Optimal rates for cooling chicken semen from +5 to -196 C

Optimal rates for cooling chicken semen from +5 to -196 C were determined. Semen was diluted 1:5 with Beltsville Poultry Semen Extender and held at 5 C for 2 hr before adding dimethylsulfoxide (DMSO), 4% v/v. The semen-DMSO mixture was allowed to equilibrate for 2 hr at 5 C before it was placed in a plastic freeze straw of .5 cc capacity. A two step controlled-rate procedure was used to test the various cooling rates (1 to 50 C/min) through certain temperature ranges (+5 to -20 C and -20 to -180 C) and to determine the optimal semen temperature for making the transition of cooling in alcohol (Step 1) to cooling in liquid nitrogen (LN2) vapor (Step 2). Fertilizing capacity of spermatozoa was maximal when semen samples were cooled from +5 C to -20 C at a rate of 1 C/min (Step 1), then transferred to LN2 vapor (Step 2) and cooled to -80 C at 30 C/min before the samples were plunged in liquid nitrogen. After storage in LN2 for 24 hr and subsequent thawing at +2 C, the fertilizing capacity of the sperm was 47%.