EXPECTED PROGENY DIFFERENCES: THE BEDROCK OF GENETIC IMPROVEMENT

Editor’s Note: A comprehensive version of this article including index mathematical calculations can be found online at GoBrangus.com.

by Darrell Wilkes, Ph.D., IBBA executive vice president

After 40-years of speeches and writing about EPDs, it is apparent from the calls we receive from breeders that some of the most fundamental truths about EPDs are still not well understood. Every survey that’s been done in the past decade has confirmed that commercial cattlemen utilize EPDs in their bull selection process. Bull customers expect you, as a seedstock producer, to understand EPDs and to be able to answer their questions on this subject. Therefore, perhaps one more article on EPDs will sharpen your explanations to your customers.

At the end of this article, I will actually show how to compute an EPD from scratch. Don’t worry, it’s easy. You can do the math in your head.

When EPDs were introduced to the beef industry in the late 70s, the scientists said that EPDs were seven to nine times more powerful than individual weights and measures with regard to making genetic change. Since then, the equations have been improved, and computers are much more powerful, which makes complex equations easier to handle. We have also added the value of genomic testing to EPDs. If EPDs were seven to nine times more powerful than individual weights and measures back then, I would argue that they are at least 10-times more powerful than individual data today, probably 12-15 times more powerful.

Although obvious, it helps to restate the fact that EPDs can be used to compare animals across an entire breed or other population of cattle whose data are kept in the same database. Individual weights and measures ONLY have relevance within a single herd, and within a legitimate contemporary group.

There simply is no dispute: If you want to make purposeful and I used to ask my students: Would you be significant genetic better off to buy a bull with a 110 YW ratio change in a out of a herd with an average yearling population of weight of 1,200, or would you be better cattle, EPDs are off to buy a bull with a ratio of 100 out of the single most a herd with an average yearling weight powerful piece of 1,320? Both bulls weigh 1,320. Which of information one has a higher breeding value for available to you. yearling growth? I’d let them argue about Astute breeders this for several hours before they finally know this and concluded [correctly] that the best way to are trying to select the bull with the highest breeding move away value for yearling growth is just to flip a from publishing coin. EPDs can fix that. individual weights and measures in their sale catalogs and advertising. It is a delicate balancing act because many commercial customers still want to see the actual data; such as birth weight, weaning weight, etc. Individual weights and measures are still printed in advertisements even by people who understand EPDs and know darn well that they should not be confusing things by publishing raw data – such as actual rib eye area or an amazing %IMF value. In a perfect world, we would never publish individual weights and measures again. But, the world isn’t perfect. Realizing that success in the seedstock business requires genetics AND marketing, I cannot begrudge anybody for bragging about an amazing individual record. As time goes on, however, I hope we see less of that.

The first and most fundamental TRUTH about EPDs is that they absolutely do NOT predict an actual weight or measure. A common call into the office goes something like this, “If I buy a bull with a +25 for Weaning Weight (WW), what will my calves weigh at weaning?” An EPD will not give you an answer to that question. Here’s how I answer that question:

“I have a neighbor who is a great stockman. He rotates pastures like a pro and has virtually no weeds. No matter the weather, grass always seems to be growing on his place. His water tanks are clean. He buys top end mineral and his mineral feeders never run out. His cattle are never sick. He even has a low fly population. I hate that guy (though I’d like to be like him). He buys Brangus bulls with +25 EPD for Weaning Weight. He weans a 97% calf crop and they weigh 600 lb.

I’m not that good. My pastures are mostly weeds and brush (from poor management). My water source is mostly mud puddles. I figure plain old salt is the only mineral they need. I doctor or bury 15 percent of my calves and the darn cows are always thin. But I buy “better” bulls, with WW EPD of +35 and then I wean calves weighing 475 lb. Those darn EPDs don’t work.”

EPDs absolutely “work.” If the good manager, as described, bought some bulls with +35 EPD for WW and ran them in common with his +25 WW bulls, we would expect to see a 10 lb. difference in weaning weight between the two sets of calves. However, if the poor manager upgraded his bull purchases to bulls with +50 WW EPD, he may see no difference at all in the average weaning weight of the calves sired by his +35 bulls simply because his environment is so limiting that genetic differences have no opportunity to be expressed. (continued on page 22)

(continued from page 21)

The moral of the story is that every phenotype (i.e. weight or measure) is the net effect of genetics and environment. The first equation that a genetics student learns is:

P = G + E phenotype = genetics + environment

For the simple reason that you cannot know the environment in which a breeding animal will be asked to produce, it is impossible to predict an actual weight or measure from an EPD.

