For the cows, some difficulty in reaching the fetuses with the ultrasound probe for sexing was encountered. Thus, sex ratios at pregnancy determination may not coincide perfectly with the sex ratio of calves at birth. In comparison, pregnancy rates for sexed semen with cows were less than half of those for conventional frozen semen in cows. We emphasize that there were large sire and technician effects on pregnancy rates with sexed semen in the cows. Pregnancy rates with the more experienced technician and the more fertile bulls were nearly double those of the less experienced technician using lower fertility bulls.
Further research is required into the effects of sexed semen on pregnancy rates, especially in cows.
We clearly have been able to significantly alter the sex ratio of fetuses and calves. Altering the sex ratio will allow commercial producers to better use growth and carcass genetics in production of male market offspring, while capitalizing on maternal genetics and environment provided by the cow from designated maternal lines. Acknowledgements This research was supported in part by XY, Inc., the Colorado Agricultural Experiment Station, and the Colorado State University Office of the Vice President for Research and Information Technology. We are particularly indebted to the inseminators, Y.G. Chung, D. Lefever, K. McSweeney, and M. Schreiber. We also thank Dr. M. Holland and Z. Brink for ultrasonography, and the staff at the John E. Rouse Colorado State University Beef Improvement Center for excellent assistance.