Use of Adjustment Factors Allow Comparisons Across Beef Breeds

BY J. BENTON GLAZE, JR., PH.D. Extension Beef Specialist University of Idaho

Since the release of the first national sire summary in 1971, beef cattle producers have been using expected progeny differences (EPD) to assist them in making selection decisions. Expected progeny differences are estimates of the differences in performance of future progeny from two or more sires from the same breed. Expected progeny differences are generated from breed association data banks and are published/presented in sire summaries, beef sire catalogs, beef sire directories, bull sale flyers, and on websites. Expected progeny differences are specific to the breed from which they were generated and cannot be compared across breeds. This is due in part to the computations of EPD being done separately for each breed, the different base adjustments used by the breed associations, the differences in genetic trends for traits within the breeds, and the limited amount of information from comparisons of animals from different breeds.

A host of beef producers have benefitted from using EPD to select animals to meet defined or desired production goals. However, since EPD are specific to the breed from which they were generated and cannot be compared from one breed to another, commercial beef producers using crossbreeding have found challenges in using EPD. To address these challenges, the concept of across-breed adjustment factors was employed. For several years, researchers at the United States Department of Agriculture’s Meat Animal Research Center (USMARC) in Clay Center, Nebraska have evaluated breeds, collected data, and developed tables of adjustment factors that account for differences between breeds. These adjustment factors are updated annually and released through the Beef Improvement Federation (BIF) (https://beefimprovement.org).

Across-breed adjustment factors for beef cattle growth traits (birth weight, weaning weight, yearling weight) and maternal milk were first calculated and published in 1993. Carcass trait

(marbling score, ribeye area, fat thickness) adjustment factors have been available since 2009 and carcass weight adjustment factors were added to the mix in 2015. Growth trait and maternal milk adjustment factors are available for seventeen beef breeds and carcass trait adjustment factors are available for fourteen beef breeds. The most recent list of evaluated breeds and their across-breed adjustment factors are presented in Table 1. An example of how to use the adjustment factors to calculate an across-breed EPD follows.

To calculate an across-breed EPD, you need a current EPD from a breed association sire summary and a current acrossbreed adjustment factor for the same breed and trait. Consider a Hereford bull with a yearling weight EPD of +100.0 lbs. and a Gelbvieh bull with a yearling weight EPD of +100.0 lbs. The across-breed adjustment factors (presented in Table 1) for yearling weight are -41.8 lbs. for Hereford and -18.0 lbs. for Gelbvieh.

To calculate an across-breed yearling weight EPD for the Hereford and Gelbvieh bulls, simply add the adjustment factor to the bull’s original EPD. The across-breed yearling weight EPD is 100.0 lbs. + (-41.8) lbs. = 58.2 lbs. for the Hereford bull and 100.0 + (-18.0) lbs. = 82.0 lbs. for the Gelbvieh bull.

The expected difference in progeny yearling weights when both bulls are mated to cows of another breed would be about 24 pounds (82.0 lbs. - 58.2 lbs. = 23.8 lbs.). In other words, there is a fair amount of difference in what these bulls can provide in terms of yearling growth potential to their progeny. As producers switch between breeds and select bulls for their commercial/ crossbreeding programs, they often compare EPD values for economically important traits. Taken at face value, these bulls look identical for yearling weight growth. However, as previously mentioned, these bulls cannot be compared directly and their within-breed EPD must be adjusted to make a fair and accurate comparison. Once the adjustment is made, it is clear that the Hereford bull lacks (approximately 24 lbs. less) the yearling growth potential of the Gelbvieh bull. The expected yearling weight difference of progeny when both of these sires are mated to cows of another breed would be 24 lbs. In other words, the progeny of the Hereford bull are expected to weigh, on average, 24 lbs. less at a year of age than the progeny of the Gelbvieh bull.

Another way to look at the situation stated above is to consider a Hereford bull with a yearling weight EPD of +100.0 lbs. and a Gelbvieh bull with a yearling weight EPD of +75.0 lbs. As mentioned previously and shown below (Table 1), the acrossbreed adjustment factors for yearling weight are -41.8 lbs. for Hereford and -18.0 lbs. for Gelbvieh. To calculate an acrossbreed yearling weight EPD for the Hereford and Gelbvieh bulls, simply add the adjustment factor to the bull’s original EPD. The across-breed yearling weight EPD is 100.0 lbs + (-41.8) lbs. = 58.2 lbs. for the Hereford bull and 75.0 + (-18.0) lbs = 57.0 lbs. for the Gelbvieh bull.

The expected difference in progeny yearling weights when both bulls are mated to cows of another breed would is small (58.2 lbs. - 57.0 lbs. = 1.2 lbs.). In other words, these two bulls would provide approximately the same level of yearling growth potential to their progeny. These calculations suggest that if a producer is currently using a Hereford bull with a yearling weight EPD of +100.0 lbs. in his crossbreeding program and wants to switch to a Gelbvieh bull, he would need to find a Gelbvieh bull with a yearling weight EPD of +75.0 lbs. to maintain the same level of yearling weight growth genetics in his herd.

It is well known that traditional, within-breed EPD have associated accuracy values that range from zero (low accuracy; EPD likely to change) to one (high accuracy; EPD not likely to change). These accuracy values reflect the amount of data/information use in the EPD calculations and how much the EPD might change over time as more data/information is included in the calculations. While not listed specifically, accuracies for across-breed EPD are similar to the original within-breed EPD accuracies. The across-breed EPD accuracies contain a small amount of additional error that is associated with the estimation of the adjustment factors. Generally, the accuracy of the across-breed EPD can be gauged by taking into account the accuracies of the within-breed EPD.

Since the 1970’s, beef cattle producers have benefitted from using EPD to select animals to meet defined or desired production goals. However, there are some challenges producers encounter as they work to identify the best sires to use/rotate in commercial/crossbreeding programs. Across-breed EPD can assist producers as they seek to select sires to improve/optimize economically important traits in their herds.