Yield gaps in oil palm A quantitative review of contributing factors

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European Journal of Agronomy

jo u r n al hom e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / e j a

Review article

Yield gaps in oil palm: A quantitative review of contributing factors

Lotte S. Woittiez

^a,∗

, Mark T. van Wijk

^b

, Maja Slingerland

^a

, Meine van Noordwijk

^a,c

, Ken E. Giller

^a

aPlantProductionSystems,WageningenUniversity,P.O.Box430,6700AKWageningen,TheNetherlands

bInternationalLivestockResearchInstitute(ILRI),OldNaivashaRoad,POBox30709,Nairobi00100,Kenya

cWorldAgroforestryCentre(ICRAF)SoutheastAsiaRegionalOffice,Jl.Cifor,SituGede,SindangBarang,Bogor16115,Indonesia

a r t i c l e i n f o

Articlehistory:

Received14March2016

Receivedinrevisedform23August2016 Accepted1November2016

Availableonline13January2017 Keywords:

Palmoil Perennial Yield Intensification Physiology Management

a b s t ra c t

Oilpalm,currentlytheworld’smainvegetableoilcrop,ischaracterisedbyalargeproductivityanda longlifespan(≥25years).Peakoilyieldsof12tha⁻¹yr⁻¹havebeenachievedinsmallplantations,and maximumtheoreticalyieldsascalculatedwithsimulationmodelsare18.5toilha⁻¹yr⁻¹,yetaverage productivityworldwidehasstagnatedaround3toilha⁻¹yr⁻¹.Consideringthethreatofexpansioninto valuablerainforests,itisimportantthatthefactorsunderlyingtheseexistingyieldgapsareunderstood and,wherefeasible,addressed.Inthisreview,wepresentanoverviewoftheavailabledataonyield- determining,yield-limiting,andyield-reducingfactorsinoilpalm;theeffectsofthesefactorsonyield,as measuredincasestudiesorcalculatedusingcomputermodels;andtheunderlyingplant-physiological mechanisms.Wedistinguishfourproductionlevels:thepotential,water-limited,nutrient-limited,and theactualyield.Thepotentialyieldoveraplantationlifetimeisdeterminedbyincomingphotosyn- theticallyactiveradiation(PAR),temperature,atmosphericCO2concentrationandplantingmaterial, assumingoptimumplantationestablishment,plantingdensity(120–150palmsperhectares),canopy management(30–60leavesdependingonpalmage),pollination,andharvesting.Water-limitedyields inenvironmentswithwaterdeficits>400mmyear⁻¹canbelessthanone-thirdofthepotentialyield, dependingonadditionalfactorssuchastemperature,windspeed,soiltexture,andsoildepth.Nutrient- limitedyieldsoflessthan50%ofthepotentialyieldhavebeenrecordedwhennitrogenorpotassiumwere notapplied.Actualyieldsareinfluencedbyyield-reducingfactorssuchasunsuitablegroundvegetation, pests,anddiseases,andmaybeclosetozeroincaseofsevereinfestations.Smallholdersfaceparticular constraintssuchastheuseofcounterfeitseedandinsufficientfertiliserapplication.Closingyieldgapsin existingplantationscouldincreaseglobalproductionby15–20Mtoilyr⁻¹,whichwouldlimitthedrive forfurtherareaexpansionataglobalscale.Toincreaseyieldsinexistingandfutureplantationsina sustainableway,allproductionfactorsmentionedneedtobeunderstoodandaddressed.

©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

Contents

1. Introduction...58

2. Plantationlifecycleandvegetativegrowth...59

2.1. Plantationlifecycle...59

2.2. Vegetativegrowth...59

3. Fruitdevelopment...60

3.1. Bunchnumber...60

3.1.1. Numberofdevelopinginflorescences ... 60

3.1.2. Sexdetermination,inflorescenceabortion,andsexratio...61

3.1.3. Bunchfailure...61

3.2. Bunchweightandoilcontent...61

∗Correspondingauthor.

E-mailaddresses:lotte.woittiez@wur.nl,lotte.woittiez@gmail.com(L.S.Woittiez),m.vanwijk@cgiar.org(M.T.vanWijk),maja.slingerland@wur.nl(M.Slingerland), m.vannoordwijk@cgiar.org(M.vanNoordwijk),ken.giller@wur.nl(K.E.Giller).

http://dx.doi.org/10.1016/j.eja.2016.11.002

1161-0301/©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

3.2.1. Inflorescenceandbunchdevelopment ... 61

3.2.2. Regulatingmechanismsofbunchweightandoilcontent...62

4. Magnitude,causes,andmanagementofyieldgaps...62

4.1. Thedifferentyieldgapsinoilpalm...62

4.2. Potentialyieldandyield-determiningfactors...62

4.2.1. AvailableradiationandPAR...62

4.2.2. CO2concentration...64

4.2.3. Temperature...64

4.2.4. Plantingmaterial...64

4.2.5. Plantingdensity...64

4.2.6. Culling...65

4.2.7. Pruning...65

4.2.8. Pollination...65

4.2.9. Croprecovery...65

4.3. Water-limitedyieldandyield-limitingfactors...65

4.3.1. Rainfall...65

4.3.2. Soil...65

4.3.3. Topographyandslope...66

4.3.4. Waterlogging...67

4.4. Nutrient-limitedyieldandyield-limitingfactors ... 67

4.5. Actualyieldandyield-reducingfactors...67

4.5.1. Weeds ... 67

4.5.2. Pests...67

4.5.3. Diseases...69

4.6. Interactionsbetweenstressfactors...69

5. Currentcausesofyieldgapsandfutureoutlook...70

6. Conclusions...72

Acknowledgements ... 72

References...72

1. Introduction

Africanoilpalm(ElaeisguineensisJacq.)hasitscentreoforigin inthehumidlowlandtropicsofWestAfrica.Wildoilpalmsare uncommonin primaryforestsbutrathergrowindisturbedand verywetlocations,suchasswampsandriverbanks,wheresun- lightisabundantandwateravailablethroughouttheyear(Zeven, 1967).Theoilpalmisamemberofthemonocotyledonouspalm family(Arecaceae).Thewoodystemcarriesasingleterminalgrow- ingpoint,fromwhichleavesappearatregularintervalsinadouble spiral(Rees,1964).Eachleafsupportsasingleinflorescence,which canbeeithermaleorfemale.Theharvestedproductisafruitbunch comprising1500–2000fruitlets.Crudepalmoil(CPO)isextracted fromtheorange-yellowmesocarp,andpalmkerneloil(PKO)from thewhitekernel.

Overthelast 100years, oilpalm haschanged fromasmall- holder agroforestrycropand ornamentalpalminto theworld’s mostimportantvegetableoilcrop.Currentworldwideproduction isestimatedat63Mtcrudepalmoilperyear,or36%ofthetotal worldvegetableoilproduction(USDA,2014).Expansionofoilpalm plantationshasbeensuggestedasakeycauseofdeforestationin bothIndonesia(Carlsonetal.,2012;Stibigetal.,2014)andMalaysia (Miettinenetal.,2011;Stibigetal.,2014),althoughotherdrivers suchasloggingalsoplayamajorrole(Laurance,2007;Lambinetal., 2001).Theincreasingdemandforpalmoiloverthecomingdecades willprobablybemetboththroughexpansionoftheareaplanted andincreasedproductivity(Carteretal.,2007;Corley,2009).

