RECENT RESEARCH RELATED TO GENDER PRESELECTION
Over the years, numerous methods have been suggested to alter the sex ratio of offspring. One of the most common is based on the notion that male sperm swim faster than female, but they die sooner. Thus, if you inseminate early, the male sperm die resulting in more heifer sperm fertilizing the eggs. To my knowledge, there is no conclusive, repeatable data to support the idea that male and female sperm vary in either swimming speed or longevity.
A recent article published in Animal Reproduction Science presented some interesting data to support the timing theory. Using a device which detects changes in vaginal/cervical mucous around the time of estrus, these researchers varied the time of insemination and supposedly skewed the sex ratio of resulting offspring with 90% accuracy. However, the data set presented was extremely small (13 to 14 animals/treatment). Until these results are duplicated in other laboratories and/or on a much larger scale, I'd consider this technology for altering sex ratio as strictly experimental.
An abstract presented at the 1998 Animal and Dairy Science meeting from Dr. Ray Nebel's lab at Virginia Tech, compared the gender ratios for 822 calvings as affected by time from first mount to insemination. First mount was identified by the computerized "Heat Watch" system and time to A.I. and gender ratios were retrospectively analyzed. Dr. Nebel's data (figure 1) would not support the theory that timing of A.I. has any affect on gender ratios.