rate on survival of stallion spermatozoa in a milk-based extender, at 0 to 96 hours after reaching the desired temperature. The samples were warmed to 37 degrees C and were evaluated by computer-assisted analysis of sperm motility. In Experiment 1, rate of cooling between 37 and 20 degrees C was evaluated. Sperm motion was not affected by cooling at plunge, -0.42 or -0.28 degrees C/minute. However, storage of spermatozoa at 5 degrees C after slow cooling below 20 degrees C was superior to storage at 20 degrees C. In Experiment 2, 3 cooling rates from 37 degrees to 5 degrees C were evaluated. Cooling at either -0.05 or -0.7 degrees C/minute was superior (P<0.05) to plunging spermatozoa to 5 degrees C. Cooling at -0.05 degrees C/minute rather than -0.7 degrees C/minute maximized the percentage of motile spermatozoa and their curvilinear velocity. In Experiment 3, cooling rates from 20 to 5 degrees C were evaluated, with all samples cooled at -0.7 degrees C/minute from 37 to 20 degrees C. Sperm motion was similar (P>0.05) after cooling below 20 degrees C at -0.012, -0.05 or -0.10 degrees C/minute, and the 2 slower rates were superior (P<0.05) to cooling at -0.3 degrees C/minute. It was concluded that stallion spermatozoa can be cooled rapidly from 37 to 20 degrees C, but should be cooled at