Veterinarian View | May/June 2025

Breeding Soundness Examinations in Bulls

contributed article by Dr. Vince Collison, Collison Embryo and Veterinary Services,

Rockwell City, Iowa

collisonembryoservices.com 

We are at the time of year when producers are getting a Breeding Soundness Exam (BSE) for their bulls. This is also often referred to as semen testing, but there is more to it than that. I will cover the things that are evaluated in a BSE for bulls and point out some of the limitations.

Scrotal Circumference

This is probably the first place to start on a BSE exam. When testing younger yearling bulls, it is a good indicator of their sexual maturity and circumference is highly correlated to fertility. This is probably the best way to measure testicle size.

When measuring the scrotal circumference, it is very important to push both testicles down into the scrotum in order to properly record the largest circumference. The guidelines for minimum circumference are this: less than or equal to 15 months of age is 30 centimeters, 15 to equal to 18 months of age is 31 centimeters, 18 to equal to 21 months is 32 centimeters, 21 to equal to 24 months is 33 centimeters, and anything over 24 months is 34 centimeters. Anything less than these measurements would be questionable for fertility since semen volume is associated with larger testicles.

If you keep your own replacement females, there is a correlation with higher fertility in heifers if they are sired by bulls with larger scrotal circumference. Also, at the time we measure scrotal circumference we are checking for even size of the testicles and any abnormalities with the testicles that will impair fertility.

Seminal Vesicles

We will rectally examine the seminal vesicles and prostate at the time we massage the prostate prior to collection. Infections of the seminal vesicles are fairly common and can be detected by unevenness in size and hardness. These vesicles provide a fair amount of fluid in the ejaculate and will contaminate the ejaculate with a lot of white blood cells when infected. Bulls with infected seminal vesicles can be hard to treat and usually have a guarded prognosis for recovery.

Progressive Motility

On the microscopic exam of the semen, we look for progressive motility of the sperm. The minimum recommended motility is 30 percent. Most samples will have considerably higher motility than 30 percent. We consider samples in the 80 to 100 percent range to be very good, the 60 to 80 percent range to be good, anything in the 30 to 60 percent range to be fair.

When evaluating motility, we like to use a heating plate for our glass slides, so the sperm are not affected by any temperature changes and give us the most accurate evaluation of true motility. Motility evaluation is somewhat subjective. There are machines that can give accurate percentage readings, but visual examination is fairly accurate with experience evaluating samples.

Sperm Morphology

This is where the sperm are evaluated for the percent of defective sperm. The percentage of normal sperm recommended is to be at least 70 percent normal sperm at minimum. We base this off of a count of 100 spermatozoa throughout 10 different microscope fields. This is a cumulative of all different defects of the head, midpiece and tail. Many defects will affect motility, such as detached heads and bent or coiled tails.

When I look at a fairly thick sample and don’t see very good or maybe no motility, there are usually a lot of defects present that will affect motility. There are also some defects of the head that are hard to evaluate with just a light microscope. Some of these defects can be caused from gossypol when feeding cottonseed or from over-conditioning resulting in too much fat in the scrotum.

These conditions will affect the DNA of the sperm and cause little crater defects on the surface of the head which will be too small to see with a light microscope. To see this type of a defect would probably require an electron microscope. So, there are some limitations with semen evaluation in practice, but those type of defects are fairly rare, and most common defects can be seen with a typical light microscope.

One of the more common defects we see are proximal droplets on the midpiece of the sperm. Usually this is an issue with younger bulls and will usually resolve with maturity. For this defect we typically recommend a retest and will usually see improvement with time.

Normal Physical Condition

It is important to address the overall physical condition of a bull. We like to make sure there are sound feet and legs. It is important that the eyes are healthy. When a bull gets pinkeye, they will often stop servicing cows due to the pain. We also want the bull to be able to extend its penis properly and not have any abnormalities, physical trauma, or infections such as warts.

One thing to remember is that a BSE is only a snapshot of how that bull tests on that day. A producer has to be aware of anything that can change that status, such as a broken penis during breeding. Frostbite of the scrotum can also cause issues. Semen issues from frostbite can usually resolve, but it can take about 60 days for normal sperm to be regenerated. This is why on any bull that cannot pass its semen evaluation, we like to wait 60 days before doing any retesting, so it has time to regenerate normal sperm.

These guidelines are from the official BSE Evaluation form from the Society for Theriogenology and are based on recommendations from experts in the field of bull fertility. We like to use this system as it is a vetted form. You will see many other systems for BSE exams, but we feel this is the best to cover all aspects of bull fertility exams.

learn more www.collisonembryoservices.com 

Dr. Vince Collison is co-owner of Collison Embryo and Veterinary Services PAC

in Rockwell City, Iowa.