When does a breeding bull’s fertility actually begin?
By puberty and the first breeding soundness exam, much of that bull’s reproductive potential has already been established. Research over the past two decades suggests one of the most influential periods in a bull’s reproductive life occurs during the first few months after birth, when endocrine signals are programming how the testes will develop for the rest of the animal’s life.
As Dr. John Kastelic, professor emeritus of theriogenology at the University of Calgary, explains on the most recent episode of “The Bovine Vet Podcast,” the industry’s focus is beginning to shift.
“We’re now moving beyond just getting cows pregnant. We need to generate calves of greater genetic merit,” says Kastelic.
That shift has brought renewed attention to a narrow developmental window that may ultimately determine the quantity and quality of sperm a bull can produce throughout his lifetime.
The First Months Shape a Lifetime of Fertility
For years, bull development research focused largely on post-weaning nutrition. The assumption was straightforward: feed bulls aggressively after weaning and reproductive performance would improve.
Kastelic’s research tells a different story.
Working with both beef and Holstein bull calves, his research group examined how nutrition during the first months of life affects reproductive development. Rather than simply accelerating growth, enhanced nutrition altered the hormonal environment responsible for testicular development.
In Holstein calves offered an enhanced plane of nutrition, his team observed an earlier rise in luteinizing hormone secretion and significantly higher circulating concentrations of insulin-like growth factor-1, hormones that play critical roles in stimulating testicular development. Those endocrine changes shifted the trajectory of reproductive maturation well before puberty occurred.
“The bulls on a 130% of normal intake program were just on a completely different slope,” Kastelic says. “We profoundly changed the slope of testicular development in those bulls.”
The biological changes translated into measurable improvements. Bulls receiving the enhanced nutritional program reached puberty at an average of 324 days of age, compared with 369 days for calves on the lower nutritional plane, advancing puberty by roughly six weeks. More importantly, the benefits persisted into adulthood.
By reproductive maturity, bulls fed the higher nutritional plane developed testes approximately 20% to 30% larger and possessed 20% to 30% greater epididymal sperm reserves, indicating substantially greater sperm-producing capacity.
For AI organizations, greater sperm production translates directly into additional semen doses. For seedstock producers, it means developing bulls that more fully express their genetic reproductive potential.
Later Nutrition Can’t Make up the Difference
Perhaps the most remarkable finding from Kastelic’s research was what happened when they attempted to compensate for poor early nutrition.
Calves that were nutritionally restricted through the critical developmental period were later switched onto the higher nutritional plane. Despite the improved diet, they never fully caught up reproductively.
“If we underfed bulls up to 25 weeks of age and then fed them the 130% diet, we could not rescue them. They were essentially destined to be underachievers, so to speak,” Kastelic explains. “Clearly it was that 8- to 25-week period which was absolutely critical.”
The findings suggest reproductive development follows a biological timetable. Once this developmental window closes, later nutritional improvements cannot fully recover the reproductive capacity that was lost.
“After 25 weeks the door is essentially closed and all you can do at that point is harm them,” Kastelic warns.
That does not mean nutrition loses importance after six months of age. Instead, management priorities shift toward maintaining healthy growth while avoiding excessive body condition that may negatively affect long-term soundness.
More Than Simply Feeding More
One important distinction from Kastelic’s work is that the response was not achieved simply by increasing calories.
The studies evaluated balanced increases in both dietary energy and protein while ensuring adequate mineral and vitamin intake. The objective was to support endocrine development rather than maximize body weight alone.
The economics were also surprisingly favorable. In the Holstein studies, the enhanced nutritional program cost approximately C$280 per bull, yet produced substantially greater predicted lifetime sperm output. Equally important, researchers found no evidence that improved early nutrition compromised semen quality. Semen morphology, fertility in in vitro fertilization trials and seminal plasma characteristics remained comparable between nutritional treatments.
“With a modest amount of additional feed we could profoundly alter the testicular development of that calf into maturity,” Kastelic says. “The potential to produce a lot more doses of sperm as a mature bull is huge.”
Fertility Starts Long Before Breeding Season
The research changes more than nutritional recommendations. It changes when conversations about fertility should begin.
“We should be thinking about that calf even prior to conception,” Kastelic says.
That means considering maternal nutrition during gestation, ensuring timely colostrum intake after birth, supporting balanced nutrition during the first six months of life and avoiding management practices that compromise long-term reproductive development.
The breeding soundness exam remains one of the most valuable tools, but it can only evaluate the reproductive capacity a bull has developed. It cannot restore developmental opportunities that were missed months earlier.
As Kastelic summarizes, “If you’re producing bulls you intend to be future sires, that supplemental feeding from 8 to 25 weeks has a huge impact for allowing that bull to reach his genetic potential for testicular development and sperm production.”
Ultimately, the work reinforces a principle that extends well beyond reproduction.
“Good management means doing a lot of small things and doing them well,” Kastelic asserts.
To hear more from Dr. Kastelic about the and science and practice of bull fertility, tune in to the latest episode of “The Bovine Vet Podcast.”
