One of the main areas that I have worked on in my consulting business is genetic selection and the development and use of genetic evaluation technologies. In the more than 20 years I have been working in the field, it is interesting many of the same myths and challenges continue to appear, all during a period of unprecedented technological change and advancement in the field of evaluation. While we can’t tackle every myth, we can tackle some basics and maybe knock two or three mistruths from the proverbial selection tree.
What is an EPD?
This is the first question and the sort of myth that often comes to the forefront. Expected Progeny Difference (EPD) is nothing more than a prediction of an animal’s genetic merit relative to other cattle in the population. It is calculated using all the available information on that animal once we have done some quality control on the information. If we can objectively measure differences in a trait between animals, then we can calculate what portion of that difference is due to genetics. The genetic portion of the difference is what can then be passed on to offspring. EPDs are usually expressed in the units we use to measure the trait. There are a variety of EPDs published but a simple example to explain how they work would be to look at birthweight and weaning weight EPDs if we had two bulls with EPD for BW and WW is in Table 1 below.
If we used these bulls across a similar group of cows we would expect calves from Bull A to be five pounds lighter at birth than those from Bull B. We would expect calves from Bull B to be 20 pounds heavier on average at weaning than those from Bull A. At my house that might mean calves would be 65 and 70 pounds at birth respectively, and your house they may be 90 and 95 pounds, but they will be on average five pounds different between the two sires. Additionally, some calves from Bull A might be heavier than some calves from Bull B, but on average they will be lighter. At weaning, again we would expect an average difference of 20 pounds between calves from these bull — maybe 480 and 500 at my place and 780 and 800 at yours.
Myth #1: EPDs are wrecking good cattle
Probably the major challenge that I deal with is that EPDs are somehow ruining cattle. In reality, it takes people to do that and we were creating trends long before EPDs were on the scene. Remember the belt buckle cattle of the 1950s? This change and the swing back to the monsters of the 90s was well underway before EPDs came on the beef cattle scene in Canada.
EPDs describe genetic differences between cattle based on the information available on those cattle and often breeders tend to select for bigger and bigger numbers. EPDs could just as easily be used to select for cattle that are smaller or cattle that are in the middle of the population. We have horsepower numbers on tractors, but most people do not have a 600-hp loader tractor. EPDs are no different than a horsepower or fuel efficiency rating and often somewhere in the middle is the ideal for functionality. The best tip is that if you have cattle that have worked well, look up the sire on the relevant association website and buy more cattle with EPDs like that.
Myth #2: You can’t compare with your neighbours
This myth is a bit more complex than just leaning over the fence to see how your neighbour farms, and it is common knowledge that we can all run our neighbour’s place better than our own. Because of the way that EPDs are calculated, people comparing performance of cattle managed in the same groups are often confused that their cattle are being directly compared to others. This is 1,000 per cent NOT the case. In fact, cattle performance is only ever directly compared to animals in the same contemporary group and then only after we have made several adjustments for age, breed composition, sex, and prior management. Once cattle that have been managed together and have their genetic component teased out, we can compare the relative genetic merit across herds. In other words, we remove management effects by only making direct comparisons of cattle that have the same management. So the truth is that we can’t compare your operation to your neighbour, and in fact we don’t compare you to your neighbour, or your bull supplier to other bull suppliers.
Myth #3: EPDs aren’t accurate
I often run into bull sellers who tell their customers that the EPDs aren’t accurate and that you should not trust the numbers. Just use the “actual” birthweight is a common way of phrasing this argument. First, when buying a herd sire, it is important (and sometimes difficult) to remember we are not purchasing the performance of the herd sire. We are in fact purchasing the DNA he will pass on to his offspring. In other words, we are buying the future performance of his offspring.
Part of what may fuel this myth is that EPDs do come with an associated accuracy value, but this does not reflect “how accurate they are.” In predicting future performance of offspring, a low-accuracy EPD is much better than an actual performance measure for the trait on the bull we are looking at. For example, let’s look back at Bull A. His WW EPD of 35 has an accuracy value of 0.33. This is a reflection of how much information we have available to include in calculating his EPD. Bull B with an accuracy of 0.44 has more information available for the calculation of his WW EPD than Bull A does.
Rather than just his own weaning weight and/or index, Bull A will likely have his pedigree information, his birthweight information and weaning weight information within his contemporary group (is he heavier or lighter than average) and will have information from his relatives in other herds contributing to his EPD. This is much more powerful than just having his actual or adjusted weaning weight, when we try to predict the DNA that he carries to pass on to his offspring.
A lot of the new technologies have come on the scene directly examine the animal’s DNA and include that information in the EPD prediction as well. If we look at Bull B we can see he has higher accuracy values than Bull A. This could be due to directly measuring his DNA through a DNA test and including that information in the calculation as well. We are trying to determine what DNA the sire has that he can pass on, not how well he was managed himself.
Myth #4: EPDs don’t work
There is a lot of research out there that shows that EPDs can aid in selection, and selecting for a trait using an EPD instead of an adjusted phenotype is up to nine times more effective. This power is further enhanced with the addition of genomics. To give a more understandable example, if we were concerned about birthweight and selected bulls based on their reported birthweight (eg: 75 pounds) versus selecting bulls for our herd using birthweight EPD, we will change birthweight nine times faster using the EPD. This means that they do work, but we need to be aware of what we are trying to build. It is like using an air nailer vs. a regular hammer. Both can drive nails, but we better have a plan when we upgrade to the air nailer.
Some of the confusion I have seen in the popular press of late about the effectiveness of EPDs seems to stem from the fact a lot of breeds have EPDs that can’t be directly compared with other breeds or that EPDs are not calculated on crossbred populations. In some cases this is true, however the use of crossbred data in calculation of EPDs is growing. In fact several of the major Canadian breeds participate in a multibreed evaluation that includes millions of records from crossbred and purebred cattle and many of the EPD can be directly compared. Additionally, certain breeds excel at certain aspects of beef production. Using the EPDs from the breed you have chosen will still result in changes (hopefully improvements) to your cow herd. You are trying to select DNA from within the breed of your choice to reach the specific goals of your cowherd. For the commercial industry, ideally this will be in a planned crossbreeding system.
Because EPDs are continually getting better at describing the DNA of potential seedstock, it is more important than ever to have some clear goals of what you want to accomplish in your cow herd. Do I need more performance? Lower-maintenance cows? More yield? Increased marbling? Tighter udders? Am I happy where I am at?
EPDs are a tool that is available for use in genetic selection of beef cattle, but the application of the tool is up to the craftsman who is building the cowherd.
Table 2 (top of page) shows some of the breed average EPDs for young animals in six major Canadian breeds. Results can not be compared between breeds, and many breeds have additional EPD available for use. These provide a reference for selection of cattle within the breed for the first six months of 2018. Some slight rounding has been done on some of the traits. For more information on a specific breed please contact the association or visit their respective website.