BVSINDIICEL.AGR.SCI.7,1993:1119Energy systems for ruminantsFRIK SUNDSTLDepartment of Animal Science, Agricultural University of Norway, N-1432 s-NLH, NorwaySUMMARYAsurveyismadeofsomecurrentenergyevaluationsystemsforruminants.Estimationshaveshownthatnetenergylactation(NEl)accordingtovanEsandScandinavianfeedunit(SFU)accordingtoMlleretal.seemtogivethebestfitwhencomparingpredictedandobservedmilkyield.Despitethefactthatallsystemsapparentlyhavetheirshortcomings,itisconcludedthatthereisstillaneedforanenergyevaluationsystemforruminants.Furthermore,therewouldbegreatpracticaladvantagesifacommonsystemforenergyevaluationinEuropecouldbeagreedupon.Forrationformulationandoptimalizationthevariouscomponents(substrates)ofthefeedswillbeofincreasingimportance.Keywords:comparison,energy,ruminants,systems.of their energy can then be released for use-ful work (free energy = G) according to thefollowing equation:G = H TSwhere H = enthalpy (heat content in the sys-tem), T = absolute temperature, S = entropy(degree of disorganization).Theunderstandingoftheprinciplesforbioenergetic processes is fundamental in thescience of nutrition since all the processesthat occur in the animal body when the feedis digested and metabolized leads to energychanges.INTRODUCTIONThe word energy originates from Greek andmeansinwork(energon).Theworkofthecellistocontractitself,activelytrans-portmoleculesorionsandtosynthesizemacromolecules from smaller molecules. Thesourceofenergyforthisworkischemicalenergy stored in the foods consumed by ani-mals or humans.Energy bonds between atoms or moleculesrepresent a potential source of energy whichis released when the bond is broken.When chemical compounds are transformedfrom a higher to a lower energy level, partsYFIRLITOrkumatskerfifyrirjrturdrgreininnieryfirlitumorkumatskerfifyrirjrturdr.Vimatessumkerfumhefurkomiljsanettorkatilmjlkurmyndunar(NEl)samkvmtvanEsognorrnfureining(SFU)samkvmtMllero.fl.gefabestarniursturegarborinersamantluograunverulegmjlkurframleisla.Lgerherslamiklarffyrirorkumatskerfifyrirjrturdrrttfyrirallhafiaueinhverjagalla.EinnigyriamikillvinningurefhgtvriasameinastumeittkerfifyrirllEvrpulnd.Einstakir hlutar fursins og niurbrotsefni munu framtinni hafa meira gildi vi furtreikningaogfurger.12BVSINDIThis system has found the widest applica-tioninNorthAmericaanditisstillusedtosomeextent,alsoinSouthAmerica,Africaand Asia.Energy is used to express the fuel valueoffeedsforanimalsanditcomprisesthethree main groups of nutrients viz.:- Carbohydrates- Protein- FatThis enables us to determine quantitativerelationships between the nutritional supplyand the nutritional effects as a basis for theprediction of yield (production).Todayitisknownthattherearemanyfactors influencing the utilization of the en-ergy in an animal, but, provided that the re-quirement for specific nutrients such as pro-tein, vitamins and minerals is met, the nutri-tionalrequirementisaquestionabouten-ergy.Hay unitThe hay unit was described by Albrecht vonThaerin1809,althoughitisnotclearwhoreallydevelopedthesystem.Thiswasthefirstattemptatintroducingareplacementvalue, i.e. how much of a feedstuff is neededto replace one unit of a reference feed with-out any change in the production.ENERGY SYSTEMSAnumberoffeedevaluationsystemshavebeen developed over the last 200 years. It isnotpossibleheretodealwiththeseinde-tail,butthemoreimportantonesaremen-tionedbelow.Scandinavian feed unitTheScandinavianfeedunit(SFU)origin-atesfromDenmarkabout1880where1kgconcentrateswascalledafeedunit.Theconcentrateatthattimewasamixtureofcereals(oatsandbarley)butlater(191516) the Nordic countries