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Bulls require a breeding soundness evaluation | Back to Vet Advice
Dr. David Hamilton
At this time of the year, we begin to prepare for the
upcoming breeding season. The nutrition and
management of the heifers and cows is obviously
very important to success but the bull is even
more important. If he does not do the job, we are faced
with a number of open cows or late calves next spring.
It has been estimated that in beef production, fertility is
10 times as important as carcass quality, and five times as
important as rate of gain. The challenge to beef producers then is to achieve high pregnancy rates within a short
breeding season. Unfortunately it has also been estimated
that one in five bulls are subfertile due to poor semen
quality or the inability to serve cows efficiently. Subfertile
bulls that cause disastrously low pregnancy rates can be
financially devastating to individual producers, however
the greatest economic loss to the beef industry is from
delayed conception. One estimate puts the loss at 50-60
pounds of weaning weight for every 21-day cycle that a
cow fails to conceive.
It is therefore of utmost importance that bulls be evaluated for breeding soundness before they are turned out with the females.
A bull requires three attributes to be fertile:
- good libido (sex drive) and serving capacity
- physical soundness
- good semen quality
There is no standardized test routinely used to evaluate
serving capacity in yearling bulls. And testing the serving capacity of mature bulls is not very practical for most operations. Therefore producers must assess this aspect of
fertility by their own observations, especially with virgin
yearling and two-year old bulls.
Also remember that there are many factors including
injuries and the development of penile deviations that can
incapacitate older bulls that have been sound breeders in
the past. So watch your bulls carefully during the breeding
season to be sure they are effectively breeding the cows.
In order for a bull to be an effective breeder he must
have sound conformation of the feet and legs. Often bulls
have to travel long distances to detect females in heat and
breed them. During the breeding act, the full weight of
the bull is borne on the hind legs and feet. Although some
bulls with questionable conformation may get by for a
couple of years, it is important to realize that this defect
may be passed onto their offspring. This is especially
important if replacement heifers are retained for breeding.
Common foot and leg problems such as a corkscrew claw,
corns, weak pasterns, post leggedness and sickle hocks
have a hereditary basis.
Since bulls rely mostly on vision to detect cows in heat,
it is important that bulls have normal eyesight. They also
require a normal shaped scrotum to allow for proper heat
exchange. For optimal semen production the testicles
should be cooler than body temperature. The neck of the
scrotum plays a critical role in the counter-current heat
exchange mechanism of the testicles. The presence of fat
in the neck of the scrotum, or a short neck, interferes with
this heat exchange mechanism, resulting in warmer testicles and defective sperm.
Part of the physical examination of the bull involves
examination of the testicles and measurement of the
scotrum. Scrotal circumference measurements are highly
correlated to testicle weight, daily sperm production and
high semen quality. Also, scrotal circumference is moderately to highly heritable. Large testicles are positively correlated with early onset of puberty in his heifer offspring,
early breeding of heifers, and high female lifetime productivity. In addition, larger testicles are important to insure the production of normal sperm and adequate numbers of
sperm for the high breeding pressure of a short breeding
season. There are minimum standards for scrotal circumference for bulls of various breeds and ages that bulls must
meet or exceed to pass a breeding soundness evaluation.
The physical examination involves rectal palpation of
the accessory sex glands and the inguinal rings. The most
common abnormality is seminal vesiculitis, an infection of
the seminal vesicular gland that impairs semen quality.
The final component of a pass for breeding soundness
is semen quality. Semen is collected by electroejaculation.
When the penis is extended it can also be examined for
any abnormalities while the semen is evaluated for volume, density, motility, and per cent normal sperm. The semen has to meet minimum standards for each of these
characteristics for the bull to pass. Bulls with too many
abnormal sperm will be subfertile and possibly infertile.
During the physical examination it is not uncommon to
find abnormalities such as penile warts, hair rings, persistent
frenulums (virgin bulls) or preputial injuries. Once identified
these conditions can usually be treated successfully.
Based on the physical exam, scrotal circumference,
semen motility, density, and sperm morphology, your veterinarian will score the bull as being either Satisfactory, Questionable, Unsatisfactory or defer the decision until
more information is available. He or she can explain this
process in more detail.
In summary, the contribution of a bull to reproductive efficiency and to the production of beef is of great
importance because the bull represents half of the genetic
composition of his resulting progeny. As a result, selecting
superior bulls and yearly evaluations for breeding soundness will certainly pay dividends for your beef operation
down the road.
-- Dr. David Hamilton is a large animal practitioner at the Morden
Veterinary Clinic at Morden, Man., and a member of the Western Canadian Association of Bovine Practitioners, which co-ordinates this column. Reprinted from Canadian Cattlemen magazine, May 2008