by: Heather Smith Thomas

Many factors play a role in fertility and breeding ability in bulls, including semen quality, soundness, desire to breed cows, etc. It's wise to make sure every bull passes a breeding soundness examination before putting him with cows. This evaluation looks at five things: physical soundness (feet/legs, eyes, etc.), reproductive tract soundness, scrotal circumference that meets minimum requirement, percentage of sperm cells that are normal, and acceptable motility.

Mike Sanderson, Kansas State University explains that the veterinarian palpates the testicles to make sure there is nothing wrong--that they feel normal and can freely move in the scrotum. “We palpate the internal reproductive organs and make sure they seem ok. We also make sure the penis extends and that there is nothing wrong with it--no adhesions from old injuries.” If an injury has completely healed and the penis is able to extend far enough to breed, the bull will pass the test. Body condition is also assessed.

“We measure scrotal circumference, because this is an important indication of how much sperm the bull will produce, and may determine how many cows he can potentially breed. We can't say that if his testicles are a certain size we can put him on x number of cows, but the bigger the factory, the more potential. Research has also shown that scrotal circumference is related to age of puberty in the bull's daughters. Bulls with larger testicles tend to have daughters that reach puberty sooner,” says Sanderson.

“If it's a bull that we've tested multiple years, we can look back and see if scrotal size has stayed the same. Then we know when it starts to decline—when he may start to go downhill in fertility,” he says.

“We look at semen motility and how well the sperm swim—and percentage of sperm cells that are normal. We need to know what kind of abnormalities we see. These can be difficult to interpret but sometimes give clues about what might have happened in the past if this bull has a problem, and how likely he is to get over it—versus how likely it might be the beginning of infertility,” says Sanderson.

History of the bull and past records can be helpful when assessing him for the next breeding season. “All of those things, more than just the semen check, are very important. There are limitations to the breeding soundness exam/semen check because what we see is what the bull is producing today and that doesn't tell us anything about what the bull will be producing tomorrow or next week,” he explains.

These exams have proven helpful, however. “We can identify bulls that will not do well this breeding season—so we can replace them--but it doesn't guarantee that a bull who is in wonderful shape today will be fertile by the time breeding season starts or two weeks into it,” he says.

“We talk about sub-fertile bulls and try not to use the term infertile because very few bulls are actually infertile. A bull may be sub-fertile, however—not as fertile as he needs to be.” He's not paying his way if he only sires a few calves—especially if he's keeping the more timid bulls from breeding.

Morphology And Motility -- Duane Mickelsen, DVM (retired from Washington State University), has been doing fertility studies in beef cattle for many years, and thousands of breeding soundness exams. He says morphology (looking at the form and structure of sperm and comparing numbers of normal and abnormal sperm) is probably the most important factor associated with fertility.

Bulls are flunked if they fail on any one of three standards—scrotal circumference less than 30 centimeters, less than 70% morphologic normal sperm, or less than 30% motility under field circumstances. “A bull failing any one of these three standards is reason to flunk that bull. But in field situations it can sometimes be tricky to accurately check motility,” says Mickelsen.

“I take a heated box when I do these examinations. Many veterinarians collect a sample and then run from the chute to their vehicle to evaluate semen, and if the slides are cold, motility is hindered. This evaluation may not be accurate. Thus motility is the poorest standard to go by,” he says.

“Some veterinarians go mainly by motility, yet it has the poorest correlation with fertility. If they don't look at morphology (which means counting about 100 sperm and determining percent of normal compared to percent with abnormalities), they may not get an accurate picture of potential fertility. Motility can be so variable that I don't pay as much attention to that as morphology. A cold slide or other factors can kill all the sperm in a sample, whereas morphology is more accurate and useful. I don't pay any attention to live or dead, just what's normal,” he says.

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