duckweed

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DUCKWEED:Atinyaquaticplantwithenormouspotentialforagricultureandenvironment

CHAPTER 1: Introduction

AQUATIC HABITATS

Aconsiderableproportionoftheworld'ssurfaceiscoveredbysalinewaters,andthelandareasfromwhichthesaltsoftheseamostlyoriginatedarecontinuously leachedofmineralsby the runoff of rainwater.Aquatichabitatsaboundthesemaybetemporaryfollowingrainsorpermanentlargelythroughimpedimentstodrainage.Fromthebeginningoftime these aquatic habitats have been harvested for biomass in many forms (food, fuel and building materials) byanimalsandman.FromthetimeoftheIndustrialRevolutionandwiththeonsetofintensivelanduseenormouschangesoccurred.Agriculturistsharvestedbothwateranddrylandsforbiomassandmineralswereappliedtostimulatebiomassyields,theaquatichabitatsoftenbecameenriched(orcontaminated)andwaterbodiesweremoretemporarybecauseofwateruse inagricultureorwere lost throughdrainageor theestablishmentofmajordams for irrigation,humanwatersuppliesand/or hydroelectric powergeneration.On theotherhandotherhumanactivities, createdaquatic areas forsuch purposes as the control of soil erosion, for irrigation, storage of water, sewage disposal and industrial wastestorageortreatmentandforrecreationaluse.

Aquatic habitats have, in general, degenerated throughout theworld becauseof pollutionbyboth industry andotheractivities.Humanactivitieshave, ingeneral, resulted inmuchhigher flowsofmineralsandorganicmaterials throughaquaticsystems,oftenleadingtoeutrophicationandahugedropinthebiomassproducedinsuchsystems.Thelackofdissolvedoxygen inwaterbodies, through itsuptakebymicrobes fordecompositionoforganiccompounds,producesdegreesofanaerobiosisthatresultsinmajorgrowthofanaerobicbacteriaandtheevolutionofmethanegases.

Despitethisintheareasofhighrainfall,particularlyinthewettropics,thereremainmajoraquacultureindustries,whichvary fromsmall farmerswith 'manure fed'pondsproducing fish through to largeandextensivecultivationof fishandshellfish that are replacing the biomass harvested from the seas. The distribution of global aquaculture is shown inFigure 1. Fish production from ocean catches appear to be reduced, but production from farming practices areincreasingwhichclearlydemonstrateshowimportantaquacultureis(andwillbecome)inproteinfoodproduction.Thistrendisillustratedbythetrendinworldprawn(shrimp)productionshowninFigure2.

Figure 1: Distribution of global aquaculture (Source: FAO 1989)

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Figure 2: The changing pattern of world prawnproduction for human consumption (FAO 1989)

Althoughtraditionalorstaplecropscanbeproducedfromwaterbodiesandinmanysituationstraditionalpeopleoftenharnessed these resources, theaquatic habitat hasbeenconsidered too costly and todifficult to farmother than forextremelyhighvaluecropssuchasalgaeharvestedforhighvaluematerialssuchasbcaroteneoressentiallongchainfattyacids.Intensiveaquaculture(hydroponics)ofcropsinhighlymechanisedfarmshavebeendevelopedbutrequirehighlysophisticatedmanagementsystemsandareexpensive.

Throughouttheworld,andparticularlyinAsia,farmershaveharvestednaturallyproducedaquaticplantsforanumberofpurposesincludinganimalfeed,greenmanureandfortheirfamilyfeedresources.Thebestknownoftheseincludethefreefloatingplantswaterlettuce(Pistia),waterhyacinth(Eichhcornia),duckweed(Lemna)andAzollaandsomebottomgrowingplants.

Azolla,whichisamemberofthefernfamilygrowsextensivelyinassociationwithnitrogenfixingbacteria,whichallowsittoproduceonwaters low inNbut containingphosphorus.AzollahasbeencomprehensivelydiscussedbyvanHove(1989).

In recent years a commonly occurring aquatic plant, "duckweed", has become prominent, because of its ability toconcentratemineralsonheavilypollutedwatersuchasthatarisingfromsewagetreatmentfacilities.However,ithasalsoattractedtheattentionofscientistsbecauseofitsapparenthighpotentialasafeedresourceforlivestock(Skillicornetal.,1993Leng,etal.,1994).DuckweedgrowsonwaterwithrelativelyhighlevelsofN,PandKandconcentratesthemineralsandsynthesisesprotein.Theseare thenutrientswhichareoftencriticallydeficient in traditional foddersandfeedsgiventoruminantsandtopigsandpoultryparticularlywheretheformerdependonagroindustrialbyproductsandcropresidues.

The growing awareness of water pollution and its threat to the ecology of a region and agriculture per se has alsofocussed attention on potential biologicalmechanisms for cleansing water of these impuritiesmaking it potable andavailableforreuse.Ingeneral,wateravailabilityisbecomingaprimarylimitationtoexpandinghumanactivitiesandalsothecapacityofagriculturallandtofeedtheeverincreasingpopulationoftheworld.

Anotherpressurethathasstimulatedinterestinaquaticplantshasbeentheoveruseoffertilisers,particularlyinEuropethathasledtocontaminationofgroundwatersuppliesthatcannolongerbetolerated.

ECOLOGICAL CONSIDERATIONS

Intheearly1960'sanumberofscientistswarnedofthependingshortageoffossilfuels,theexpandingpopulationandthepotentialformassstarvationfromaninabilityofagriculturetoproducesufficientfood.

Theprophesieshaveprovedwrongintheshortterm,largelybecauseoftheextentofthethenundiscoveredfossilfuel,butalsobecauseoftheimpactofthedevelopmentofhighyieldingcropvarieties,particularlyofcerealgrain.The"GreenRevolution"whilstincreasingcerealcropyieldsfasterthanhumanpopulationincreasehashadserioussideeffectssuchasincreasederosionandgreaterwaterpollutioninsomeplacesandahugeincreaseindemandforwaterandfertiliser.Fertiliser availability and water use are often highly dependent on fossil fuel costs.Water resources inmany of theworld'saquifersarebeingusedatratesfarbeyondtheirrenewalfromrainfall(seeWorldWatch1997).

At thepresent time it appears that potentially theapplicationof scientific research couldmaintain themomentum forincreasedfoodproductiontosupportanincreasingworldpopulation,butitisratherobviousthatifthisistooccuritmustbewithoutincreasedpollution,andwithlimitedincreasesintheneedforwaterandfertiliserandthereforealsofossilfuel.

GLOBAL WARMING, FOSSIL FUEL AND NUTRIENT RECYCLE NEEDS.

Globalwarminghasnowbeenacceptedasinevitable.Itisnowamajorpoliticalissueinmostcountries.Governmentsarenowconsidering theneed to reduce thecombustionof fuelswhichcontributemost to thebuildupofgreenhousegasesandthustheincreaseinthethermalloadthatispresentlyoccurring.Asecondproblemforfossilfueldevouringindustriesisthepotentialforscarcer,andtherefore,morecostlyoilresourcesinthenearfuture.AsFleay(1996)inhisbook"Thedeclineintheageofoil"haspointedouttherehavebeennomajordiscoveriesofoilinthelasttenyears.Thissuggeststhatwehavealreadydiscoveredthemajorresources.Manyoftheoilwellsareapproachingorhavepassed

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the point atwhich half the reserves have been extracted.At this stage the cost in fuel to extract the remaining fuelincreasesmarkedly.Theneedtoreducefuelcombustionandthepotentialforlargeincreasesincostsofextractionofoilfromthemajordepositsallindicatemajorincreasesinfuelcostsandtheneedtostimulatealternativeenergystrategiesforindustryandagriculturealike.

Fuelisamajoreconomiccomponentofallindustries,andinparticular,industrialisedagriculture.Thereforefoodpricesare highly influenced by fuel prices. The energy balance for grain production has consistently decreased withmechanisationasisillustratedbythefuelcostsforgrainproductionwhichisapproaching1MJinasfossilfuelusedinallactivitiesassociatedwithgrowing thatcrop to1.5MJout in thegrain.Amajorcomponentof thecostsare in traction,fertiliser,herbicidesandwateruse,particularlytheenergycostsofirrigation.

