After collection, semen is diluted with semen extender and stored prior to insemination (Table 1). For horses, swine, poultry, sheep, and goats, semen is stored at temperatures above 0°C and is referred to as fresh semen. For cattle, semen is stored at temperatures below 0°C and is referred to as frozen semen. The decision to use fresh or frozen semen is based primarily on fertility. In most species, fertility is considerably lower with frozen than with fresh semen. Therefore, fresh semen is the preferred choice. In cattle, frozen semen is equivalent to fresh semen in terms of fertility. As a result, frozen semen is used with cattle because it allows for much longer periods of storage prior to insemination, compared to fresh semen.
Extenders used in the preservation of semen perform several basic functions, including providing nutrients for sperm metabolism, neutralizing metabolic wastes, stabilizing sperm membranes, preventing drastic changes in
Table 1 Preferred collection and storage conditions for semen from cattle, chickens, horses, sheep and goats, swine, and turkeys
Type of semen
Maximum storage length
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5°C with aeration
>1 year 24 48 hours 24 36 hours 2 5 days 4 6 days 12 24 hours the osmolarity of semen, and retarding bacterial growth. The chemical compositions of semen extenders are similar across species. Common ingredients in extenders include glucose, sodium bicarbonate, phosphate buffers, organic zwitterionic compounds, sodium citrate, potassium chloride, albumin and other proteins, and antibiotics. Extenders used for preparing frozen semen include these ingredients as well as cryoprotectants, which protect spermatozoa during freezing and thawing.
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