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Who’s your daddy?

A study that used DNA testing to determine the siring success of individual bulls produced some surprises

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DNA testing can help you know which bulls are siring calves and make better management decisions.

“There was a lot more variation in what bulls were actually doing and which bulls were siring calves than we thought going into this,” Stacey Domolewski said during a recent Beef Cattle Research Council webinar.

As part of a three-year project with the Western Beef Development Centre, Domolewski worked with six commercial producers in Saskatchewan. Both bulls and calves were DNA tested in order to precisely determine the number of offspring per bull.

“The average was around 21 — that was around the bull-to-cow ratio recommended for mature bulls,” said Domolewski, the beef council’s science and extension co-ordinator, who recently completed her master’s degree at the University of Saskatchewan.

But that average was misleading — the number of calves sired per bull ranged from a low of one to a high of 53.

The research team calculated a bull prolificacy index (BPI) which allowed them to account for pregnancy rate and the number of bulls in a pasture (which influences the bull’s ability to sire calves). This was calculated by the number of calves sired, divided by the number of calves that they are expected to sire, and the bull-to-cow ratio.

“If there were two bulls in a pasture with 50 cows, each bull would be expected to sire 25 cows,” said Domolewski. “A BPI greater than one meant that the bull was siring more calves than we expected; a BPI of one meant that the bull is siring the number of calves expected; and a BPI of less than one meant that the bull was siring fewer calves than expected.”

Two-year-old and mature bulls sired the most calves but, again, there was a lot of variation.

“What I found most interesting is that even in the mature bull group, we have a lot of variation,” said Domolewski. “The lowest down here was siring a quarter of the calves we expected him to sire. The highest was almost three times what we expected.”

In the second year, the two-year-olds had the most variation, but also sired the most calves.

“This just kind of complicates things a bit,” she said. “It isn’t quite as cut and dry. Just because a bull is older, it doesn’t mean that he will sire more calves.”

Some producers in the study thought that more bulls in a pasture might produce more calves, but that may not be the best route to go.

“What we saw is in the pasture with a lower number of bulls is that bulls tended to sire roughly the same amount of calves,” she said.

More bulls meant more variation. A pasture with nine bulls saw one bull that sired three times more than expected, and one bull that sired three times less.

A sire who is only producing one or two calves costs as much to feed and care for as one that is much more prolific, noted Domolewski.

“That calf essentially comes into the world owing you $1,400,” she said. “That’s just the bull cost, not the cost of maintaining the cow herd for a year.”

That, of course, is only if the bull only sires one calf. If a bull sires 35 offspring, then the cost drops to about $40 per calf. And while culling underperforming bulls decreases costs, researchers are also digging deeper by looking at weaning weights to see if bulls were siring a lower number of calves, but passing on great traits.

One of the operations in the trial followed its calves until weaning. On that ranch, bulls that had the highest bull prolificacy index had the highest total pounds of calves weaned, which meant it makes sense to keep them around.

“That extra calf or two was made up for in the total number of calves weaned,” said Domolewski.

About the author

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Alexis Kienlen

Alexis Kienlen lives in Edmonton and has been writing for Alberta Farmer since 2008. Originally from Saskatoon, Alexis is also the author of two collections of poetry, a biography, and a novel called "Mad Cow."

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