Advanced Topics in EPD Use
By Dr Sara Via
This folder includes articles on several topics that extend EPD use beyond the basic application to the genetic improvement of a single trait.
The most important from the breeding perspective is how to use EPDs to select on multiple traits, since it is unlikely that any breeder will want to improve a herd for only one trait. One of the complications with multiple traits comes from "genetic correlations", which are genetically based associations among traits. These are caused because often a given gene will affect several traits. A simple example is the length of front and hind legs in 4-legged animals. The same genes affect both of these traits, so it would be hard to select an animal with short front legs and long back legs (though not impossible, witness the kangaroo…).
For animal breeders, the fact that genes can affect more than one trait causes great difficulty, because when you select on one of the correlated traits, the other one also changes even if you don't want it to. For example, fleece weight and body weight are positively genetically correlated. Great, you say! Right, because by selecting larger animals, one also gets more fleece. But, the catch is that fleece weight and fiber diameter are also positively genetically correlated, so increasing fleece weight will lead to increased fiber diameter, unless you are very careful.
The next group of articles concerns the estimation of EPDs in a pedigree context. Finally, I include two articles on using EPDs in livestock judging (wave of the future at alpaca shows??).
Multiple Trait Selection
1_ Multiple Trait Selection for Pork Improvement. NSIF, Fact Sheet #10. Another simple fact sheet from NSIF, which clearly describes the problems of multitrait selection. It describes what determines the rate of selection response, the importance of carefully considering which traits to select, covers three methods for multitrait selection (of which index selection is the most effective), and ends with a discussion of how to determine the traits to include in an index.*
2_Select on One Trait or Multiple Traits? NSIP, Fact Sheet #9. Another great review of the same concepts, with some special tips for wool producers.*
3_ Genetic Relationships_Dr. Scott Greiner, Extension Specialist, Virginia Tech. Dr. Greiner adds a bit more about genetic correlations among traits to the discussion of selecting on multiple traits.*
4_Selection Decisions: Tools for Economic Improvement Beyond EPD, Dr. Mark Enns, Colorado State University, Chapter 11, Beef Sire Selection Manual, National Beef Cattle Selection Consortium. A more technical but much more complete description of multitrait selection and the construction and use of selection indices.***
Putting EPDs into Pedigrees
5_ Understanding Performance Pedigrees, Beef Improvement Federation Fact Sheet, #2. This article reviews EPDs, and shows how putting EPDs on a pedigree can help to evaluate a young animal whose own EPD has a very low accuracy.*
6_ Expected Progeny Differences, Part IV, Use of EPDs, Dr.John Evans, Department of Animal Breeding, and Dr. David S. Buchanan, Department of Animal Science, OK State Extension Fact Sheet. Again, after a brief introduction to EPDs, this article discusses how to calculate a "Pedigree EPD", in which the EPDs of relatives are weighted by their relationships to a given individual to give a better idea of its genetic value before it has progeny of its own. One could use this technique, for example, when considering whether to purchase a Jr. Herdsire before his first EPD values are available.**
Using EPDs in Livestock Shows
7_ Understanding and using performance data in judging classes. NSIF Fact Sheet #17. Basic description of how EPD values could be used as an additional source of evaluating individuals in the show ring. Examples for swine.*
8_Using Expected Progeny Difference (EPD) and Scenarios in Livestock Judging Contests, Dr. Chris T. Boleman, Texas Cooperative Extension. Another basic description of how to use EPD data in the showring, with examples for cattle. Interesting use of "scenarios", which are different situations in which the cattle are raised, or different ways in which they could be utilized. Different traits are of value in different scenarios.*