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Freezing Bovine Semen. III. Effect of Freezing Rate on Bovine Spermatozoa Frozen and Stored at -79° C.^1,2,

Dairy Breeding Research Center, Department of Dairy Science, The Pennsylvania State University, University Park

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 optimum 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 +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.

¹ Authorized for publication on August 14, 1957, as Paper No. 2186 in the journal series of the Pennsylvania Agricultural Experiment Station.

² Supported in part by a grant from the Pennsylvania State Association of Artificial Breeding Cooperatives.

³ A portion of the data contained in this paper is from a thesis submitted by one of the junior authors (L.A.M.) to the Graduate School of The Pennsylvania State University in partial fulfillment of the requirements for the degree of Master of Science.