For the last 50 years, there have been numerous press releases and rumors that sexed semen is “just around the corner.” Well, in the 1980s, a breakthrough in sex sorting technology was made by USDA researchers in Livermore, California, and Beltsville, Maryland. The patents for this technology were licensed to XY Inc. in Fort Collins, Colorado, which performed a considerable amount of research during the 1990s to optimize efficiency of these sorting procedures.

Commercialization of sex-sorted semen in the U.S. started in 2003 with a license granted to Sexing Technologies (ST). In February 2003, the first ST sexing laboratory started operations in Navasota, Texas. In February 2006, a second laboratory was established in Plain City, Ohio, at Select Sires Inc. In August 2006, an ST laboratory opened in Madison, Wisconsin, and was affiliated with ABS Global.

During mid-2007, an ST sorting laboratory was open in Ithaca, New York, for Genex Inc. and an XY licensee in Canada for Semex Inc. In January of 2008, ST started a sorting lab outside of Calgary, Alberta Canada for Alta Genetics Inc. In mid-2008, ST opened another sorting laboratory in Fond du lac, Wisconsin. Presently, five of the six major North American A.I. companies have an ST laboratory near a production facility to sort semen. We now can truly proclaim sex-sorted semen is a reality!

Flow-sorting technology
The only repeatable technique to sex-sort sperm uses a machine called a flow cytometer to detect varying differences in DNA content over multiple breeds from 3.6 to 4.1 percent between X- and Y-bearing sperm. The first step in this procedure is to dilute sperm to a very low concentration and stain them with a harmless DNA-specific fluorescent dye. The sample is then sent through the flow cytometer at 60 mph under pressures of 40 psi.

Stained sperm are aligned single-file in a fluid stream. Then cells of interest are identified at a particular droplet and sorted if classified as a cell of interest. Droplets holding the cell of interest are deflected into a catch tube. As the cells enter the laser beam profile, they emit light proportional to the amount of DNA. For the sorting to be successful each sperm head must be precisely oriented so the DNA content can be accurately determined.

Because an X chromosome is larger it emits slightly more light than a Y-bearing (male sperm) cell. Detectors measure the amount of fluorescence or light emitted and assign positive or negative charges to each droplet containing a single sperm. Charged deflector plates then split the single stream into three streams: positively charged droplets containing X-bearing sperm go one direction; negatively charged Y-bearing sperm are deflected in the opposite direction; uncharged droplets containing dead, multiple cells in one drop or droplets with unidentified, unresolvable sperm pass straight through. This procedure separates sperm of the two sexes with approximately 90 percent purity.

Technology limitations
As you can imagine from the description above, sex sorting of sperm is a highly invasive procedure compared to the process used to freeze conventional semen. 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 bull sperm heads, only about 60 to 70 percent are correctly oriented and half of these are female. Thus, only 15 percent of the sperm going into the machine are recovered as a viable, sex-sorted product.

Although 5,000 sperm of each sex sorted per second sounds like a lot, this translates into approximately one hour and seven minutes of sorting to process enough semen for a standard 20-million sperm straw. Thus, due to the slow sorting speed and the reality that only 10 to 15 percent of the sperm entering the sorting machine are recovered as marketable product, commercialization is only possible with very low sperm numbers per straw (approximately 2 million sperm per straw). Additionally, the cost of flow cytometry equipment (approximately $400,000 per machine) and highly skilled labor required to sort sperm dictates that sex-sorted semen be sold at a higher price than the same bull packaged traditionally.

Conception rates with sex-sorted semen
Sex-sorted semen has always been recommended for use in heifers because of the known compromise in conception rates largely due to the reduced sperm number per straw. Initially, reports published from a limited number of services warned of approximately a 30 percent reduction in conception rates in virgin heifers. In January of 2008, insemination and calving information were retrieved from 198 dairy herds that had used Select Sires’ gender sex-sorted semen from January 2005 to January 2008 (Figure 1*). The unadjusted conception rate for 41,398 breedings to gender-sorted semen was 45 percent.

Across all herds, 74 percent of gender-sorted semen was used at first service, 18 percent at second service and 8 percent at third and subsequent services. The conception rate was 47 percent, 40 percent and 34 percent, respectively. These actual field results revealed that gender-sorted semen was achieving approximately 80 percent of conventional semen or only a 20 percent reduction in conception rates.

Genetic merit of sex-sorted semen
Inefficiency of sorting and unknown acceptance by dairy producers of sex-sorted semen dictated that sires initially made available were not the most elite sires for genetic merit. There are numerous considerations that must be counted when a sire is placed in a program for sex sorting. The semen quality, quantity and fertility along with the calving ease of his offspring are major considerations for bulls selected for semen sorting.

