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
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.