Can your bull be too fat to make babies? That was one topic talked about at this year’s CattleCon.
Discussing bull reproductive health were Saulo Menegatti Zoca, DVM, Extension beef cattle specialist in reproductive management with University of Tennessee’s Institute of Agriculture, and Pedro L.P. Fontes, DVM, assistant professor in beef cattle reproductive physiology at the Department of Animal & Dairy Science with the University of Georgia.
“The bull only has one job in the operation: get cows pregnant,” Menegatti Zoca said. Recent data found that more than 90% of females are serviced by bulls in the U.S. And even with AI, producers often still use a clean-up bull, so the number is actually closer to 97%.
Most bull calves need 12 months to reach puberty – which is defined by a scrotal circumference of 28 cm and ejaculate containing 50 million sperm. Those sperm need at least 10% progressive motility – the forward movement of sperm, specifically in a mostly straight line or large circles.
Motility is a crucial factor for fertility as it allows sperm to reach and fertilize an egg.
“How we manage animals at this age can either boost or delay growth,” Menegatti Zoca said.
A “mature” bull is physically sound, passes a breeding soundness exam (BSE) and has 30% motility in his sperm with 70% normal morphology. Generally, mature bulls have scrotums with a 30 cm circumference at 15 months and 34 cm at 24 months and older.
Unfortunately, about 20% of bulls are expected to fail their BSEs, due to poor semen quality or poor physical condition.
Menegatti Zoca reported that about 67% of producers “semen-test” their purchased, leased or borrowed bulls, but only 31% test their resident bulls. In that case, a BSE is an insurance policy. You also hope all your owned bulls pass.
Ultimately, utilizing routine BSEs has shown that a subfertile bull can cause more economic damage than buying a new bull.
Why wouldn’t they pass a BSE? Diseases are a common issue, including leptospirosis, which can be prevented through biosecurity measures and vaccination, and venereal diseases. These can spread either bull to cow or cow to bull, but not cow to cow, unless you use a contaminated applicator.
Other viral reproductive diseases that can cause issues are bovine viral diarrhea virus, which has a vaccine, or bovine herpes, which has no treatment but does have a vaccine preventative.
An injury can lead to a BSE failure, as can scrotal temperature – it needs to be 2º to 3º lower than the body for sperm to develop.
The final factor is nutrition.
“Is there a relationship between sire over-conditioning and semen quality?” Fontes asked. “If he is over-conditioned, we see that BSE drop significantly.” He cited one study looked at backfat thickness, and with extreme thickness, more BSE failures were found.
How much fat is too much? Fontes said when it reaches 1.4 cm/0.55 inches, that’s where it becomes a bigger sperm morphology (the size and shape of the sperm) issue.
Their research has found that high energy diets did alter sperm cells, with decreases in motility and increases in secondary defects. Additionally, more fat in the scrotum is bad, because it holds in heat.
Some producers use high energy diets to help push bulls into puberty, but Fontes said that high plane of nutrition only hastened puberty achievements when it was offered during the first six months of a bull’s life. After that, it’s just extra food for gains but not necessarily reproductive maturity.
Those high energy diets can affect embryos too. With high adiposity (the degree of body fat accumulation), high leptin (a hormone produced by fat cells that plays a key role in regulating body weight) and high glucose, triglycerides and cholesterol, there can be functional and epigenetic changes to sperm.
These include DNA methylation (which can change the activity of a DNA segment), non-coding RNA profiles and chromatin (the “packaging material” for DNA) structure issues. It can all result in blastocyst/embryo production drops due to a high fat diet.
“The bull plays a big role (as does his diet) on embryo development – it’s not solely the fault of the female” if things go wrong, Fontes said.
For a bull to have potential high fertility, they need to be healthy. They need a scrotal circumference that’s sufficient for daily sperm production during the breeding season. And they need to produce an ejaculate with a great proportion of sperm with high levels of desirable traits.
To achieve all that, proper nutrition is critical.