In recent times, a movement has begun to examine a more sustainable future for agriculture, particularly in thedevelopingcountries.TheneedindevelopingcountriesofAsiaandAfricawheremostoftheworld'spopulationlivesandwherepopulationgrowthisthehighestisto:

decreasepopulationgrowthmaintainpeopleinagricultureandproduceanincreasingamountoffoodinasustainableway

Thissuggeststhatsmallfarmersneedtobetargetedandthatfarmingshouldbeintegratedsothatfertiliserandotherchemicaluseisminimisedtogetherwithloweredgaseouspollution.Atthesametimeacountrymustensureitssecurityof foodsupplies. In the1998 financialcrisis inAsia, thesmall farmerwasseriouslyeffectedbecauseof the relativelyhighcostoffuel.Thisisboundtohaveseriouseffectsonfoodproductioninthenextfewyearsiffertiliserapplicationsarerestricted.Thiswillshowupasadeclineincropyieldsoverthenextfewyears.

Theproblemofdecreasingworldsuppliesoffuels,increasedlegislationtodecreaseuseoffossilfueltoreducepollution,and theeconomicdisincentive touse fertilisers indevelopingcountries indicates to thiswriter that there isamassiveneedtoconsideramoreintegratedfarmingsystemsapproach,ratherthanthemonoculturesthathavedevelopedtothepresenttime.

Integratedfarmingsystemsuserequirethreemajorcomponentstominimisefertiliseruse:

acomponentwherenitrogenisfixed(e.g.alegumebank)acomponenttoreleasePfixedinsoilsforplantusewhenthisislimitingasystemofscavenginganyleakageofnutrientsfromthesystem.

Italsorequiresincorporationofanimalsintothesystemtoutilisethemajorbyproductsfromhumanfoodproduction.

Duckweedaquacultureisanactivitythatfitsreadilyintomanycrop/animalsystemsmanagedbysmallfarmersandcanbe a major mechanism for scavenging nutrient loss. It appears to have great potential in securing continuous foodproduction, particularly by small farmers, as it can provide fertiliser, food for humans and feed for livestock and inadditiondecreasewaterpollutionandincreasethepotentialforwaterreuse.

Theproductionanduseofduckweedisnotrestrictedtothisareaandthereisimmensescopetoproduceduckweedsonindustrialwastewaters,providingafeedstockparticularlyfortheanimalproductionindustries,atthesametimepurifyingwater.

In this presentation, duckweed production and use, particularly in small farmer systems, is discussed to highlight itspotentialinfoodsecurity,particularlyincountrieswherewaterresourcesaboundandhavebeenmisused.Ontheotherhand,duckweedaquaculturethroughitswatercleansingabilitiescanmakeagreateramountofpotablewateravailabletoapopulationlivingunderaridconditions,providingcertainsafeguardsareapplied.

CHAPTER 2: The plant and its habitat

INTRODUCTION

Duckweedisthecommonnamegiventothesimplestandsmallestfloweringplantthatgrowsubiquitouslyonfreshorpollutedwaterthroughouttheworld.Theyhavebeen,botanicalcuriositieswithaninordinateamountofresearchaimedlargely at understanding the plant or biochemical mechanisms. Duckweeds have great application in genetic or

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biochemical research.Thishasbeenmoreor less in thesamewaythatdrosophila(fruit flies)andbreadmouldshavebeenusedasinexpensivemediumforgenetic,morphological,physiologicalandbiochemicalresearch.

Duckweedsare small, fragile, free floatingaquatic plants.However, at times theygrowonmudorwater that is onlymillimetresdeep towaterdepthsof3metres.Their vegetative reproductioncanbe rapidwhennutrientdensitiesareoptimum.Theygrowslowlywherenutrientdeficienciesoccurormajor imbalances innutrientsareapparent.Theyareopportunisticinusingflushesofnutrientsandcanputongrowthspurtsduringsuchperiods.

DuckweedsbelongtofourgeneaLemna,Spirodela,WolfiaandWolffiella.About40speciesareknownworldwide.Allof thespecieshave flattenedminute, leaflikeoval to round "fronds" fromabout1mm to less than1cmacross.Somespeciesdeveloprootlikestructures inopenwaterwhicheitherstabilise theplantorassist it toobtainnutrientswheretheseareindiluteconcentrations.

Whenconditionsareideal,intermsofwatertemperature,pH,incidentlightandnutrientconcentrationstheycompeteintermsofbiomassproductionwiththemostvigorousphotosyntheticterrestrialplantsdoublingtheirbiomassinbetween16hoursand2days,dependingonconditions.Anideaoftheirrapidgrowthisillustratedbythecalculationthatshowsthat if duckweed growth is unrestricted and therefore exponential that a biomass of duckweed covering 10cm2mayincreasetocover1hectare(100millioncm2)inunder50daysora10millionfoldincreaseinbiomassinthattime.

Obviouslywhenbiomassdoublesevery12days,by60days this couldextend toa coverageof32ha. Innatural orfarmingconditions,however, thegrowth rate isalteredbycrowding,nutrient supply, light incidenceandbothair andwatertemperatureinadditiontoharvestingbynaturalpredators(fish,ducks,crustaceansandhumans).

Inaddition to theabove limiting factors therealsoappears tobea senescenceand rejuvenation cyclewhich is alsoapparentinAzolla.

Vegetativegrowth inLemnaminorexhibitscyclesofsenescenceand rejuvenationunderconstantnutrientavailabilityandconsistentclimaticconditions (Ashbey&Wangermann,1949).FrondsofLemnahaveadefinite lifespan,duringwhich,asetnumberofdaughter frondsareproducedeachof thesedaughter fronds isofsmallermassthantheoneprecedingitanditslifespanisreduced.Thesizereductionisduetoachangeincellnumbers.Latedaughterfrondsalsoproducefewerdaughtersthanearlydaughters.

Atthesametimeasasenescencecycleisoccurringanapparentrejuvenationcycle,inwhichtheshortliveddaughterfronds(withhalfthelifespanoftheearlydaughters)producefirstdaughterfrondsthatarelargerthanthemselvesandtheirdaughterfrondsarealsolarger,andthiscontinuesuntilthelargestsizeisproducedandsenescencestartsagain.Thishasrepercussionsastherewillbecyclicalgrowthpatterniftheplantsaresourcedfromasinglecolonyandareallthesameage.Undernaturalconditionsitispossibletoseeamatofduckweeds,apparentlywaneandexplodeingrowthpatterns.

Thecyclicnatureofasynchronisedduckweedmat(i.e.allthesameage)couldbeoveratleast1monthasthelifespanoffrondsfromearlytolatedaughterscanbe33or19drespectivelywitha3folddifferenceinfrondrateproduction(SeeWangermann&Ashby,1950).

Thephenomenaofcyclicalsenescenceandrejuvenationmaycauseconsiderableerrorsofinterpretationinstudiesthatexamine,forexample,theresponseofafewplantstodifferingnutrientsourcesovershorttimeperiods.

Inpracticethiscyclemayberesponsiblefortheneedtorestockmanyproductionunitsafterafewweeksofharvesting.In Vietnam, with small growth chambers the duckweed required reseeding every 46 weeks (T.R. Preston personalcommunication)tobeabletoproduceaconstantharvestablebiomassgrowingondilutedbiogasdigestorfluid.Thereisalso thepossibility in such systemsof abuildup in theplant of compounds that eventually become toxicor at leastdiminishtheirgrowthrate.

Rootlengthappearstobeaconvenientrelativemeasureoffrondage.

Thesenescencerejuvenationcycle is increasedbyhightemperaturesthroughadecrease in individual frond lifespanbutthereisaconcomitantincreaseindaughterfrondproductionsothatthebiomassoffrondsproducedinashorterlifespanisthesame.

Therejuvenationcycleappearstobeunaffectedbyeitherlightdensityortemperature.

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Photo 1: The various species of Lemnaeca relevant to thispublication

Thecyclicalchangesappear tobemediatedbychemicalssecretedby themother frondandgrowthpatternsmaybemodified greatly by harvesting methods which mix water, wind effects and shelter as well as light intensity andtemperature.

The increased death rate of duckweedmats exposed to direct sunlight has been recognised inwork in Bangladeshwhereworkersareset tocoolduckweedmatsbysplashing themwithwater frombelow thesurfaceand inVietnam,Preston(personalcommunication)observedthattheincidenceofshowersstimulatedveryrapidgrowthofduckweedinsmallponds.

