Interpreting scrotal circumference in yearling and mature bulls

Breeding soundness examinations (BSE) of bulls reduce the risk of poor breed-up of cow herds. 

One component of a thorough bull evaluation is measurement of scrotal circumference, which is done by pushing the testicles firmly to the bottom of the scrotum and placing a flexible measuring tape snugly around the widest point. 

Scrotal circumference

A bull’s scrotal circumference provides a good indication of total testicular weight and the amount of sperm-producing tissue. This measurement is particularly informative when assessing bulls one to two years of age but also has value when testing mature bulls. 

Testicle growth is very rapid from the time of weaning until when bulls are relatively sexually mature at about 15 months of age. After this, testicles continue to slowly grow so 90 percent of final testis size is achieved by the time bulls are two years old. 

Scrotal circumference of bulls around yearling age is a better predictor of the onset of puberty than age or weight, and greater yearling testicular size also predicts earlier puberty of both his bull and heifer offspring.

The most common reason a yearling bull fails to pass a semen evaluation is immaturity and low levels of testosterone. One study found 20 percent of 11-month-old bulls, 51 percent of 13-month-old bulls and 61 percent of 15-month-old bulls had a passing level of normal sperm. 

This rapid increase in the percentage of normal sperm in the months around one year of age is supported by improved sperm motility over the same time frame. 

Heritability and nutrition 

The heritability of scrotal circumference is moderate to high, but in addition to the contribution of genetics, nutrition of young bulls can also influence scrotal circumference, age at puberty and testicular development. 

The earliest phase of nutrition is the preweaning diet composed of a bull calf’s dam’s milk and then milk plus forage. 

This preweaning period is very important as early nutritional restrictions can cause permanent reduction in testicular development which can delay puberty despite adequate postweaning nutrition. 

After weaning, feeding high-energy diets is associated with larger yearling scrotal circumference and earlier puberty compared to lower energy diets. Although, some of the larger scrotal circumference may be due to scrotal fat. 

High-energy rations continued past 12 months of age may result in excessive scrotal fat which interferes with the ability of the testicles to stay cool. This leads to lower sperm production and more sperm defects. 

High-energy diets that don’t result in excessive scrotal fat do not appear to negatively affect fertility. 

While yearling bulls receive the greatest focus, guidelines from the Society for Theriogenology – a veterinary organization for reproduction – require the scrotal circumference of bulls older than two years of age be greater than 34 centimeters (cm). 

Most importantly for older bulls, any decline in scrotal circumference from one breeding season to the next could be associated with degenerating testicles and be a cause for concern. 

Two Canadian studies following hundreds of cow herds reported the percentage of normal sperm was lower in bulls with a scrotal circumference less than 34 cm versus more than or equal to 34 cm. 

In addition, cows exposed to bulls with a smaller scrotal circumference were less likely to be diagnosed pregnant and to take longer to become pregnant after calving. 

Bull fertility is a critical driver of herd fertility, and the measurement of scrotal circumference is an important component of a thorough breeding soundness examination.  

Dr. Bob Larson is a professor of production medicine and executive director of Veterinary Medicine Continuing Education at Kansas State University in Manhattan, Kan. This article was originally published by the American Angus Association on May 9, 2025.