Sinceoilpalmexpansionmayleadtothedisplacementofbio- diverserainforests(Gaveauetal.,2014), increasedproductivity, combinedwithtargetedexpansionintodegradedareas(Fairhurst andMcLaughlin,2009),arethepreferredstrategies tomeetthe growingdemandforpalm oil.Increasingproductivitydoesnot, perse,leadtoreductionindeforestationunlesssupportingpoli- ciesareinplaceandareproperlyenforced(Angelsen,2010),but isanecessarysteptowardsreducingpressureonland.Athorough understandingandquantificationofthecontributionofdifferent

productionfactorstooilpalmyieldisurgentlyneededtoestimate thescopetoincreaseproductivityinexistingstands,andinongoing (re)plantingprograms.

Yieldgapanalysishasbeencommonlyusedasatooltoexplore thepossibilitiesforimprovinglandproductivity(Lobelletal.,2009;

vanIttersumetal.,2013;seealsowww.yieldgap.org).The‘yield gap’isdefinedasthedifferencebetweenpotentialandactualyield (vanIttersumandRabbinge,1997),withtheupperlimitofproduc- tivityperhectarebeingthe‘potentialyield’.Thispotentialyieldis definedasthetheoreticalyieldatagiventemperature,ambient atmosphericCO₂concentration,andincomingphotosynthetically activeradiation(PAR),withoptimumagronomicmanagementand withoutwater,nutrient,pestanddiseaselimitations(vanIttersum andRabbinge,1997).Itreferstocurrentgermplasmortothebest currentlyavailablematerial.

Yieldgapanalysishasbeencarriedoutforarangeofannual cropssuchaswheat(AggarwalandKalra,1994;Belletal.,1995;

Anderson,2010),cassava(Fermontetal.,2009),rice(Yangetal., 2008;Laborteetal.,2012),andcerealsingeneral(Neumannetal., 2010).Alimitednumberofperennialcroppingsystemshasbeen subjectedtoyieldgapanalysis,includingcoffee(WairegiandAsten, 2012),highlandbanana(Wairegietal.,2010),andcocoa(Zuidema etal.,2005).Perennialcropssuchasoilpalmarestructurallydif- ferentfromannualcropsinseveralways.Inannualcrops,growers cantakeadvantageofnewseedswitheachgrowingseason.Bycon- trast,theyieldpotentialforperennialcrops,withalifespanofup toseveraldecades,isfixedforeachplantingcycle.Eventsearlyin theplantationlifetime,especiallyinthenurseryandatplanting, mayhavestrongeffectsonyieldinlateryears,whichcomplicates theinterpretationofyielddata(BreureandMenendez,1990).In addition,oilpalmfruitbunchestakeseveralyearstodevelop,and thereisatimelagof20–30monthsbetweentheonsetofstress factorsandtheirimpactonyield.Thismakesitdifficulttoseparate andquantifytheeffectsofindividualfactors(Adametal.,2011).

Quantitative dataonyieldresponsesof oilpalm todifferent productionfactors,particularlyplantingdensity,irrigation,andfer-

tiliseruse,areavailablefromtrialscarriedoutbycompaniesor researchstations.Results ofmany suchtrialsarereportedonly in thegrey literature and canbe difficulttoaccess, but Corley and Tinker(2016) providea very complete overview.Recently, Fairhurstand Griffiths(2014)performedayield gapanalysisin oilpalmfromapracticalplanters’perspective,withastep-by-step guidanceontheidentificationandresolutionofyieldconstraintsin thefield.However,anassessmentoftheunderlyingcausesofyield gapsinoilpalmproductionsystemsworldwideislacking.Inthis review,weexploreexistingknowledgeonoilpalmproductivity fromaplantphysiologicalperspective,toprovideacoherentpic- tureoffactorscontributingtoyieldgapsinoilpalm.Westartwith adiscussiononplantationlifecycle,vegetativegrowth,andleaf areadevelopmentinSection2.InSection3weprovideadetailed assessmentofbunchproduction,focusingonbunchnumberand bunchweight,thetwomaindeterminantsofyield.InSection4 wereviewtheyieldgapconceptandthedifferentproductionlev- els(i.e.potential,water-limited,nutrient-limited,andactualyield), anddiscussthedifferentfactorsthataffectgenerativeproductiv- ityinoilpalm,includingclimaticfactors,nutrition,andthemain pestsanddiseases.InSection5weconsiderthemostimportant constraintstoyieldintheoilpalmproducingregionsaroundthe world,withfocusonbothlarge-scalecommercialandsmallholder systems.FinallyinSection6weidentifytheexistingknowledge gapsandproposedirectionsforfutureactionandresearch.

2. Plantationlifecycleandvegetativegrowth

Inthissectionwediscusstheoilpalmproductionsystem,the differentyieldprofilesduringtheplantationlifetimeandthevege- tativegrowthoftheoilpalm,withafocusonleafareadevelopment.

2.1. Plantationlifecycle

Oilpalmsarecommerciallygrowninplantationsystems,witha densityof120–150palmsperhectare.Pre-germinatedseedsare raised in polybagsin a nursery for 6–12 months(Rankine and Fairhurst,1999a),afterwhichtheseedlingsareplantedinthefield atfinaldensitywithlimitedoptionsforreplacingplantsthatdo notsurviveorprovetohaveless-desirablepropertiesbeyondthe first12months.Plantationshaveanaveragelifetimeof25years,of which21–23areproductive.Fouryieldphaseshavebeendescribed (Fig.1):1)theimmatureor‘yieldbuildingphase’,upto2–3years after planting (YAP), before harvestable production begins and when thecanopy isnot yetclosed;2)theyoungmaturephase or‘steepascentyieldphase’,4–7YAP,whenleafareaandyield increaselinearly;3)thematureor‘plateauyieldphase’,8–14YAP, whenyieldandleafareaarestable;and4)aphaseofyielddecline, 15–25 YAP(Ng,1983; Gohet al.,1994; Fairhurstand Griffiths, 2014).

Thefirstyearofharvesttypicallyyields10–15tfruitbunches ha⁻¹ (withand oiltobunchratio of10–15%) under favourable circumstances; initial yields of >20t fruit bunches ha⁻¹ have beenachievedincommercialplantings(Raoetal.,2008).Under favourableconditions,bunchproductionpeaks6–7YAP,withtyp- icalpeak yields of35tfruit bunchesha⁻¹ (Ng,1983; Donough etal.,2009).Maximumyieldsof60tfruitbunchesha⁻¹havebeen obtainedwithselectedclonalplantingmaterials(Ngetal.,2003).

Duringthematurephase,bunchproductionstabilisessomewhat belowthepeakachievedatsixYAP,withtypicalcommercialyields of 25–30tfruit bunchesha⁻¹ inwell-managed plantations(Ng, 1983;Donoughetal.,2010).Inthephaseofyielddecline,leafpro- ductionrateandbunchnumbersdecrease,butincreasedbunchsize partlycompensatesforthereductioninbunchnumber(Hardon et al.,1969;Goh et al.,1994;Jacquemard andBaudouin,1998:

0 2 4 6 8 10 12

0 5 10 15 20 25

Yield (tonne oil ha-1year-1)

Years Aer Planng Potenal yield

Nutrient/water limited yield Actual yield

Fig.1. Developmentofoilpalmyieldovertimeinthreehypotheticalplantations (afterNg,1983;Gohetal.,1994;FairhurstandGriffiths,2014).Thelightgrey (bottom),darkgrey(middle)andblackgrey(top)linesshowtheyieldprogress atdifferentproductivitylevels:actualyield(average3.5toilha⁻¹year⁻¹),nutri- ent/waterlimitedyield(average6.1toilha⁻¹year⁻¹)andpotentialyield(average 8.9toilha⁻¹year⁻¹),respectively,withalargegapbetweenthethreelevels.The yieldbuilding(noyield),youngmature(increasingyield),mature(plateau)and yielddeclinephasecanbediscerned.