agreed to use 1kgbarleyasbasisfortheunit.ThankstoNielsJohannesFjordandNilsHanssontheSFUwasdevelopedfurtherandmanyfeed-ingexperimentswerecarriedouttodeter-minethereplacementvalueforvariousfeedstuffs.TodaytheScandinavianfeedunitisesti-matedinthesamewayasFatteningfeedunit(FFU),butahigherNJFvalue(14.2)isused for digestible protein (see later).Total digestible nutrientsTotal digestible nutrients (TDN) was devel-opedalmost150yearsagobecauseitwasshown that the proximate analyses (Weende)were not sufficient to characterize the nutri-tive value of feeds.ThecontentofTDNperkgandperkgDM of a feedstuff is calculated as follows:TDN, kg =kg digestible crude protein (DCP)+ 2.25 kg digestible ether extract (DEE)+ kg digestible carbohydrates (DCHO)Kellner systemTheKellnersystemwasdevelopednearly100 years ago by O. Kellner and later by G.Fingerling in Germany. Because fat deposi-tion is the purest form of energy retention,respiration experiments were carried out withsupplementsofpurenutrients(protein,fat,starchandcellulose)toadultsteers.Itwasfoundthattheenergy(kJ)depositedasfatper gram of digestible nutrients was:- Starch 9.87 (100)- Straw cellulose 10.00 (101)- Protein (gluten) 9.37 (94)- Fat in fat rich seeds 23.85 (241)- Fat in grain 20.92 (212)- Fat in forages 18.83 (191)Thesefactorsforenergydepositionmaybecallednetjoulesfattening(NJF),con-vertedfromKellnersnetcaloriesforfattening(1kcal=4.184kJ).Relativeval-ues are given in parenthesis.Whenfeedswereusedinsteadofpurenutrients,theenergyvalueswerefoundtobelowerthanthoseestimatedbyapplyingENERGY SYSTEMS FOR RUMINANTS13the factors given above. The observed valueinpercentageoftheestimatedvaluewascalled the value number (W=Wertigkeit):Observed value 100W =Estimated valueThe value numbers are then used for cor-rection of the estimated values.Forforagesthiscorrectionwasfoundtoberelatedtothecrudefibercontentofthefeed. This is called fiber deduction and basedonexperimentswithhayandstraw.Origi-nallyitwassetto5.7NJFpergtotalcrudefiber, but after the change from true proteinto crude protein, a fiber deduction of 6.3 NJFhas been used.Fromthenetenergyfatteningthefol-lowing units are derived:Starch equivalent (1 kg starch) =9870 NJF (SE)Fattening feed unit (1 kg barley) =6900 NJF (FFU)The problems associated with the use of afatteningcattleunitforotherspeciesofanimals and other types of production are toagreatextentovercomebyuseoftheNJF-equivalents.Themainweaknessofthissystemisthecorrectionfortheconcentrationofthediet.TheWandthefiberdeductionarerathervariableandifnorespirationexperimentiscarriedout,thechoiceofcorrectionfactorhas to be based on guesswork.Toovercomethisproblem,ourDanishcolleagues have developed a method for cal-culating the cattle feed unit (CFU) which isbased on digestible energy and corrected forthe content of digestible crude fiber (Mlleret al., 1983):CFU/kg DM =0.426 + 0.101 MJ DE 0.502 DCFwhereDEisdigestibleenergy,MJ/kgDM,and DCF is digestible crude fiber, kg/kg DM.Itisassumedthat1feedunitisneededper 2.5 kg of milk (7.89 MJ net energy).Because1kgbarleyDMisestimatedtoequal1.13CFU,1kgofbarleywith14%moisture is equivalent to 7.65 MJ net energyfor lactation.Metabolizableenergy(ME)isdefinedasgrossenergyminusfaecalenergy,uri-nary energy and energy in combustible gas-ses (methane). ME represents the maximumamountofenergythatisavailableforananimal.MEisusedasmeasurefortheen-ergyvalueoffeedsinSwedensince1967andaccordingtoSprndly(1989)itiscal-culated as follows:Forages 50% legumes: ME=0.106x2.93Straw: ME=0.114x0.47where x is per cent organic matter degradedin rumen liquor (VOS).Forotherfeeds,ME(kJpergdigestiblenutrient)iscalculatedmainlyaccordingtoAxelsson (1941):ProteinRoughages 18.0Concentrates 18.9FatRoughages 32.7Cereals 34.9Protein-concentrates 36.9CarbohydratesDisaccharides 15.1NFE 15.5Crude fiber 12.5Theenergyrequirementsoftheanimalsare also given in ME without any correctionfor differences in utilization.OtherequationsforestimatingMEforruminantsaregivenbySchiemannetal.