Each year we get comments from producers about unusual strings of bull or heifer calves being born, sometimes sired by the same bull. The standard explanation that I use is that anything can happen with relatively few observations on one farm. Averaged over many births, the odds are that percentages of bull or heifer calves will be about 50:50. Most people seem satisfied with such an answer but if such results are from artificial insemination, there are often lingering questions about whether the bull studs have been sorting semen to favor one gender over the other. Although sorting semen into X- and Y-bearing spermatozoa is possible, the process is slow, tedious, expensive, and not yet commercially available. However, recent work from Louisiana State University indicates that bulls themselves can differ in their potential to sire male or female offspring.
The 1996 Louisiana Dairy Report includes results of a semen study conducted by Dr. John Chandler and his graduate student, Hilde Steinholt. They used fluorescent DNA image analysis techniques to compare two semen ejaculates from each of 10 bulls to a standard preparation containing 50% Y-bearing spermatozoa. The analysis revealed a range in percentages of Y-bearing spermatozoa from a low of 24% to a high of 84% for the 20 individual ejaculates. Using replicated observations to measure variation in assay precision, they found that both ejaculates of one bull contained significantly greater (84% & 82%) than the expected 50% Y-bearing spermatozoa. Interestingly, one ejaculate of each of two other bulls was either significantly above (76%) or below (24%) the expected 50% with Y chromosomes while the other ejaculate from each of those bulls did not differ >from a 50:50 split of X- and Y-bearing spermatozoa. Now if you happened to be using a bull with over 80% Y-bearing spermatozoa, you might need to get used to high percentages of bull calves being born. Conversely, using 100 straws from an ejaculate with only 24% Y-bearing spermatozoa would raise your expectations for a high percentage of heifer calves.
Somewhat similar observations were observed in two prior studies using human semen with ranges of Y-bearing spermatozoa ranging from about 42% to 57% in one study and 27% to 68% in the other. These findings may help explain some unusual gender ratios that can occur. They also raise basic questions about mechanisms of physiological control over such processes as sperm transport and spermatogenesis that lead to such variable ratios between individuals or even among ejaculates of the same individual. If control mechanisms can be identified then perhaps we will one day be able to use procedures to enable bulls to produce semen that is already 'sorted' for production of either heifer calves or bull calves. It likely will not occur at a rate of 100% but if we could expect consistent results at 80% or better, there would likely be a market. Finally, don't pester your AI representatives about the proportions of Y-bearing spermatozoa among their offerings --- they don't really have an economical way of providing that information routinely.