EPDs are relative to the population from which they are computed. That sounds “mathy,” but the concept is simple. EPDs are always computed for a specific defined population of animals and they can only be compared head-to-head with other animals in the same population or, more specifically, animals within the same computational database. The population is usually a breed, although it need not be restricted to a single breed. In the case of Brangus, our database contains data on Brangus (black and red), Ultra (black and red), Angus (black and red), Brahman and the various crosses among Brahman and Angus that are used in the approved mating schemes to produce Brangus. All of these breeds and composite animals are compared head-to-head in the Brangus genetic evaluation.

Brangus EPDs cannot be compared directly to any other breed (i.e. Simmental, Limousin, etc.) as they are on a different scale. They can only be accurately compared to other EPDs computed within the Brangus database. There are over 1.5 million animals in the Brangus database. Even though we did not have EPDs back in the 40s, 50s, etc., until the late 70s, we can now compute EPDs for every animal in the database – and we do so every month. Yes, we compute EPDs on animals that have been dead for 70 years. Why? Because they are part of the pedigree of current animals, which contributes to their genetic evaluation.

The correct way to evaluate EPDs is to use the EPD to compare the expected progeny outcome of one bull versus another.

The correct way to evaluate EPDs is, of course, to use the EPD to compare the expected progeny outcome of one bull versus another. The word “expected” is emphasized in the preceding sentence to make another obvious point. An EPD is an estimate; it is not a guarantee. “Accuracy” values indicate the reliability of the published EPDs for an animal and can be defined as the relationship between the published or listed EPD and the “true” value of the number. The EPD of a young animal will have a relatively low accuracy value. For most animals, we will never know their true EPD. This would be an EPD with an accuracy of .99 (an accuracy of 1.0 is mathematically unachievable). This can only happen when a bull sires thousands of reported progeny in dozens or hundreds of herds. It is a rare circumstance.

Another popular and appropriate way to utilize EPDs is to compare the EPD of the animal(s) you’re evaluating to the breed average or a percentile rank within the breed. So, if you buy bulls in the top 25% of the breed for WW, for instance, will your calf crop weigh in the top 25% of the Brangus population? Again, it depends on the environment. Remember, P = G + E. If you provide an above-average environment for WW, your WW will be heavier than the top 25% of adjusted weaning weights. If you provide a belowaverage environment, your calves will weigh less than those at the top 25% mark. Remember, P = G + E.

I give fair warning that the next statement I offer will seem shocking and might ruffle some feathers. Virtually every EPD on a young animal is “wrong.” WHAT? After preaching how great EPDs are, you say that they’re wrong? Yes. There is a very, very high probability that every EPD on an animal will change as they add progeny. Some changes will be extremely small. Others might be fairly large. As the accuracy increases and more data is added to an animal’s record, the EPD will change. But as the accuracy increases, the magnitude of changes decline. By the time an AI sire gets to an accuracy of .75 or .80, the odds of a significantly sized change in his EPDs are low.

How do you explain this to a commercial bull customer? Here’s how I explain it: If you buy a single bull with a WW EPD of +25, even if he has genomically enhanced EPDs (GEEPDs), the accuracy will be below .50. His “true” EPD, if it is ever learned, is likely between +15 and +35. However, if you buy five bulls at a sale and the average WW EPD is +25, I can virtually guarantee that the true average WW EPD is very, very close to +25. Your commercial customers are (or should be) focused on population genetics. They are interested in moving their averages steadily toward their goals. If a WW EPD of +25 is the average they seek, they should buy bulls that are close to this value.

Before I show how to compute an EPD from scratch, it is worthwhile to restate the basic truths of EPDs. 1) EPDs are significantly more powerful in achieving purposeful genetic change than individual weights and measures. Those who pretend otherwise are slowing their own progress. 2) EPDs do not predict actual weights and measures because environments are vastly different. 3) The proper use of EPDs is to compare one animal to another, head-to-head, or to compare an animal of interest to the breed population ranking. 4) Individual animal EPDs, especially on young virgin animals, will almost certainly change over time as more data are added. But, the AVERAGE EPD on a group of young virgin animals is very, very close to the (unknowable) TRUE EPD.

Computing an EPD from scratch

1. A group of bull calves has an average adjusted

WW of 600 lb. a. Bull A, nicknamed “Stud”, weaned at 700 lb. b. Bull B, nicknamed “Handsome”, weaned at 620 lb.

