Variation among Beef Bulls in the Ratio of X- to Y-Chromosome Bearing Spermatozoa

A study was conducted to evaluate variation in the ratio of X- to Y-bearing sperm of individual ejaculates and to determine any relationship between skewed sex ratio and either routine mor-phological evaluation or computer assisted sperm analysis (CASA). Semen was collected from bulls weekly for 6 consecutive weeks, sperm DNA was recovered and quantitative, and real-time PCR was used to determine the ratio of X- to Y-bearing sperm in each ejaculate. The overall mean of X-bearing sperm within ejaculates was 54.7 % over the 6 weeks of semen collections. The percen-tages of X-bearing sperm were similar (P> 0.5) across all collections. Between bulls, there were differences (P < 0.05) in the mean percentage of X-bearing sperm. No significant correlations were found between CASA parameters and percentage of X-bearing sperm across bulls, so analysis was done within each bull. Different combinations of CASA and/or morphological parameters were found to correlate with the percentage of X-bearing sperm but the prediction equations were spe-cific for individual bulls and unlikely to be of use across bulls. These results confirm that the ratio of X- to Y-bearing sperm may be skewed in some ejaculates of bull semen. Some sperm parameter