Finally, an examination of the reproductive tract should be done. The stallion's testicles should be measured. Ultrasound
provides the most easy and accurate means of obtaining testicular size. This information then is used to correlate how much
semen that stallion is producing as compared to how much semen he should be producing, according to Dr. Dixon Varner of the
College of Veterinary Medicine at Texas A&M University. Daily semen output is compared to testicular size, and this information
allows veterinarians and breeding managers to follow a stallion's reproductive ability. Evaluation of the testicles also can
reveal inflammation—suggesting an inflammatory process, tumors or atrophy. Testicular degeneration leading to atrophy is perhaps
the most common cause of reduced fertility in older stallions. As the testicles degenerate, there are fewer and fewer seminiferous
tubules producing sperm, and an overall reduction occurs in the quantity and quality of the sperm. The testicles themselves
become soft, then smaller and finally firm. Though aging is the most-common cause of testicular degeneration, it also can
be caused by an increase in heat in the testicles (trauma, infection, systemic viruses), adverse reactions to some antibiotics,
exposure to toxins, chemicals and other factors.
If an evaluation of the breeding practices, the quality of broodmares, their preparation and handling, and a complete physical
examination of the stallion has not yielded any reasons for poor breeding efficiency, then the next step is a careful semen
analysis. Semen should be evaluated for gross appearance and should be milky white to turbid without clots and without smell.
Any pink or red color in the ejaculate indicates hemospermia and definitely will correlate to decreased fertility. Yellow
color and a dilute sample can indicate urine in the ejaculate and also correlates to breeding problems.
This is a problem that usually can be corrected at the time of collection though natural service (in breeds that prohibit
artificial insemination) remains a concern with stallions that tend to urinate prior to ejaculation, so more work-up might
be required with these horses. Volume of the ejaculate should be recorded exactly. Average volume is from 60 ml to 120 ml,
with a gel-free fraction of 50 ml to 100 ml. There is tremendous variability between stallions however and consistency is
more important than specific numbers.
Motility should be evaluated immediately upon collection using a pre-warmed slide. This is still a subjective evaluation,
and the average stallion will show around 80-percent motility with 60 percent being progressively motile.
All stallions will show some percentage of dead sperm, but this number should be watched. If a stallion shows a decrease in
the number of motile sperm or an increase in the number of dead sperm, then a problem is developing and should be attended
to. The sperm should be evaluated as to longevity as well. Check the sperm at five-minute intervals [while on a warm (body
temperature) slide] for 30 minutes and you should see consistent motility percentages. A rapid reduction in motility signals
poor ability of the sperm to exist long enough to reach and penetrate the egg and points toward poor fertility.
Sperm pH should also be measured. Normal values are between 7.0 and 7.7. Abnormal pH and the presence of white blood cells
can indicate the presence of an infection. Sperm sample slides should be stained with a nigrosin-eosin stain and evaluated
for morphology. The gross appearance of the individual sperm should be noted, and abnormalities such as double heads or tails,
abnormal mid-pieces and other irregularities should be recorded.
Sperm concentration is a very concrete number that can provide tremendous information to the veterinarian and breeder, but
it sometimes can be used incorrectly as a measure of breeding ability. It is felt that a sperm concentration of 500X10(6)
provides maximum pregnancy rates. But there are stallions that remain effective despite counts much lower and stallions whose
count is above average but whose pregnancy rate is below average. These are the older stallions that cause concern for veterinarians
asked for an evaluation and recommendation as to future breeding ability. There are additional tests that can identify damage
to sperm and problems on a deeper, cellular level. Dr. Varner says the sperm chromatin assay is a very valuable test for evaluation
of an older stallion.
"Sperm quality is dependent on the amount of damage to sperm DNA or DNA fragmentation," Varner says.