Sexing Embryo Technologies
Genetic improvement is a goal on every dairy or ranch. More efficient cows that produce more milk and meat from the same inputs give more profit. The greatest impact on genetic improvement has been from artificial insemination and culling management, followed by successful embryo transfer programs. Incorporating embryo sexing into an embryo transfer program can accelerate genetic gains if the right mating decisions are made.
Most farms consider embryo sexing as a way to reduce the number of bulls from their ET programs, and to make more efficient use of their recipient animals. The goal is to maximize the number of female calves from the top cows with less recipients. Transferring only female embryos into recipients can increase the total number of female calves produced each year from a farm. In a group of 30 cows, a normal year would produce 15 females (50%). If these same 30 cows receive female embryos, assuming a 67% pregnancy rate, 20 will produce female calves (30 x 0.67). If the remaining 10 recipients are mated normally they would be expected to produce 5 female calves (50%). The total calf production from these 30 cows would be 25 female and 5 male calves. This female calf crop will allow a farm to grow from within, or to be more selective in choosing replacement heifers, increasing the potential rate of genetic gain.
Embryo sexing is applied on farm as a later step of traditional embryo transfer programs. Every cell in the embryo (actually, every living cell in an animal) contains a complete copy of all of the DNA needed to produce a complete individual. Each cell from a male embryo contains the male (Y) chromosome. After collecting and sorting the embryos, a small biopsy of 2 - 4 cells is cut off of each embryo using a specially designed inverted microscope and an electronically controlled micromanipulator which holds an ultra-sharp splitting blade. If the embryo is at the blastocyst stage, these cells can be selected from the trophoblast cells that will make up the placenta, without even touching the inner cell mass. The biopsy is placed in a specially prepared test tube while the embryo is returned to a numbered, separate dish filled with holding media. The biopsied embryo can be frozen, or remain in culture while the next steps are carried out.
The test tubes, each carrying a biopsy from one embryo, are incubated with a special solution containing primers which will multiply specific segments of DNA, including fragments on the male (Y) chromosome, using a DNA amplification process called Polymerase Chain Reaction (PCR). The DNA of these chromosome fragments are incubated for one hour in a thermocycler to be multiplied many times.
The DNA of the embryo needs to be multiplied to have the mass required to see it during the final phase of the sexing technology: gel electrophoresis. The contents of each test tube containing the multiplied DNA is placed into a separate well in a prepared gelatin pad. An electric current is introduced across the gel which causes all fragments of DNA to move with the current. The DNA fragments move according to electrical charge and size of the fragment. The male DNA fragments are smaller and separate themselves from other DNA in the well. After 20 minutes bright bands of male and non-male DNA will show up when the gel is illuminated with ultraviolet light. By reading the specific positions of the bands, we can now tell which embryo is male and which embryo is female. The accuracy of this test is 98%.
After the Embryos are Sexed - Transfer or Freeze?
Until recently, all sexed embryos were transferred fresh to synchronized recipients. The demand for recipients is reduced when only the female embryos are transferred. For example, if 4 donors are flushed and 24 embryos are collected (average of 6 embryos per donor), approximately 12 synchronised recipients are required, as 50% of the embryos sexed should be female. The savings to the dairy or ranch in reducing the recipient pool maintenance costs can be substantial.
Sexed embryos are now being successfully frozen and thawed with a pregnancy rate which is equivalent to regular frozen embryos. Embryos which have had their zona pellucida cut and their embryonic cells exposed have different requirements for freezing than non-biopsied embryos. By experimenting through the summer using different freezing and culture medias, and refining the biopsy procedure, a simple technique has been developed that allows biopsied embryos to be frozen with normal pregnancy rates at thawing.
The Economics of Embryo Sexing:
Each new technology has to be evaluated. This technology has remarkable accuracy, and can produce more females on the farm. It is also possible that embryo sexing can be less costly than normal embryo transfer programs alone.
Apart from the potential increases in genetic gain and marketability (of both sexed embryos and recipients carrying a fetus whose sex is known), embryo sexing enables producers to influence the overall cost of embryo transfer by reducing the required number of recipients.
Also, total transfer costs are reduced. Historically, if more embryos are recovered than the number of available recipients, the extra embryos were frozen. To have these embryos transferred later involves synchronizing another group of recipients, and the costs to transfer the frozen embryos.
Some export markets now are exploring purchasing only embryos of a desired sex (females for breeders, or males for A.I. units) to fill their orders, and as such, breeders using these technologies have a marketing advantage.
A full spreadsheet for the economic evaluation of embryo sexing is available at the office for anyone who would like to look at their specific farm situation.
Embryo transfer programs are developing to reflect both the emerging biotechnologies and the demands of breeders for economical production of pregnancies. Wisely applying biotechnological advances on the farm is an ongoing challenge.