Effect of cooling rates on post-thaw sperm motility, membrane integrity, capacitation status and fertility of dairy bull semen used for artificial insemination in Sweden

We studied the effects of 2 different cooling rates during equilibration of semen from room temperature to 4 degrees C, at 4.2 degrees C/min (control split sample) or at 0.1 degrees C/min (treatment split sample) on in vitro sperm viability post thawing and fertility after AI. Forty batches of split -frozen semen from 14 dairy bulls (Swedish Red and White breed) aged 14 to 16 mo or 66 to 79 mo were evaluated post-thawing for sperm motility (visual and computer-assisted sperm analysis [CASA], membrane integrity (fluoresce nt microscopy and now cytometry post-loading with the combined fluorophores Calcein AM/EthD-1 and SYBR-14/PI); acrosomal status (with Pisum sativum ag glutinin [PSA] staining); and capacitation status (CTC-assay). Fertility va lues (56-d nonreturn rate) of the slow cooling batches (treatment) were 0.4 % units higher than for faster cooled (control) batches, but the difference was not statistically significant. Fertility values for the older bulls we re 1.6 % units higher than for the group of younger sires. No statistically significant correlations were found between semen viability parameters ass essed in vitro and 56-d nonreturn rater Visually assessed sperm motility, m embrane integrity, capacitation and acrosomal status post-thawing did not d iffer significantly between cooling procedures, however the percentage of m otile spermatozoa and the kinetic characteristics of spermatozoa - average path velocity (VAP), straight path velocity (VSL) and curvilinear velocity (VCL) - assessed by CASA differed significantly between cooling procedures. The results indicate that most of the in vitro sperm viability parameters post-thawing and the fertility results for bulls after AI did not differ si gnificantly between the 2 semen cooling procedures tested. (C) 1999 by Else vier Science Inc.