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Semen extender is a liquid diluent which is added to semen to preserve its fertilizing ability. It acts as a buffer to protect the sperm cells from their own toxic byproducts, and it protects the sperm cells from cold shock and osmotic shock during the chilling and shipping process (the sperm is chilled to reduce metabolism and allow it to live longer). The extender allows the semen to be shipped to the female, rather than requiring the male and female to be near to each other. Special freezing extender use also allows cryogenic preservation of sperm ("frozen semen"), which may be transported for use, or used on-site at a later date.
Semen extenders should not be confused with drugs or nutritional supplements designed to increase the volume of semen released during an ejaculation. The efficacy and utility of such products is dubious. 
The addition of extender to semen protects the sperm against possible damage by toxic seminal plasma, as well as providing nutrients and cooling buffers if the semen is to be cooled. In the case of freezing extenders, one or more penetrating cryoprotectants will be added. Typical cryoprotectants include glycerol, DMSO and dimethylformamide. Egg yolk, which has cryoprotective properties, is also a common component.
In the equine, Kenney extender (named after its developer, Dr. Robert M. Kenney of the University of Pennsylvania, New Bolton) has been used for many years, and contains a non-fat dried milk solid (NFDMS) and glucose. Dual-sugar extenders typically have similar ingredients, with an additional sugar, sucrose. Other extenders (e.g. INRA '96) may also contain milk components.
Antibiotics are almost universal in semen extenders, especially those being used for shipping or freezing semen. Ticarcillin (often used in combination with clavulanic acid under the designation timentin), amikacin sulfate, penicillin, and gentamicin are commonly used. The latter - gentamicin - has been noted to reduce sperm motility in the equine. In human semen extenders, antibiotics are required for regulatory reasons, so their use is almost universal in clinics, even though antibiotics can be detrimental to sperm. This is because in procedures such as IVF with frozen sperm the sperm do not need to swim up the reproductive tract on their own, and the detrimental effects of the antibiotics are not problematic. When private donors ship chilled semen outside of the formal regulatory environment, and fertilization is accomplished by allowing sperm to swim through the reproductive tract without the help of procedures such as IVF, then it is possible to achieve better results without antibiotics.
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- Kenney (often marketed under a brand name - e.g. E-Z Mixin')
- INRA '96
- Universal dual-sugar
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- Gent Extenders Minitube
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- Spervital extenders (http://www.spervital.nl)
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-  Equine-Reproduction.com - FAQ-AI article
-  Equine-Reproduction.com - Frozen Semen FAQ article
- "Increasing Semen Volume".
- Vera-Munoz O., Amirat-Briand L., Diaz T., Vásquez L., Schmidt E., Desherces S., Anton M., Bencharif D., Tainturier D.; (2009) Effect of semen dilution to low-sperm number per dose on motility and functionality of cryopreserved bovine spermatozoa using low-density lipoproteins (LDL) extender: Comparison to Triladyl and Bioxcell; Theriogenology 71:6; 895-900
- The Equine Research Hall of Fame - Robert M. Kenney
- Aurich C., Spergser J., (2007) Influence of bacteria and gentamicin on cooled-stored stallion spermatozoa; Theriogenology 67:5; 912-918
- "Semen extenders used in the artificial insemination of swine. A review". Spanish Journal of Agricultural Research (2003) 1 (2):17-27. Archived from the original on 2012-03-04. Retrieved 2009-05-05.
- "The SpermPrep TYB Media". Retrieved 2009-05-05.