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April 01, 2011 (Vol. 31, No. 7)

Real-Time Cytometric Assay for Gender Bias

New Methodology Makes It Faster and Easier to Selectively Bias Births of Female Dairy Cows

  • The 2011 Final Report of the Foresight Global Food and Farming Futures Project predicted that global population size will increase from nearly seven billion today to eight billion by 2030, and probably to over nine billion by 2050. With these increases in population, global food production will need to increase without the use of substantially more land and with diminishing impact on the environment.

    Technologies that support increased, sustainable productivity and which are practical for the poorest farms to adopt can increase the supply of diverse foods at affordable prices. Dairy products are one of the most efficiently produced and safely transported protein foods. However, to meet the demands of population growth, the current 277 million worldwide dairy cow population needs to expand. This is not an easy feat: dairy farmers are already in constant need of more female calves to grow their herds and replace their aging cows.

    Artificial insemination (AI) is widely used for breeding dairy cattle and has made bulls of high genetic merit available to all. A normal ejaculate from a dairy bull will contain 5 to 10 billion sperm, which can be diluted and used to inseminate 300 to 1,000 cows. The natural bias leans slightly toward bull calves as with most mammal species. While the basic principles controlling the sex of mammalian offspring are well known, only recently has technology allowed the advent of commercialization of sexed semen.

    Female sperm have 3–4% more DNA than male sperm. Traditional sexed semen techniques rely on this difference in DNA content to separate female, male, and undefined sperm on a fluorescence-activated cell sorter. The process sorts sperm with high (approximately 90%) purity but is inefficient and requires extensive manipulation of sperm prior to cryopreservation.

    In addition cell sorters are expensive, both in terms of instrument acquisition and day-to-day operation. Extensive training and a dedicated operator are typically required. Recovery of female sperm after sorting is only about 15%. This, combined with slow sorting speeds, has limited the application of sex-sorted semen to low-dose inseminations of about 2 million sperm, instead of the 10–20 million sperm used in conventional doses. Female birth rates do increase (up to 80%) but with drawbacks—lower conception rates, process variability, a steep learning curve, and high instrumentation costs. Easy-to-perform, cost-effective complimentary alternatives are needed to make widespread commercialization of sexed-sperm techniques viable.

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