It seems logical that the discussion of bull selection would occur closer to breeding season. However, semen catalogs start hitting the mailbox after the first of the year, so I wanted to get a head start on helping you work through all those numbers. What follows is a simple way to make sense of EPDs (expected progeny differences) and how to apply them.
It is widely accepted that the EPD is the most valuable tool to make genetic progress. The practical application is clear. If your bull supplier does not have EPDs, move on to someone else.
Much has been written about EPDs – do an internet search for “beef EPD” and hundreds of results will appear. Yet, there are a few things you need to understand. First, EPDs are not a prediction of a given trait. For example, a weaning weight EPD of 47 does not mean that you can expect an additional 47 pounds of weaning weight. Second, EPDs are not transferable across breeds. So you cannot compare the weaning weight EPD of a Hereford to that of a Simmental. There is a conversion table that geneticists at the U.S. Department of Agriculture Meat Animal Research Center develop each year (https://beefimprovement.org/wp-content/uploads/2017/03/2016-Across-Breed-EPD-Table.pdf). It allows you to adjust EPDs to compare across breeds. As of this writing, the 2017 conversion table is not yet available.
Due to the explosion in the number of EPDs for single traits, many breed associations have developed economic indexes. An EPD index combines several EPDs into one single economic value. A good explanation of EPDs and indexes is provided by Donna Lofgren, Purdue University (http://www.ansc.purdue.edu/ibep/faq/epd-index.htm) and Darrh Bullock, University of Kentucky (https://beefimprovement.org/wp-content/uploads/2013/07/2017BIFProceedings.pdf). Breed association websites are also resourceful.
All this said, there’s an easy method of comparing EPDs, whether for a single trait or for an index: percentile tables. Most breeds publish a percentile table that tells you where an animal ranks within the breed. The numbers go from 1 to 99. A bull that has an EPD or index in the 10th percentile tells you that he is in the top 10 percent of the bulls in the breed for that trait.
Following are the EPDs for a SimAngus sire and the percentile table for SimAngus bulls. Note that you must use the Hybrid Simmental table, as this is a SimAngus bull, not a purebred or full bred. Also if the breed association publishes tables for non-parent bulls (yearlings with no progeny), use that table to rank yearling bulls. The Simmental association gives you the percentile when you search for specific animals. If that is not provided you should have the percentile table available, so that you can find where the animal ranks. Although the percentiles are shown, for the purpose of demonstration I will use the percentile table to find out where this bull ranks. For example, this bull has a calving ease (CE) EPD of 10.2. Using Table 2, I know that he lies somewhere between the 60th and 55th percentile. This means that I probably would not want to use him on first-calf heifers. Looking at the indexes, all purpose (API) and terminal (TI), he has a value of 139 and 82, respectively. Using the table, he is in the top 20 percent to 25 percent of the breed; for API he lies in the top 5 percent to 10 percent and therefore will put positive pressure on the traits included in these indexes.
You also see accuracy (ACC) listed. As this is a young sire with few progeny, his accuracies are low, meaning that the values are likely to change – this will be true for all yearling bulls. An older bull that has been used extensively and has many progeny will have accuracy of 0.80 and above.
Making sense of all of these numbers is difficult. By using a percentile table, I can look at the EPD of interest, find it in the percentile table and then determine if he will make the genetic change I want in my herd. EPDs and Indexes are extremely valuable and are the best tools available to make genetic progress. The percentile table helps you in sorting bulls and all the data that is available.
For further assistance, go to the websites of your breed association and or semen supply companies.