Symposium review: Genetic relationships between different measures of feed efficiency and the implications for dairy cattle selection indexes

摘要

A greater number of dairy economic selection indexesare incorporating a measure of feed efficiency (FE) asa key trait. Definitions of FE traits have ranged fromdry matter intake (DMI) to residual feed intake (RFI),noting that RFI is effectively DMI adjusted for variousenergy sink traits such as body weight (BW) and milkenergy (MilkE). Other definitions of FE fall betweenthese 2 extremes such as feed saved (FS), which combinesRFI and the portion of DMI required to maintainBW. The choice between different FE traits can createconfusion as to how to meaningfully compare their heritabilities,estimated breeding values (EBV) and theircorresponding reliabilities, and how to differentiallyincorporate these EBV into selection indexes. If RFIand FS are merely linear functions of DMI, BW, andMilkE with known genetic variances and covariancesbetween these 3 traits, there may be no need to directlycompute RFI or FS phenotypes to determine theirheritabilities, genetic correlations, EBV, and respectivereliabilities for individual animals. We demonstratehow the estimated total genetic merit is invariant tothe specification of a FE trait within a selection index.That is, economic weights for a selection index involvingone particular FE trait readily convert into the economicweights for a selection index involving a differentlinear function of that FE trait. We use these differentspecifications of FE to provide insight as to the effect ofthe degree of missingness (e.g., paucity of DMI relativeto milk yield records) on the EBV accuracies of thevarious derivative FE traits. We particularly highlightthat the generally observed higher EBV accuracies forDMI, then for FS, and lastly for RFI are partly drivenby the greater genetic correlations of DMI with BWand MilkE and of FS with BW. Finally, we advocatea genetic regression approach to deriving FS and RFIrecognizing that genetic versus residual relationshipsbetween FE component traits may differ substantiallyfrom each other.