SURVIVAL OF BULL SPERMATOZOA SEEDED AND FROZEN AT DIFFERENT RATES IN EGG YOLK-TRIS AND WHOLE MILK EXTENDERS

Six factorially arranged experiments were designed to study effects of seeding, freezing, and thawing rates in whole milk and egg yolk-Tris extenders commonly used for commercial cryopreservation of bull sperm. In these extenders, semen normally is supercooled to -13 or -14-degrees-C unless the sperm are seeded. When sperm were supercooled or seeded, either mechanically or with immobilized silver iodide, and frozen to -196-degrees-C, the postthaw percentages of motile sperm were 59, 57, and 64%, respectively. Freezing rates of -15, -25, and -35-degrees-C/min gave similar sperm survival rates and were superior to -5-degrees-C/min. For milk, the critical freezing temperature extended to -75-degrees-C before transfer to liquid nitrogen gave good results. For egg yolk-Tris extender, transfer to liquid nitrogen was less critical once -50-degrees-C had been attained. Thawing of sperm in water baths at 25 and 45-degrees-C gave similar results, and both temperatures were superior to 5-degrees-C. The postthaw percentage of motile sperm in egg yolk-Tris was equal or superior to that of sperm frozen in milk. A freezing rate of -15-degrees-C/min to -100-degrees-C and thawing at 25-degrees-C consistently gave good results.