Factors that affect bull fertility

Everything from winter shelter to genetics can affect a bull’s fertility

Some bulls are more fertile than others, due to genetics, scrotal size or structure. But poor management can also reduce a bull’s fertility.

Since the average bull is expected to breed 25 to 30 cows in a natural breeding situation, his fertility is very important. Dr. John P. Kastelic, professor of cattle reproductive health at the University of Calgary, says the goal of breeding soundness evaluations is to identify and eliminate bulls with poor fertility.

Bulls must meet minimum standards for scrotal circumference, sperm motility and sperm morphology (shape of sperm cells), says Kastelic.

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“The bull also must be physically sound, and have no obvious structural problems,” he adds.

But the breeding soundness exam does not detect infectious disease. Some diseases might be obvious, such as foot rot or seminal vesiculitis. However, a bull could have trichomoniasis or vibriosis and you wouldn’t know unless you specifically tested for those pathogens, says Kastelic.

Nor does a breeding soundness exam tell you whether the bull has adequate desire to breed cows, or whether he’s able to complete a service.

“When you turn a bull out with cows, you should observe him to see if he is actually doing his job.”

What the numbers mean

When checking semen, there are usually some abnormal sperm; the veterinarian notes the proportion of normal versus abnormal sperm and the proportion of each type of abnormality. Some have more effect on fertility than others, but each abnormality indicates something about what is going on with that bull. The most important thing — no matter what the abnormality — is that a bull must have at least 70 per cent or more normal sperm in order to pass.

“Some producers look at numbers; if one bull is 85 per cent and another bull 75 per cent they think one is 10 per cent better than the other,” says Kastelic.

While finding some abnormal sperm is expected, a bull should have 70 per cent or more normal sperm to pass the semen check.
photo: Heather Smith Thomas

But as long as a bull is above 70 per cent normal sperm, he is considered acceptable, Kastelic says, although he adds they don’t want to see more than 20 per cent head defects.

If a valuable bull doesn’t pass the semen test, Kastelic says they try to predict whether he might improve, and whether to retest him later. Or if an entire battery of bulls has the same abnormalities, it’s worth looking at why. For example, after cold weather, there can be many abnormal sperm.

The same can happen to a group of bulls on a ration containing a lot of cottonseed, with high gossypol levels. Their sperm will have abnormalities with certain characteristics. Then the veterinarian and rancher might discuss whether to retest the bull or bulls later, to see if sperm quality improves.

Scrotal circumference

The number of sperm a bull can produce is closely tied to scrotal circumference.

“Bulls with bigger testes produce more sperm, and to some extent produce better quality sperm. There are differences between breeds, however. Limousin and Blonde d’ Aquitaine may average only 32 to 33 centimeters circumference, yet have really good semen. Whereas in breeds like Angus and Simmental that tend to have larger scrotal circumference, a bull with only 32 centimeter circumference will generally have poor semen quality,” he says.

If a bull’s scrotal circumference is obviously outside the average for his age and breed, there may be a problem. For example, a yearling bull with huge testes (such 42 centimeters) may have an inherent defect, he says.

Shape of the scrotum is also important. The normal, healthy, fertile bull has a pear-shaped scrotum with an obvious neck above the testes.

“You don’t want a bull with a short scrotum, with no discernable neck, with testes tucked up against the belly. This can be hard to judge in cold weather, but if it’s a warm day, the testes drop down and you should see an obvious scrotal neck,” he says.

The number of sperm a bull can produce is closely tied to scrotal circumference.
photo: Heather Smith Thomas

At the other extreme, some Bos indicus bulls have a long scrotal neck. In general, you don’t want to see the bottom of the scrotum below the point of the hock, or the testes may be easily injured, Kastelic says.

Kastelic says they sometimes see a bull with a minor rotation of one testicle, but it’s generally not a cause for concern.

“In other instances, a bull may have testes mismatched in size or shape. Ideally they should be similar in size. If there is substantial difference, this may mean the big one is swollen or the other one is degenerating,” says Kastelic.

When palpated, normal bull testes feel firm, but slightly soft, says Kastelic. They shouldn’t be rock hard or compressible.