21).Oilpalmscontinuetoproducefruitbunchesuntildeath,but replantingisrequiredat20–25YAPwhenpalmsbecometootall foreconomicharvestingorwhenyieldsdeclineduetothelossof palmstopestsanddiseases.

2.2. Vegetativegrowth

The averageyearlyabove-ground dry matterproduction per hectareformaturepalms(>10YAP)plantedwithtriangularspac- ingatplantingdensitiesof120–150palmsha⁻¹rangesfrom19t DMha⁻¹yr⁻¹inNigeria(ReesandTinker,1963)to32tDMha⁻¹ yr⁻¹inMalaysia(Corleyetal.,1971a).Drymatterproductioncan bedescribedbythefollowingequation:

DMP=PAR×f×RUE (1)

where DMP=dry matter production (kgm²yr⁻¹), PAR=yearly photosynthetically active radiation (MJm²yr⁻¹; 50% of total incomingsolar radiation(Monteith,1972)),f=fractionofradia- tioninterceptedbythecanopy,andRUE=radiationuseefficiency (kgDMMJ⁻¹PAR)(Monteith,1977;Corley,2006).Estimatedval- uesforRUEare0.6–1.3gMJ⁻¹PAR(ReesandTinker,1963;Squire, 1986; Squireand Corley,1987).RUEdoesnot changewithage inoilpalm(SquireandCorley,1987)butisdecreasedindrycli- matesandonpoorsoilsandenhancedbyfertiliseruse(15–30%

increaseinresponsetotheapplicationofN-P-K)(Squire,1986).

Radiationinterception(f)dependsmainlyontheleafareaindex (LAI),i.e.theareaofleavespersurfacearea(m²m⁻²),althoughleaf orientationwithrespecttolightanglecanmodifyeffectiveinter- ception.TheLAIincreaseslinearlyfromplantinguntil5–6YAPand peaksaround10YAP,whentheleavesreachtheirmaximumsize (GerritsmaandSoebagyo,1999).ThemaximumLAItypicallyvaries between4and6dependingongenotype(GerritsmaandSoebagyo, 1999;Breure, 2010), environment(Corleyetal.,1973), planting density(Corleyetal.,1973;GerritsmaandSoebagyo,1999),prun- ing(SquireandCorley,1987),fertiliseruse(Breure,1985;Corley andMok,1972),andgeneralagronomicmanagement.Inplanta- tionswhereoldleavesarenotremoved,LAImayexceed10(Squire andCorley,1987).AtanLAIof4.5interceptionofPARisatleast 80%,increasingupto90–95%atanLAIof6–7(Gerritsma,1988;

Breure,1988).YieldsarereducedwhenLAIexceedsavalueof6 duetocompetitionamongpalms(Breure,2010).

Fig.2.Schematicrepresentationofinflorescenceandbunchdevelopment,showingkeydevelopmentalstagesandtheeffectsofstressonpotentialbunchnumber(after UexküllandFairhurst,1991;Corleyetal.,1995;Adametal.,2005).Timestartsatleafinitiation(pointzero)andprogressesuntilbunchripeness,andisindicatedinmonths sinceleafinitiation(bottomx-axis)andleafnumber(upperx-axis,assuminganaveragephyllochronlengthof1.9month⁻¹).They-axisshowsthenumberofpotential bunchesperhectare.Thetwolinesshowtheprogressoftwohypotheticalbatchesofpotentialbunches,startingatoneperpalminaplantationwithaplantingdensityof 142palmsperhectare.Overtimethenumberofpotentialbunchesdecreasesasthebatchespassthroughseveralcriticalphases.Severestress(bottomline)leadstolarger reductionsinbunchnumberthanmildstress(topline).Thebarsrepresentthestress-sensitiveperiods:sexdetermination(left),inflorescenceabortion(middle)andbunch failure(right).

Inolderplantations,mostofthestandingbiomassiscontained inthetrunk(ReesandTinker,1963).Ofanestimatedgrosspri- mary production of 160tDMha⁻¹yr⁻¹ in 10-year-oldpalms in Malaysia,around70tha⁻¹yr⁻¹wasallocatedtotrunk,root,and rachisrespiration,and55tha⁻¹yr⁻¹ allocatedtoleafletrespira- tion,leaving30–35tha⁻¹yr⁻¹ ofdrymatterproduction(Corley, 1976b).Estimatesofstandingrootbiomassat15YAPfromdifferent experimentswerelistedbyHensonandChai(1997),rangingfrom 9tDMha⁻¹(Corleyetal.,1971a)to20tDMha⁻¹(TeohandChew, 1988).Underconditionswithoutwaterlimitation,about10–12%of assimilatesareallocatedtotheroots(HensonandChai,1997),but underwaterlimitedconditions,assimilateallocationtorootsmay beupto35%(Dufrèneetal.,1990;vanNoordwijketal.,2015).

Inproductivepalmsplantedatstandarddensities,about45–50%

oftheabovegrounddrymatterproductionisallocatedtogener- ativegrowth(maleinflorescencesandfemaleinflorescencesand bunches;Corleyetal.,1971b).Ithasbeenproposedthatallocation ofassimilatestoinflorescencesandbuncheswillnotoccuruntil demandsforvegetativeproductionaremet(the“overflow”model;

Corleyetal.,1971b).Yetlaterresearchhasshownthatbothvegeta- tiveandgenerativegrowtharesource-limitedandthatcompetition occursbetweenthedifferentsinks,althoughpriorityisgiven to vegetativegrowth(CorleyandTinker,2016:103).

3. Fruitdevelopment

Anumberofkeystagescanbedistinguishedduringinflores- cenceandfruitbunchdevelopment(Fig.2;foradetailedreview,

seeAdametal.,2005).Oilyielddependsonthenumberofhar- vestedbunches,thebunchweight,andtheoilcontentofthefruit (Breureetal.,1990).Thesefactorsarediscussedindetailbelow.

3.1. Bunchnumber

Thenumberofripebunchesavailableforharvestisdetermined by1)thenumberofinflorescencesinitiated(whichinturndepends ontherateofleafproduction;GerritsmaandSoebagyo,1999);2) sexratio(Corleyetal.,1995;Adametal.,2011;Heeletal.,1987);

3)abortionoffemaleinflorescencesbeforeanthesis(Pallasetal., 2013);and4)failureofdevelopingbunchesbetweenanthesisand bunchripeness(Combresetal.,2013).