Duckweedsappeartohaveevolved,soas,tomakegooduseoftheperiodicflushesofnutrientsthatarisefromnaturalsources. However, in recent times they are more likely to be found growing in water associated with cropping andfertiliser washout, or down stream from human activities, particularly from sewageworks, housed animal productionsystemsandtosomeextentindustrialplants.

TAXONOMY

For themanypurposesrelated in thispublication, theselectionofduckweed to farmwilldependonwhatgrowsonaparticularwaterbodyandthefarmerhaslittlecontroloverthespeciespresent.Thevariousduckweedshavedifferentcharacteristics.The frondsofSpirodelaandLemnaare flat,ovaland leaf like.Spirodelahas twoormore threadlikerootsoneachfrond,Lemnahasonlyone.WolffiellaandWolfiaarethalloidandhavenorootstheyaremuchsmallerthanSpirodelaorLemna.Wolfia frondsareusually sickle shapedwhereasWolffiella isboat shapedandneitherhasroots.Differentiationandidentificationisdifficultandisperhapsirrelevanttothediscussion.Thisismainlybecausethespeciesthatgrowsonanywateristheonewiththecharacteristicrequirementofthatparticularwaterandthedominantspecieswill changewith thevariations inwaterquality, topography,managementandclimate,mostofwhicharenoteasilyoreconomicallymanipulated

MORPHOLOGY AND ANATOMY

Thestructureof the frondsofduckweed issimple.Newordaughterfrondsareproducedalternativelyandinapatternfrom two pockets on each side of the mature frond inSpirodela and Lemma. In Wolffiella and Wolfia only onepocket exists. These pockets are situated in Spirodela orLemnaclose towhere the rootsarise.Each frond,as theymature,mayremainattachedtothemotherfrondandeachinturn,undergoesthisprocessofreproduction.

In all four genea each mother frond produces aconsiderablenumberofdaughter frondsduring its lifetime.However,aftersixdeliveriesofdaughterfronds,themother

frondtendstodie.Coloniesproducedinlaboratoryornaturallyarealwaysspottedwithbrowndeadmotherfronds.

Thebulkofthefrondiscomposedofchlorenchymatouscellsseparatedbylargeintracellularspacesthatarefilledwithair (orothergases)andprovidebuoyancy.SomecellsofLemnaandSpirodelahaveneedle like raphideswhicharepresumablycomposedofcalciumoxalate.

TheupperepidermisintheLemnaishighlycutinizedandisunwettable.Stomataareontheuppersideinallfourgenea.AnthocyaninpigmentssimilartothatinAzollaalsoforminanumberspeciesofLemnacae.BothSpirodelaandLemnahavegreatlyreducedvascularbundles.

Roots in both Spirodela and Lemna are adventitious. The roots are usually short but this depends on species andenvironmentalconditionsandvaryfromafewmillimetresupto14cm.Theyoftencontainchloroplastswhichareactivephotosynthetically.However,therearenoroothairs.

Theplantreproducesbothvegetativelyandsexually,floweringoccurssporadicallyandunpredictably.Thefruitcontainsseveralribbedseedswhichareresistanttoprolongeddesiccationandquicklygerminateinfavourableconditions.

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Photo 2: Duckweed accumulation in the crocodile lagoon inHavana Zoo, Cuba.

DISTRIBUTION

TheLemnacaefamilyisworldwide,butmostdiversespeciesappearinthesubtropicalortropicalareas.Thesereadilygrowinthesummermonthsintemperateandcoldregionstheyoccurinstillorslowlymovingwaterandwillpersistonmud. Luxurious growth often occurs in sheltered small ponds, ditches or swamps where there are rich sources ofnutrients.Duckweedmatsoftenaboundinslowmovingbackwatersdownstreamfromsewageworks.

In the aquatic habitat of crocodiles and alligators, duckweeds often have luxurious growth on the nutrients from theexcrement of these reptiles and the local zoo can often provide a convenient source of duckweed for experimentalpurposes(seePhoto2).

Somespeciesappear to toleratesalinewatersbut theydonot concentrate sodium ions in their growth.Theapparentlimitforgrowthappearstobebetween0.5and2.5%sodiumchlorideforLemnaminor

When the aquatic ecosystem dries out or decliningtemperatures occur, duckweeds have mechanisms topersistuntilconditionsreturnthatcansupportgrowth.Thisoccurs through latesummer flowering,or theproductionofstarch filled structuresor turinwhicharemoredense thanthefrondssotheplantssinktothebottomofthewaterbodyandbecomeembeddedindriedmud.

The four species of Lemnacae are found in all possiblecombinationswitheachotherandotherfloatingplants.Theyaresupportedbyplantsthatarerootedinthepond.Theyeffectthelightpenetrationofwaterresourcesanddependingontheircoverageoftheareatheycanpreventthegrowthofalgaeorplantsthatgrowemersedinwater.Theyprovidehabitatandprotectionforanumberofinsectsthatassociatewiththeplantbuttheyappeartohavefewinsectsthatfeedonthem.Themainpredatorsappearstobeherbivorousfish,(particularlycarp),snails,flatwormsandducks,otherbirdsmayalsofeedonduckweedsbutreportsare few in the literature.Themuskratappears toenjoyduckweedsandtheauthorsuggeststhatmanyotheranimalsmayoccasionallytakeduckweedssuchaspigsandruminants.

TheappearancesofduckweedspeciesnotpreviouslyseeninareasofEuropehavebeenattributedtoglobalwarmingand/orastrongindicationofrisingwatertemperaturethroughouttheworldfromglobalwarming(Wolff&Landolt1994).

HISTORY OF DUCKWEED UTILIZATION

Thisisamostdifficultareatoreviewsincemuchoftheinformationisbywayofthepopularpressorisonlymentionedinscientificpapers.However,afteralecturegivenattheUniversityofAgricultureandForestryinHoChiMinhCityinwhichthe potential of duckweed biomass for animal production was discussed, as a novel concept, the writer was mostchastened to find that duckweedwasusedextensively by local farmers as feed for ducks and fish and therewasaflourishingmarketforduckweed.

TheduckweedbasedfarmingsysteminVietnamdependedlargelyonmanureandexcrementbeingcollectedinasmallpondwhere someeutrophication takes place thewater from this pond runs into a larger pond about 0.5mdeep onwhichduckweedgrowsinathickmat.Thiswasharvestedonadailybasisandimmediatelymixedwithcassavawaste(largelypeelings)andfedtoduckswhichwereconstrainedinpensonthesideofthepondorlagoon(seePhoto3).Theduckswereproducedforthelocalrestauranttrade.

InTaiwan,itwastraditionaltoproduceduckweedsforsaletopigandpoultryproducers.

TherearereportsthatWolfiaarrhiza,whichisabout1mmacrosshasbeenusedformanygenerationsasavegetablebyBurmese,LaotionsandNorthernThailandpeople.Thai's refer to thisduckweedas"Khainam"or "eggsof thewater"and it was apparently regarded as a highly nutritious food stuff. It could have been a valuable source of vitaminsparticularofvitaminAtothesepeople.ThiswouldhavebeenparticularlyimportantsourceduringthelongdryseasonofNorthern Thailand when green vegetables may have been scarce. It is also a good source of minerals, again itsphosphorous content could have been vital in areaswhere there aremajor deficiencies, such as occurs inNorthern

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Photo 3: Duckweed growing as part of an integrated farmingsystem in Vietnam

Thailand.

Therearereferencesintheliteraturetoduckweedasbothahumanfoodresourceandasacomponentofanimalandbirddietsintraditional/smallfarmersystemsinmostofSouthAsia.

CHAPTER 3: Nutrient requirements of duckweed

INTRODUCTION

Like all photosynthetic organisms, duckweeds grow withonly requirements for minerals, utilising solar energy tosynthesise biomass. They have, however, the capacity toutilise preformed organicmaterials particularly sugars andcangrowwithoutsunlightwhenprovidedwithsuchenergysubstrates. In practice the ability to use sugars in themediumasenergysourcesisirrelevant,asinmostaquaticsystemstheydonotexist.However,theycouldbeofsomeimportance where industrial effluent's need to be purifiedand duckweed is considered for this process (e.g. wastewater from the sugar industry or waste water from starch

processing).