(1971) and van Es (1978).British systemTheBritishsystem(ME)wasdevisedbySirKennethBlaxterinthesixties(Blaxter,1962;ARC,1965andmodifiedbyMAFF,1975).The energy values of feeds are expressedin terms of metabolisable energy.ME can be derived at in different ways:1. Determination in sheep.2. Digestibleorganicmatterdata(DOM),14BVSINDITable1.Preferredvaluesfortheefficiencyofutilizationofmetabolisableenergyofnormaldietsbyruminants(ARC,1980).1.tafla.Valingildiyfirntingubreytiorkufrifyrirjrturdr.Metabolizability (qm)a)Function 0.40 0.50 0.60 0.70Maintenance(km) 0.643 0.678 0.714 0.750Growthandfattening(kf)Alldiets 0.318 0.396 0.474 0.552Pelleteddiets 0.466 0.470 0.474Lactation(k1) 0.560 0.595 0.630 0.665a)qm=ME/GE, k1 = 0.35 qm + 0.420, km = 0.35 qm + 0.503,kf = 0.78 qm + 0.006, kg=0.0435ME/DM.assuming that OM = 19 MJ/kg and thatME/DE = 0.81.3. Rostock equation.4. Predictionfromchemicalandinvitroanalyses.Theenergyrequirementsoftheanimalsareexpressedinabsolutetermsasneten-ergy. The interface between the two involvescalculationswithacoefficientofMEutili-zation varying with type of production, levelofintakeandconcentrationofthediet(Ta-ble 1).Thissystemisconsideredanetenergysystem, because the utilization of the ME isused for estimating the requirements.workers proposed an energetical feed unit(EnergetischeFuttereinheit)whichissetto2.5 kcal (10.5 kJ) for ruminants (EFr).Onekgofbarley(14%moisture)isac-cording to this system equivalent to 6.20 MJNEFr (590 EFr).Rostock systemThe Rostock system (NEF=Nettoenergie Fett)was described by Schiemann et al. (1971) inthe book, Energetische Futter-bewertung undEnergienormen.A large number of experiments were car-riedoutwithbalanceddietstoruminants.For cattle (Rind) the following equation wasgiven (page 207):NEFr (kJ) = 7.2x1 + 31.5x2 + 8.4x3 + 8.4x4wherex1x4aregramsofdigestiblecrudeprotein, ether extract, crude fiber and N-freeextracts, respectively.Strictly,theseequationsarevalidforconcentrations(DE/GE)between0.670.80only.ForpracticalfeedingSchiemannandco-American feed evaluation systemsThe American feed evaluation systems may,apartfromTDN,bedividedintwoprinci-pally different ones:The California net energy system (NEm, NEq)as described by Lofgreen and Garrett (1968)isbasedoncomparativeslaughterexperi-ments with sheep and beef cattle.The net energy for maintenance and growthmay be calculated as follows:ME (Mcal/kg feed) =DE (Mcal/kg feed) 0.82F = g dry matter/kg W0.75Log F = 2.2577 0.2213MENet energy for maintenance (NEm) = 77/FNet energy for growth (NEg) =2.54 0.0314FTheBeltsvillenetenergysystemforlacta-tion (NEl) is described by NRC (1978).The calculation of the net energy accord-ingtothissystemisverysimplebecauseNElislinearlyrelatedtotheDEorMEcontent of the diet:NEl = 0.84 DE 0.77 (Flatt, 1988)ENERGY SYSTEMS FOR RUMINANTS15Table2.MEandNEvaluesofbarleyfordairycattlecalculatedusingstandardizedcompositionanddigestioncoefficients(vanderHoningandSteg,1984).2. tafla. Breyti- og nettorka byggi fyrir mjlkur-krreiknutfrstluuefnainnihaldiogmeltanleikastulum.ME NECountry/system MJ kg1DMDenmark/SFU 8.91France/UFL 13.13 8.31FRG/NEl13.36 8.55GDR/EFr12.92 7.25Hungary/NEl12.9 8.96TheNetherlands/VEM 12.65 7.76Sweden/ME 13.44Switzerland/NEl12.62 