By Steve Boyles
Expected Progeny Differences (EPD) and have been proven to be the most reliable tool to generate change from selection. Expected Progeny Differences are predictions of genetic merit of an individual as a parent. As the name would imply, they are predictions of the differences in individuals’ offspring performance. Historically, most beef breed associations conducted a genetic evaluation twice annually, meaning that EPD were updated twice a year. However, with the advent of genomic information, new data are continually available. This has necessitated weekly genetic evaluations, and thus updated EPD are available on a weekly basis for the majority of beef cattle breeds.
How Do You Use EPD?
Simply knowing an animal’s EPD for a given trait has no meaning without something to compare it to. This comparison can be between animals or an animal and a point of reference, such as the average of a particular breed. Breed averages are rarely 0. Knowledge of breed average is helpful in determining how an animal ranks within a given breed for a particular trait. Most breeds publish a percentile rank table which allows producers to determine how an animal ranks for a particular trait within a particular breed.
The difference in EPDs between two bulls is the average difference in performance of their offspring if the bulls were mated to the same cows and the calves were reared in the same environment. Following is an example. EDP for Weaning Weight: Bull A = 50 and Bull B = 60
On average, we expect the offspring of Bull B to weigh 10 pounds more than the offspring of Bull A. This does not mean that every calf from Bull B will weigh more than every calf sired by Bull A. There will be variation in the weights of calves produced by both bulls, but with large enough groups of offspring the average difference will be reflected by the difference in sire EPD.
What Are Accuracies?
Accuracy is the theoretical correlation between an animal’s EPD and their true genetic merit and can range between 0 and 1. Expected Progeny Differences are predictions and thus are not known with complete certainty. They are updated, and become more accurate, when additional data becomes available. For example, a young non-parent animal may have a record for their own weaning weight. If the animal becomes a parent and has offspring with recorded weaning weights, their offspring inform their EPD. This increases accuracy. Another source of data that increases accuracy is genomic data.
Contemporary Groups
The foundation for a contemporary group includes animals born in the same year, season, herd, and who were treated equally. In other words, if a subset of animals is fed differently (given preferential treatment) they should become a separate contemporary group. It is critical to report data on all animals in a contemporary group. Not doing so leads to biased estimates of genetic merit.
Direct vs. Maternal EPD
Some phenotypes are influenced by both the genetics of the individual (direct) and genetics of the dam (maternal). Examples include weaning weight and calving ease. The EPD for weaning weight direct is simply called weaning weight whereas the maternal EPD for weaning weight is called milk. Milk EPD can be thought of as the comparison of a bull’s grand-progeny that are products of his daughters. Calving ease also has a maternal genetic component. Calving ease direct EPD represent the probability of how easily a bull’s calves will be born when he is bred to heifers. Calving ease maternal EPD are a misnomer in the sense that they reflect total maternal merit. Total maternal is the sum of maternal EPD and half of the direct EPD and represents the probability of unassisted births of a bull’s daughters during their first parturition. Although calving ease maternal EPD are not labeled as such, the majority of beef breed associations publish total maternal calving ease.
Multiple-trait Analysis
Many traits are genetically correlated to each other. Birth, weaning, and yearling weight are all genetically correlated with each other and as a consequence are evaluated in the same multiple-trait model. This has two primary benefits. First, it enables early growth traits to inform the EPD of later growth traits before the later growth traits are observed. Secondly, it mitigates the impact of selection that has occurred earlier in life (sequential selection) on EPD. In the case of yearling weight, it is conceivable that animals with low weaning weights were culled prior to the collection of yearling weight. Accounting for this selection decision is critical to avoid bias in traits measured later in life.
Another example of a multiple-trait model is calving ease and birth weight. Birth weight is a useful indicator of calving ease and is thus included in the same model as the economically relevant trait of calving ease. This means that resulting calving ease EPD incorporate birth weight observations, and selecting on both calving ease and birth weight EPD results in overemphasizing birth weight.
Summary
Expected Progeny Differences enable genetic selection decisions for multiple traits. Core to accurate EPD are well formed contemporary groups. Expected Progeny Differences change over time as additional information is available. These changes are more frequent with weekly genetic evaluations. Genomic data that is integrated into EPD allows accuracy of non-parent animals to increase.
Source : osu.edu