Most research on nutrient requirements have centred on the need for nitrogen, phosphorus and potassium (NPK).However, like all plants, duckweeds need an array of trace elements and have well developed mechanisms forconcentrating these fromdilutesources.From theexperienceof theNonGovernmentOrganisationPRISM(based inColombia,Maryland,USA,seechapter6)inBangladesh,itappearsthatprovidingtracemineralsthroughtheapplicationofcrudeseasaltwassufficienttoensuregoodgrowthratesofduckweedsinpondedsystems.However,considerableinterest hasbeenshownby scientists in the capacitiesof duckweed to concentrate, in particular, copper, cobalt andcadmiumfromwaterresourceswherethesehaveeconomicsignificance.

Mineral nutrients appear to be absorbed through all surfaces of the duckweed frond, however, absorption of traceelementsisoftencentredonspecificsitesinthefrond.

Therequirementstofertiliseduckweedsdependsonthesourceofthewater.RainwatercollectedinpondsmayneedabalancedNPKapplicationwhichcanbegivenasinorganicfertiliserorasrottingbiomass,manureorpollutedwaterfromagriculture or industry. Effluent's from housed animals are often adequate or are too highly concentrated sources ofmineralsandparticularlybecauseofhighammoniaconcentrationmayneedtobediluted to favourduckweedgrowth.Runoff water from agriculture is often high in P and N but the concentration may need to be more appropriatelybalanced.SewagewastewatercanbehighorlowinNdependingonpretreatmentsbutisalmostalwaysadequateinKandP.IndustrialwastewaterfromsugarandalcoholindustriesforexamplearealwayslowinN.

Littleworkhasbeendonetofindthebestbalanceofnutrientstoprovidemaximumgrowthofduckweed.Theduckweedhasbeenprovidedwithmechanismsthatallowittopreferentiallyuptakemineralsandcangrowonverydilutemedium.Themainvariablesthateffectitsgrowthunderthesecircumstancesarelightincidenceandwaterandairtemperatures.

Thegrowthrateandchemicalcompositionofduckweeddependsheavilyontheconcentrationofmineralsinwaterandalsoontheirrateofreplenishment,theirbalance,waterpH,watertemperature,incidenceofsunlightandperhapsdaylength.Itsproductionperunitofpondsurfacealsodependsonbiomasspresentatanyonetime.

WATER TEMPERATURE

Duckweedsgrowatwatertemperaturesbetween6and33C.Growthrateincreasewithwatertemperature,butthereisan upper limit of water temperature around 30 C when growth slows and at higher temperature ceases. In openlagoons indirectsunlightduckweed isstressedbyhigh temperaturecreatedby irradicationand inpracticeyieldsareincreased bymixing the cooler layers of water low in the pond and splashing to reduce surface temperature of theduckweedmatt.

WATER, pH

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Table 1: The composition of duckweed harvested from anatural water source or grown on waters with minerals

DuckweedsurvivesatpH'sbetween5and9butgrowsbestovertherangeof6.57.5.EfficientmanagementwouldtendtomaintainpHbetween6.5and7.InthispHrangeammoniaispresentlargelyastheammoniumionwhichisthemostreadilyabsorbedNform.OntheotherhandahighpHresultsinammoniainsolutionwhichcanbetoxicandcanalsobelostbyvolatilisation.

MINERAL CONCENTRATIONS

Duckweedsappeartobeabletoconcentratemanymacroandmicromineralsseveralhundredfoldfromwater,ontheother hand highmineral levels can depress growth or eliminate duckweedswhich grow best on fairly dilutemineralmedia.Thereisamassofdataontheuptakebyduckweedofmicroelementswhichcanbeaccumulatedtotoxiclevels(foranimal feed).However, theirability toconcentrate traceelements fromverydilutemediumcanbeamajorassetwhere duckweed is to be used as an animal feed supplement. Trace elements are often deficient in themajor feedavailabletothelivestockofsmallresourcepoorfarmers.Forexample,incattlefedmainlystrawbaseddietsbothmacroandmicromineraldeficienciesarepresent.

Duckweeds needmany nutrients andminerals to support growth. Generally slowly decaying plantmaterials releasesufficient trace minerals to provide for growth which is often more effected by the concentrations of ammonia,phosphorous,potassiumandsodiumlevels.ThereisbyfarthegreatestliteratureontherequirementsofduckweedforNPKandtheabilityoftheplanttoconcentratetherequirementsofmicronutrientsfromtheaquaticmediumisusuallyconsiderednottobealimitation.IntheworkinBangladeshbyPRISM,crudeseasaltwasconsideredtobesufficienttoprovidealltracemineralrequirementswhenaddedtowaterat9kg/hawatersurfaceareawhenduckweedgrowthrateswerehighataround1,000kgoffreshplantmaterial/day.

WATER DEPTH

Depthofwaterrequiredtogrowduckweedunderwarmconditionsisminimalbutthereisamajorproblemwithshallowpondsinbothcoldandhotclimateswherethetemperaturecanquicklymovebeloworaboveoptimumgrowthneeds.However,toobtainasufficientlyhighconcentrationofnutrientsandtomaintainlowtemperaturesforprolongedoptimalgrowth rate a balance must be established between volume and surface area. Depth of water is also critical formanagement, anything greater than about 0.5 metres poses problems for harvesting duckweeds, particularly byresource poor farmers. Whereas, where water purification is a major objective in the production of duckweed, it isimpracticaltoconstructpondsshallowerthanabout2mdeep.

Incident sunlight and environmental temperatures are significant in determining the depth of water as undoubtedlyduckweedisstressedbytemperaturesinexcessof30Candbelowabout20Cgrowthrateisreduced.

In practice, depthofwater is probably set by themanagement needs rather than thepool of available nutrients andharvestingisadjustedaccordingtochangesofgrowthrate,climatechangesandthenutrientflowsintothesystem.

REQUIREMENTS FOR NPK AND OTHER MINERALS

Duckweedsevolvedtotakeadvantageofthemineralsreleasedbydecayingorganicmaterialsinwater,andalsotouseflushesofmineralsinwaterastheyoccurredwhenwetlandsflooded.Duckweedsnowappeartohavethepotentialtobeharnessedasacommercialcropforanumberofpurposes.

Wateravailabilityislikelytolimitterrestrialcropproductionparticularlyofcerealsinthecomingyears(seeWorldWatch1997).Waterpurificationandreuseparticularlythatwaterarisingfromsewageworks,industrialprocessingandrunofffrom irrigationappears tobemandatory in the future,both toreducepollutionofexistingwaterbodiesand toprovidereusablewaterformanypurposesincludingthatrequiredbyhumansinsomeplacesasdrinkingwater.

Nitrogen requirements

Duckweedsappeartobeabletouseanumberofnitrogenouscompoundseitherontheirownorthroughtheactivitiesofassociatedplantandanimalspecies.Theammoniumion(NH4+)appearstobethemostusefulNsourceanddependingon temperatures duckweeds continue to growdown to extremely low levels ofN in thewater.However, the level ofammoniaNinthewatereffectstheaccretionofcrudeproteinintheplant(seeTable1).

Thevalueofduckweedasafeedresourcefordomesticanimalsincreaseswithincreasingcrudeproteincontent.In

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enriched (Leng et al. 1994) Crude

ProteinFat Fibre Ash

Source (%DM) (%DM) (%DM) (%DM)

Naturallagoon

2535 4.4 810 15

Enrichedculture

45 4.0 9 14

Figure 3: The influence of the concentration of N in culturewater on crude protein in duckweed (Spirodela spp) grownon diluted effluent from a piggery. The P levels in watervaried from 1.26.1 mg P/litre (Leng et al., 1994).

Figure 4: Relationship between root length and proteincontent in duckweed (Lemna minor) (Le Ha Chau 1998)

studiesattheUniversityofNewEngland,Armidale,Australia,thecrudeproteincontentofduckweedgrowingondilutedeffluentfromhousedpigsincreasedwithincreasedwaterlevelsofNfromabout15%crudeproteinwithtracelevelsofN(14mgN/l)to37%atbetween1015mgN/l.Above60mgN/latoxiceffectwasnoticedperhapsduetohighlevelsoffreeammoniainthewater.Whilstfewexperimentshavebeenundertakenontheoptimumlevelofammoniarequired,theseresultsgiveaguidelineforthelevelsofNtobeestablishedandmaintainedinduckweedaquaculturetoobtainaconsistentlyhighcrudeproteinlevelinthedrymatter.

