Sperm Sexing In Cattle
Sperm sexing is an area of bovine reproduction technology which is utilised to improve and increase farm productivity by taking advantage of the difference in the value of males and females in the dairy and beef industry. It is a useful and accurate application in which the desired X- or Y- bearing chromosome is sorted and stored in a straw for later use. Selection of the desired sex is a potential determining factor to increase the genetic progress and farmer profitability in either beef or dairy cattle. This is largely due to male calves in dairy farms having little economical value. However, in beef farms, the male calf is the product of interest due to its increased potential to produce meat.
In the late 1980s, the process of sorting semen by gender was developed and the technology has been gradually improving since then. There are more advantages to using sexed semen in the dairy industry than beef industry, but both benefit from the implementation, whether the purpose be to add to the genetic and economic value of replacement heifers for dairy production features, or to produce genetically superior beef steer calves for market. Recent technology has been developed over the years to predict and/or manipulate the calf sex proportion. There are many ways in which the application of sperm sexing impacts cattle production, as demonstrated through the advantages and disadvantages of using it.
What is Sexed Semen?
Sexed semen is semen in which the fractions of X-bearing (female) and Y-bearing (male) sperm have been modified from the natural mix through sorting and selection. The separation of the X sperm from the Y is possible due to the differences of the DNA content of these spermatic cells. The highly purified groups of sorted semen are then frozen for future use in AI, to improve AI programs or embryo transfer.
For practical purposes, sexed semen first became commercially available in North America in 2006, using the Beltsville method of flow cytometry/ cell sorting. No other method has proven effective for sexing semen. Several million doses of sexed semen have been produced to date at the industry standard of 90% purity. Purity can be adjusted to exceed 95%, but sort rates decrease greatly at >90% purity, and thus achieving such purity becomes very expensive. Sorting is based on flow cytometrical cell sorting for DNA content of sperm. Flow cytometry is the measurement of cells as they flow by a detector. The principle of this method relies on the fact that X-bearing (female) sperm contain 3.8 percent more DNA than Y-bearing (male) sperm. Prior to sorting, the sperm cells are stained with a fluorescent dye and pass through the flow cytometer as drops of liquid containing a single sperm cell per droplet. Due to the difference in amount of DNA, the X-bearing sperm radiate brighter than the Y-bearing sperm when exposed to light, allowing the cytometer’s laser and detector to determine the gender of the sperm cell based on the amount of light it releases. A positive or negative charge is then applied to the droplet containing the single sperm cell. Positively charged drops are diverted one way and negatively charged drops deflected the other, whilst uncharged droplets pass straight through. The uncharged drops may contain damaged material, or cells that were not arranged in the proper direction. The sorted sperm are then frozen in .25 cc straws to be used for AI.
This method is fairly accurate with 90% of the sperm containing the desired sex. Sexed semen will contain a lower concentration of sperm per straw (approximately 2 million) than non-sexed semen (approximately 20 million) because the sorting process is relatively slow. The sorting process also helps make the sexed semen more successful in low dosage than regular sperm as well as sorting off dead sperm. Normal ejaculate always has a number of dying, dead or damaged sperm cells, so when it is sorted,...
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