3.1.1. Numberofdevelopinginflorescences

Leafinitiationratedeterminesdirectlythepotentialnumberof inflorescences,asasingleinflorescenceisinitiatedintheaxilof eachleaf.Anaverageoilpalmcarries45–50unopenedleavesin varyingstagesofdevelopmentand32–48openedleaves(Breure, 1994).TheyoungestfullyopenedleafisdenotedasLeaf1,with unopenedleavesbeingnumberednegatively(Fig.2).Leafinitia- tionrateisdeterminedprimarilybypalmage(Broekmans,1957), withopeningratesdecliningrapidlyinthefirst10YAP(Gerritsma and Soebagyo, 1999).Typically40–45leaves palm⁻¹year⁻¹ are produced at two YAP, 25–35 leaves year⁻¹ at six YAP, 20–25 leavesyear⁻¹at12–14yearsYAP(Broekmans,1957;Gerritsmaand Soebagyo,1999)and17–20leavesyear⁻¹at21YAP(Broekmans, 1957;Rafiietal.,2013).Leafinitiationratemayvarybetweendif-

ferentplantingmaterialsby±1leafpalm⁻¹year⁻¹(Gerritsmaand Soebagyo,1999)orthreedaysperphyllochron(thetimeelapsed betweentheappearanceoftwoconsecutiveleaves)(Lamadeetal., 1998).Leafinitiationratesofindividualpalmsrespondpositively tolight availability:initiationrates increasedby19%two years afterthinningofpalms11–15YAPathighdensity(186palmsha⁻¹; Breure,1994).Sinklimitationin13yearoldpalms,resultingfrom completeremovalofdevelopingfruits,reducedphyllochronlength from17daysto15days(Legrosetal.,2009b),possiblybecauseof increasedcarbohydrateavailabilitytoyoungleaves.Thissuggests thatoilpalmisabletorespondtoabundantcarbohydratesupply byincreasingitsrateofinflorescenceinitiation(Pallasetal.,2013).

Therateofleafopeningisreducedrapidlyinresponsetodrought (Changetal.,1988),resultingin theaccumulationofunopened leavesinthecentreofthepalmcrown(Broekmans,1957;Nouy etal.,1999).Droughtmayalsoreduceleafinitiationrates(Chang etal.,1988;Breure,1994).

3.1.2. Sexdetermination,inflorescenceabortion,andsexratio Incontrasttootherpalms,suchascoconut,thatcarrymaleand femaleflowersinthesameinflorescence,sexisdeterminedatinflo- rescencelevelinoilpalm.Theearliestmorphologicaldifference betweenmaleandfemaleinflorescencesistheincreasednumber ofbractsinitiatedonmalerachillae(Leaf-6;Corley,1976a;Heel etal.,1987;Adametal.,2005).Thetimingofsexdetermination variesamongexperiments,researchsitesand plantingmaterial, rangingfrom29to30monthsbeforeharvest(Broekmans,1957) to20 monthsbeforeharvest(Breureand Menendez,1990; Fig.

2).Corleyetal.(1995)foundthatthetimingofsexdetermina- tionvariesamongclones:eitheratbractinitiation,Leaf-29,orjust beforefirstrachillainitiation,Leaf-10,orboth.ThisledCorleyand Tinker(2016:121)tospeculatethatsexdifferentiationoccursat Leaf-29butisreversibleuptoLeaf-10(Crosetal.,2013).Thephys- iologicalmechanismsunderlyingsexdeterminationandtheroleof carbohydratebalanceandplanthormonesremainpoorlyunder- stood(Corley,1976a;CorleyandTinker,2016:120;forareview ontheeffectsofenvironmentalfactorsonsexdeterminationsee Adametal.,2011).

Sexratio(i.e.theratiooffemaleinflorescencenumbertototal inflorescencenumber)isaffectedbybothsexdeterminationand thepreferentialabortionoffemaleormaleinflorescences;thetwo effectsaredifficulttoseparate(Corley,1976a).Intheabsenceof severestress,theaveragesexratiois0.9–1.0inthefirstfourYAP (Henson andDolmat, 2004), 0.6–0.9until 12YAP (Jones, 1997;

HensonandDolmat,2004),andthensteadilydeclines(Corleyand Gray,1976).Severewaterdeficit,suchasoccursinthedryseasonin WestAfrica,canreducethesexratioto0.1–0.2(Broekmans,1957;

Bredasand Scuvie, 1960; Corley,1976a). Sexratio, particularly inflorescenceabortion,isaffectedbyfruitingactivity(Corleyand Breure,1992).Thecombinedeffectsofenvironmentalandinter- nalsignalsresultinannualoscillationsinsexratioandyield(Cros etal.,2013).Developinginflorescencesaremostsensitivetoabor- tion4–6monthsbeforeanthesis,whichcoincideswiththeonset of floralorgandevelopmentand elongation(Broekmans,1957).

Whereasseveralauthorsreportedapreferentialabortionoffemale inflorescencesduring(partof)thesensitiveperiodofinflorescence development (Bredas and Scuvie, 1960; Breure and Menendez, 1990; Pallasetal., 2013), othersobserved preferential abortion offemaleinflorescencesonlyinspecificlines(Corleyetal.,1995), preferentialabortionofmaleinflorescences(Legrosetal.,2009b), or equalabortionrates for inflorescencesof bothsexes (Henry, 1960).Inflorescenceabortionratesof25–40%weremeasuredin youngmaturepalmsthatexperiencedprolongeddryseasonsin Nigeria,decreasingto5–10%inpalms>15YAP(Broekmans,1957).

Muchsmallerabortionrates of2–13%weremeasuredin palms

of4–17YAPplantedondeeppeatsoilswitha highwatertable in Malaysia,and noclearagetrendwasobserved (Henson and Dolmat,2004).Areductioninsourceavailabilitythroughdefoli- ationdownto16leavesincreasedinflorescenceabortionratesin Leaves+2to+12from10%to40%,onaverage,inclonalpalmsof 9YAPinMalaysia(Corleyetal.,1995).Whilethesexratioatthe momentofpeakabortiondidnotchangesignificantlyinallclones butone,theaveragepercentageofleafaxilswithmaleinflores- cences increasedfrom50%in thecontrolto60% inthepruned palms,intheperiod11–25monthsafterdefoliation.Conversely, adecreaseinsinkactivityinducedbyfruitpruninginpalmsof14 YAPinSumatraincreasedthefractionoffemaleinflorescencesin thetroughand thepeak seasonfrom0.15–0.6inthecontrolto 0.25–0.8inthepruned palms.Simultaneously,theabortedfrac- tionsdecreasedfrom0.2–0.6to0.1–0.2,andthenumberofmale inflorescencesinthetroughseasonincreasedfrom0.1to0.5(Legros etal.,2009b).Thresholdsofspecificassimilateavailabilitythattrig- gersexdeterminationandfloralabortionresponsesremaintobe identified,duetothelargevariationinresponseamongplanting materials,researchsites,andexperiments(Breure,1987;Corley andBreure,1992;Corleyetal.,1995;Crosetal.,2013).

3.1.3. Bunchfailure

Bunchfailure,theabortionofabunchbeforefullripening,occurs 2–4monthsafteranthesis(Sparnaaij,1960).Bunchfailuremaybe causedbypoorpollinationoracuteandsevereassimilateshortage, usuallycaused bylackofwater orradiation(CorleyandTinker, 2016:125;Combresetal.,2013).Bunchfailureratesbetween1.5%

(Corley,1973b)and>25%(Sparnaaij,1960;CorleyandTinker,2016:

124–125)havebeenobserved,buttheavailabledataisscarce,and thephenomenonremainspoorlydescribedandunderstood(Corley andTinker,2016:124–125).

3.2. Bunchweightandoilcontent

Bunchweightandoilcontentarelessresponsivetostressthan bunch number, but have a major impact on yield. We briefly describeinflorescenceandbunchdevelopment,andthendiscuss theregulationofthevariouscomponentsofbunchweightandoil content.