Demand for sex-sorted semen by dairy producers has increased both the number and quality of sires that have sex-sorted semen available (Table 1*). Use of sex-sorted semen solely to produce more heifers without regard to genetic merit may not be a profitable strategy under most commercial dairy management conditions. The genetic merit of sex-sorted sires available today has increased from when sex-sorted semen was initially offered to a point that genetic merit need not be compromised.

Gender bias
Research has consistently demonstrated that DNA X-sorted technology used to sex semen produces about 90 percent purity; therefore, the subsequent gender ratio should be approximately 90 percent heifers and 10 percent bulls, if a sufficient number of calvings are counted. Odds and ratios advise that in about 26 percent of the herds, 80 percent or less of the offspring will be heifers. Seven percent of the time, 70 percent or less of the offspring will be heifers. These are simple mathematical probabilities of which the herd owner should be aware before purchasing sex-sorted semen. Reality is that DNA sex-sorted semen technology is achieving approximately 90 percent heifer calves when evaluated across a larger number of calvings.

Economic evaluations of sex-sorted semen
The optimal use of sex-sorted semen depends on many economic and biological factors. There is no reliable “rule of thumb” that can dictate proper use across the variety of herds and economic scenarios possible. Dairy producers could use sex-sorted semen to produce more herd replacements, to produce heifers to sell to other dairy producers or both. Increasing replacements from within reduces the risk of introducing infectious diseases by increasing biosecurity.

Sex-sorted semen will produce more heifer calves that have lower birth weight than bull calves and will reduce rate and cost of difficult calvings. Difficult calvings occur in approximately one out of every 10 calvings of first-lactation heifers. With sex-sorted semen, culling of poor-performing, growing heifers is possible, avoiding losses associated with bringing them into the herd only to have them removed early in lactation. In some specialized dairy sectors such as organic dairies and herds using crossbreeding programs, the value of replacements may remain significantly above the cost of rearing, making sex-sorted semen very valuable.

The return on investment for the dairy producer depends on a complex interaction between the initial conception rate with non-sexed semen, the percent reduction in conception (if any) due to use of sexed semen, the price differential between sexed and conventional semen, the value differential between bull and heifer calves and the enterprise that the extra heifers will be used for (herd replacements, to contract, etc). Most of these factors will change considerably from herd to herd, which differentially affects the value of sex-sorted semen to each respective producer.

Keys to success
Use of sexed semen will require a breeding gun designed to accommodate the smaller diameter quarter-cc straws. Straws are to be thawed and handled identical to their half-cc counterparts. However, the smaller diameter will make sex-sorted semen more sensitive to cold shock and errors in semen handling. To maximize the 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 contacting straws.

• Only highly experienced technicians should be using sex-sorted semen.

• Use only in well-managed, virgin heifers that are in moderate or better body condition.

• Inseminate heifers eight to 12 hours after observed estrus (AM/PM Rule).

• Use of estrus synchronization and breeding to observed estrus is encouraged, but use of timed A.I. in the absence of observed estrus is discouraged.

Impact of sex-sorted semen
Most dairy producers have used sex-sorted semen to produce more heifers by using it on every first service of virgin heifers that are detected in estrus regardless of the genetic merit of the heifer. Because of market prices for replacement heifers, high milk prices, reproductive inefficiency of dairy herds and the shortage of heifers for replacements, sex-sorted semen has been used to just get more heifers.

The real value in the future will be to source replacement heifers from the best cows in the herd. Who do you really want your replacements from? Sex-sorted semen provides an opportunity to advance the genetic merit of cows in the herd by selecting the best heifers, or maybe even cows, to be the dams of herd replacements. In conjunction with high genetic merit sires, sex-sorted semen will produce more rapid genetic advancement than obtaining replacements from the whole herd. For this to work, the dairy must be able to rank its dams by genetic merit, so sire ID is a must. Genomics testing will be another tool that producers can use to identify the gene profile of dams for sex-sorted semen.

There is no question sex-sorted sperm for gender selection is a reality today. The optimal use of sex-sorted semen depends on many economic and biological factors. There is no steadfast rule of thumb that can dictate proper use across the diversity of herds and economic scenarios possible. Sex-sorted semen can provide a more plentiful supply of genetically superior replacement heifers. Herds with better genetic information (sire ID and genomic testing in the near future) have the ability to capture more value from sex-sorted semen. Herds that desire to guarantee a more reliable and better quality of internally grown heifers will use sex-sorted semen to source more heifers and improve biosecurity. PD

*Figures and tables omitted but are available upon requet to editor@progressivedairy.com