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.