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The only reliable and relatively fast method of sex regulation in cattle is X and Y sperm high-speed sorting by flow cytometry. In October 2003 a field experiment started in Poland. The aim of the experiment was to examine the efficiency of sperm sexing and the fertility rate in field conditions. The semen of three Holstein bulls was used for the experiment. The semen was sexed and frozen according to modified XY, Inc. (Fort Collins, CO, USA) method. Only the X-fraction of spermatozoa was sorted and frozen. Sexing was performed at a speed of 15–20 × 106 spermatozoa/h and frozen in doses of 2.5 × 106 spermatozoa/straw. Progressive motility of spermatozoa was 90–95% immediately after sorting and 50–70% after freezeing/thawing for all three bulls. The X-fraction sorting purity was checked by re-analysis and it ranged from 90% to 96%. In total, 316 inseminations at 10 farms were performed up to June 2004. Fertility data of 178 inseminations were collected by ultrasonographic (USG) examination during the same time. Average fertility rate was 37.08%. However, significant variations of fertility rates were observed between farms: it ranged from 22.22% to 84.21%. Four calves, all females, were born after insemination with the X-fraction until June 2004. The experiment will continue.

311 CATTLE SEX REGULATION BY SEPARATION OF X AND Y SPERMATOZOA PRELIMINARY RESULTS OF FIELD EXPERIMENT IN POLAND