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Parentage testing determines bull efficiency

by Wyoming Livestock Roundup

Breeding season is an important part of ranch life, says Stacey Domolewski, but she asked ranchers, are bulls really siring as many calves as they should?

On Nov. 16, the Beef Cattle Research Council (BCRC) held a webinar titled, “Are Your Bulls Actually Siring Calves?” Stacey Domolewski, BCRC Science and Extension coordinator, presented information from a study she worked on during her master’s degree from the University of Saskatchewan College of Agriculture and Bioresources.

“Everyone has different opinions on what they find important in a herd sire. Some producers focus on structure. Others focus on expected progeny differences (EPDs) or carcass values, and some producers look at how well the bull fits their environment,” Domolewski stated.

The study

“This is a three-year study, and I complied the first year of data. Another student did the second year, and the third year is in progress,” noted Domolewski.

In the study, six commercial cattle producers across Saskatchewan had their bulls and calves tested using DNA parentage testing, she said.

Tissue samples were taken from the calves while hair samples were taken from bulls to determine which bull sired each calf. Producers in the study provided bull age, breeding soundness and purchase prices for the bulls.

Domolewski stated, “We wanted to determine whether it’s worth spending money on bulls with certain traits if the bull doesn’t actually sire calves.”

She presented the first-year data results, which showed a lot of variability in the number of calves the studied bulls were actually siring compared to what they were expected to sire.


“We were surprised by how uneven siring was. Looking across the six ranches the first year, one bull only sired a single calf, and another sired 53 calves,” she said.

Domolewski and fellow students developed a Bull Prolificacy Index (BPI) to help compare the bulls across the different operations.

“BPI allows us to account for the bull-to-cow ratio and number of bulls in a pasture, which influences a bull’s ability to sire calves,” she added.

A bull with a BPI of greater than one indicated the bull sired more calves than expected, while a bull with a BPI of less than one sired fewer calves than expected.

“The theory across the board was mature bulls would sire the most calves, followed by the two-year-old bulls, then yearling bulls. The first year of data supported this theory,” Domolewski said.

Mature bulls sired the most calves in the first year, and two-year-old bulls sired the most calves the second year, with some variability.

“The data from the first two years of the study goes to show that older bulls don’t always sire more calves,” she stated.

Study factors

Breeding soundness, number of bulls in a pasture and the economics of DNA parentage testing are the factors Domolewski’s study took into account.

“All of the bulls in the study had to have a breeding soundness exam, which looks at scrotal circumference, percent normal sperm and physical health. The bulls were required to pass the breeding soundness exam to be turned out with cows,” she said.

Essentially, researchers found bulls that passed the breeding soundness exam still showed variability in BPI.

“The minimum values set for the breeding soundness exam are set because previous research shows if the value is below the specific benchmark, fertility is majorly effected. If the value is above the set benchmark, there aren’t any major effects. It wasn’t surprising to find there wasn’t much correlation between BPI and passing the breeding soundness exam,” she stated.

They also looked at the number of bulls in a pasture because producers said they would just add more bulls if the job wasn’t being done well.

Domolewski said the pastures with a lower number of bulls tended to sire roughly the same amount of calves, and pastures with a higher number of bulls had more variation in the number of calves sired by each bull.

Testing economics

“We looked at the economics of DNA parentage testing and the different ways testing could create value through dystocia selection, changing management practices, increased weaning weights and managing bulls better,” Domolewski noted.

With dystocia, testing could help producers determine what bulls cause calving difficulty, allowing producers to cull or sell them before the next breeding season.

“Testing can be expensive, so producers selecting for less dystocia can just test calves from difficult births, instead of all the calves, to figure out which bull is the problem,” Domolewski stated.

Using testing can also change management practices because test results might highlight more than one bull causing a problem.

“When producers get results showing more than one bull causing a problem, they should then look at the cow herd and management practices going into the breeding season,” she added.

Domolewski also said, DNA parentage testing could be used to increase the total number of pounds weaned.

“We looked at whether there were bulls siring a low number of calves but passing on desirable traits, which would make them more economical,” she added, noting bulls with higher BPIs had higher total pounds of calf weaned.

Testing can also help producers manage their bulls better.

For example, only testing calves born in the first three weeks of the calving season can shortening the time frame for test results.

“We found by testing the calves born in the first 21 days, the lowest performing bulls could be identified. This goes to show producers testing calves born in the first 21 days can use the results to make management decisions,” Domolewski stated.


Domolewski stated the study results also show there are bulls not doing their job and some doing more than expected, but adding more bulls isn’t the answer.

“I can’t say a lot about the long-term trends, but it will be interesting to see the third year of data and what producers using these practices will see in the long run,” Domolewski stated.

Heather Loraas is assistant editor of the Wyoming Livestock Roundup and can be reached at

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