Simple Dilution – Cooled-Shipped Semen
The general guidelines for extending semen for cooled-shipping are to have between 5% to 20% seminal plasma (Jasko, Hathaway, et al., 1992), a concentration of 25 x 106 to 50 x 106 sperm/mL (Varner et al., 1987), a volume less than 50-mL (Loomis, 1992) and 1 x 109 progressively motile sperm in the final breeding dose.
Semen extended with 5 to 20% seminal plasma had better motility at 24, 48 and 72 hours of cooled storage (Jasko, Hathaway, et al., 1992). This equates to a dilution ration (semen: extender) from 1:4 to 1:19. Thus, the minimum recommended dilution ratio is 1:4 for cooled-shipped semen. The final extended concentration of sperm should be between 25 to 50 x 106 sperm/mL. Dilution of semen to 25 x 106 sperm/ml resulted in highest total and progressive motility after 24 h compared to 50 x 106 , 100 x 106 and no dilution, with the 50 x 106 having the second best motility values (Varner et al., 1987). Lastly, The standard recommendation iis to ship ≥ 1 x 109 progressively motile sperm in the insemination dose (Brinsko, 2006) and minimize the insemination volume ( keep it< 50-mL).
There are situations when these guidelines cannot be met such as when a stallion has poorly concentrated semen or poor progressive motility. In these situations, the semen should be concentrated using a technique such as cushion centrifugation, to reduce the total volume while maintaining an acceptable number of progressively motile sperm in the breeding dose. A simplified table (Table 1) was developed to assist in determining the minimum dilution ratio (semen: extender) for cooled-shipped semen based on the raw sperm concentration of semen and the progressive motility cutoff values for centrifugation by sperm concentration in the ejaculate.