Sex-sorted sperm for gender selection is now a reality.
Continuing research will likely lead to product improvement and the potential to utilize sex-sorted semen on lactating cows.
However, fertility results indicate sex-sorted semen is best used on virgin heifers currently.
Sexed semen has been “just around the corner” for quite some time. Through the years, countless techniques have been investigated with no potential application in the real world. However, in the 1980’s a breakthrough in semen sexing technology was made by USDA researchers in the Lawerence Livermore Laboratory in California. The patents for this technology were licensed to XY, Inc of Fort Collins, CO, which spent the 1990’s to optimize efficiency of these sorting procedures.
One of only a few repeatable techniques to sex sort sperm at a high level of purity uses a flow-cytometer to detect a 3 to 4% difference in DNA content between male and female sperm and sort them with upwards of 90% purity. The first step in this procedure is to dilute sperm to a very low concentration and stain them with a fluorescent dye. The sample is then sent through the flow-cytometer at 60 mph under 30 to 60 psi of pressure. As sperm pass through the internal laser beam, the dye fluoresces. Because of the larger X chromosome, female sperm emit slightly more light than male sperm, with the smaller Y chromosome.
Detectors measure the amount of fluorescence and assign positive or negative charges to each droplet containing a single sperm. Charged deflector plates then split the single stream into 3 streams: positively charged particles containing one sex go one way; negatively charged particles containing the other sex are deflected in the opposite direction, while uncharged droplets containing multiple sperm or an unidentified sex pass straight through. Confirmed with tens of thousands of offspring born in world-wide research trials, the procedure separates sperm of the two sexes with approximately 90% purity. However, that still leaves 10% of the undesired sex available to compete for fertilization.
Commercialization of sexed semen in the U.S. began in 2003 when Genetic Resources International (GRI, Navasota, TX) was granted a license. In late 2004, Select Sires partnered with GRI to begin collection and processing of sex-sorted semen with four proven sires. In 2005-2006, Select Sires conducted a nationwide field test comprising over 27,000 services. Test herds were randomly selected from herds with average or better reproductive efficiency in order to accurately determine success for the “average producer.” Nearly 7,000 offspring have resulted so far with a gender ratio of 89% heifers from sexed semen compared to 48% heifers with conventional semen (2005-2006 Select Sires Field Data).
Several major limitations have stifled implementation of sex-sorted semen. Reduced conception rates have been a primary hurdle. Sex sorting of sperm is a highly invasive procedure that negatively impacts sperm viability and longevity compared to normally cryopreserved semen. Additionally, the procedure is extremely slow and inefficient. To properly sort, sperm must be precisely oriented as they pass through the laser and fluorescence detectors in the flow cytometer. Due to the flat shape of bovine sperm heads, only about 30% are correctly oriented and half of these are female. Thus, only 15% of the sperm going into the machine are recovered as a marketable, sexed product.
The high rate of sperm loss precludes use of the “most elite” sires for production of sexed semen. Although the 3,000 to 5,000 sperm of each sex sorted per second sounds like a lot, this translates into ~1.3 hours of sorting to process enough semen for a standard 20 million sperm/straw dosage. Due to the slow sorting speed, commercialization is only possible with very low sperm numbers per dose (~2 million). Adding to the problem, the high cost of flow cytometry equipment (~$250,000 per machine) and requirements for intensive amounts of highly skilled labor dictates a high cost for sexed semen. Because of the low sperm numbers per dose and compromised sperm viability, Select Sires only recommends use of sexed semen in well-managed, highly-fertile, virgin heifers. While many research herds have realized very acceptable conception rates, averages indicate well-managed herds that achieve 60 to 65% conception rates in virgin heifers with normal semen can expect 45 to 55% conception rates with sexed semen.
With over 27,000 inseminations evaluated, Gender SELECTed semen produced a conception rate that was 84% of that achieved with conventional semen (Gender SELECTed semen: 42% conception rate; Conventional: 50%). These favorable field results led to commercialization of sexed semen by Select Sires in the fall of 2005. Orders for sexed semen have exceeded company expectations.
What is the return on investment for sexed semen?
The return on investment for sexed semen depends on complex interactions between factors such as; conception rate with non-sexed semen, reduction in conception (if any) due to use of sexed semen, price differentials between sexed and conventional semen, expected gender ratio for sexed vs. conventional, and the value differential between bull and heifer calves. Most of these factors vary considerably among herds, making the breakeven value of sexed semen different for each respective producer. Select Sires provides a sexed semen calculator in Microsoft Excel format to account for more than 20 variables that can affect return on investment. Based on today’s product, the best return on investment will be achieved with virgin heifers while following the “Keys to Success” (see below) to insure optimum probability for conception.
Keys to success
Sexed semen requires a breeding gun designed to accommodate the smaller diameter ¼ cc straws. Straws should be thawed and handled identically to traditional ½ cc straws. However, the smaller diameter and compromised semen quality makes sexed semen much more sensitive to cold-shock and errors in semen handling. To maximize potential for success:
- Thaw straws in 95° F water bath for 45 seconds.
- Semen thawing and handling environments should be warm and draft free.
- Warm all semen handling equipment including guns, sheaths, and paper towels prior to use.
- Only highly experienced technicians should use this product.
- Use only in well-managed, virgin heifers that have achieved greater than 60% of their mature weight by 14 months and are in moderate or better body condition.
- Inseminate heifers 8 to 12 hours after observed estrus (AM/PM Rule).
- Use of estrus synchronization and breeding to observed estrus is encouraged; however, timed-AI in the absence of observed estrus is discouraged.
Other methods of sorting semen
New sex-sorting technologies have recently emerged, including gender specific antibodies, centrifugation, and free flow electrophoresis. In evaluating sex-sorting technologies, investigate the technology fully to make informed decisions. If the technology is not based on flow-cytometry and the patents developed by USDA, ask for scientific evidence that the procedure can, in fact, sort sperm. Statistically, hundreds of births must be assessed to know whether the procedure can effectively produce offspring of the desired sex. Similarly, conception data should be based on thousands of services and palpated pregnancies, as simple nonreturn data may distort success rates. To date, only flow cytometry provides both sorting purity and commercial adaptation.
Sex-sorted sperm for gender selection is now a reality. Continuing research will likely lead to product improvement and the potential to utilize sex-sorted semen on lactating cows. However, fertility results indicate sex-sorted semen is best used on virgin heifers currently.