Scrotal circumference matters for fertility
Published on March 21, 2020
Measuring scrotal circumference is part of any breeding soundness exam because size of the testes determines how many sperm are produced daily. Some breeds, however, have smaller circumference than the average across breeds, but the bulls are still very fertile.
University of Calgary Professor and Veterinarian John P. Kastelic says the main focus of the breeding soundness exam is the reproductive tract and bulls have to meet minimum standards for scrotal circumference.
Goals are key
“We want bulls to meet minimums, but it also depends on our goals,” Kastelic says. “If we are just producing feeder cattle, scrotal circumference is slightly less important, and we could get by with a bull that barely meets minimum requirements.”
“If we want to keep daughters, however, scrotal circumference is more important because it’s an indication of the future fertility of those daughters,” says Kastelic.
“Bulls with bigger testes produce more sperm, and to some extent produce better quality sperm,” he explains. “There are differences between breeds, however.”
“Limousin and Blonde d’ Aquitaine may average only 32 to 33 centimeters circumference, yet they have really good semen, versus breeds like Angus and Simmental that tend to have larger testes,” he explains. “A bull with only 32 centimeter circumference will generally have poor semen quality. Thus, it is also important to consider the breed.”
If a bull’s scrotal circumference is outside the average for his age and breed, there may be a problem.
“For instance, a yearling bull with huge testes, such as 42 to 44 centimeters, may be likely to have subsequent testicular degeneration. We worry about those bulls because they may have some inherent defect,” Kastelic says.
“We should be wary of a bull whose scrotal circumference is way off the charts early in life and those with an extremely large frame score,” he says. “Often, these really tall animals have delayed puberty.”
He continues, “When a male goes through puberty, there’s an increase in testosterone and increased length of long bones with a growth spurt. Then when puberty is reached, growth slows. The growth plates at the end of the long bones fuse at puberty, with no more lengthening of those bones and therefore, no more height.”
“Bulls that grow tall, with extreme frame, may have reasonable mature scrotal circumference, but they often have reduced yearling circumference because of delayed puberty,” Kastelic explains. “Any young bull with an extreme frame score or one with massive yearling scrotal circumference makes me worry about possible fertility issues.”
The shape of the scrotum is also important. According to Kastelic the normal, healthy, fertile bull has a pear-shaped scrotum with an obvious neck above the testes.
“We don’t want a bull with a really short scrotum and the testes tucked up against the belly,” he says. “This can be hard to judge in cold weather, but if it’s a warm day, the testes drop down farther and we should see an obvious scrotal neck.”
“At the other extreme, some bos indicus bulls have a very long scrotal neck,” he notes. “In general, we don’t want to see the bottom of the scrotum below the point of the hock or the testes may be easily injured in rough terrain. We want to avoid extremes.”
“Sometimes, we see a bull with a minor rotation of one testis,” he notes. “One side of the scrotum may be a little turned. Generally, this is no cause for concern.”
“In other instances, a bull may have testes mismatched in size or shape. If there is substantial difference, this may mean the big one is swollen or the other one is degenerating,” says Kastelic.
“We palpate the testes. Normal testes feel firm, but slightly soft. They should not be rock hard, but also not too soft and compressible,” he says. “The tail of the epididymis is at the bottom and toward the midline of each testis 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,” Kastelic says.
“Overfat bulls may have a straight-sided scrotum with no narrowing at the top, indicating too much fat in the neck of the scrotum,” according to Kastelic. “These bulls generally have poor fertility due to the insulating effects of the fat, inhibiting heat loss and proper cooling.”
Kastelic explains their testes are always too warm for optimum sperm production and viability. Fat in the neck of the scrotum also hinders the ability of a bull to raise and lower his testes properly for optimum temperature control.
Kastelic advises against using bulls with a V-shaped scrotum that tapers down 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,” he explains.
“Fat deposits in the scrotum can usually be detected when measuring scrotal circumference,” he says.
“Before applying the measuring tape, the testes should be pressed firmly into the lowest part of the scrotum so they are side by side, and all scrotal wrinkles that might interfere with accurate measurement eliminated,” e says.
“Thumb and forefingers of one hand should be placed above the testes with the hand extending down from the side of the scrotum, around the back and finger tips wrapping around and pointing backward,” he explains. “Avoid sticking the thumb between the testes or excessively forcing testes to the bottom of the scrotum, as that would falsely increase the measurement.”
The looped tape should be slipped up over the scrotum and pulled tight around the largest portion, with only moderate tension.
“In a bull with normal testes, the testes will not be compressed much at all by the tape,” says Kastelic. “By contrast, a bull with a thick, fat scrotum or soft testes may compress rather easily.”
Measurements are usually recorded to the nearest half centimeter, with at least two measurements taken until a consistent reading is obtained.
Heather Smith-Thomas is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.