ABBA Genetic Evaluation Review

August 23rd, 2019 by shelbys@brahman.org

ABBA Genetic Evaluation Review July 2019

Raluca Mateescu, Professor of Animal Genomics, University of Florida

Larry Kuehn, Research Geneticist, U.S. Meat Animal Research Center

A review of procedures implemented by Livestock Genetic Services to conduct the American Brahman Breeders genetic evaluation has been performed. There have been a number of changes to the procedure used for the genetic evaluation which would result in changes in the reported EPDs of many animals. However, based on the materials reviews, these changes also resulted in an increase of EPDs accuracies which is a great benefit to Brahman producers. These changes in accuracy are the result of improving the models to better match the unique attributes for Brahman cattle.  Previous data adjustments were overestimated effects of younger cows on birth weight and weaning weight, resulting in biased EPDs when these traits were evaluated.  Removal of this potential for bias increased the accuracy of the evaluation significantly, but it also prevented records from young cows being adjusted incorrectly.

Brief review of the implemented changes in the genetic evaluation:

  1. Data Adjustments. This is a very important step before the genetic evaluation is performed and probably the main reason for changes in EPD values on existing animals from previous genetic evaluations. The move from BIF standard adjustment to Brahman specific adjustments is highly recommended because it results in more accurate EPDs. For example:
    • the BIF adjustment for the birth weight was overestimating the effect for younger cows (younger cows had higher adjusted birth weights)
    • the BIF adjustment for weaning weight unfairly rewarded younger animals with higher adjusted weaning weights.
    • The yearling weights are adjusted by splitting the trait into post wean gain and weaning weight.
    • the BIF adjustment for scrotal circumference overestimated the age adjustment for Brahman cattle
  2. Contemporary Groups. New contemporary groups have been defined and should more accurately account for differences in management.
  3. Data culling – allowing more or less data in.
  4. Genetic parameters – defining two traits as correlated or not correlated. These changes, particularly after the new data adjustments, likely resulted in the observed changes in accuracy.

The benefit of the newly implemented changes is evident in the approximately 10% increase accuracy for weight traits: birth weight (old mean accuracy 49%, new mean accuracy 50%) and weaning weight (old mean accuracy 42%, new mean accuracy 51%).  These higher accuracies for most traits allowed the EPDs to spread further (higher max and lower min EPD) in most traits, which will result in greater opportunities for genetic improvement and the potential for faster genetic change.  Yearling weight did not increase in accuracy but did show an increased spread (max – min).  We believe this increased spread resulted from a new model for yearling weight where it is expressed as weaning weight plus postweaning gain rather than yearling weight as a single trait.  The accuracies means/correlations are likely for postweaning gain rather than this combined trait.  Decreased accuracy was also observed for milk and scrotal circumference likely due to new variance components that we are guessing have lower heritability.  Neither of these decreases are concerning to us given the data adjustments described for both trait complexes.

As a further examination, we asked Livestock Genetic Services to examine correlations of the EPDs from both the old system and the new system with the performance of progeny born after January 1, 2018.  These progenies were not used in the EPD derivation on either system; therefore, the correlations were a way to examine which EPDs were more predictive of progeny performance and, therefore, more accurate for selection.  Livestock Genetic Services provided these correlations for bulls that had progeny after the cutoff on birth weight, weaning weight, and yearling weight.  In all cases, the new EPDs from Livestock Genetic Services had higher correlations to progeny performance than the EPDs from the old system.  This test is a further confirmation of the increase accuracy of the new EPD system.

In conclusion, based on the materials reviewed, the new EPDs reported are expected to be slightly different then the ones reported previously, however, these new EPDs should be a more accurate reflection of the true genetic merit of these animals.

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