EPDOLOGY

EPD-OLOGY

by Darrell Wilkes, Ph.D., International Brangus Breeders Association (IBBA) executive vice president

Beginning with this issue of the Brangus Journal, we present the first of a monthly series on the basics of Expected Progeny Differences (EPDs). We receive emails and phone calls every week from members and non-members seeking a clearer understanding of EPDs and how to use them.

This is an open invitation for members to submit your questions about EPDs. Send your emails to dwilkes@ gobrangus.com. Don’t be bashful. There is no such thing as a dumb question.

IBBA is blessed with a number of members who understand EPDs at a very deep level. I (Darrell Wilkes) don’t want to be the only one answering your questions. So, we will recruit your fellow breeders to respond to your good questions about EPDs. Don’t be surprised to see your neighbor or friend as the author of an EPDology article.

EPD Lesson #1: The most important equation in animal genetics: P = G + E

Even though there is a lot of very complex math that goes into EPD calculations, the most important equation is very easy to understand. Understanding this simple equation is the key to appreciating the true value and power of EPDs.

The letter “P” represents “phenotype”. “G” stands for “genetics” and “E” stands for “environment”. Hence, the full equation is: Phenotype = Genetics + Environment. This is the very first lesson in any textbook about animal genetics – or at least it should be.

Phenotype represents anything that we can measure or observe. In this context, it is not limited to what an animal looks like. It also includes its birth weight, weaning weight, hip height, ultrasound data, etc. Anything that can be measured or weighed qualifies as a phenotype.

Cattle breeders observe extreme variation in phenotypes within their herd every year. Take weaning weight for example. When adjusted to 205 days of age and corrected for cow age, it is very rare to find a herd where weaning weight from top to bottom differs by less than 100-150 lbs. That is a huge difference. The most obvious question is, how much of that difference is due to genetics and how much is due to environment? How much is due to the maternal effects of the dam, and how much is due to the genes that the calf actually inherited? The honest answer is: we don’t know for sure how much is due to genetics and how much is due to environment. The best we can do is estimate the effect of each factor. In essence, that’s what EPDs do. They sort out the non-heritable environmentally-caused variation from the variation that is likely due to genetics.

A simple example of two young bulls from two different herds helps to illustrate the lesson. The example is real. It comes from another breed, but the example is legitimate.

Bull Calf A, from Herd A Bull Calf B, from Herd B

Weaning Weight 985 lbs. Weaning Weight 638 lbs.

WW EPD +66 WW EPD +60

What? The early-life EPDs on these two bulls suggests that their progeny will only differ by 6 lbs of weaning weight when bred to similar cows and raised in the same environment. How can this be when the actual weaning weights differed by 347 lbs? This cannot be right. The EPDs must be wrong.

Each of these bulls was purchased by an A.I. stud and semen was sold into dozens of herds. Hundreds of progeny weaning records were submitted. Oddly, Bull B ended up with a high accuracy WW EPD of +63 and Bull A ended up with a high accuracy WW EPD of +63.

Clearly, Herd A provided a much better environment for weaning growth than did Herd B. Perhaps Bull A lived in a creep feeder and was a milk thief – nursing every cow that would stand for it. Bull B never saw a creep feeder and didn’t realize that he could steal milk. The point is, even though the environments were grossly different and the phenotypes were grossly different, the actual genetics for weaning growth were virtually identical. Very importantly, this outcome was actually predicted by the initial WW EPDs which were only 6 lbs apart.

Bottom line: EPDs sort out the environmental effects from the genetic effects. They sort out the G from the E in

The Most Important Equation in Animal Genetics.

Next month, unless a reader sends a more compelling question, EPDology will explain the difference between additive genetic variance and non-additive variance. It is important because the additive component is the only one that is predictably passed from one generation to the next.