ELSEVIER COMPARISON OF TWO PROCESSING SYSTEMS FOR BULL SEMEN WITH REGARD TO POST-THAW MOTILITY AND NONRETURN RATES E. Kommisrud.’ T. Graffcr and T. Steine Norwegian Cattle Association, N-2300 Hamar, Norway Received for publication : May 26, 1995 Accepted : zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP November 6, zggs ABSTRACT Two semen processing systems based on either skim milk-yolk extender or BiladylB were compared with regard to post-thaw motility and nonretum rates. The skim milk-yo& extender contained penicillin and streptomycin, while BiladylB consisted of tylosin, gentamycin, spectinomycin and liicomycin, The dilution of semen invohed a 2-step procedure for both extenders. The second dilution with the skim milk-yolk extender took place at 5°C while the Biladyl@ extender was processed at room temperature. In this study 197 ejaculates from 45 Norwegian Cattle bulls were used. The ejaculates were split-sampled and diluted with the 2 extenders. A total of 4 1,445 fust inseminations was recorded, 2 1,035 with semen diluted with skim milk-yolk and 20,4 IO with Biladyl@& The 60-d nonreturn rates were 73.1 and 71.9% for skim milk-yolk and BiladylQ respectively. The results for skim milk-yolk were significantly better than for Biladyl@ (P<O.Ol). No difference in post-thaw motility for these extenders was found. Key words: bull, frozen semen, extender, motility, nonreturn rates INTRODUCTION The main purpose for preserving semen is to maintain the fertilizing capacity of spermatozoa while diluting the ejaculate to maximize the use of superior genetic bulls. Moreover, the risk of spreading pathogens through semen needs to be effectively reduced. Traditionally, in Norway, a skim milk-yolk based extender has been used for the cryoprcscrvation of bull scmcn (3). However, current international standards with regard to the antibiotic components of semen extenders have made it necessary to look for alternatives to the penicillin/streptomycin-containing skim milk-yolk extender. It has been demonstrated that the combination of gentamicin, tylosin, lincomycin and spectinomycin is more effective for the control of mycoplasmas and ureaplasmas than the standard I Correspondence and reprints requests. Theriogenology 45:1515-1521, 1996 0 1996 by ElsevierScience Inc. 0093~691x/w$15.00 PII so093-691x(96)00119-4