A well-known historical limitation to selection of bulls for use in crossbreeding programs has been the inability to directly compare EPDs of bulls of differing breeds. Such comparison is useful to strategically utilize bulls of different breeds in a complimentary fashion. For several years now, geneticists at the U.S. Meat Animal Research Center in Clay Center, Nebraska have annually calculated and published Across Breed EPD Adjustment Factors to enable producers to “standardize” within-breed EPDs to a common base, and therefore allow for utilization of EPDs across breeds. A condensed version of the 2010 across breed adjustments can be found below. These adjustments are based on comparative breed research conducted at the USMARC. It is important to note that the adjustment factors found in the table do not represent a direct comparison of breed differences (more on that later).
To calculate across breed EPDs, add the adjustment factor found in the table to the within-breed EPD published in the most recent genetic evaluation for the animals of interest. As an example, assume a Simmental bull and a Charolais bull are being compared for use as a terminal sire on mature Angus-based cows. The Simmental bull has a YW EPD of +60 and the Charolais bull has a YW EPD of +30. To fairly compare the YW EPDs of these two bulls of different breeds, the EPDs must first be adjusted to a common base using the across-breed table. Using the table, the Simmental bull would have an across-breed YW EPD of +78.4 (60 + 28.4 = 78.4) and the Charolais bull an across-breed YW EPD of +80.8 (30 + 50.8 = 80.8). Comparison of the calculated across-breed EPDs for these two bulls suggests they would transmit similar genetics for yearling growth as the difference in their across-breed YW EPDs is minimal (+78.3 vs. +80.8- the two bulls are within two pounds of each other for YW EPD).
Across-breed EPDs are most useful in managing uniformity when multiple breeds are rotated in a crossbreeding system to avoid large fluctuations in traits such as birth weight and milk. Uniformity from one generation to the next when using sires of different breeds can be improved by selecting bulls with similar across-breed EPDs. A common challenge to overcome in crossbreeding systems is to avoid large differences in traits such as calving difficulty, cow size, and milk production resulting from use of breeds that are largely divergent for these traits. Across-breed EPDs are a tool to manage these potential differences, while favorably utilizing the basic genetic differences between breeds that exist as well as optimizing heterosis. By using the across breed EPD adjustment factors it can be determined that an Gelbvieh bull with a Milk EPD of +12 and a Simmental bull with a Milk EPD of +10 are similar to an Angus bull with a Milk EPD of +20.
The three different sires of different breeds listed above would be estimated to transmit similar genetic potential for milk production to their daughters since their across breed Milk EPDs are similar.
Without across-breed adjustment factors, EPDs for animals of different breeds cannot be accurately compared. The across-breed adjustment factors take into account breed differences, as well as differences in the established base year (year in which average EPD in breed equals zero) used in the calculation of EPDs for each breed. For these reasons, the adjustment factors themselves are not reflective of breed differences. To reflect breed differences, each individual breed average EPD needs to be adjusted to a common base. The following table does just that- adjusts the breed average EPD for each trait to a common base using the across breed adjustments. Hence, the EPDs in the following table are directly comparable, and reflect genetic merit differences across breeds as they exist today.
In summary, Across-Breed EPD Adjustments are a tool to manage genetics across breeds. The accuracy of across-breed EPDs is primarily associated with the accuracy of the within-breed EPDs for the individual animals being compared. Using the adjustment to formulate benchmarks and windows of acceptability for sire selection are logical uses. For example, establishing Milk EPD parameters for crossbreeding programs using Angus, Simmental, and/or Gelbvieh. With the across-breed tools, one can establish a range of EPDs within each breed which will contribute similar milk genetics to a breeding program.
Source: Dr. Scott P. Greiner, Extension Animal Scientist, VA Tech