Freezing Bovine Semen. III. Effect of Freezing Rate on Bovine Spermatozoa Frozen and Stored at −79° C

Abstract

These experiments show the effect of various freezing rates on the decreases in the percentages of motile spermatozoa during freezing and storage at --79 C. Spermatozoa diluted in heated fresh skimmilk showed a progressive decline in motility, irrespective of freezing rate when ampules were removed at --10, --15, --20, --35, --50, and --79 C. Greatest losses in motility with 12 freezing rates occurred between --10 and --35 C. Samples thawed immediately after reaching --79 C. showed the highest motility when a rate of 10 C. per minute from --15 to --50 C. was used. However, following storage for ten days at --79 C. there was a smaller loss of motile spermatozoa with a rate of 5 C. per minute. Very rapid freezing (70 C. per minute) was extremely detrimental; 75% of the total motile spermatozoa present before freezing were lost during" freezing ~ and storage for ten days, as compared to only 40% with a freezing rate of 10 C. per minute from --15 C. Similar, but less pronounced, harmful effects were observed when semen diluted in whole milk and yolk-citrate was frozen rapidly. In a storage trial for 0, 2, 4, 8, 16, and 32 wk., the optinmm freezing rate for semen diluted in fresh skimmilk or homogenized milk was 1 C. per minute from +5 to --15 C., 5 C. per minute from --15 to --50 C., and rapidly (approximately 20 C. per minute) from --50 to --79 C. The optimum rate for semen diluted in 1:4 egg yolk-citrate was 3 C. per minute from -t-5 to --15 C. and 5 or 10 C. per minute from --15 to --50 C. The greatest decline in spermatozoan survival occurred during the first 4 wk., regardless of diluents or freezing rates.