7.76UK/ME 13.37USA,Israel/NEl13.96 8.22Average 13.18 8.26SD 0.45 0.46NEl = 0.68 DE 0.36 (van der Honing andAlderman, 1988)NEl = 0.84 ME 0.44 (Flatt, 1988)whereDE,MEandNEareallexpressedasMcal/kg DM.The Dutch net energy lactation systemThe Dutch net energy lactation system (NEl)is described by van Es (1975 and 1978).Basedonalmostallavailabledatafromenergy balance experiments with dairy cows,van Es proposed a system for expressing theenergy value of feeds for milk production.From the content of ME, corrected for theconcentrationofthedietandthelevelofintake, the net energy content of the feeds isestimated.At an energy concentration (q=100 ME/GE)of57,autilizationof60%isassumed.Athigher and lower q the utilization is increasedor decreased by 0.4 percentage unit, respec-tively.The utilization of the ME is also reducedby1.8%permultipleabovemaintenance(M) intake. For an average intake of 2.38 Mthe utilization is reduced by 2.484%.TheequationforestimationofNElisasfollows:NEl =0.60 [1.0 + 0.004 (q 57)] 0.9752 MEOnefeedunitmilk(VEM)isequivalentto 6.9 kJ, i.e. 1 g of barley.COMPARISONOFFEEDEVALUATIONSYSTEMSEstimated values of feedsAnobviousexercisetodowhencomparingdifferentfeedevaluationsystemsistoesti-mate values of some feeds and see how theyrelate to each other.Table 2 shows the content of ME and NEofbarleywithastandardcompositionanddigestibilitycalculatedaccordingtodiffer-ent feed evaluation systems (van der Honingand Steg, 1984). According to these calcula-tions,MEwaslowest(12.6MJ/kgDM)forthe Dutch and Swiss systems (which are es-sentially the same) and highest for the Ameri-cansystem(14.0MJ/kgDM).Theneten-ergyvaluewaslowestforEFrfollowedbytheNEl(DutchandSwiss)andhighestforthe Hungarian NEl system and SFU.Theabsolutevaluesofthefeedsareoflimitedinterest.Ofmoreinterestaretherelative values, the replacement values.Ekern (1982) compared fattening feed units(FFU),Dutchnetenergylactation(VEM),and metabolisable energy for some commonfeedstuffsinNorway.Someoftheresultsare shown in Table 3.Asonewouldexpectthethreesystemscomparedreasonablywellforconcentratedfeeds,butforlessconcentratedfeeds(for-ages)theFFUsystemseemedtounderesti-mate and the ME system to overestimate thefeeding value relative to VEM. Similar cal-culations were made by van der Honing andAlderman (1988) showing that the estimatedenergycontentoflowqualityroughages,relativetobarley,wasconsiderablylowerfor FFU than for other systems (Table 4).16BVSINDITable3.TheenergyvalueofsomefeedsexpressedinFFU,VEMandME(Ekern,1982).3. tafla. Orkugildi nokkurra furtegunda fitunarfureiningum, mjlkurfureiningum og breytiorku.FeedevaluationsystemDM FFU VEMa)ME,MJa)% PerkgfeedHay,earemerg.flowering 85 0.48 (48)b)0.63 (63) 7.6 (69)Silage,grass,earemerg. 22 0.16 (64) 0.19 (74) 2.3 (79)Swedes 11 0.10 (78) 0.13 (100) 1.4 (99)Barleygrain 86 1.00 (100) 1.00 (100) 11.1 (100)Soybeanmeal,extracted 88 0.94 (92) 0.97 (95) 11.0 (99)Relative,VEM=100Hay 76 100 110Silage,grass 86 100 106Swedes 78 100 101Barley 100 100 100Soybeanmeal 97 100 103a)Withoutcorrectionforfeedinglevel.b)Figuresinparenthesisarerelativevaluesof1kgdrymatterwithineachunit(barley=100).Table4.Energyvaluesaccordingtodifferentfeedevaluationsystemsrelativetobarley(vanderHoningandAlderman,1988).4.tafla.Orkugildireiknutfrmismunandiorkumatskerfummiavibygg.TDN MEswEFrFFU VEMUSA Sweden Rostock Finland NetherlandsIcelandNorwayGrass,freshearlycut 91.1 89.4 92.2 78.8 91.1Grasssilage,unwilted 81.8 74.1 84.7 64.9 73.6Hay,goodquality 78.2 73.5 78.6 60.8 71.9Hay,moderatequality 70.0 64.5 70.3 49.5 63.2Barleystraw 53.6 46.7 54.4 25.3 44.9Barleygrain 100.0 100.0 