It’s important to make sure every bull passes a breeding soundness examination (BSE) before putting him with cows.  This evaluation looks at five things: physical soundness (feet/legs, eyes, etc.), reproductive tract soundness, scrotal circumference, percentage of sperm cells that are normal, and acceptable sperm motility.

Another aspect might be regulatory factors regarding certain diseases, particularly trichomoniasis.  Testing for “trich” is not part of routine breeding soundness exams, but is recommended for non-virgin bulls or bulls used in community pastures.  Testing is required in some states.

Dr. Robert Larson (Kansas State University) says some people call the BSE a semen evaluation, but looking at semen under a microscope is just one component. “Many bulls fail for other reasons; they may have issues with lameness, injuries, and other problems that might limit their ability or willingness to mate.  A good physical examination is crucial, giving particular attention to feet, legs and structural soundness,” Larson said.

Dr. Ahmed Tibary (Washington State University) says it is fairly common to check young virgin bulls before their first breeding season, to make sure they are mature enough and fertile enough to breed cows, but many producers don’t bother to check older bulls; they just assume the bull is fine again for this year.

“This is often where we see problems.  After a hard winter with cold weather and wind, we find a lot of abnormal sperm,” he said.  Also, a bull may have had an injury or infection and is not as fertile or capable of breeding as he was last year.

Soundness

The physical exam evaluates health and soundness and whether the bull would be able to mount and breed a cow or has any conformation faults or injuries.  The veterinarian will do a thorough examination of reproductive organs and external genitalia.  “The prepuce/sheath is inspected and the scrotum examined for conformation, symmetry or presence of lesions.  Testicles are palpated for normal consistency,” said Tibary.

A common problem is injury to the prepuce.  “We also may see lesions on the scrotum, if the winter was cold.”

Scrotal size

“Size and shape of the scrotum is important but some breeds have different standards for scrotal size per age of the bull,” Tibary said. Some breeds have smaller circumference than the average, but the bulls are still very fertile.

Scrotal circumference is important because size of testes determines how many spermatozoa are produced daily.  This is usually what determines how many cows a bull can breed.  If a bull has at least the minimum circumference standard, he can be used with at least 25 cows. 

Larson says evaluation for yearling bulls versus mature bulls is a little different.  “For yearlings, scrotal circumference is a good indication of their stage of maturity and how they compare to other bulls of that age.  For adult bulls we don’t expect scrotal size to continue growing, but I check records from the year before—particularly if it’s a bull with a semen problem--to see if there’s a change.  Sometimes larger circumference will indicate a problem, but if his scrotal circumference is now smaller, this is evidence that something bad is happening,” said Larson.

“For yearlings, the Society for Theriogenology set a minimum at 30 centimeters but most breeders consider minimum to be 32 centimeters to make sure young bulls are actually reaching puberty and ready to breed,” he said.

Internal sex organs

“Trans-rectal palpation of internal sex organs enables us to find one of the most common problems, which is inflammation of the seminal vesicle, also called the vesicular gland.  This results in poor semen quality,” said Tibary.

Many things might cause this type of infection, such as a blood-borne infection or an ascending infection up through the reproductive tract.  “A common predisposing factor in young bulls is that around the time they are reaching puberty and being fed high energy/high protein rations, they may experience subclinical acidosis; bacteria from the gut may leak into the bloodstream.  Other possibilities include systemic viral or bacterial infections.  Older bulls may pick up reproductive tract infections when breeding infected cows,” he said.

Semen collection

The last part of the exam is semen collection and evaluation. “It is also important to examine the shaft of the penis to see if it is normal or if there are any lesions, and whether the bull can extend it.  One of the most common injuries in bulls is preputial adhesions,” said Tibary.  An injured prepuce or broken penis may heal with adhesions to the sheath.

“Another common problem is penile warts.  These are caused by a virus and can be contagious.  I’ve seen ranches where more than half the young bulls have penile warts, which can interfere with mating.  Warts can be removed surgically but it is important to check the bull again, as they may recur,” he explains.

Semen evaluation

“The standard for semen motility according to the Society of Theriogenology guideline is 30% minimum.  We look at gross motility, which evaluates a wavy motion,” Tibary said.

The next step is to stain the semen and look at morphology (form and structure)—to determine if there are abnormalities.  “We note the proportion of normal sperm versus abnormal sperm.  We must examine at least 100 sperm and determine proportion of normal sperm and the proportion of each abnormality—and there are many different abnormalities,” he explains.

“Some have more effect on fertility than others, but each abnormality tells us something about what is going on with that bull.  For a breeding soundness examination, the most important thing—no matter what the abnormality—is that a bull must have at least 70% or more normal sperm in order to pass,” said Tibary.

Another aspect is whether the first ejaculate is actually typical of what the bull will produce, especially if he has not been breeding cows (such as a young bull or a bull that’s been apart from cows over winter).  “This first ejaculate is often referred to as a ‘rusty’ load, with accumulation of old sperm that doesn’t look good.  Some bulls must be collected several times before they clean out, to get a true picture of their semen.”

Mating ability

Bull fertility is important, and a BSE ahead of breeding season is crucial, but injuries and illness can happen after breeding season begins.  Even if the bull was fine at the time of his BSE, producers should monitor bulls throughout the season. 

Larson says you should watch every bull mate, to make sure he can do it.  Some problems and injuries may not be evident until the bull is trying to breed a cow and fails, and some bulls may mount the cow but not breed her.  You’d want to know if a bull is having a problem.  This gives you a chance to replace him before the breeding season is over.

[Heather Smith Thomas and her husband both grew up on ranches and have been raising cattle together for 56 years.  Their children, several grandchildren and one great-grandson currently live on their ranch near Salmon, Idaho.  She has been writing about cattle care and horses for many years and has written more than 10,000 articles and 24 books.]