Sexed semen: The next breeding approach
Author:
Dr E M Muhammed
MVSc Scholar
Department of Animal Breeding, Genetics and Biostatistics
College of Veterinary and Animal Sciences
Mannuthy, Thrissur, India, Pin-680651
Email:dremmuhammed@gmail.com

    Artificial insemination and the technology for freezing of semen had tremendous impact in dairy breeding. The successful implementation of cross breeding was primarily based on these two technologies. The subsequent introduction of Multiple Ovulation and Embryo Transfer technology (MOET) enhanced the genetic progress in dairy animals. Use of sexed semen may be considered as the next breeding Sexed semen is semen in which the fractions of X bearing and Y bearing sperm have been modified from the natural mix through sorting and selection. The goal of sexed semen is to produce a calf of a specific sex. Supply of replacement heifers in dairy farms is a major issue in commercial ventures. Use of sexed semen would provide an increased supply of replacement heifers, thereby reducing dairy heifer purchase and sale prices. Since milk production is a sex limited trait and males are not usually maintained by farmers for breeding purpose use of sexed semen will increase with in herd growth and production. Another important advantage is in bull selection. Bull selection is mainly carried out on the basis of progeny testing, i.e. breeding bulls are selected on the basis of the performance of their female progenies.

    The use of sexed semen will increase the effectiveness of progeny testing by reducing the number of pregnancies required for sire evaluation. Currently two methods appear feasible for the production of sexed semen. The commercial viable method for sexing of semen is based on flow cytometrical cell sorting for DNA content of sperm. The method works by staining sperm with a DNA binding fluorescent dye .The bovine Y chromosome bearing sperm contain 3.8% less DNA than the X chromosome bearing sperm. This allows their separation by a fluorescence activated cell sorter. The method is fairly accurate with 85-95% of the sperm containing the desired chromosome. The second reliable method for sexing is the sexing using polymerase chain reaction (PCR) amplification of DNA sequences specific for the Y or for the X chromosome .This method relies on the availability of well characterized primers which enable amplification of sex specific regions or single copy sequence in a multiplex reaction with a positive amplification control.

      The price of dairy heifers is very sensitive to supply and demand. The plentiful availability of economically priced, genetically competitive sexed semen allows for more dairy heifer calves to be born than are needed to replace culled cows. The decoupling between breeding to obtain replacement heifers and breeding to start new lactations will alter the structure of the dairy sector which may facilitate enhanced milk production from the heifers .Moreover milk price, pregnant replacement heifer price and cull cow price are positively related The cost to raise a dairy replacement heifer calf to first calving is very high, so that replacement heifer price is also high. The use of sexed semen will increase the supply of dairy replacement heifers to meet this strong demand. The decrease in heifer prices will cause the average value of a cow in the herd to decrease .Moreover the availability of female dairy sexed semen will enable farmers to keep fewer heifers and cows which is needed to produce the next generation of replacement heifer calves. Use of sexed semen will also help us to reduce inbreeding as the average level of inbreeding for the major dairy breeds continues to increase. Thus the use of sexed semen will lead to sustainable production greatly enhancing the ability to make progress in cross breeding, marker assisted selection, and genomic selection.