100.0 100.0 100.0Wheatbran 84.9 86.087.6 79.8 82.4Canemolasses 82.8 81.3 81.8 75.0 77.9Soybeanmeal 100.7 109.0 92.8 98.6 101.9Fat 251.2 260.1 412.0 286.1 313.0Observed values of feedsThe response of the feeds in production tri-als should ideally be in accordance with theirestimated energy values.BasedonexperimentsinDenmarkbyO.Aaes (personal communication), the predictedmilkyieldsinfourexperiments(169cows)were compared with the observed milk yieldsfor the animals. The results are presented inTable 5.Foraprotein-richfeedsuchassoybeanmealMEshowedthehighestandFErthelowestrelativevalue.Thevalueoffatwashigher according to VEM and especially ac-cordingtoEFrthanfortheothersystems.ENERGY SYSTEMS FOR RUMINANTS17Table5.ObservedandpredictedmilkyieldsinfourDanishexperimentswithatotalof169cows(Thuen,1990).5.tafla.Raunverulegogtlunytfjrumdnskumtilraunummesamtals169kr.Energy Observed Predicteda)system milkyield milkyield SDkg/d kg/dME 25.2 31.0 8.1FFU 25.2 28.8 6.2VEM 25.2 26.8 4.5a)Predictedfromtotalenergyintakeandthere-quirement for maintenance and weight changes.Table6.Observedmilkyield(ECM),predictedmilkyield,standarderrorofpredictioncorrectedforliveweightchange(SEP(C)),energysupplyasaproportion(%)ofrequirement(RAT),standarddeviation(SD(RAT))andcoefficientofvariation(CV(RAT)).6.tafla.Raunverulegnyt(ECM),tlunyt,staalfrvikspgildumleirttfyrirbreytinguungafti(SEP(C)),agengilegorkasemhlutfall(%)afrfum(RAT),staalfrvik(SD(RAT))ogfrviksstuull(CV(TAT)).Feedevaluation No.of Observed Predictedsystem animals milkyield milkyield SEP(C) RAT SD(RAT) CV(RAT)Individualdata(J.Berg,personalcommunication)FFU 835 20.8 22.5 4.6 105 12.5 12.0SFU(corr.) 835 20.8 20.9 2.9 101 8.9 8.8ME 835 20.8 22.8 4.5 106 11.4 10.8NE 835 20.8 20.3 3.6 98 10.6 10.8Treatmentmeans(E.Thuen,personalcommunication)FFU 708 20.33 22.15 3.0 106 6.7 6.4SFU(corr.) 708 20.33 21.43 2.3 104 6.8 6.6ME 708 20.33 22.90 3.3 108 6.3 5.9NEl708 20.33 20.27 1.8 100 5.4 5.4Energyvalueforliveweightchange:4.5kgmilk/kgLWC.tle. The short duration of these experimentsdoesnotallowtoowideconclusionstobedrawn.Asindicatedbefore,theEFrsystemseems to overestimate the value of fat whichmay be associated with the analytical proce-dure used as pointed out by M. Riis Weisbjerg(personal communication). This fact may haveplayedacertainroleintheFinnishstudybecause many experiments included a com-parison between barley and oats fed accord-ing to yield.Inarecentstudy,J.Berg(unpublishedresults)wasnotabletoconfirmthegoodresults with EFr in predicting the milk yieldsofthecows.BestfitbetweenobservedandpredictedmilkyieldwasfoundforSFU,corrected for feeding level, and NEl (VEM)whereasFFUandMEgavepoorerresults(Table6).SimilarresultswereobtainedbyE. Thuen (personal communication).Bestcorrespondencebetweenpredictedand observed yield was obtained for net en-ergylactation(VEM)followedbyFFU.Metabolisableenergygavethepoorestpre-diction of the milk yield.Based on five change over experiments andfive continous feeding experiments with dairycowsinFinlandP.Huhtanen(unpublishedresults)concludedthatEFrwasmostaccu-rate in estimating the differences in feedingvalues.ThiswassurprisingbecausetheRostock system is based on fattening of cat-ASSOCIATIVE EFFECTS BETWEENFEEDSOne of the main problems in feed evaluationforruminantsisassociativeeffects.Manystudieshaveshownthatthepresupposition18BVSINDIwill still be a need for an energy system bywhichtherelativevalueoffeedsmaybeexpressedinasingleunit.Especiallyforplanners, economists, politicians and in tradesuchfigureswillberequired.Ifnot,feedswill be compared kg by kg, which will be agreat step