3.2.1. Inflorescenceandbunchdevelopment

Bothmaleandfemaleinflorescencesconsistofapeduncle,car- ryingspikeletsonwhichtheflowersareset,eachsubtendedbya singlebract.Themalepeduncleandspikeletsare40and10–30cm inlength,respectively,andeachofthe100–300spikeletscarries 400–1500maleflowers3–4mminlength.Thefemalepeduncleis shorter(20–30cm)andthickerandcarriesaround150spikelets, each 6–15cm inlength.Aspikeletcarries5–30flowersthatare subtendedbyabractintheshapeofasharpspine(Jacquemard andBaudouin,1998).Thenumberofspikeletsandthenumberof flowersperspikeletincreasewithpalmagebutreachaplateauat 10–12YAP(CorleyandGray,1976).Thenumberoffemaleflow- ers thatdevelopsintofruitletsrangesfrom30–60%(Corleyand Tinker, 2016: 49) to80% (Harun and Noor, 2002) when insect pollinators are present. In palms 10–15 YAP, bunches contain 1500–2000fruitlets.Thebunchmaturationtime(fromanthesisto bunchripeness)variesfrom140to180days,dependingonboth geneticandenvironmentalfactors(Lamadeetal.,1998;Henson, 2005).Fruitmaturationstartstwoweeksafteranthesisandoccurs inseveraldistinctphases(Ooetal.,1986).Oilstartstoaccumulatein theendospermoffruitletsabout12weeksafteranthesis,andfour weeks latertheendocarpandendosperm(whichtogetherform thekernel)havehardened(Ooetal.,1986;Sambanthamurthietal., 2000).Oildepositioninthemesocarpbeginsaround15weeksafter anthesisandcontinuesuntilfruitripeness,5–6monthsafteranthe-

sis(Ooetal.,1986),whenfruitletmesocarpoilcontentisabout60%

andwatercontenthasdecreasedfrommorethan80%tolessthan 40%(BaforandOsagie,1986;BilleNgalleetal.,2013).

3.2.2. Regulatingmechanismsofbunchweightandoilcontent Themain componentsthat determinebunchweightarethe number of spikelets, number of flowers per spikelet, fruit set, weightper fruitlet,and weight ofnon-fruit bunchcomponents (Broekmans,1957).Bunchfreshweight(with53%drymatter,on average;Corleyetal.,1971b)increaseswithpalmage,startingat 3–5kgat24MAPandincreasingtoover30kgby25YAP(Limand Chan,1998,citedby Corleyand Tinker,2003:113;Sutarta and Rahutomo,2016).Allcomponentsofbunchweightrespondpos- itivelytoincreasedassimilateavailability(BreureandMenendez, 1990;CorleyandBreure,1992;Pallasetal.,2013).Removalof75%

oftheinflorescencesinpalmsof4–7YAPincreasedtotal bunch weightto12.7kgfrom7.6kgincontrolpalms,resultingfroman increaseinallcomponentsmentionedabove(CorleyandBreure, 1992;BreureandCorley,1992).Fruitsetisdeterminedmainlyby pollinationefficiency(4.2.8).

Oilcontentisprimarilyaffectedbyplantingmaterial(4.2.4).A singlegenedetermineskernelshellthickness,whichinturnaffects thethicknessofthemesocarpandthereforefruitbunchoilcontent (BeirnaertandVanderweyen,1941).Wildtypeoilpalm(dura)has athickshellandatypicaloilextractionrateof16–18%,whereas thetenerahybrid,acrossbetweenduraandtheshell-lesspisifera mutant,hasanintermediateshellthicknessandoilextractionrates of22–30%(Jalanietal.,2002;RajanaiduandKushairi,2006).Fer- tiliseruseaffectsbunchoilcontent(OchsandOllagnier,1977),with increasedtissuechlorideconcentrationsleadingtoanincreasein kernel-to-fruitfrom7.8to9.3%,andareductioninmesocarp-to- fruitfrom81.7to79.2%inpalmsof8YAPinPapuaNewGuinea (Breure,1982).Oilcontentisnegativelycorrelatedwithrainfall, andpositivelycorrelatedwithavailableradiation;highrainfallin Malaysiain1996resultedina0.8–1.5%decreaseofoilextraction rate(OER)comparedwith1993(HoongandDonough,1998).Itis positivelyrelatedwiththeconcentrationofMginleaftissue(Ochs andOllagnier,1977)butsometimesnegativelycorrelatedwiththe applicationofpotassiumchloride(OchsandOllagnier,1977;Zin etal.,1993),probablyasaconsequenceofincreasedClconcentra- tionsintheplanttissueresultinginincreasedkernel-to-fruitratio (Breure,1982).

4. Magnitude,causes,andmanagementofyieldgaps

Oilpalmisgrowninlarge-scalemonocultureplantationsoras asmallholdercrop,withfruitbunchesastheprimaryoutputand crudepalmoil(CPO)andpalmkerneloil(PKO)asthefinalproducts.

Productivityisbestmeasuredasoilyield(tha⁻¹),calculatedfrom theyieldoffruitbunches(tha⁻¹)andtheextractionrate(%).Inthis reviewyieldsareexpressedeitherintha⁻¹ fruitbunches(with 53%DM)orintha⁻¹oil.PKOisnotconsidered,asitisaby-product whichisextractedandtradedbyalimitednumberofmills.Kernel extractionrateisusuallyabout5%(Carteretal.,2007).

4.1. Thedifferentyieldgapsinoilpalm

Inproductionecology,threeproductionlevelsarecommonly distinguished: the potential yield (Yp) determined by yield- defining factors(PAR, temperature,ambient CO₂ concentration, and crop genetic characteristics); the water-limited (Yw) and nutrient-limitedyield (Yn)determined byyield-limitingfactors (waterand nutrition); and the actualyield (Ya) determinedby yield-reducingfactors(weeds,pest,diseases)(vanIttersum and Rabbinge,1997).Yieldgapanalysisistheanalysisofthedifference betweenYp(assuminggenotype andmanagement areoptimal)

and Ya in a particularphysical environment (vanIttersum and Rabbinge,1997;forrecentreviewsonyieldgapanalysisseealso Lobelletal.,2009;vanIttersumetal.,2013).

We definethepotentialyieldasthe yield ofacultivar,when growninenvironments towhichitisadapted; withnutrientsand waternon-limiting;andwithpests,diseases,weeds,lodgingandother stresseseffectivelycontrolled(Evans,1993).Thetheoreticallimitto geneticgainincropyieldcanbecalculatedusingsimulationmod- els(Lobelletal.,2009).Thisnumberissometimesalsoreferredto asthe‘potentialyield’inoilpalmliterature(Breure,2003;Corley, 2006),andcanbeusedtosetatargetforbreedersandtoexplore futurescenarios,suchasforlanduse.Oilpalmmanagementliter- aturereferstothe“siteyieldpotential”(Tinker,1984;Gohetal., 2000),definedastheyieldobtainedonaspecifiedsite,withnatural watersupply,nutrientssuppliedatoptimumrates,andagronomicand diseasecontrolmeasuresimplementedtoahighstandard(Corleyand Tinker,2016:322).Thisissimilartowhatwecallthewater-limited yield,butincludesmanagementdecisionstakenatplanting,specif- icallyplantingmaterialanddensity.Forthoroughreviewsonthe approachtoyieldgapanalysisfromtheoilpalmmanagementper- spective,seeGohetal.(1994);Griffithsetal.(2002),andFairhurst andGriffiths(2014),amongothers.