InmostpracticalsituationstheapproachtogrowingduckweedistofindthedilutionofwaterwhereNisnotlimitinggrowthandsupportshighlevelsofcrudeproteinintheplant.Thisisusuallydonebyanarbitrarytest.Serialdilutionsofthewatersourcewithrelativelypurewater(rainwater)iscarriedoutandduckweedseededintoeachdilutionandweightchangerecordedafter,say,4weeks.InthiswaytheappropriateNconcentrationisestablished.

Ausefulindicatorofwhetherconditionsinthepondareappropriateforgrowthofduckweed(Lemnaspp)ofhighproteincontentinthelengthoftheroots.Manyexperimentalobservations(RodriguezandPreston1996aNguyenDucAnhetal1997LeHaChau1998)haveshown

thatovershortgrowthperiodsthereisaclosenegativerelationshipbetweenrootlenghtandproteincontentoftheduckweedandwiththeNcontentofthewater.DatatakenfromtheexperimentofleHaChau(1998)areillustratedinFigure4.Inmostsmallscalefarmsituationsitisnotfeasibletodeterminetheproteincontentoftheduckweedthatisbeingusednorcanthenutrientcontent(especiallynitrogen)ofthewaterbeestimatedeasily.Todeterminetherootlengthofduckweedisasimpleoperationandandrequiresneitherequipmentnorchemicals.Bymonitoringthischaracteristic,theusercanhaveanindicationofthenutritivecorrectivemeasureswhenthelenghtoftherootsexceedsabout10mm.

InduckweedaquacultureasourceofNessentialandinmanystartupsystems,basedonwatereffluentfromsewageorhousedanimals,theprojecthasbeenconsideredbypretreatmentsthatdenitrifythewaterandreduceammoniaconcentrations.Mostformsofaerationinsewageworksarehighlyefficientindenitrificationofwastewaters,butthisprocesscompoundspollutionpeoblems.forinstancewheretheeffluentishighinPthispromotesthegrowthofalgaethatfixN.InAustraliathecontaminationofriversystemswithphosphorusoftenledtomassivebloomsofbluegreenalgaethatraetoxictohumansandanimals.

AlthoughthereisanassociationofNfixingcyanobacteriawithduckweeds,thesearecertainlynotimportantfromastandpointoffarmingduckweeds.DuongandTiedje(1985)wereabletodemonstratethatduckweedsfrommanysourceshadheterocystouscyanobacteriafirmlyattachedtothelowerepidermisofolderleaves,insidethereproductivepocketsandoccasionallyattachedtotheroots.They

calculatedthatNfixationviathesecoloniescouldamountto3.77.5kgNperhectareofwatersurfaceintypicalLemnablooms,buttheassociationofcyanobacteriawithLemnatrisulcawas10timesmoreeffective.

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Figure 5: The effect of N level in culture water on growth of duckweed and its crudeprotein content.The experiments were conducted on duckweed collected from abillabong down stream from a sewage works. The sewage water used in theincubation was taken from that flowing into the sewage works prior todenitrification processing. The pond were 2.5m. (Stambolie & Leng, 1994)

(a) Crude Protein induckweed (b) Changing biomass on

waterc)Level of N in water

Figure 6: Yield and crude protein content of duckweed biomass growing on sewagewaste water (Sutton & Ornes 1975)

Probably,undermostpracticalsituationsammoniaistheprimarylimitingnutrientforduckweedgrowthandtheestablishmentoftheoptimumlevelformaximumgrowthofduckweedsneedsresearch,particularlyinthevarietyofsystemstheplantmaybeexpectedtogrow.TheeffectsoftimeandloweringofNcontentofsewagewateronyieldandcrudeproteincontentofduckweedisshowninFigure5andFigure6.

Fromrecentresearchitappearsthatduckweedrequireabout2060mgN/ltogrowactivelyandfromtwostudies[(thoseofSutton&Ornes,(1975)comparedwiththoseofLengetal.,(1994)]itisapparentthatthereisacomplexrelationshipbetween,theinitialcompositionoftheduckweedusedinresearchandthelevelofnutrientsrequired.

StambolieandLeng(1993)showedwithduckweedsharvestedfromabackwaterofariverandwithaninitiallowcrudeproteincontent,itwasonlywhentheduckweedproteinincreasedtothehighestlevelthatrapidgrowthofbiomasscommenced(i.e.at3weeksafterintroductiontothewater)(Figure6)eventhoughbythattimetheNcontentofthewaterhaddeclinedtolevelsthatwerebelowtheoptimumthatappearstobenecessaryformaximumproteinlevels(Figure3).

IntheworkofSuttonandOrnes(1975),however,duckweedofahigherproteincontentwasinitiallyusedandgrowthrateagainpeakedataboutthethirdweek(Fig.6)butbythistimethecrudeproteincontenthaddeclinedtobelow15%.Thisapparentoppositeresultcanberationalisedifthereisastressfactorinvolvedwhichrequires3weekstoovercome,andunderthesecircumstancesitsgrowthmaybeconsidered

tobeoptimisedatabout20mgammoniaN/lbuttoobtainmaximumcrudeproteincontentitrequiresammonialevelstobeabout60mgN/l(seeFigure5).Afurtherimplicationisthatwhereahighproteincontentispresentinduckweedatthecommencementofagrowthstudy,theduckweedcangrowthroughmainlysynthesisofonlycarbohydrate.However,thevariableresultsusingduckweedsharvestedfromthewildandtheslow"adaptation"tonewconditionsisobviouslyaconfusingfactorininterpretinganydataofthe

requirementsforduckweedfornutrientsinsuchshorttermstudies.

Themostimportantissueisthatduckweedincreasesitsproteincontentaccordingtotheammonialevelinanotherwiseadequatemediumuptolevelsof60mgN/l(seeFigures3,4,&5).Forfoodorfeedpurposesthereisavastdifferenceinthe value of duckweed biomass depending on its protein content (see later). Rapid growth of duckweed is alsoassociatedwithhighproteinaccretionandlowfibrecontentandfibrecontentincreaseswhererootgrowthoccurs.

Phosphorus requirements

DuckweedsappeartoconcentrateP up to about 1.5% of their dryweight and as such are able togrow on high P waters providedthe N concentrations aremaintained. The plant alsoappearstobeabletodrawonthepool of P in its biomass for itsbiochemicalactivitiesandonceP

hadbeenaccumulateditwillcontinuetogrowonwatersdevoidofP.OntheotherhandthePinduckweedappearstobehighlysolubleandisreleasedrapidlytothemediumondeathoftheplant(Stambolie&Leng1994).

TherelationshipbetweenPcontentofsewagewaterandPcontentofduckweedsgrowingonsuchwaterareshownin

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Figure 7: The relationship between the quantityof P in duckweed and the concentration of P inwater. Filled squares are results from Suttonand Ornes (1975) the filled circles (uppervalues) are results from research in Australia(Stambolie & Leng 1994).

Figure6.Lengetal (1994) foundahigherconcentrationofP induckweedathighwaterP levels than that foundbySuttonandOrnes(1975).ThetimecourseofuptakeofPbyduckweedinstaticsewagewaterisshowninFigure6.Thedifferencesinaccumulationlevelsinthetwostudiescitedpossiblyresidesintheratesofgrowthwhenthesamplesweretaken.ThecapacityofduckweedtoconcentratePisclearandmaximumPlevelsintissues(1014mgP/kgdryweight)areachievedwithwaterPlevelsaslowas1.0mgP/litre.

The important issuehere is that duckweeds concentratePwhenwaterlevelsareenrichedwithPanditappearstobereadilyavailableoncetheplantisdisruptedordies.ThePlevel induckweedissufficientlyhightobeavaluablesourceofthisnutrientforbothplantsandanimals.

Potassium requirements

Vigorously growing duckweed is a highly efficient K sink, but only lowconcentration of K in water are needed to support good growth whenothermineral requirements are satisfied.Most decaying plantmaterialswouldeasilyproducetheKrequirementsofduckweed.

Sulphur requirements

LittleworkhasbeendonetoexaminetheSrequirementsofduckweeds.Themechanismsforsulphateuptakehavebeenstudiedsinceuptakeofsulphate is the first step in the biosynthesis of Samino acids. Suchbiosynthesisneedstheintegrationofpathwaysprovidingcarbonbuildingblocksandreducedsulphur(Datko&Mudd,1984).ItispossiblethatSlevelsareattimeslimitingtogrowthorproteinaccretionbecauseofthehighlevelofSaminoacidsintheplantwhengrowthrateishighandammoniainthemediumisnonlimiting.Saltsofsulphateappearstomeettherequirements.AsSissoreadilyleachedfromsoilsit isanunlikelycandidate for deficiencies in systems thatmay be established to farm duckweed exceptwhere huge dilutions of thewaterareneededtoobtainasuitableNlevel.

