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Measuring a bull’s scrotal circumference

Pay attention to minimum measurements for different breeds, but bigger isn’t always better

All producers buying bulls should understand the meaning of a bull’s scrotal circumference. It is good to review some of the common parameters and also clear up a few misconceptions I’m sure are out there. A tremendous amount of information has been gathered and we should use this when making decisions with regards to our bull selection. Scrotal measurement is one of the best indicators of semen output and hence fertility.

In measuring scrotal circumference the veterinarian pulls down the testicles and measures across the widest part of the scrotum. The tape is then pulled tight, compressing the scrotal skin. This measurement (in centimetres) is then a correlation to the testicular mass.

It is very repeatable as measurements made by different veterinarians might only vary by one to two centimetres or so. One must remember the technique used by U.S. veterinarians is much different than in Canada. In the U.S. they simply lay the tape around, pull it snug, but do not compress the scrotal skin. Often breeders importing U.S. bulls will find their measurements four to five cm larger. Caution then for anyone buying U.S. bulls looking to increase scrotal size; this very well could be a misnomer by Canadian standards.

Variability between measurements made by veterinarians or producers, however is a concern. Measuring tapes with spring mechanisms on them standardize the pull on the tape. Previously the Coulter tape was used. Problems in production and breakages have seen this tape removed from the market. The Reliabull Tape, a newer tape invented by a professor in Saskatoon, works off the same principle as the metal tapes with a spring mechanism and also works well. I personally have found this tape very trouble free, repeatable and durable. It definitely standardizes the force on the tape for veterinarians and producers alike.

Breed minimum measurements

Beef breeds have established accepted minimum scrotal circumferences in order to sell bulls through sales. These vary from 29 cm for Limousin and Salers to 32 cm for Simmental at one year of age. The breed associations generally have the accepted standard for yearlings and two-year olds. The two-year-old minimum is generally two to four cm over the yearling minimum. The Western Canadian Association of Bovine Practitioners have broken this down further with minimums for 13, 14 and 15 to 20 months. This is because a bull’s scrotum grows quite rapidly in the 10- to 15-month range so a real difference exists when comparing 12-month-old bulls with 15-month olds. A western Canadian survey looked at average scrotal circumference of the breeds and a few were adjusted upwards slightly reflecting the increase in scrotal circumference from selection pressure.

These breed minimums are very important as in most cases none or very little semen is produced from bulls measuring below the breed minimums. These animals should be culled from breeding programs. At the very least, breeding capacity would be reduced significantly. The different breeds definitely have the distinctions. For example, a 30-cm Limousin yearling can produce good semen whereas a 30-cm Simmental, even if we try to get semen, there is virtually a 100 per cent chance it is sterile. Therefore the breed minimums are very much worth noting. With other species such as bison, 26 to 29 cm for two-year olds is quite acceptable. Rams on the other hand often measure around 40 cm. You can see how through evolution different scrotal sizes have occurred for different species.

With all the data that has been collected on bulls over the years, very accurate breed averages have been established. These are constantly being pushed upwards as purebred breeders select for increased scrotal size in their herd sires. Since scrotal size is highly heritable breeders can keep driving up scrotal size. Replacement heifers Heifer will also be earlier maturing, as fertility is definately enhanced by these larger-testicled sires. These yearling averages vary from 30.3 cm for Limousin to 36 cm for Simmental. Some producers only purchase bulls that are at least close to the average for the breed especially if breeding capacity is a concern.

Bigger not always better

While larger scrotal sizes are desirable bigger is not always better. Research has shown that anything over 38 cm does not result in any more semen production. I get very nervous when yearlings of any breed have scrotal size much above 42 cm. Invariable problems are encountered with abnormally large scrotal size. Extremely large mature bulls can even develop problems with their suspensory apparatus making temperature regulation difficult.

When selecting your next herd sires, get to know breed minimums, averages, make sure qualified veterinarians have done the measurements and realize the differences between Canada and the U.S. measuring techniques. Your herd’s fertility depends on it.

About the author

Columnist

Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.

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