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Early supplementation of beef bulls leads to larger testes, breeding potential

by Wyoming Livestock Roundup

The challenges of the beef industry extend into a wide array of issues, but John Kastelic, a veterinarian at the University of Calgary explains cattle production has changed in the last several decades, which means production must change slightly. 

“There are a lot of challenges in the beef industry,” Kastelic said. “We better be willing to move ahead and do things somewhat differently to meet those challenges.”

A look back

Historically, Kastelic mentions that beef cows calved in May, and their sons were sold at two years of age. 

“That has radically changed today. We’ve got cows calving earlier and earlier in the year,” he says. 

Additionally, many bulls are sold as yearlings, and as production sales are scheduled earlier, buyers select their seedstock earlier in the year. 

“Once people buy bulls, they stop going to bull sales, so seedstock producers have to be ready to sell earlier,” he continues. “It becomes a real challenge to get bulls to pass a breeding soundness exam early in the year so they are able to get into those early sales.”

Bull requirements

Kastelic explains bulls are expected to do several things during their lives. 

“A bulls has to identify cows in estrus, be able to mount those cows, successfully breed cows and deliver a large number of normal, fertile sperm,” he says. “If any of these are deficient, he will not be a functional breeder.” 

For example, if a bull has top-quality sperm motility, if he has a sore back and is unable to mount cows, pregnancy rates will be very low. 

Physical elements

One of the important considerations for bulls is related to scrotal circumference. 

“In general, larger testes produce more and better sperm, up to a scrotal circumference of about 35 to 37 centimeters,” Kastelic says. “We want to feed the bulls to reach genetic potential for testes size.” 

As a result, several studies have been conducted to demonstrate the impact of feeding on early bull development. 

“When we talk about developing testes, we need to talk about early versus later feeding and what we should and should not do,” Kastelic comments. “We want our bulls to be functional breeders for many, many years, so developing them is important.”


Kastelic says numerous studies have been conducted on the effects of feeding dairy bulls, and most of it was done after weaning, with limited work done in the early stages of bull development, so Kastelic’s team did a number of experiments to fill the void. 

“In this experiment, we fed three groups of bulls from 10 weeks to 70 weeks,” he explains. “These were beef bulls that were weaned early. We had one group who was fed a medium diet, which was our control group.”

The three experimental groups were low and high nutrition diets. Group one was fed low nutrition from 10 weeks to 26 weeks, and after that, half went on to a high nutrition diet and half went to medium nutrition. The third group was fed a low nutrition diet and, after 26 weeks, went to a medium-nutrition diet. 

The medium nutrition diet met 100 percent of requirements for both energy and protein, while the high nutrition diet met 130 percent of energy and protein needs. The low nutrition diet was only 70 percent of energy and protein requirements.

“All bulls received all the minerals and vitamins they needed,” Kastelic adds.


“Early on, we had a diversion in terms of scrotal circumference,” Kastelic says. “The bulls on low nutrition had testes that grew at a slower pace than were the bulls fed medium and high nutrition.” 

He continues, “The bulls that were initially fed on lower nutrition had a permanently lower scrotal circumference. Even the bulls that were put on high nutrition after 26 weeks, they never caught up to the other bulls.” 

Those bulls on a low plane of nutrition to 26 weeks, then fed a medium nutrition diet had the smallest testes throughout the study. 

“The bulls that were restricted early on had testes that were roughly 20 percent smaller,” Kastelic says. 


After the conclusion of the first study, a second study was conducted looking at one group of bulls fed a medium nutrition diet throughout the entirety of their lifespan, from 10 weeks through 74 weeks. 

“We had a second group, which we supplemented early on. They were fed a high nutrition diet from 10 weeks until 30 weeks, and then we had a medium nutrition after that,” Kastelic explains.

Again, in the second study, even though bulls were only supplemented to 30 weeks, high-nutrition bulls had larger testes across the board. 

“At slaughter, the supplemented, high nutrition bulls had testes about 20 percent larger, and perhaps more importantly, they had roughly 30 percent more daily sperm production,” he explains.

“By feeding bulls better early, we created bulls with larger testes that produced more sperm,” Kastelic summarized. “These effects remained, even though we went back to common nutrition after 30 weeks.”

Additionally, Kastelic noted that supplemented diets after 30 weeks had a limited effect on reproductive development, including a limited ability to overcome earlier deficits. 

“We need to feed our bulls really well prior to 30 weeks at the time when we can most influence them,” he says.

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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