TheuptakeofNPSKbyduckweedfromsewagewaterisshowninFigure7andtheexperimentaldesignforsuchstudiesareshowninPhoto4.

Sodium requirements

Seasalt(9kg/ha/d)hasbeenappliedaspartofafertiliserprograminpilotstudiesofduckweedfarminginBangladesh(see the discussion of PRISM'swork inChapter 6). Thiswork suggested a good ability of duckweed to accumulatesodium as therewas no apparent problemswith salination. It appeared possible that duckweed removed up to 9kgsalt/ha/dwhengrownunderfairlyoptimalconditions,suggestingapotentialforduckweedtorehabilitatesalinelandandwater.

In studies undertaken at the University of New England itsoonbecameapparentthattherequirementforsaltandthecapacity of the plant to concentrate sodium was notsignificant in relation to salt levels that accumulated inlagoonsfedbyopencutcoalmines.However,theexercisepointed a way for the potential use of such waters forduckweedproductionasduckweedstoleratedthesaltlevelsand grew substantially when additional nutrients wereprovided.Using small galvanised iron tanks (seePhoto 4)theeffectofgrowingduckweedonsalineminewaterswithor without extra added nutrientswas studied.Growth rateand protein content of duckweed is shown in Figure 8together with the effects on mineral levels in the water.Duckweed grew on the water with or without addedfertilisersbut theuptakeofsodiumwas low.Thequalityofthe duckweed (indicated by its crude protein content)wasmaintained for some time by fertiliser application. The

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Photo 4 : Small scale containers used for duckweed growthstudies. Sewage water, collected at a site where it is flowinginto the works, was transported, diluted and used for thegrowth trials. Duckweed was seeded onto the "ponds" sothat half the surface area was covered and harvests weremade when the "ponds" were 100% covered. Duckweed washarvested by placing a stick across the diameter of the pondand taking half of it.

phosphorousrequirementforgrowthwasapparentlylow.

Thisdataisintroducedheretoshowthatevensalinewaterscan be used to grow duckweed, although research isneeded to investigate theneeds foradditionalnutrientsonsalinewaters.

Figure 8a. The effects of growing duckweed on salinemine waters with (D ) or without (o) added NPK fertiliserto optimum levels on crude protein and salt content.(Sell et al., 1993 Sell, 1993).

Figure 8b. The effects of growing duckweed on salinemine waters with (D )or without (o) added NPK fertiliserto optimum levels on dry matter harvest and P content.(Sell, et al 1993, Sell, 1993).

8(b) Sodium content 8(a) Crude protein of duckweed

8(c) Duck weed dry matter harvested: cumulative and residual pool

8 (d) Phosphorus content of mine waste water growing duckweed

8 (c) Dry matter 8 (d) Phosphorus content

Conclusions on mineral requirements

The well developed system of concentrating minerals in duckweed allows them to grow under a wide range ofconditions. The concentrating ability of duckweed for trace elements has been estimated to be many hundreds ofthousands times. A simple comparison indicates some of the potential for duckweed to accumulate nutrients bycomparingwaterlevelsandtissuedrymatterlevelsofanumberofminerals.

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Table 2: Some mineral compositions of duckweed and their potential to remove minerals from water bodies(calculated from the literature).

Concentrationin Potentialremoval

Culturemedium

Duckweedtissue

at10tonDM/ha

Element (mg/l) (mg/kgDM) (kg/ha/y)

N 0.75 60,000 600

P 0.333.0 5,00014,000

56140

K 100 40,000 400

Ca 360 10,000 100

Mg 72 6,000 60

Na 250 3,250 32

Fe 100 2,400 24

Alongwith this advantage ofmineral removal is obviously the potential detrimental effects of accumulation of heavymetals.

Heavy metal accumulation by duckweeds

Allmembersoftheduckweedfamilyconcentrateheavymetalsinparticularcadmium,chromiumandleadwhichmayattimesreach levels in theplantwhicharedetrimental toboth thehealthandgrowthof theplant inaddition tocreatingproblemswheretheplantisusedinanypartofafoodchaineventuallyleadingtohumanconsumption.

Theaccumulationofheavymetalsbyduckweed isnotnormallyaproblem for thosewishing touseduckweeds fromnaturalwaterresourcesoreffluentfromhumanorintensiveanimalhousingasthesemetalsarenormallyatextremelylowconcentrations.

Duckweeds,however,arecontaminatedbysuchheavymetals from industriessuchas tanning (chromium) leachatesfrommining(e.g.cadmium)andgreatcareisneededwherewateriscontaminatedtobesurethatheavymetalsdonotgetintothehumanfoodchain.

Ontheotherhand,duckweedsmayfinduseinstrippingheavymetalsfromindustrialwater.Alsotheircontentofheavymetalscanbeusedtoindicatepotentialpollutionlevelsofwaters.

Cadmiumappearstobeabsorbedbybothlivinganddeadduckweedplantsandthecadmiumisactivelytakenupbytheplant (Noraho & Gour 1996). Cadmium at high concentrations, that is the concentration that prevents vegetativereproduction(EC50)wasfoundtobe800ppbbutduckweedsgrowninmediumof2.2ppmstillaccumulatedmostofthecadmiumover7dandwhenfedtocrayfishincreasedcadmiuminthehepatopncreata26foldandinmusclesalmost7fold(Devietal.,1996).Itistherefore,extremelyimportanttobesureoflowcadmiumlevelsinwaterpriortoanylargescaleuseofduckweedsasfeedfordomesticanimalsorhumans.

Many reports are available on the uptake of metal ions by duckweeds and the numerous interactions that occur.DuckweedswilluptakeandconcentrateCd,N,Cr,Zn,Sr,Co,Fe,Mn,Cu,Pb,AlandevenAu.Toattempttodefinetheratesofaccumulationisnot importanthere,excepttopointoutthatasthelevelsofthesemineralsrisetohigherthannormalingeneraltheymaydirectlyinhibitgrowthoftheplantandanyanimalthatconsumesignificantquantities.Atlowlevelaccumulationtheplantsbecomeaveryusefulsourceoftracemineralsparticularlyforlivestockandfish.

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Problemsofheavymetalcontaminationobviouslyarisewhereduckweedsgrowon industrialandminingwastewherethecontaminatingelementsareknownandthereforetheproblemshouldbeapparentfromthebeginningofanystudy.

Inconclusion,itisonlywhereheavymetalsarewashedoutineffluentsfromindustryandminingthatthereispotentialforduckweedstobecometoxictolivestock,andinthesesituationsduckweedsharvestedfromsuchsourcesshouldbedisposedofdifferently.Amostusefuldisposalmethodbeingasamulchfornonfoodcropssuchastrees.

MEETING MINERAL REQUIREMENTS

Fertilisers

AcommercialbalancedNPSKwithseasalttoprovidetracemineralscanundoubtedlybeusedwithrelativeunpollutedwaterstomeetthegrowthrequirementsofduckweeds.IntheMirazapurproject,(seechapter6)muriateofpotash(KCl),ureaandsuperphosphate(supplyingP+S)weresuccessfullyusedtoproduceduckweedsoninundatedlandsthatalsocollected theeffluentwaters from the localhospital.Fertiliserswereappliedonadailybasis,which togetherwith theneedforregularharvestinghadahighlabourcost.Seasaltwasaddedasasourceoftraceminerals.

Manure

Slowdecompositionofmanureandotherorganicmaterials aregoodways to continuously supplyawater bodywithnutrients required for duckweeds to grow. The skill here resides in somehow controlling the nutrient inflow into thegrowthponds.Inmanyinstancesthiscanbeestablishedbytrialanderror.ThishasbeenapparentlyhighlysuccessfullyinVietnamwhere,commercialproducersofduckweedusedruminantandpigexcrement.Thiswascollectedinasmallsettlingpond,thewaterfromwhichthenpassesthroughaseriesofduckweedpondsbeforeeitherenteringtheriverorbeingusedforirrigation.Thesesystemsappeartoworkbecauseoflongestablishedexperiencewithgrowingduckweed.Theseriesofpondsallapparentlyproducedagoodharvestofduckweed.