Accurate analysis of yield gaps depends on the correct assessment of thevarious production levels(Fig.3). Theyield- determining,yield-limitingandyield-reducingfactorsrelevantin oilpalmandtheirquantitativeeffectsonproductivityarediscussed indetailbelow.

4.2. Potentialyieldandyield-determiningfactors

Thepotentialoilyield,asdefinedbyfruitbunchyieldandoil content,is determined byPAR,temperature, ambientCO2 con- centration, and crop geneticcharacteristics, under perfect crop management(vanIttersumandRabbinge,1997;Table1).Wedis- cuss thedifferent factorsthat determinethe potentialyield in furtherdetailbelow.

4.2.1. AvailableradiationandPAR

Asaperennialwithapermanentleafcanopy,oilpalmisableto interceptradiationthroughouttheyear,whichisoneofthemain reasonswhyitsproductivityissolargecomparedwithotherveg- etableoilcrops.Inthetropics,availableradiationismostlylimited bycloudiness.Therangeoftotaldailyincomingshort-waveradi- ationandsunshinehoursperdayinoilpalmgrowingregionsare showninTable1.

Aminimumof15MJm⁻²day⁻¹ total solarradiation(equiva- lentto∼7.5MJm⁻²day⁻¹PAR)or5.5hday⁻¹ofsunshineisoptimal foroilpalmgrowth,indicatingalesseryieldpotentialinpartsof AfricaandtheAmericas(Paramananthan,2003).Modellingwork byKraalingen etal.(1989) indicatedthat each hourperdayof brightsunshineresultsin15–20kgbunchdrymatterproduction palm⁻¹year⁻¹inexcessofthebunchdrymatterproducedunder cloudycircumstances,assumingaplantingdensityof110palms ha⁻¹.Thuspotentialyieldsinregionswitheightsunshinehoursper daywouldbe>60%largerthaninregionswiththreesunshinehours perday(Kraalingenetal.,1989).Lightsaturationinoilpalmleaves typicallyoccursataphotosyntheticphotonfluxdensity(PPFD)of

>1100–1200␮molm⁻²s⁻¹,roughlyequivalentto250Wm⁻²PAR (Dufrèneet al.,1990).A light-saturatednetassimilationrateof about20␮molCO₂m⁻²s⁻¹wasmeasuredat1100␮molm⁻²s⁻¹ PPFDinleaf8and9ofpalmsplantedinIvoryCoast(Dufrèneand Saugier,1993),whichissimilartotheaveragerateof17.8␮molCO₂ m⁻²s⁻¹ foundinpalms12–13YAPinMalaysia(Henson,1991b).

ReductionofavailablePARduetohaze,causedbyforestburning, isacommonissueinIndonesia.Forestburningoccursmostlydur- ingthedryseasonwhenavailableradiationisatitspeak,andis

Table1

Yield-determiningfactorsinoilpalmsystems:potentialyield(Yp).

Yield-determining factors

Rangeinoil-palmgrowingareas Yieldeffectsmeasuredincasestudies Selectedreferences

Radiation:solar radiation

• Allregions:average15to23MJtotal radiationm⁻²day⁻¹

• Africaandpartsofthe

Americas:<10MJm⁻²day⁻¹during thewetseason

• Modelledincreasesof1.7–2.1tfruitbunchesha⁻¹yr⁻¹per additionalMJm⁻²day⁻¹

• Modelled15–20%annualyieldlossaftertwomonths reductionfrom15to12MJtotalradiationm⁻²day⁻¹dueto haze

(Paramananthanetal.,2000) (Henson,2000)

(Goh,2000) (Calimanetal.,1998)

Radiation:sunshine hoursday⁻¹

• Asia:5.3–6.9

• Americas:2.2–7.7

• Africa:3.6–6.3

• Productivityconstraintsif<5.5hday⁻¹

• Oneadditionalhrday⁻¹yieldsanadditional15–20kgbunch DMpalm⁻¹yr⁻¹comparedwithproductivityundercloudy conditions

(Hartley,1988:100-101) (Kraalingenetal.,1989) (Paramananthan,2003)

CO2concentration • 1960:317ppm

• 1980:339ppm

• 2000:370ppm

• 2015:399ppm

• ModelledbunchDMproduction(tha⁻¹yr⁻¹)insitewithout waterdeficit:

(Ibrahimetal.,2010) (Henson,2006) (TansandKeeling,2015)

Temperature Lowestmonthlyminimum:17.7^◦C (Bahia,Brazil)

Highestmonthlymaximum:34.6^◦C (Aracataca,Colombia)

• Undefinedstrongyieldreductionsatminimummonthly averagetemperaturesoflessthan18–19^◦C

• Seedlinggrowthinhibitedat15^◦C,seventimesslowerat 17.5^◦Candthreetimesslowerat20^◦Cthanat25^◦C

• Immatureperiodincoldconditionsupto1yearlonger

(Hartley,1988:102-103;110) (Henry,1958)

(Olivin,1986)

Plantingmaterial • Teneraclones

• Tenerasemi-clones

• DxPteneraseed

• Duraseed

• Seedofunknownorigin

• Teneraclones:15.7toilha⁻¹yr⁻¹at7YAP

• Tenerasemi-clones:11.1toilha⁻¹yr⁻¹at5YAP

• DxPteneraseed:8.9toilha⁻¹yr⁻¹

• Duraseed:∼35–50%reducedbunchoilcontent

• Seedofunknownorigin:reductionspotentiallyverylarge dependingonpercentagepisiferainpopulation(zeroyield frompisiferapalms)andpotentialofparentmaterials

(Simonetal.,1998) (Ngetal.,2003) (Rajanaiduetal.,2005) (Sharma,2007)

Plantingdensity • 110–156palmsha⁻¹infavourable environments

• 160–170palmsha⁻¹inunfavourable soils

• Optimumfixedplantingdensity:140–160palmsha⁻¹; optimumLAI:5.5–6.0

• 1-2%reductionincumulativeplantationyieldwhen density±10palmsfromoptimum

• Ondeeppeat:higheroptimumdensities(>160palmsha⁻¹)

• Yieldincreaseof4tfruitbunchesha⁻¹yr⁻¹from9to16YAP inresponsetothinningfrom160to120palmsha⁻¹at8YAP comparedwithnothinningorafixeddensityof143palms ha⁻¹inThailand

(CorleyandTinker,2016:282) (Breure,2010)

(Corley,1973a) (Breure,1977) (Gurmitetal.,1986) (Gohetal.,1994) (Uexkülletal.,2003)

Culling • Good:20–30%ofseedlingsremoved

• Poor:incorrectorinsufficientculling

• Noculling:20–30%abnormalseedlingsproducing40–100%

lessyieldthannormalseedlings

(Tam,1973) (Gillbanks,2003)

Pruning • 50–60leavesat0–3YAP

• 40–50leavesat4–10YAP

• 32–40leavesat>10YAP

• Over-pruningpalms8–12YAPplantedat138palmsha⁻¹in Malaysia:<2,12,19,24,and25tfruitbunchesha⁻¹with8, 16,24,32,and40leavespalm⁻¹,respectively

• Under-pruning:directbutunquantifiedyieldlossdueto reducedharvestingefficiency

(Hartley,1988:441-442) (Henson,2002) (CorleyandHew,1976)