“The tail of the epididymis is at the bottom and toward the midline of each testes and can also be palpated and checked. Occasionally, a bull will be missing the tail of the epididymis on one side and can only ejaculate sperm from the other side,” he says.

Over-fat bulls may have a straight-sided scrotum with no narrowing at the top. “These bulls generally have poor fertility due to the insulating effects of fat — inhibiting heat loss and proper cooling.”

Kastelic also advises against using bulls with a V-shaped scrotum that tapers to a pointed tip.

“The pointed, wedge-shaped scrotum tends to hold testes closer to the body wall. Bulls with this scrotal shape usually have undersized testes and produce fewer sperm.”

Age and fertility

A bull’s fertility peaks at two to four years of age, on average, but most mature bulls have acceptable fertility for a number of years.

“If we check bulls at 12 months and are rigorous with scrotal circumference and sperm morphology requirements, only about 60 per cent will be considered breeding sound. Many bulls that are not acceptable at that age have sperm morphology issues but improve in just a month or two.”

A bull six to 16 months of age is growing rapidly and so are his testes. There can be great variations in testes size among bulls the same age within a certain breed during those early months. These differences can be a clue regarding future fertility.

“This trait — early or late testicular growth — is highly heritable. Scrotal circumference has proven to be a more accurate way to predict when a bull will reach puberty than age or body weight, regardless of his breed or breed cross,” explains Kastelic.

If you choose bulls that pass their breeding soundness exam at 12 months, you are selecting for early puberty in their offspring.

Regarding yearling scrotal circumference, some consideration must be given to the bull’s nutrition in early life, including the age of the bull’s dam. Bull calves from first-calf heifers tend to have smaller scrotal circumference as yearlings than bulls from older dams.

“Dr. Don Lunstra at Clay Center, Nebraska, came up with a correction factor for yearling scrotal circumference according to age of dam. If the bull had a heifer mother, he was given a one to 1.5 centimeter adjustment in yearling scrotal circumference to compensate for not having adequate nutrition from his dam,” Kastelic says.

Young bulls fed 130 per cent of required energy and protein at six to 25 weeks of age will reach puberty a month or so earlier than bulls fed 70 per cent of the ration.

“The biggest benefit, however, is that their mature scrotal circumference and testes size will be bigger. If you want a bull with early puberty, large scrotal circumference and more likely to be able to breed more cows, he needs a good start nutritionally.”

Temperature Stress

“Minimizing environmental challenges, keeping up vaccinations to minimize diseases, and maintaining bulls in reasonable body condition is also important. Adequate nutrition and shelter during winter to help avoid frostbite are part of bull management,” says Kastelic.

Extreme cold, especially with wind, can damage scrotal tissue and have a negative impact on fertility. The damage may be temporary or permanent.

“If half or more of the scrotal skin is damaged, and in particular if there are adhesions and the testes cannot be raised or lowered, this usually has a bad prognosis.”

That bull will have reduced semen quality for up to six weeks or longer. If damage was temporary and the tissues have healed with no adhesions, the bull will usually have good semen later.

Remember, the sperm that’s checked during semen testing started production 60 days before the test.

“If it was a rough winter or you get a late winter storm, semen quality plummets. A sperm defect called distal mid-piece reflex is common in bulls going through puberty but we also see this in bulls that were recently stressed.”

Cold stress, heat stress or issues such as foot rot can trigger the defect, he adds.

“In a mature bull, with a short-term stressor like a nasty spring blizzard, these changes are temporary. That bull has a very good prognosis for improvement. This damage to sperm occurs in the epididymis and resolves fairly quickly.”

Some years back, Dr. Albert Barth, who was with the Western School of Veterinary Medicine in Saskatoon, examined bulls in May from a community pasture. The bulls had been through a severe blizzard a couple of weeks before. The semen quality of those bulls was very poor, so three weeks later they checked those bulls again. Their semen quality had improved dramatically, says Kastelic.

Hot weather, or fever from a disease such as pneumonia or foot rot, can also cause problems.

“In hot weather, semen quality of some bulls really falls apart and other bulls are more tolerant of the heat without detrimental effects. We are trying to find out what makes bulls susceptible or resistant to increased temperatures. We often can’t resolve the problem unless it’s an AI bull and we can put an air conditioner in his stall.”

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