AfurtherextensionofthismethodwasseeninBangladesh.Thesystemwasbasedonasimpletoiletblock,whichmaybejustaholeinaconcreteslabfromwhichhumanexcrementcouldbedirectedbygravitythroughaplasticpipeintoabasket (usually split bamboo) situated at the centre of the pond and from which nutrients slowly diffused into theduckweedpond(seePhoto6).

Manure and biogas

The effluents from biogas digestors, suitably diluted are very effective media for growing duckweed. These can beextremelysimplesystems,easilyincorporatedintoasmallfarmingareasbasedonhomebiodigestors,constructedfromplastic (seePhoto6) through to industrialsizebiodigestorsmadeofmetal. Inallcases theexcrementpluswashingsfromanimalsheldunderpennedconditionsarecollected,heldinsomeformofsettlingpondtoremovesolidsandthenintoanenclosedcontainerwhichallowsanaerobicmicrobestogrowandconverttheresidualcarbohydratestocarbondioxide andmethane. The gaseous effluent containingmethane and carbon dioxide is collected and combusted forvariouspurposesincludinghouseholdcooking.Thewaterleavingthebiodigestorretainsthemineralsandwithsuitabledilutionisagoodmediafortheduckweedfarm.

Photo 5: A young boy harvests duckweed as a protein source for ducks in Vietna

Figure 9: Schematic representation of present duckweed framing in Vietnam.

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Photo 6. Duckweed mats fed from faecal materials through a small basket which collects the solids in themiddle of the pond

BiodigestoreffluentfromanimalproductionisusuallypHneutralandhasarelativelyhighammoniacontent.Themineralcomponentofthedieteffectsthelevelsofnutrientsinthewaterandthereforetheneedtodilutetheeffluentdependsontheanimal's diet.Ammonia treated straw results in aneffluent fromcows fed this that is high in ammonia.A simplesystemisshowninFigure10.

ThesystemadvocatedbyDr.Preston,(Photo7)isrelativelysimplytoapplyonsmallfarms.Acowandcalf,mainlyfedcrop residuesprovidebothurineand faeces toabiodigestorsuitablydilutedwithwashwater.Thebiodigestor in thiscaseisasimplepolyethylenetubeplacedinagroundpit.Thewashingsfromthestallenterthedigestorandhaveahalftime of 1015 days during which timemost of the organicmatter is converted to carbon dioxide andmethane. Theeffluentisdilutedandrunintonarrowplasticlinedchannelsorconcretecannelsinwhichtheduckweedsareseededandgrowforseveralweeks,harvesting theduckweedoccursevery fewdays.Theduckweed is then fedeither freshasasupplementtopigsandpoultryorissundriedforthesameuse.

Figure 10: Diagrammatic representation of flow of nutrients through simple biogas digestor to feed duckweed.

Photo 7: Duckweed growing on small plastic lined containers fed by biogas digestor fluid

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Another system that has been proposed uses the effluent (washings) from large numbers of housed animals underintensivemanagementorfromabattoirs.Theeffluentischannelledthroughalagooncoveredwitha510cmthickplasticfilmandmethaneiscollectedfromaconvenientsitebeneaththeplastic.Theeffluentbeingrunintoponds,dilutedandduckweedproducedontheeffluent(Figure11)

Miscellaneous systems

Wherever there is an effluent of pollutedwater associatedwith industry or agriculture there is potential to purify theeffluentwaterswithduckweeds.Eachprocess,however,requiresparticularattentionanditisbeyondthescopeofthispresentationtomakerecommendationhereforsuchsystems.SomeexamplesofwhereduckweedcleansingsystemsmighthaveapplicationarelistedTable3.

Figure 11: Schematic Diagram showing abattoir or intensive animal production waste processing and biogasflow

Table 3: Some examples of where duckweed might be used to cleanse wastewater of mineral pollution andproduce a feedstock of duckweed biomass

1. Effluentsfrom:

DairiesPiggeriesCattleandsheepfeedlotsUrbansewageIndustrialwastefrom:

Brewing&alcoholproduction(solubles)MilkprocessingSugarfactoriesStarchfactoriesWoolscouringAbattoirs&tanneries

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Foodprocessing

1. Runofffrom:

Agriculturalpractices

CottongrowingSugarindustryBeefandsheepgrazingindustry

HorticultureandnurseriesMining

SodiumrunoffwaterHeavymetalsinotherminingactivities

Parksandsportingfacilities

Sewage

PossiblythebestcasecanbeputfortheuseofduckweedstoremovePfromhumansewagewhichismostlycollectedstrategicallyatapointsiteinatownshipandalthoughtreatedtovaryingdegreesisoftenfinallyexportedviariverstothesea. There are now a number of commercially viable duckweed based sewage systems that have been developed.These systems are expensive because of the obvious need for high technology to ensure success in treatment. Itappears,however,thatboth

chemicalandmicrobiologicaltreatmentplantsaremuchmorecostly.Ontheotherhandtheuseofaquaculturedoesnotnecessarily replace such systems and they can often be incorporated into or added on to a number of sewagepurificationplants.However,insmallcommunitiesinthetropicsthecultivationofduckweedonlagoonsmaybetheonlytreatmentnecessaryforsimplesewagetreatment.

RECORDED YIELDS OF DUCKWEEDS

Theliteraturecontainsagreatdealofinformationonthepotentialgrowthratesofduckweeds.Inmanyearlystudiesthegrowthratesweremeasuredundercontrolledconditionsforshortperiodsoftime.Intheabsenceoflargescalefielddataobtainedover12monthperiodsthesedatahavebeenusedtoestimatepotentialproductionrates.ThedataneedstobetreatedwithreservationsasthedatainFigures4and5pointtoseriousproblemsindoinggrowthtrialswithduckweedunderlaboratoryconditions.

The results in Table 4 are from researchwith near optimum conditions for duckweed growth. Landolt and Kandeler(1987)concludedthatundersuchconditionsa73tonofdrymatterarepossiblyproducedperhectareperyearor20gDM/m2/d.Resultsupto180tonDM/ha/yhavebeenrecorded.Underlessthanoptimumconditionsitismorerealistictotargetbetween5and20tonDM/ha/y(Table5).

Inpracticetheyieldsofduckweedoftendependontheskillofthefarmerinsolvingtheproblemofhowtobalancethemineral requirements of duckweeds and to identify with time the need for continuing and varying mineralsupplementation.WatersthatarehighinPandKandtraceelementneedminimalbutrepeatedinputsofanammoniasource,keepingammoniaataroundthe60mgN/lwhengrowthandproteinaccretionisgreatest.

Table 4: Field results of duckweed growth in nearoptimal conditions

Location DMYield

Source

(t/ha/yr)

LouisianaUSA 4455 Mesteyeretal(1984)

LouisianaUSA 2738 Mesteyeretal(1984)

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LouisianaUSA 182.5 National Academy ofScience(1976)

Southern StatesUSA

54 Said in Mbagwu andAdeniji(1988)

Southern StatesUSA

20 National Academy ofScience(1976)

Israel 3651 Oranetal(1987)

Israel 39 Heppher in Landolt et al(1987)

Table 5: Field results of duckweed growth in suboptimal conditions.

Location DMYield Source

(t/ha/yr)

Thailand 10 HassanandEdwards(1992)

Thailand 11 HassanandEdwards(1992)

Thailand 10 BhanthumnavininLandoltetal(1987)

Israel 1017 Porathetal(1979)

Russia 78 Rejmankova in Landolt et al(1987)

Uzbekistan 715 Taubaev et al in Landolt et al(1987)

Germany 22 NationalAcademyofSciences(1976)

Germany 16 SchultzinLandoltetal(1987)

India 22 Rao et al in Landolt et al(1987)

Egypt 10 ElDininLandoltetal(1987)

LouisianaUSA

9 CulleyandEpps(1973)

LouisianaUSA

20 Russoffetal(1980)

FloridaUSA 513 ReddyandDeBusk(1985)

FloridaUSA 1721 ReddyandDeBusk(1985)

FloridaUSA 13 DeBusk et al in Landolt et al

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(1987)

FloridaUSA 19 Stanley et al in Landolt et al(1987)

FloridaUSA 23 Culley et al in Landolt et al(1987)

FloridaUSA 1427 MeyersinLandoltetal(1987)

FloridaUSA 214 SuttonandOrnes(1977)

Yieldofduckweedwilldependonhowthefarmermonitorstheduckweedsystemandwhetherahighproteinmealistheobjective. IndustrialwaterscanonlybedischargedwhentheP levelshavebeendepletedbyconsistentcroppingandfertilisationwithureaasammonialevelsdecreaseandbecomelimitingtogrowth.Howoftentofeedureaintothesystemandhowlongcanduckweedgrowthbemaintainedcanonlybeunderstoodfromresearchinthelocality.

