In this study we found that indices and bull rankings for economic indices were most sensitive to scaling effects that increase the expression of genetic differences due to the spread in breeding values for milk production traits under intensive feeding systems. In contrast, the economic index was very robust to changes in many other assumptions, including accounting for regional differences in seasonal feed cost. A simple theoretical basis for adjusting economic values to account for scale differences in expression of breeding values spread is presented under the assumption of a genetic correlation of one between milk production in low versus high input systems. Further research is required to determine if the true genetic correlation is sufficiently less than one, such that separate breeding values need to be estimated to correctly rank bulls for high versus low input systems.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume Genetic Improvement Programs: Breeding objectives, economics of selection schemes, and advances in selection theory, , 002, 2014
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