Not all bulls are able to breed cows successfully, and not all bulls that breed cows have semen that can result in successful fertilization and pregnancy.
“If a bull can’t fertilize cows and you turn him out to breed, there are likely to be a lot of nonpregnant cows at the end of the year,” says Carl Dahlen, North Dakota State University Extension Service beef cattle specialist. “Because the No. 1 determinant of profit potential in a beef cow-calf operation is the birth of a live calf, turning out infertile bulls can have a tremendous impact on profitability in beef production systems.”
Despite that, the National Animal Health Monitoring Survey showed that less than 20 percent of producers in the U.S. perform breeding soundness exams on their bulls prior to spring turnout.
“Breeding soundness exams can uncover potential problems with young bulls that were just purchased and older bulls that already have sired calf crops,” Dahlen says.
The breeding soundness exams include an examination of the bull’s physical structure, reproductive organs and semen.
The physical examination is important because bulls with proper structure are more likely to hold up under the rigors of the breeding season than bulls with structural problems.
Structural problems of the feet and legs or movement in general are a big issue because bulls are asked to cover a lot of ground and need to be free of physical problems to breed cows successfully. Bulls will lose body condition during a breeding season, so they must enter the breeding season with adequate condition.
Vision also is an important part of the breeding season for bulls. Seeing the mounting behavior of cows in heat helps the bull identify who is ready to breed from across the pasture.
Evaluation of the reproductive organs is another important part of the breeding soundness examination. The penis, testicles, prostate and other internal glands/structures are evaluated. These organs need to be free of injuries or defects for a bull to breed cows successfully.
Scrotal circumference is heavily scrutinized in young bulls because it serves as an indicator of semen volume. As young bulls grow, the standards for adequate scrotal circumference also increase. For example, a bull that is less than 15 months old should have a circumference of at least 30 centimeters, whereas a bull more than 24 months old should have a scrotal circumference of at least 34 centimeters.
Bulls with inadequate scrotal circumference often are taken out of production sales or sold at discounted rates, with their inadequate scrotal circumference mentioned at the time of sale.
A sample of semen is evaluated for motility, morphology and concentration. Motility is the movement of sperm, and, ideally, the sample will have a rapid swirling movement. If sperm are not moving in a synchronized manner (think synchronized swimming), they may not be able to swim successfully through the female reproductive tract to the site of fertilization.
If a bull has less than 30 percent of sperm with proper motility, it is not recommended for breeding, whereas a bull with greater than 70 percent proper motility receives a very good rating for the motility portion of the breeding soundness exam.
Morphology is an evaluation of the structure of the sperm. Ideally, the sperm will have heads and tails of proper shape. Common defects include tapered or detached heads and folded or coiled tails. Sperm that has the incorrect structure will not result in successful fertilization. A minimum of 70 percent of the sperm cells need correct morphology for a bull to pass a breeding soundness exam.
Just because a bull sired calves last year does not mean he can do it again this year. Injuries during the nonbreeding months, as well as effects of extreme cold weather and frostbite, can render once-fertile bulls infertile. The process of making sperm, spermatogenesis, takes 60 days, so frostbite or other injuries that occur in March may be lingering in May.
“Perform breeding soundness exams close to the time of breeding to ensure recovery from winter injuries but enough time in advance of turnout to find new bulls if the exam finds fertility problems,” Dahlen advises.
An important indicator of breeding season success is stocking rate, or how many cows a bull is required to breed in a breeding season. The nationwide average stocking rate is 25 cows per mature bull or 15 cows per yearling bull. Stocking rates of up to 50 cows per bull are used in some systems, but high stocking rates may lead to cows not becoming pregnant on their first heat of the breeding season and subsequently calving late the following year.
The breeding soundness exam does not evaluate libido or willingness to breed, however. This is very important to keep in mind, especially when using young or virgin bulls. Young bulls may have all of the qualifications to pass the exam, but if they aren’t actively breeding cows, producers must find a different option.