2. The heritability of WW is assumed to be .30 – meaning that 30 percent of the observed phenotypic variance is due to the type of gene effects that are predictably passed from parent to offspring.

Believe it or not, that’s all we need to calculate EPDs for Stud and Handsome.

Calculating Stud’s EPDs

For Stud, his selection differential is 100 lb. (i.e. 700-600 = 100). We assume that 30 percent of this observed difference from the average is due to additive genetic variance, which means that 30 lb. of the 100 lb. deviation is believed due to the type of gene effects that are passed predictably from parent to offspring. When Stud sires progeny, a random sample of half of his genes are passed to each offspring. Therefore, we assume that each progeny will benefit by 15 lb. compared to his contemporary average. Hence, his EPD for WW is +15.

Calculating Handsome’s EPDs

For Handsome, his selection differential is 20 lb. (i.e. 620-600 = 20). With 30 percent heritability, we assume that 6 lb. are due to additive genetic variation. Hence, his EPD is +3.

If we select both bulls, and collect weaning weights on their progeny, we expect the progeny averages to differ by 12 lb. So, there you have it. That’s how to compute an EPD.

Now, as a thoughtful cattle breeder, start listing all the information that is missing in the above analysis. > There is no consideration for the milk production of the dams of Stud and Handsome. > We have no estimate of the growth genetics of the two dams. Is this the first above-average calf that Stud’s dam has ever weaned, or does she hit a home run every year? If she weans a big calf every year, how much is due to milk and how much is due to “growth genes?” It would be good to know how the paternal sisters of Stud’s dam perform (i.e. Stud’s aunts). That might help us sort out the effects of milk vs. growth genes. > We don’t know the sires of these two bulls. What if the Stud’s sire has a history of siring calves that weigh less than the herd average? What if the Handsome’s sire has a history of siring calves that are above the herd average? Shouldn’t we factor this into our simple EPD? Of course, we should. If this were the case, we would reduce the EPD spread between these two bulls. We would logically conclude that Stud is not as good as he appears, and Handsome is better than he appears. Or, vice versa, we should increase the spread; but by how much? > No account is taken for the siblings of Stud and Handsome within this herd. What if Stud’s siblings equaled the herd average, and Handsome’s siblings exceeded the herd average? Wouldn’t this trigger our cowboy common sense to conclude that the two bulls are not as far apart as they appear based only on their individual weights? Sure, it would; but by how much? > It would be nice to have yearling weights. There is a genetic correlation between WW and YW owing to the effects of certain genes that, grossly, increase rate of gain across the board. We might end up adjusting the weaning weight EPDs based on yearling weight. What if

Handsome catches up to Stud by the time they are yearlings? Does that affect our estimate of their genetic difference for weaning growth? Of course, it does; but byhow much?

Every thoughtful cattle breeder could fill up one or two more pages listing all the information that should have been considered in our simple EPD calculation. Even if we had all the information listed above, but only had data from this single herd, the opportunities for genetic improvement are limited by selecting individuals from this herd to parent the next generation. Wouldn’t we be better informed if the sires of Stud and Handsome had progeny records in other herds? Wouldn’t we be better informed if we knew that the aunts of Stud consistently wean heavier calves than expected based on their growth pedigrees? Implying that heavy milk runs in the family and a large portion of Stud’s weaning weight advantage is likely due to a heavy milking dam and not so much for direct growth?

Having contemplated all these questions and realizing that we are missing an incredible amount of information in our simple EPD calculation, consider the fact that we have only been talking about ONE TRAIT – weaning weight. Try adding another 12-15 traits, from Mature Weight to Heifer Pregnancy to Rib Eye Area, some of which are genetically correlated, and some aren’t, and then try doing the math as to which bull in our example fits your breeding program best.

EPDs factor in all relevant information and emphasize it in the right proportion. For any thoughtful person to believe that an individual weight or measure contains as much information as an EPD, or should be used as a rationale for excusing a poor EPD or depreciating a good EPD, is kidding themselves. For any thoughtful person to believe that an individual weight or measure contains as much information as an

EPD is kidding themselves.

EPDs are powerful. When used properly, they represent a sure-fire way to move a herd in whatever direction a breeder chooses. What makes EPDs valuable are the decades of data that back them up. No matter how many innovations are made with genomics and no matter how many improvements are made to the statistical models used to compute EPDs, the bedrock of this incredibly powerful technology will always be individual animal weights, measures and scores (phenotypes) purposefully collected by serious breeders and turned in to your association so that the information can be fully utilized.