With the breeding season approaching, University of Arkansas professor of breeding and genetics Brett Barham offered Ozarks Farm & Neighbor readers a primer on those valuable Expected Progeny Differences. Breed associations are providing a steadily increasing mountain of data on sire EPD's, and although the newer ones are of interest to many producers, there are a few standbys that most ranchers in this region rely upon. The most important one for producers who sell weaned calves, Barham said, is the weaning weight. Also high on the list, especially for bulls that are going to be turned out on first-calf heifers, are birth weight and calving ease.
"The official definition of calving ease," he said, "is the probability that calves out of a particular bull will be born unassisted — that would be the definition for calving ease 'direct'. The definition of calving ease 'maternal' would be the probability that daughters out of this particular sire will calve unassisted, so there's a slight difference between the two." In most breeds, a higher number means a higher probability the dam will not require assistance. "You want a lower number on the birth weight," said Barham, "and a higher number on the calving ease." EPD's will also carry an accuracy score, which is based on the number of progeny records that went into that calculation; AI sires tend to have the highest scores.
"Unfortunately," Barham noted, "most producers don't use AI, so therefore they're stuck buying yearling or two-year-old bulls that really don't have any progeny reported on them yet." Those bulls' EPD's, for the most part, are based on those of their parents and other relatives, and their accuracy values are usually quite low. "That doesn't necessarily mean that they are not a good selection tool," Barham said. "That just means that statistically speaking, we weren't exactly sure that if the bull is +5, that he really is +5 and not +7 or +2."
Those numbers compare animals within the same breed for a specific trait. For instance, if one bull has an EPD of +5 for weaning weight and the other one has an EPD of +10, the second bull's progeny would be expected to average five more pounds apiece at weaning. Since each breed calculates its EPD's slightly differently, you can't compare, say, the EPD's for the same trait of an Angus bull and a Charolais bull.
However, Barham said USDA has what it calls an "across breed EPD conversion table" with conversion factors that "put them all on the same page." Barham said within the first few pages of each breed's sire summary is a wealth of valuable information; included is a table listing what's called the "possible change value," which is calculated using the accuracy figure. Barham said, "Accuracy is always reported on a 0-1 scale, 1 being perfectly accurate — which statistically speaking is impossible; you can get up in the high 90's — all the way down to 0, which means no accuracy at all. They'll have those accuracy values broken down, and will have a possible change value associated with those accuracies."
He gave an example: "Let's say you look at the table, and the accuracy for a weaning weight that you have on this bull is 0.2; the possible change value may be +/- 5 lbs. So basically, what that's saying is that his reported EPD may be +15, the Possible Change Value is +/- 5 lbs, so his true EPD is probably somewhere between 10-20 lbs. The higher the accuracy, the smaller that range gets." Among the other EPD's some ranchers make use of are carcass traits; these would be of value to a cattle producer who's retaining ownership and feeding his own calves.
Some breeds now make a disposition EPD available, and this could be important to a producer with herd handling problems; one of the EPD's the breeds are trying to accumulate data on is stayability, how long a sire's daughters tend to remain productive cows. Barham said there are also "indexed" EPD's that combine several traits into one number, each factor adjusted for importance.
Barham said he tries to get bull buyers to either contact him or go to a breed's web site to look up the most recent EPD's, so they can compare. "If you go into the bull and he's got a weaning weight of +35," he said, "what does that really mean? How does he fit in the average of the Angus bulls? Does that mean he's a good bull compared to other Angus?…
Most producers that you talk to, if you asked them if they wanted to go buy a below average bull for weaning weight, they would probably tell you no, but many of them probably are unknowingly buying a below average bull, because they don't know what below average is."