Fruitsetand pollination

• Pollinatingweevilpresentinall regions

• Averagefruitset70–80%

• Quadraticasymptoticrelationbetweenfruitsetandbunch weightwithanaveragebunchweightof24,20and14kgat 90,50and20%fruitset,respectively

• Quadraticrelationbetweenfruitsetandoiltobunchratio withanaverageO/Bof25,20and13%atafruitsetof75,40 and20%,respectively

(HarunandNoor,2002) (Syedetal.,1982) (RaoandLaw,1998) (Henson,2001)

Harvestingfrequency • Plantations:7-day,10-dayor14-day harvestinginterval

• Smallholders:usually14or15-day harvestinginterval,sometimesupto 30days

• Yieldincreaseof5–20%whenreducinglengthofharvesting roundfrom14to10days

(Donoughetal.,2013) (Leeetal.,2013) (Corley,2001) (Donough,2003)

Croprecoveryinthe field

• Varyingfromnearcomplete recoverytolessthan70%offruit

• Reportedyieldlossesofupto5tfruitbunchesha⁻¹dueto poorcroprecovery

• Yearlylossesunderstrictharvestingregimeat7-day interval:200kgfruitbunchesha⁻¹unharvestedbunches and65kgha⁻¹uncollectedloosefruits

• Incompletecollectionofloosefruit:onaverage>5%yield loss

• ∼30%lessoilyieldfromunripebunches

(FairhurstandGriffiths,2014:

Chapter6)

(Donoughetal.,2013) (Corley,2001) (Wood,1985)

Fig.3.Differentoilpalmproductionlevelsandthecontributingfactors.

likelytoreduceyieldssignificantly(Table1).InAfrica,dustfrom theHarmattanandsmogcauseperiodicreductionsinradiation.

4.2.2. CO2concentration

UndercurrentcircumstancestherateofphotosynthesisinC3 cropssuchasoilpalmislimitedbytheavailabilityofCO₂.Yield increasesof10–30%inresponsetodoublingatmosphericCO2con- centrationshavebeenobservedinotherC3cropssuchaswheat (Kimballetal.,1993;Fuhrer,2003),andmaybeexpectedinfuture inasoilpalmiswelladaptedtohightemperature-environments (DufrèneandSaugier,1993).Increasesinphotosyntheticratesinoil palmseedlingsfrom5to12␮molm⁻²s⁻¹havebeenobservedin responsetochangesinatmosphericCO₂concentrationsfrom400 to800ppm(Ibrahimetal.,2010).Whetherincreasedratesofphoto- synthesisaretranslatedintoimprovedyieldsdependsonmultiple factors,particularlythesource/sinkbalance(e.g.PaulandFoyer, 2001)andtheairtemperature(4.2.3).Maturepalmsareusually source-limited(Breure,2003)makinganactualyieldresponseto risingCO₂ concentrationslikely,ifthetemperatureremainssta- ble.Noresearchhasbeencarriedouttodateontheactualeffectof availableCO2onoilpalmyieldinmatureplantations.Theexpected effectsofclimatechangeonworldwidepalmoilproductionare reviewedbyCorleyandTinker(2016,Section17.3).

4.2.3. Temperature

Thetemperaturerangeintheoilpalmgrowingregionsisshown in Table1.The uppertemperature limitfor efficientphotosyn- thesisinoilpalmleavesis>38^◦C,providedthatvapourpressure deficitissmall(Dufrèneetal.,1990;DufrèneandSaugier,1993;

Paramananthan,2003).Temperatureandmaintenancerespiration inplantsarestronglypositivelyrelated,withanaveragefactortwo increaseinmaintenancerespirationatevery10^◦Ctemperaturerise (Amthor,1984;Ryan,1991).Whetherthisestimateholdsforoil palmremainsunclear,andyieldresponsestoincreasingtemper- atureshavenotbeenquantified(Henson,2004,2006).Oilpalmis sensitivetocold(Table1).IncoolerregionssuchasinBahia(Brazil) andTela(Honduras),strongreductionsinyieldoccurduringthe secondhalfofthecoldseasonandthebeginningofthewarmer season,andinSumatralowtemperaturesathigherelevationswere foundtoextendtheimmatureperiodbyatleastoneyear(Hartley, 1988:110).

4.2.4. Plantingmaterial

Estimatesoftheoreticalceilingoilyields(withfutureplanting materialsunderthebestpossibleenvironmentalandmanagement condition)rangefrom10.6(Breure,2003)and14.0(Henson,1992) to18.5toilha⁻¹ yr⁻¹ (Corley,1998,2006)onaverageover the plantationlifetime.Whilethelargerestimatesmaybebasedon someunrealisticassumptions(Breure,2003),bestyieldsachieved insmallplantationsorexperimentalfieldsalreadyfallwithinthe estimatedrange (Table1).Non-clonalplantingmaterials,raised fromseed,consistofapopulationofoffspringfromaduramother andapisiferafather(DxP),andindividualsvaryintermsofpotential forvegetativegrowthandproductivity(Okwuagwuetal.,2008).

PotentialyieldsofDxPplantingmaterialshaveincreasedbyanesti- mated1.5%peryearthroughbreedingwithspecificmale/female parentcombinationsthat showanearlytrackrecord ofperfor- mance:thistrendinyieldincreaseisexpectedtocontinue(Soh, 2004;Corley,2006).Breedinghasparticularlyimprovedphotosyn- theticconversionefficiency(CorleyandLee,1992)andbunchoil content(CorleyandLee,1992;Prasetyoetal.,2014;Soh,2015).

Varietieswithimprovedtoleranceforcold(Chapmanetal.,2003) anddrought(Raoetal.,2008)arebeingfurtherdeveloped.

Clonesfromcarefullyselectedortetscanoutyieldconventional seedmaterialby20–30%,duetoacombinationofbetteruniformity, increasedfruitbunchyieldandgreateroiltobunchratio(Khaw andNg,1998;Simonetal.,1998;Kushairietal.,2010;Soh,2012;

Table1).Althoughfieldexperimentshaveconfirmedthesuperior yieldsofselectedclonesundercircumstancesofrigorousculling, keyissueswithmultiplicationofembryosandsomaclonalvaria- tionlimitthecurrentplantingofclonesatcommercialscale(Soh, 2004;Sohetal.,2011).Therecentfindingoftheepigeneticfac- torunderlyingthemantlingphenotype(afloralmalformationthat resultsinfailuretoformfruitletsorreducedfruitletoilcontent) islikelytoboosttheplantingandperformanceofclonaloilpalm (Ong-Abdullahetal.,2015).

4.2.5. Plantingdensity

Plantingdensityisanimportantdeterminantofpotentialyield (Corley,1973a;Breure,1977,1982;Uexkülletal.,2003).Anopti- mumplantingdensity(Table1)balancestherequirementforrapid canopyclosureintheimmaturephasewithalargenumberofpalms (i.e.bunches)intheyoungmaturephaseandlimitedinter-palm

competitionforlightinthematurephase.Ondeeppeat,vegeta- tivegrowthisreducedanddenserplantinghasbeenrecommended (Table1;Gurmitetal.,1986).High-densityplantingfollowedby selectivethinningat8–9YAPisaneffectivestrategyforyieldmax- imisation(Uexkülletal.,2003;Palatetal.,2012;Table1).