backwards.generally made that energy values of feedsareadditiveinmanyinstancesdoesnotholdtrue.Bothpositiveandnegativeasso-ciative effects between feeds may occur. Theextentofinteractionwithindietsisoftenhard to predict and therefore difficult to in-clude in estimations (Mould, 1988; Be, 1989;P. Huhtanen, unpublished results).FUTURE FEED ENERGY EVALUATIONIN RUMINANTSThe progress in the science of ruminant nu-tritionovertherecentyearsisremarkable.Moresophisticatedtechniqueshavebeenapplied, e.g. cannulation of the animals di-gestive tract and better analytical procedureshavebeendeveloped.Tracerkinetics,infu-siontechniquesandarterio-venousdiffer-ence measurements are other advancementsmade in ruminant research.Due to these new and improved techniquestheinsightintothevariousmetabolicproc-esses,bothinthedigestivetractandthebody proper, has expanded. The various re-actionstakingplacecannowbefollowedquantitativelytoamuchlargerextentthanonly 3040 years ago.Byuseofmodellingoncomputersthecomplicated pattern of biochemical and physi-calreactionsinwhichfeedcomponentsareinvolvedcannowbesimulatedwithanin-creasingdegreeofprecision.Thedietaryinputs may be the concentrations of solublesugars,starch,structuralcarbohydrates,li-pid, protein and nonprotein nitrogen and valuesfor the rate of digestion and potential rumendegradability of structural carbohydrate andprotein (Thomas, 1990).This flow of new information has lead tothe question: do we need an energy evalua-tion system for ruminants in the future?For ration formulation and optimalizationof diets for ruminants, there is no doubt thatnewsystemswillhavetobeorientedto-wardssubstrates(asittosomeextenthasbeen in the past).Formanyotherpurposes,however,thereREFERENCESARC(AgriculturalResearchCouncil),1965.TheNutrientRequirementsofFarmLivestock,No2.Ruminants.HerMajestysStationaryOf-fice,London.ARC(AgriculturalResearchCouncil),1980.TheNutrientRequirementsofRuminantLivestock.CommonwealthAgriculturalBureaux,Slough:351pp.Axelsson,J.,1941.DerGehaltdesFuttersanumsetzbareEnergie.ZeitschriftfrZchtungs-kunde16:335347.Blaxter,K.L.,1962.TheEnergyMetabolismofRuminants.Hutchinson,London:329pp.Be,U.B.,1989.UtilizationofNH3-treatedstrawbysheep:Effectsofsupplementationwithdif-ferentamountsofgroundbarley.NorwegianJournalofAgriculturalSciences3:329336.Ekern,A.,1982.Vurderingavenergienifrtilmelkekyr.In:Husdyrforsksmtet,NLH.Aktu-eltfraStatensfagtjenesteforlandbruket,Nr.1:7479.Flatt,W.P.,1988.Feedevaluationsystems:His-toricalbackground.In:FeedScience(ed.E.R.rskov).WorldAnimalScience,B4,Elsevier,Amsterdam:122.Lofgreen,G.P.&W.N.Garrett,1968.Asys-temforexpressingnetenergyrequirementsandfeedvaluesforgrowingandfinishingbeefcattle.JournalofAnimalScience 27:793806.MAFF(MinistryofAgriculture,FisheriesandFood),1975.EnergyAllowancesandFeedingSystemsforRuminants.TechnicalBulletin33,HerMajestysStationaryOffice,London.Mller,P.D.,P.E.Andersen,T.Hvelplund,J.Madsen&K.V.Thomsen,1983.Ennybe-regningsmetodeforfodermidlernesenergiverditilkvg(FEK).BeretningfraStatensHusdyr-brugsforsgnr.55.Mould,F.L.,1988.Associativeeffectsoffeeds.In:FeedScience(ed.E.R.rskov).WorldAnimal Science, B4, Elsevier, Amsterdam: 279292.ENERGY SYSTEMS FOR RUMINANTS19NRC,1978.NutrientRequirementsofDairyCat-tle.5threv.edn.NationalAcademyofSci-ences,Washington,DC.Schiemann,R.,K.Nehring,L.Hoffmann,W.Jentsch&A.Chudy,1971.EnergetischeFutterbewertungundEnergienormen.VEBDeutscherLandwirtschaftsverlag,Berlin.Sprndly,R.,1989(ed.).Fodertabellerfridisslare1989.Sverigeslantbruksuniversitet,Uppsala.Speciellaskrifter39.Thomas, P.C., 1990. 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