Scrotal circumference can be a valuable indicator of a bull's fertility, if it's measured correctly.
Cow-calf producers have long known that scrotal circumference size matters when making sire selection decisions. A bull's scrotal circumference is the best estimate of testes volume, and thus the amount of potential sperm-producing tissue producers can measure, says Glenn Coulter, assistant director of the Lethbridge Research Center in Alberta, Canada.
"It's a relatively fast and inexpensive way to get an indication of the reproductive health of a bull. It's only an indication, but it's a good one," says Coulter.
Oklahoma Extension animal reproduction specialist Glenn Selk agrees. "It does an excellent job of identifying problem bulls," says Selk.
But, scrotal circumference measurements and EPDs have merit only if they are used appropriately and if they are measured correctly, Coulter adds.
Scrotal circumference leads the list as one of the three components of a breeding soundness exam (BSE). It's highly correlated (.72-.92) with daily sperm output in 12- to 18-month-old bulls. The size of the scrotum of yearling bulls is also positively correlated with sperm motility and percent normal sperm, the other two components of a BSE. Hence, scrotal circumference of a young bull directly and indirectly affects his likelihood of passing all three components of the BSE.
To be classified as a good potential breeder, a yearling bull must be in good general health and exceed a threshold of greater than 30-cm scrotal circumference; greater than 50% sperm motility and greater than 70% normal sperm, according to standards set by the Society for Theriogenology.
Scrotal circumference is also considered a better predictor of puberty than age or body weight. And, a correlation has been found between a sire's yearling scrotal circumference and the age at which his daughters reach puberty. Simply put, bulls with larger testicles sire daughters that begin cycling earlier.
Oklahoma's Selk says cattlemen who keep replacement heifers should take note of that correlation. "It's a trait that can have a long-term impact on the reproduction capabilities of a herd. Someone may really like a bull, but if he has a small scrotal circumference for his age, we've found that over time his daughters reach puberty later. They're also harder to get bred and harder to get rebred."
Coulter says measuring scrotal circumference is critical for another reason. "Measuring scrotal circumference is not only important for determining the size of the testes, but while taking the measurement the scrotum should physically be palpated."
That can be very revealing, Coulter says. "Visually, the scrotum may look normal, but until you get your hands on it, you don't know. You may find soft testes or a hard tail to the epididymis. We talk about the measurements, but palpating is equally important."
Palpation should include moving the testes upward within the scrotum toward the abdominal wall to ensure that they're not adhered to the scrotum. The testes should be uniform in size and consistency. The epididymides of both testes should be palpated to ensure the head and body of each epididymis is normal.
Orientation of the testes within the scrotum should also be noted. Two common abnormalities include one testicle being held higher than the other, and one or both testes being rotated around their long axis. However, the extent of the importance of these abnormalities is not well documented, says Coulter.
Doing It Right A consistent scrotal circumference measuring technique is essential if comparisons are to be made among bulls for selection purposes, or if minimum standards are applied as eligibility criteria for bull growth performance test stations or shows.
Coulter describes the proper technique for taking scrotal measurements as follows:
* Palpate the testes firmly into the lower part of the scrotum so they are side-by-side and scrotal wrinkles that might inflate the measurement are eliminated. (If the temperature is below freezing, bulls should be evaluated in a warmer environment.)
* Place the thumb and fingers of one hand on the sides of the scrotum, cradling the testes. Stabilizing the testes by grasping either the front and back or neck of the scrotum can cause an inaccurate measurement.
* Slip the looped scrotal measuring tape over the testes-scrotum and contract it around the largest circumference. Place moderate tension on the sliding tape with the thumb until moderate resistance is provided by the testes-scrotum.
Little compression of the testes-scrotum will occur in bulls with normal testes, while compression may be substantial in bulls with a thick, fat scrotum and/or soft testes. Firm testes having good tone and resiliency are most desirable.
* Finally, read the circumference. Then repeat the procedure to confirm the results.
The largest source of variation among individuals taking scrotal circumference measurements is the amount of tension placed on the measuring tape, says Coulter. "Moderate tension" is interpreted differently by different operators, he says.
To try to minimize that variation, Coulter and his colleagues Del Buckley and Bruce Scobie at the Lethbridge research station designed the Coulter Scrotal Tape. A spring within the handle of the tape provides exactly the same amount of tension every time a scrotal circumference measurement is taken.
"It doesn't eliminate all the problems, but it applies consistent tension each time," says Coulter.
Another consideration to minimize error is holding the testes-scrotum properly when taking the measurement, says Coulter.
Coulter says scrotal circumference measurements of herd sires should be taken annually or even semi-annually. Typically, scrotal size should increase in yearlings as they become two and three years old.
"However, there can be a tendency for scrotal size to decrease from spring to fall. This is a normal seasonal effect. If there's a decrease from year-to-year (i.e., from spring to spring), that could be an early warning sign of possible difficulties or some degeneration in the testes," he says.
Coulter stresses that while measuring scrotal circumference is important, "you can't overlook the other components of the BSE."
Selection Criteria When using scrotal circumference for sire selection, measurements aren't particularly useful until a bull is 12-14 months of age, depending on breed, says Coulter. Recommended minimum scrotal circumference measurements by age and breed are outlined in Table 1.
Coulter also suggests avoiding extremes in scrotal circumference size. "Sometimes extreme size is caused by abnormalities or disease. A large scrotum (> 40 cm) could be a warning sign of possible difficulties," says Coulter.
Scrotal circumference is a moderately heritable trait (.26-.53), however. Cattlemen selecting herd sires should therefore consider selecting bulls with a larger actual scrotal circumference and a higher scrotal circumference EPD than the minimums to facilitate a genetic change in scrotal circumference for their herd.
"At this time, scrotal circumference is a trait that is virtually free. Producers aren't paying more for large scrotal circumference bulls," says Selk. But he says that could eventually change. "People weren't paying a lot of attention to birthweight 25 years ago. Today, that's a valuable trait and people pay more for it."
The common sense approach to predicting a bull's breeding potential is to use a bull's scrotal circumference as a threshold. If the bull exceeds the scrotal size averages for his breed and age, then there is a strong indication he will be a satisfactory semen producer and successful breeder, Coulter says.