The ability to have stallion semen sorted for sex-selected foals is being offered on a commercial basis through a new alliance between Sexing Technologies and Equine Reproduction Innovations. The use of sex-selected semen in conjunction with intracytoplasmic sperm injection will be offered to horse breeders for the 2010 breeding season.

Why used sexed semen?

Since the dawn of time, people have tried to influence the gender of offspring, from humans to cattle.

In 1998, the world's first sex-selected horse, "Call Me Madam" was produced by XY Inc. of Fort Collins, Colo. While XY Inc. continued their research and production of sex-selected foals, the service was not commercially available to breeders.

In the horse industry, it is well known that some bloodlines produce better stallions and other bloodlines better mares. Given the current economic climate, it would appear to be more efficient to produce the gender of choice rather than to overproduce in hopes of getting the gender desired. Furthermore, it might be easier to market a known-sex foal.

How does semen sorting work?

The stallion determines the sex of the foal, making it relatively easy to select the sex of the offspring if one can choose the proper sperm cell. All males have both an "X" and "Y" chromosome. All females have two "X" chromosomes. Each parent contributes one chromosome to the foal. Mares can only contribute an "X" chromosome, while the stallion can contribute either an "X" or a "Y" chromosome. If the stallion contributes an "X" chromosome, the mare will (always) contribute an "X" chromosome and the resulting foal will be a filly (XX). If the stallion contributes a "Y" chromosome" and the mare (as always) contributes an "X" chromosome, the resulting foal will be a male (XY).

Using a sophisticated machine called a flow cytometer, Sexing Technologies is able to run a thin column of sperm cells through the machine. The sperm cells are treated with a fluorescent dye that glows as they are run through the machine. Because the "X" chromosome has just that one leg of DNA compared to the "Y" chromosome, it glows slightly brighter. Therefore the machine can sort the cells into "X" cells, "Y" cells or "undetermined," which are discarded.

The process is quite specific and the assurance rate currently for getting the correct sex foal is over 95%.

Effect on the horse industry

The biggest issue raised with gender selection is whether it will tip the balance of foals into an excess of one sex or the other. The answer is no--or very unlikely. For every breeder that wants to produce only colts, there will be another that wants only fillies. The point is, they are producing foals that they have a market for before breeding, rather than selecting what they want after the foals are on the ground. In that scenario, there are always byproduct foals that are not the sex desired. The feasible result could be tighter, more marketable foal crops with a decreased number of unmarketable foals produced.

How sex selected semen should be used for best results

The first foals produced by sex-selected semen were conceived using a very small amount of sperm cells (5 million vs. the normal 500 million). The sorting process will inherently divide the amount of sperm cells available to use into small amounts (X or Y sample). After the sperm cells have been sorted, you are left with a small number of sex-selected cells. In general, there are not enough cells to use for conventional insemination with a high degree of success. However, there is one procedure for breeding mares that requires only one spermatozoa: intracytoplasmic sperm injection (ICSI).

In ICSI, the egg (oocyte) of the donor mare is recovered and placed in a culture media. One sperm cell is then manually injected into the egg under a microscope. Once fertilization occurs, the dividing egg is transferred at around day 1.5 or 2 (as opposed to day 6 or 7 in embryo transfer) into a recipient mare. This is a specialized procedure that can only be done by specially trained personnel.

What is necessary to have semen sorted on a stallion?

Chilled or frozen semen can be sent to the facility for sorting the sperm cells. The sample is sorted and the semen is frozen into straws with a small number of sperm cells per straw.

Each straw is considered a one-use straw. Once the egg has been recovered from the mare, the straw is thawed and the best sperm cell selected for use in the ICSI procedure. Currently, Sexing Technologies is the only facility that is sorting equine semen samples. Both companies recommend that breeders considering using semen sorting with their stallion should send semen now in order to have it sorted and banked for the breeding season.--Cindy Reich

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