When bull shopping, it is important to understand the meaning of a
bull’s scrotal circumference.
Scrotal measurement is one of the best indicators of semen output and
fertility.
To measure scrotal circumference, Canadian veterinarians pull down the
testicles and measure across the widest part of the scrotum. The tape
is then pulled tight, compressing the scrotal skin. This measurement,
in centimetres, is a correlation to the testicular mass. It is
repeatable, and differences among veterinarians vary by about one to
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two cm.
American veterinarians use a much different technique. They simply lay
the tape around, pull it snug but do not compress the scrotal skin.
Breeders who import American bulls often find that their measurements
are four to five cm larger.
As a result, it is important to be careful when
buying U.S. bulls for their scrotal size.
Measurement variability is a concern.
Tapes with spring mechanisms standardize the pull on the tape. The
Coulter tape used to be common, but it has been removed from the market
because of production and breaking problems.
A University of Saskatchewan professor has invented the ReliaBull Tape,
which works on the same principle as metal tapes, but has a spring
mechanism.
I have found this tape trouble free, repeatable and hardy. It
definitely standardizes the force on the tape.
Suggested breed minimums have been established, which are accepted
minimum scrotal circumferences for use at sales.
These vary from 29 cm for Limousins and Salers to 32 cm for Simmentals
at one year of age. Breed associations generally set standards for
yearlings and two year olds, with the two-year-old minimum usually two
to four cm greater than the yearling minimum.
The Western Canadian Association of Bovine Practitioners has broken
this down further, with minimums for 13, 14 and 15-20 months. This is
because a bull’s scrotum grows rapidly in the 10-15 month range, which
means a big difference when comparing 12-month-old bulls with 15 month
olds.
These breed minimums are important because most bulls that measure
below the breed minimums produce little or no semen. They should be
culled.
It’s important to remember that scrotal size means different things in
different breeds. A 30 cm Limousin yearling can produce good semen, but
a 30 cm Simmental will be sterile.
In other species such as bison, 26 to 29 cm for two year olds is
acceptable. Rams, on the other hand, often measure around 40 cm.
All the data that has been collected on bulls over the years has helped
produce precise breed averages. These are constantly being pushed
higher as purebred breeders select for increased scrotal size in their
herd sires.
Because scrotal size is highly heritable, breeders can keep driving up
scrotal size. Heifer replacements will also be earlier maturing, as
fertility is enhanced by these larger-testicled sires.
Yearling averages vary from 30.3 cm for Limousins to 36 cm for
Simmentals. Some producers buy only bulls that are at least close to
the average for the breed, especially if breeding capacity is a
concern.
While larger scrotal sizes are desirable, bigger is
not always better. Research has shown that semen production does not
increase with circumferences
beyond 38 cm.
I get nervous when yearlings of any breed have scrotal size much larger
than 42 cm, because abnormally large scrotums have their own problems.
Extremely large mature bulls can even develop problems with their
suspensory apparatus, making temperature regulation difficult.
