Scrotal shape and size can be a good indication of a bull's fertility. Exact size can't be measured, but you can usually tell by visual inspection whether or not a bull is adequate or falls short in this department. Shape is important also, since a bull needs to be able to raise and lower his testicles easily for proper temperature control.
The testicles should hang down well away from the body in warm weather. There should be an obvious "neck" at the top of the scrotum, with the testicles hanging down large and pearshaped. A bull with a straightsided scrotum or a "V" shaped scrotum (tapering down to the tip) is often not as fertile as a bull with a normal scrotum.
Also beware of selecting a bull with odd shaped testicles (one obviously smaller than the other). Any abnormalities should be noted. Scabby, thickened skin, especially on the back, bottom third; may be indication of frostbite, which can cause temporary or permanent infertility, depending upon the extent of damage and scarring.
If you have any doubt about whether scrotal size is adequate, scrotal circumference is easily measured. Testicle size is an excellent indicator of a bull's fertility, since a significant correlation exists between scrotal circumference and sperm cell volume (and percent of normal sperm cells). There is also a strong genetic correlation between scrotal circumference in bulls and the fertility (as measured by earliness of puberty) of their daughters.
Bulls measured at one year of age should have a scrotal circumference of at least 32 centimeters, and preferably 34 to 36 cm. To measure a bull, confine him in a chute, and from behind him take hold of the neck of the scrotum, and gently force testes down into the scrotum, putting the measuring tape snugly around the largest circumference.
Small testicles have been associated with bulls having exceptionally good muscling and high carcass cutability. It is extremely important that high performance (fast growing) bulls be reproductively sound. A high rate of gain and/or superior muscling will not compensate for inadequate scrotal circumference and poor breeding performance. What good is exceptional rate of gain or muscling in a breeding bull if he cannot pass it on to his offspring because he is not very fertile, if he sires few calves, or perpetuates fertility problems in the ones he does produce? A bull with low fertility can be a very expensive investment and damaging to your next year's calf crop or breeding program.
Bulls with small testes not only have lower sperm production but often suffer from other problems that make them subfertile or infertile. Infertility associated with small testicles is often due to incomplete development or underdevelopment of these organs, or due to testicle degeneration. Some bulls with scrotal circumference of 29 centimeters or less may produce no sperm at all.
Some bulls with smaller than average testicles may be fertile at first (for a year or two) and then become less fertile or completely sterile because the tubules within the testicles degenerate earlier and at a more rapid rate than in a normal bull. There is often more abnormal sperm in the semen of bulls with small testes, probably because of early testicular degeneration.
All types of testicular underdevelopment are heritable. Bulls with low fertility tend to sire sons and daughters with low fertility (and daughters pass on low fertility to their sons). Selection of bulls with large scrotal circumference for their age can greatly help in avoiding this problem, avoiding bloodlines that tend to produce small, incompletely developed testicles.
Because scrotal circumference is associated with age of puberty, semen traits and testicle health, always choose a bull with adequate circumference. Early maturing bulls generally have greater scrotal circumference than late maturing bulls, and will sire daughters that reach puberty sooner. Beef bulls usually average between 34 and 36 centimeters when they are mature enough to breed as yearlings. Research trials have shown that bulls with a scrotal circumference of less than 30 centimeters should not be used for breeding, even if most of their sperm is normal.