ImpactofSelectionforResidualFeedIntakeonForageIntakebyBeefCowsandFeedEfficiencyofProgenyM.S.Kerley1DivisionofAnimalSciences,UniversityofMissouriIntroductionUseofresidualfeedintake(RFI)analysistoassessefficiencydifferencesamongcattlewasfirstpublishedbyKochetal(1963).Gainwasregressedonintakeandbodyweightwithresultingregressioncoefficientsusedtocomputeexpectedgainforeachanimalbasedupongroupaverageperformance.ThisapproachwasadaptedtocomputeRFIwherebyintakesofanimalsinatestgroupareregressedagainsttheirbodyweightanddailygain.Regressioncoefficientsforbodyweightandgainarethenusedtocomputeanexpectedintakeforeachanimal,anddifferencebetweenintakeandexpectedintakereportedasRFI.RFIisarguedasasuperiormeasurementoffeedefficiencycomparedtofeedconversionratio(feedtogainratio;FCR)becauseitisnotconfoundedbygain.Residualfeedintakeisbelievedtoprovideameasureofmetabolicefficiency.Feedconversionratio,whilecorrelatedtoRFI,isimprovedbybothgreatermetabolicefficiencyandimprovedgrowthrate.ThususingFCRasselectioncriteriawouldselectforgreatergrowthrate,andinturnincreaseframescore/matureweight.AnimalswithanegativeRFIarebelievedtobemoreefficientatusingconsumedenergyformaintenanceandgrowthfunctions.Ourgoalinanimalselectionistoidentifyandpropagateanimalsnearbiologicalmaximumforefficiencyofconsumedenergyuse,andRFIprovidesaphenotypeforthatpurpose.ButhowmuchprogresscanbeexpectedwhenselectionpressureforRFIisapplied?Intakeamongcalvessimilarinbodyweightanddailygainwillvaryby40%.Consequentlyrangeinmetabolicefficiencyamongindividualswithinapopulationissubstantial.Heritabilityofefficiencyhasbeenshowntobemoderate(~0.4),similartogrowthandcarcasstraits,meaningthatsubstantialprogresscanbemadethroughgeneticselection.RichardsonandHerd(2004)segmentedbiologicalmechanismscontributingtoenergyexpenditurebetweenRFIphenotypes.Componentsmeasuredwerebodyproteinandlipiddifference(5%),feedingbehavior(2%),activitylevel(10%),digestion(10%),andmetabolicfunctions(73%).Theprimarydifferencebetwee