Breeding Soundness Exam

Gerald Hauer
Bison Production Specialist
Bison Centre of Excellence

Reprinted from the Tracker. Volume 4, Issue 11, December 2000

Breeding soundness exams are used to determine if a bull is fertile and has the potential to be a successful breeder. This article explains the procedure used by veterinarians when examining bison bulls for breeding potential.

Testing bulls for breeding soundness is used routinely in bison and occasionally in elk to measure a bull’s potential to breed. The exam is not a guarantee that the bull will get all of the cows pregnant but it determines if he has the tools necessary to get the job done. It is used to select the most fertile bulls out of the population. Some people refer to the exam as a semen test but it is more than just collecting and looking at the semen. The examination can be broken down into several categories:

• Physical exam
• Scrotal circumference
• Sperm Motility
• Sperm Morphology

A visual exam is performed to make sure that the bull has the physical attributes necessary to breed. Are his feet and legs free of defects? Does he have two good eyes? Does he show any signs of illness or injury that would impede his ability to breed? Are his penis, scrotum, testicles and sheath properly formed?

It has been demonstrated that the size of the testicles (measured by the scrotal circumference) correlates to the number of sperm cells that can be produced. Acceptable limits have been determined for the different cattle breeds but not for bison or elk. This step of the exam is sometimes skipped in theses species because it can dangerous for the veterinarian and there are no standards with which to compare the measurements.

The final step of the exam is to collect and examine a semen sample. Semen can be collected by massaging the bull’s accessory sex organs (prostate gland, etc.) in the pelvic canal or by electro-ejaculation. Once the sample is collected it is rated for concentration, motility, and morphology. Motility is determined by placing a drop of semen on a warm microscope slide and viewing the sample under a microscope. A good sample will have most of the sperm moving in a forward and progressive motion. A poor sample will have only a few sperm moving or sperm moving erratically. Finally a small amount of semen is added to a special stain on a slide. One hundred sperm cells are counted under a microscope and the number of normal sperm cells are compared to the number of abnormal cells. This gives a percentage of normal and abnormal sperm.

In order for a bull to "pass" his test or be rated as a satisfactory potential breeder he must meet a minimum standard in each and every category. If he fails to meet the standard in any one category he will be classed as an unsatisfactory potential breeder. The standards set out as the minimum acceptable are as follows:

Physical exam: no physical weaknesses that could affect his breeding ability

Scrotal circumference: not used in bison and elk but if possible bulls with larger testicles should be used

Sperm motility: greater than 60% of the sperm are progressively motile (moving forward in a straight line)

Sperm morphology: greater than 70% of the sperm are normal cells. An important point to remember is that the percentage of normal sperm is not a score for the bull. Often I have heard that a bull scored 80% or 90% on his semen test. As long as the number is over 70% he passes this category. It is a pass/fail system-90% is not better than 70%! He could score 100% in this category and still fail the test if his motility is less than adequate.

Most bulls can be tested any time after they reach puberty. In bison this would be about 1½ to 2 years of age. If a young bull doesn’t pass his exam it may be because he is a late bloomer and he may pass at a later date. In this case a veterinarian can defer his/her decision and not actually fail the bull. Expecting a bison bull to be sexually mature at 1½ years old is asking a lot. If a young bull doesn’t pass I would suggest having him examined again in 3-6 months.

Another factor to consider in bison is their seasonality. In species that rut, it has been noted that males become more fertile before and during the breeding season. In the off season their fertility wanes. If a bison bull is unsatisfactory in January, will he be OK next August? Researchers from the veterinary college in Saskatoon are currently doing a study on bison bulls to determine what the effects of the season have on male fertility. Hopefully this will help our understanding of this issue.

After the veterinarian determines that the bull has the potential to breed, the producer must observe him in the breeding season to be sure that he is actually breeding. The bull must interact properly with the cows and the other bulls in the herd in order to get his job done. He must also be exposed to fertile females. Be sure that the nutritional requirements of the females are met to ensure a high conception rate. Remember that the breeding soundness exam doesn’t guarantee that a bull will be a successful breeder but it will help you decide which bulls have the best potential to breed your cows.