Thebeefindustryhasmadegreatimprovementsinanimalfacilities,feeding,welfare,breeding,herdmanagement,andtheuseofbiopharmaceuticals.However,thepreventionandtimelytreatmentofcalfdiarrheastillrepresentachallenge1.AccordingtotheresultsoftheU.S.NationalAnimalHealthMonitoringSystemstudy,publishedin(CalfNote03),39%ofcalfmortalityiscausedbydiarrheaduringthefirst3weeksoflife.Thecausesofcalfdiarrheathathavebeenidentifiedareinfectiousagents,includingviruses(forexample,therotavirusgroup,bovineviraldiarrheavirus,andbovinecoronavirus),bacteria(forexample,Salmonellaspp.andEscherichiacoli),andprotozoa(forexample,Eimeriazuernii),andnon-infectiousfactors,suchasstress,diet,andhumidity,3.Giventhatmostofthefinanciallossinbeefproductionisattributabletocalfmorbidityandmortality,abetterunderstandingoftheetiologyandpreventionofdiarrheainpreweaningcalvesisessentialtomaximizebeefproductionandprofitability.Antibioticshavebeenwidelyusedtotreatorpreventdiarrheaandpromotegrowthinlivestock4–6.Recently,however,accumulatingevidencehasshownthattheuseofantibioticsinanimalhusbandryisassociatedwithmanyadverseeffects.Theemergenceofantibiotic-resistantbacteriaandantibioticresiduesinmeatarerecognizedasmajorproblems7–9.Duetotheirpotentialtokillbothpathogenicandbeneficialmicrobes,theuseofwidespectrumantibioticscanpromotethecolonizationofthegutbypathogenicmicrobes,whichcancausedisease10–1.Thus,antibiotictherapyofdiarrheiccalvesoftenleadstotherecurrenceofseriousdiarrheawithindaysofstartingit.Thepresenceofanimbalanceintheintestinalmicrobial