A beef producer wants to get a bull calf from a particular cow because he wants to pass on her superior genetics to the rest of his herd.
Another beef producer wants to ensure his heifers conceive female calves because their smaller size will mean less chance of dystocia.
A dairy farmer wants to ensure his high-producing cows produce heifers to replace themselves.
Modern technology makes choosing the sex of a cow’s offspring a reality.
Rather than a 50 percent chance of getting a bull (or heifer) calf, you can expect 80 percent or better if you use sexed semen when artificially inseminating your cows.
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Sperm separation is possible because of the difference in DNA content between males and females.
Sperm cells are formed in the testicles. The immature sperm cells have the same DNA as the rest of the male body cells. They have one X and one Y chromosome.
Eventually these cells divide so the final mature sperm cell has either one X or one Y chromosome.
The cow contributes an X chromosome to each egg cell she produces. If the sperm that fertilizes the egg has an X, the fetus will be female (XX).
If the sperm cell that penetrates the egg contains a Y chromosome, the union with an X produces a male calf (XY). Since the Y chromosome contains less DNA than the X, researchers tried to separate the sperm based on weight alone.
Semen was put into a centrifuge and spun to separate the two types of sperm.
In cattle, there is a 3.8 percent difference in the DNA content between a male and female sperm due to the difference in the X and Y chromosome.
Though partially successful, this method was not reliable enough to become commercially viable.
The next technique, which is still used, involves staining the DNA.
Once stained, the semen is diluted and passed through a chamber one drop at a time. An ultraviolet light is then directed at the drops.
The difference in fluorescence allows technicians to differentiate between sperm cells that have an X chromosome and those that have a Y chromosome.
The sperm type can only be identified if there is one sperm cell in each drop.
Also, the cells must be precisely positioned to allow identification. If the drop can be read, it is sent to either a “female” or “male” test tube.
Those that can’t be interpreted go into a waste container.
It is possible to examine 100,000 sperm cells each second.
Because some cells are discarded and only live sperm are kept, the sorting rates are generally about 2,000 sperm per second for each sex with a 90 percent purity rate.
A regular artificial insemination semen straw contains at least 15 million sperm. This number can be reduced, especially if the bull has good fertility.
Researchers at Genex Corporation feel that only 1.5 million sperm are needed to produce acceptable pregnancy rates.
Even if the number of sperm per straw is lowered to this level, a machine capable of processing 15 million sperm an hour would only produce 10 straws per hour.
According to Genex, this increases costs to users by about $60 (U.S.) per straw over regular semen prices.
Further technology may enhance these sorting rates.
The company XY Inc. in Fort Collins, Colorado, uses the same technology.
Technicians there studied conception rates and found that the pregnancy rate of sexed, frozen semen was between 70 to 90 percent of that of unsexed (regular) semen.
Predicted sex was 86 percent accurate.
Canadian scientists have recently discovered a new way to separate sperm using a third technology.
The company Gensel Biotechnologies, a spinoff from the University of Guelph, uses antibodies to distinguish male and female sperm.
They have developed antibodies that selectively attach to either the X or Y chromosome.
If they add the Y antibodies to semen, it makes the sperm cells with the Y chromosome clump together. The unclumped “female” sperm (with only an X chromosome) are then available for artificial insemination.
A similar process can produce semen that contains only sperm with the Y chromosome.
This antibody technique can be applied to cattle, poultry, sheep and horses.
Gensel feels there is a tremendous market for this technology.
For a cattle producer to use sexed semen, artificial insemination is obviously necessary.
