Purebred breeders strive for the perfect reproductive bull and go to great measures to select for scrotal size.
It has long been accepted that scrotal size is a good measure of sperm production.
Larger testicles generally should produce more sperm, making the bull more fertile.
Testicular size is moderately to highly heritable (0.4-.75) and purebred producers have been successful over time increasing the testicular size of bulls in all breeds.
There are still breed differences that are acceptable and various lines within breeds will have differences in testicular size.
Read Also
Western Producer Livestock Report – August 14, 2025
Western Producer Livestock Report for August 14, 2025. See U.S. & Canadian hog prices, Canadian bison & lamb market data and sales insights.
Over my career, bull testicular size has crept up to the point where we are starting to see a detriment to fertility when yearlings get up over the 40 centimetre mark.
There appears to be an upper limit and I believe research is needed to determine maximum limits in scrotal size.
Dr. John Kastelic and Dr. Jacob Thundathil at the University of Calgary faculty of veterinary medicine have conducted research that found that nutrition, both in utero and especially early in life (before puberty), had a profound effect on testicular development.
There was a definite increase in testicular size on bull calves on an adequate nutritional diet versus those on restricted intake.
The adage of farmers saying you can’t starve the head or testicles out of an animal is false.
Bull calves on a poor nutritional diet might appear to have normal size heads, horns (if horned) and testicles but it is an optical illusion because if an animal is thin and stunted, those parts automatically seem bigger.
So bull calves and their mothers must receive good nutrition early, especially in the calf’s first four months of life. This should work well as this is the same time cows need to be on a rising plane of nutrition to rebreed.
Trying to compensate for early poor growth by overfeeding during or after puberty will have a detrimental effect on sperm production and testicular size.
Too much fat in the scrotum alters the thermoregulatory effect. I have seen too many fat bulls with small scrotal size in varying degrees of testicular degeneration. These testicles have been too warm for too long.
Some purebred producers definitely get their bulls too fat, which is why in my experience there seems to be a higher infertility rate with overfit show bulls. As back fat measurements rise, fertility goes down.
Energy and protein levels, as well as micro and macro trace minerals, are critical to testicular development.
Proper balancing of the ration with careful attention to the calcium-phosphorus ratio and adequate levels of micro minerals, such as copper, zinc and selenium, all play into the equation.
The bottom line is growing cattle must receive a balanced mineral program throughout their lives.
We sometimes fail to realize that bull calves grow so rapidly that they can outstrip their nutritional requirements.
We also need to make sure that nutrition is not being stolen by internal parasites (worms) or lice.
Many years ago it was reported that chelated minerals, because of their increased absorption, would help increase scrotal size.
I found this claim unfounded. Your nutritionist needs to balance the ration for your class of cattle and the growth rate they are on and include the vitamins and minerals in this package.
Age at puberty will affect scrotal size and some bull calves undergo rapid testicular growth from 11 to 15 months of age. Generally speaking, greater body weight leads to reduced age at puberty.
Purebred breeders want early maturing bulls that can pass their semen tests and be sold at a younger age.
Some genetic lines do not reach puberty as early and as a result, testicular development is slower and they may not pass semen exams until around 15 months old.
I have one large herd where we measure bulls at nine to 10 months of age to cull and then again about three months later at semen checking time (12 to 13 months).
The growth patterns through puberty can be variable with all other factors being similar. Testicle growth over these three months can vary from a couple of centimetres to eight cm.
Purebred producers should know the exact age of yearling bulls because even a month difference can have a huge difference on scrotal size.
In the United States, they adjust scrotal measurements to one year of age. This may help with age discrepancies, but from what I have seen their estimates are low.
This means many bulls bought from American producers have the adjusted scrotal size on an overly inflated scrotal size score.
Purebred producers should be aware of scrotal size when selecting their herd sires and keep average scrotal size around the breed average or above and they will cull few bulls.
As well, they must pay close attention to proper nutrition.
