Commercially available sex-sorted semen is now available for a limited number of sires, reports Sandy Johnson, K-State livestock specialist. Despite assorted claims from time to time, the only proven, repeatable and reliable method of sorting is done with a machine called a flow cytometer/cell sorter. The initial development of this technology was by USDA scientists. Private industry now has licensed key patents and patented other aspects of the technology. The amount of research funding needed to take this process to commercialization was only available in private industry.
Sperm cells are sorted based on the X bearing sperm containing 4 percent more DNA than the Y sperm. A dye that attaches to the DNA makes the X sperm brighter when viewed by the proper electronic equipment and a powerful computer. Cells must pass a light beam one at a time to be sorted correctly.
The machine can discern three populations 20 to 30 percent X sperm, 20 to 30 percent Y sperm and 40 to 60 percent unknown with 90 percent accuracy.
Typically one conventional dose of semen of each sex can be produced in an hour however, there is considerable variability between sires and even ejaculates. Recently the Monsanto Corporation has developed a multinozzle flow cytometer to provide gender sorted semen at 85 percent accuracy. No field data have been presented on subsequent fertility of their proprietary process.
To adapt the process for commercial purposes multiple machines costing over $300,000 each and a lower dose inseminate are used. Insemination of unsexed sperm at a lower dose produces normal fertility for some bulls and only slightly reduced fertility in other bulls. When a conventional dose of sexed sperm is used, pregnancy rates are slightly lower than with unsexed semen. The sorting process increases handling and processing time and exposes the cells to a concentrated dye. Sperm move through the sorter at 50 miles per hour and come to a stop in a collection device only to be centrifuged at high speeds to concentrate them enough to fit 2,000,000 in a .25 ml straw. Pregnancy rates to sex-sorted sperm vary with management, female age and parity. Research results ranged from 35 to 100 percent of unsorted controls. In the best conditions, pregnancy rates were 70 to 90 percent of controls.
Thus if normal conception rates are 70 percent, then 49 to 63 percent conception rates might be expected with sexed sperm. Under average conditions, pregnancy rates are 50 to 70 percent of controls. Ten percent of pregnancies are expected to be of the "wrong" sex. If 10 animals are inseminated, four to five become pregnant one to two could be of the wrong sex, with the end sex ratio similar to normal. Larger numbers of females (minimum of 20) would need to be inseminated before achieving the distortion of sex ratio desired.
The cost of currently available sex-sorted semen is about $30 more than the same bull unsorted. In the sorting process, 75 percent of the semen is wasted since only one gender is usually desired from a given bull. Sex-sorted semen is not available from the most popular bulls because owners can sell all of this semen they can produce. Because of the cost of sorting and reduction in pregnancy rates, George Seidel of Colorado State University estimates that one gender must be worth $300 more than the other at birth for sexing semen to be profitable.
Sexed semen is only recommended where an existing highly successful AI program is already in place. Fertility will be highest in virgin heifers when inseminating after a detected estrus.
Use of cows and/or fixedtimed AI will generally result in unacceptable fertility rates. In superovulated cows, the number of good quality embryos is reduced by about half with sexed-sperm compared to control.
Calves born from matings with sexed semen are completely normal with the exception of gender ratio. Abortion rates, neonatal death rates, gestation length, birth weights, weaning weights and incidence of abnormalities were similar with sexed semen compared to controls.
Currently the use of sexed semen is limited due to cost and reduced fertility. Improvements should come over time and allow for more widespread application.