4.2.6. Culling

Thequalityanduniformityoffieldpalmsdependsontheplanted material andontheselection ofindividuals duringthenursery phase,termed‘culling’(Tam,1973).Duetogeneticdiversityand stressesduringthenurseryandfieldplantingphase,largediffer- ences in productivitybetweenpalmshave beenobserved even whenrigorouscullinghasbeencarriedout(Okwuagwuetal.,2008), withthemostproductiveindividualsyieldingtwotothreetimes morethanaverage,andtheleastproductiveindividualsyielding nobunches(Yeowetal.,1982;Hartley,1988:222).Normallythe prevalenceofstuntedorabnormalseedlingsis20–30%.Abnormal seedlings,identifiedbyphenotypicselectioninthenurseryphase, givestronglyreducedyieldswhenplantedout(Tam,1973;Table 1).Allabnormalseedlingsshouldberemovedduringthenursery phaseorreplacedwithin12monthsafterplanting(Gillbanks,2003;

JacquemardandBaudouin,1998:56).

4.2.7. Pruning

Pruning,theremovalofselectedleaves,isamanagementprac- ticespecificforperennialcrops.Pruningaimstooptimisesource availabilitywhileminimisinglossofassimilatesduetorespiration insenescingleaves.Newly-openedleavesinoilpalmshowastable orslightly increasingphotosyntheticactivityuntil4–10months afteropening(inpalmsof3and 10–12YAP,respectively),after whichactivitydecreasesuntiltheleavessenesceanddie(Corley, 1983,1976b).Leavesatthebottomofthecanopyremainphoto- syntheticallyactiveandarenetsourcesuntilsenescence(Henson, 1991a), and retaining all living leaves but removing senescing leavesisthebestwaytomaximiseassimilateavailabilityirrespec- tiveofplantationage(Hartley,1988:441;Henson,2002).Pruning in immatureand youngmaturepalmsisusually limited tothe removalofsenescingordeadleaves,asreductionsinleafareahave astrongnegativeeffectonlightinterceptionandtotalassimilate availabilityduringthisphase(Gerritsma,1988;Breure,2003).Yield penaltieswhenpruningfrom>48downto32–40leavesperpalm inmatureplantationsarenotsignificant(CorleyandHew,1976) andsufficientpruningoftallpalmstofacilitatecompleteandcor- rectharvestingandquickrecyclingofnutrientsisrecommended (FairhurstandGriffiths,2014).

4.2.8. Pollination

Aquadraticfunctiondescribestherelationshipbetweenfruit setandbunchweight,withamaximumbunchweightat90%,and amaximumoiltobunchratioat75%fruitset(HarunandNoor, 2002;Table1).Seasonalepisodesofpoor(10–20%)fruitsethave beenobservedinMalaysia,causedbystrongreductionsofpollinat- ingweevilpopulationsduetoexcessiverain,absenceofsufficient maleflowersandinfectionwithparasiticnematodes(RaoandLaw, 1998).Asaconsequenceoilextractionrate(OER)fellfrom21.2 to18.8%,andkernelextractionratefrom4.7to3.5%inMalaysia between1993and1996.Aminimumoftwomalepalmsperhectare inplantationswithahighsexratioisthoughttosupplysufficient pollenandmaintainweevilpopulations(RaoandLaw,1998).

4.2.9. Croprecovery

Thegoalofharvesting,orcroprecovery,istocollectallfruit bunchesatthemomentofoptimumripeness(i.e.maximumoil contentwithaminimumconcentrationoffreefattyacidsinthe extractedoil;PORLA,1995).Infrequent,incompleteorincorrect harvestingpractices (i.e.harvestingunripeoroverripebunches)

directly reduce both the quantity of fruit and the oil quality (Donoughetal.,2010;Table1).Theharvestinginterval(i.e.the numberofdaysbetweentwoharvestingrounds)shouldbeadapted tothespeedatwhichloosefruitsdetachfromtheripebunch,to minimiselossesfromuncollectedloosefruitandoverripebunches (Gan,1998).Anoptimalharvestinginterval of10dayshasbeen proposed (Gan, 1998; Rankine and Fairhurst, 1999b; Donough etal.,2010).Harvestingofunripebunchesislikelytoaffectthe source/sinkbalanceasbunchsinkrequirementsincreasestrongly towardsthelastphaseofripening(Henson,2007),butthishasnot beenquantified.

4.3. Water-limitedyieldandyield-limitingfactors

Thewater-limitedyield(Yw;Table2)isanimportantbench- markasmostoilpalmcroppingsystemsarerain-fed(Ludwigetal., 2011).Wateravailabilitydependsonrainfallandsoilcharacteris- ticsandisstronglysite-specific(Lobelletal.,2009;vanIttersum etal.,2013).Ywcanbeapproximatedbycropsimulationmodels usingplausiblephysiologicalandagronomicassumptions(Evans andFischer,1999),byfieldexperiments,estimatesofbestfarmers’

yields,orgrowers’contests(vanIttersumetal.,2013).

4.3.1. Rainfall

Oilpalmtranspiresabout6mmwaterday⁻¹undernon-limiting conditions, and requires sufficient rainfall throughout the year (Table2).Averageactualtranspirationratesinoilpalmplantations are4.0–6.5mmday⁻¹ intherainyseasonand1.0–2.5mmday⁻¹ ondrydays(Carr,2011).Moderatetoseverewaterstressstrongly suppressesyield (Table3).Oil palmleavesdo notwilt,but the openingofnewleavesisdelayedinresponsetowaterstress,and stomatalopeningisstronglyaffectedbyairvapourpressuredeficit (VPD) and soil water availability (Smith,1989; Caliman,1992).

HensonandHarun(2005)measuredpotentialevapotranspiration ratesof1.3mmday⁻¹at1.9kPaVPDand75%availablesoilwater content,in palmsof3 YAPplantedata sitewitha regulardry seasoninMalaysia.Inanothersite,anincreasedVPDfrom0.4to 2.0kParesultedinadeclineinphotosyntheticratefrom18–19to 10–12␮molCO₂ m⁻¹s⁻¹ in palmsof1–2YAP,evenundercon- ditions of sufficient soil water availability (Henson and Chang, 1990).

Alinearrelationshipbetweenappliedwatervolumeandyield hasbeenfoundinirrigationtrialsindrierenvironments(Corley, 1996; Palat et al., 2008; Carr, 2011; Table 2). Although yield responsestoirrigationhavebeenobservedinareaswithoccasional dryspellsinMalaysia,irrigationisnotalwayseconomicallyfeasible (CorleyandHong,1982;HensonandChang,1990).Criticalwater deficitthresholdsatdifferentstagesofpalmdevelopmentandopti- mumvolumesofwatertobeappliedremaintobedefined(Carr, 2011).

4.3.2. Soil

Soilwateravailabilitydependsontheinfluxofwater(rainfall, irrigation,andgroundwater),thelossofwater(evapotranspiration, drainage,andsurfacewaterrun-off),andtheprevioussoilwater reserve.AsimplifiedcalculationwasproposedbySurre(1968)to allowforaquickassessmentofthesuitabilityofsoil-climatecom- binationsforoilpalmdevelopment.Thiscalculationisbasedonthe followingequation:

B = Res+R−Etp (2)

where Bis thewaterbalanceat theendof aperiod, Resis the soilwaterreserveatthebeginningofaperiod,Ris rainfalland Etp isthepotentialevapotranspiration(Surre,1968).Using this equation,Olivin (1968)estimatedwater-limitedyieldsinAfrica forfivescenariosofwaterdeficitonfivesoilclassesrangingfrom