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When talking about the stallion in a breeding situation, the sperm count and motility are often mentioned. The sperm is the male gamete and is responsible for fertilization of the ovum. In order for the sperm to penetrate and fertilize, they must be motile.

Motility refers to the movement of sperm. They must be actively moving, or “swimming,” to even have a chance of fertilizing. When talking about motility in semen evaluations, the critical part is the progressively motile sperm. These are the cells moving in a straight line (more or less). Total motility is just the percentage of sperm that are moving in any direction. A stallion’s fertility is often based on the progressive motility of his sperm (in addition to the concentration, morphology, and other factors).

For equine semen to be termed “progressively motile,” the sperm must demonstrate ≥75% straightness. The formula for this definition is: Percentage straightness = VSL/VAP x 100 where VSL is “velocity straight line” and VAP is “velocity average path.” These can be evaluated and calculated using the software termed CASA (Computer Assisted Semen Analysis). When examining a collection for motility, diluting the sample to 25 million sperm/ml or lower is ideal. A short video is taken by the computer and evaluates the differences in sperm movement–providing details about the sample, including total and progressive motility. Typically 50-60% progressive motility is considered “good” for stallions. Remember this is a percentage, the velocity and total sperm numbers must be taken into account in determining fertility.

Semen evaluation is done in several species, including equine, when the fertility of the male is called into question or when performing AI. Other than the Thoroughbred industry, it is not uncommon to obtain “superior” genetics through AI systems.

I’ve added the links for several videos of CASA evaluations…note the differences between the good, poor and average motilities.




Stefani Garbacik is a Livestock and Forage Extension Agent with the North Carolina Cooperative Extension. If you need any further assistance, please contact her at or (919) 731-1525.