A bull’s job is to breed cows. From a fertility standpoint, a bull should be sound and ready to go to work each breeding season.
To better understand how a bull’s condition affects his soundness, a recent research project at the University of Georgia (UGA) looked at the relationship between a bull’s subcutaneous backfat thickness and its semen quality.
“Over conditioning is very common within the seedstock world, and we’re interested in understanding to what extent that over conditioning or increased subcutaneous back fat thickness may have an influence on that bull’s fertility,” says Molly Smith, a UGA doctorate student.
She says the project included more than 700 bulls — a combination of Angus and Sim-Angus crosses from three different bull development programs. The research process included two key steps: a carcass ultrasound and a breeding soundness evaluation (BSE).
The study categorized bulls into three groups based on their subcutaneous backfat thickness, using 10% and 20% thresholds.
When examining semen characteristics, Smith says they found some intriguing results.
“We saw no differences in the amount of motility, but we did see differences in morphology,” Smith explains.
Morphology refers to the shape, size and structure of the sperm. Sperm motility is how it moves.
Specifically, bulls in the top fat thickness categories showed concerning semen quality characteristics.
“Those bulls that were in the top threshold actually had a less percent, a less amount of normal sperm cells and a greater amount of primary and secondary abnormalities,” Smith says.
BSE Impact
Pedro Fontes, UGA associate professor, recently visited with Smith about her project during a “UGA Beef Tips” podcast.
He asked Smith if the backfat affected the bull’s ability to pass the BSE.
Smith says, “Bulls that have a greater than about 0.5" of subcutaneous back fat thickness had a greater probability to fail their breeding soundness evaluation.”
Quantifying this, she adds, “10% of them failed their breeding soundness evaluation, which is greater than two times the bulls that were in our other two categories.”
Fontes clarifies the failures were specifically related to semen quality, excluding other potential reasons for failing a BSE.
“Most of the issues that you (Smith) observed were associated with morphology and non-motility. So those cells are moving well, but they look different,” Fontes says.
Performance and ADG
When evaluating performance or average daily gain (ADG), Smith reports there was no change between bull groups.
“There actually were no differences between our categories of top, middle and bottom bulls, our top bulls were not gaining any more than our middle and bottom bulls were, so they were all kind of following the same trend,” Smith reports.
Fontes interprets this finding as significant, suggesting that “it’s not a matter of allowing those bulls to express their genetic potential for growth” but rather about understanding fat deposition.
Smith agrees, saying, “It’s about essentially where that fat deposition occurs, and that’s how it matters.”
While producers might worry about restricting bull growth, the research suggests that careful monitoring of fat distribution is more important than preventing weight gain.
Fontes summarizes the key takeaway from the research is the ability of bulls to pass a BSE appears more related to body condition score and over-conditioning rather than their overall growth potential. Excessive back fat can negatively impact a bull’s reproductive potential, even if it doesn’t affect its growth performance.
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