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.
By Stefani Garbacik
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