Wheretheproductionofcleanwaterisamajorobjectivethenitbecomesnecessaryoftentobalanceothernutrientsaswellasammonia inorder toendupwithwater thathashad its totalmineralcompositiondecreasedto levels thatwillallowitsreuse.

InTable6theresidualPinwashwaterfrompiggeriesshowsthatwithskill,anumberoffertilisationswithureafollowedbyharvestingofduckweedcouldresultinverylowPlevelsoccurring.RechargingammonialevelscouldbeexpectedtostimulategrowthsothatPlevelscandropbelow1mgP/litre.

Density of duckweed and yield

Thegrowthrateofduckweedunderideallight,temperatureandpHwouldbeexponential iftherewerenolimitationintermsofmineral(includingammonia)deficienciesorexcesses.However, inpracticemanyissuesreducethebiomassyield.Oneofthemostimportantisobviouslyplantdensity.Therateofharvestingduckweedisimportantsincethereisaminimumbiomassatwhichyieldswilldecreaseandanupperbiomasswhereyieldwillbelimitedbycrowding,allothervariables being equal. In a study where most of the conditions for growth were unlimited the effect of harvestingindicatedthat,aboveabout1.2kg/m2duckweed(fresh)growthdecreasedandbelow0.6kg/m2

Table 6: Removal of nutrients by Lemna from a flow through pond fed by aerated piggery waste water (Institutode Investigaciones Porcinas, Havana, Cuba (unpublished observations)

Concentration(mg/l)

Inflowwater Outflowwater

COD 461 323

BOD 51 30

TotalN

42 21

NH3 17 2.2

TotalP

6.4 3.3

duckweed(fresh)biomasslimitedgrowthpotential.Itappearedthatif1.0kgfreshduckweed/m2couldbemaintainedby

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frequent harvesting then an extrapolated yield of 32 tonnes DM/ha/yr could be produced under other nonlimitingconditions.ThedataisshowninFigure12.

Figure 12: The range of densities of duckweed biomass on the water surface after harvesting at whichduckweed grows optimally (Stambolie and Leng, 1994). In this case the average yield was 32 tonnes DM/ha/year.The upper density (filled squares) appears to be that at which crowding limits growth and the lower density(unfilled squares) is the density at which growth is insufficient to prevent algal blooms (Stambolie 1994 reportedby Leng et al 1994)

CHAPTER 4: Integrated farming systems

WHY DO DEVELOPING COUNTRIES NEED TO EXAMINE THE POTENTIAL FOR INTEGRATED FARMINGSYSTEMS?

Integratedfarmingsystems,solongastheyimprovesoilfertility(orat leastmaintainthesamesoilfertility) inthelongtermhavemajoradvantageswhichcanimproveboththeoverallproductionoflandwithoutlosingsustainability.

TheWorldCommissiononEnvironmentandDevelopmentdefinessustainabilityas:

"ensuringthatdevelopmentmeetstheneedsofthepresentwithoutcompromisingtheabilityoffuturegenerationstomeettheirownneeds".

However,developmentopportunitiesandaspirationschangewithchangingeconomicconsiderations.Majorincreasesinthecostof foodproduction is likely toarisewherecostof fuel (fossil) increases relative to income.Fuelpricesmustsurelyincreaseinthefutureinresponseto:

increasingdepletionofworldreserves(Fleay,1996)asaresultofeconomicdecisionstakenatgovernmentleveltoreducetheircountriesfueluseandreduceglobalwarming.becauseofeconomicdownturnwhichputsenormouspressureongasolineprices.

Thecostoffoodproductioninacountryishighlydependantonfuelprices,andfoodpricesinthe1998Asianfinancialcrisisroseandmustcontinuetorise.

Thetwoimportantagriculturalcostfactorsthatwillbeeffectedmostare,mechanisation,wherefuelisdirectlyconsumedincropfarming,andfertiliseravailabilityandapplicationsincethecostofNPKishighlyrelatedtogasolineprices.Thefuelcrisis inCubabroughtaboutbyremovalofeconomicsupport fromRussiaandtheembargobytheUnitedStateshasseenareturntoanimaltractioninthepast6yearsandamassivedeclineincropyieldsthroughdecreaseduseofinorganicfertilisers.

Farms export considerablemineral nutrients in products and also effluent frommany sources. In the future and forcontinuingsustainablefoodproduction,thesemineralsmustbereplaced.Inindustrialisedfarmingsystemsthisisdone

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largelybyinorganicfertilisersproducedanddeliveredatanincreasingcostoffossilfuelcombustion.

HighleveluseofNPKhaveresultedinthesustainedyieldsoffeedcropsinindustrialisedcountriesandinthelast20yearsgreatlyimprovedyieldsindevelopingcountrieswheretheseinputshavebeenusedtogetherwithimprovedcropvarieties.

Theothermajorissuesintermsofcropproductionhasbeentheincreaseduseofwaterresources,someofwhichareirreplaceable. Levels of fertiliser andwater application are almost always in excess of plant needs andwater runoffcontaminatedwithmineralshas createdgreat problemswith salinationandeutrophication in river andpondsystemsthroughouttheworld,changingtheaquaticecologyofwholeregionsinplaces.

Integrated systems are aimed at minimising (or preventing) loss of nutrients from a farming system and in manysituationsconservingwaterforreuse(Preston&Murgueitio,1992).

Integrated farming systems to be employed by small farmers in developing countries, require considerable skills inoperationinorderforthemtobeeconomicand/orsustainable.Integrationmaybedevelopedonasinglelandholdingormay bemore easily applied where a number of farms combine their requirements to develop an integrated systemwherethemineralsleachedfromthelandbyfarmingarereturnedtothelandviagoodconservationpracticesinvolvinganumber of farms. In Australia the LandCareMovement involves usually a number of landholders to combine theirefforts to conserve awhole catchment area.Similarly in India a catchment area approach to sustainability has beenimplementedthroughICRASAT.

Integratedandsustainablesystemsmustbedevelopedinordertopreventlanddegradation,minimiseexternal(costly)inputs,conserveresourcesotherwiselostthrougheffluentsandpotentiallyincreasetheincomeandstandardoflivingofthefarmerandatthesametimemaintainthefertilityoftheland.Integratedfarmingsystems,thatarealsosustainablerequirethat:

depreciationofmineralswithinthesystemareminimisedand/oreliminated(thisItermnutrientrecapture).replacementofminerals,exportedinproducts,bysourcesgeneratedonthefarmorfrombyproductsofagroindustries(e.g.mineralsinwaterfromindustriessuchassugarproduction,fertiliserproductionorfromcommercialbiogasdigestors).

RECYCLING OF NUTRIENTS

Integrated systems were traditional in most developing countries prior to the "green revolution" and many ancientsocieties recognised and put into practice sustainable cropping systems often through application of taboos againstpractices that caused degeneration of food supplies. In more recent times (say the 1920's) this took the form ofintegrationofcropandanimalproduction.Theanimalsbeinganintermediateinconversionofcropresiduesandotherwastes to dung which was then returned to the land. In some countries composting and biogas digestors wereinstrumental inrecyclingnutrientswithinthefarm(s).Thenetresultwasthattheminerals inbiomassproducedonthefarmwererecycledbyreincorporatingthenutrientsbacktothelandviahumanoranimalexcrement.Oftentheanimalwas a draught animal which is now being replaced by tractors. However, even in these systems effluent loss wasconsiderableinwaterrunoffandinproductwherecropsandanimalproductsweresoldfromthefarm.

FIXATION OF N AND MOBILISATION OF P FROM PLANT GROWTH

Within the integrated farmingsystems,strategies toencourageN fixationand for increasingPavailabilityareprimarytargets.

Naccumulation in landormaintenanceof levels throughN fixingplants (e.g. legumes)or theextraction fromeffluentwatersbyaquaticplantsarestrategiesthatwereusedbysmallfarmersonlyafewdecadesago.SimilarlyPfertilisershaveoftenbeendevelopedfromaquaticplants.Forexample,inKashmir,aquaticbottomgrowingplantsareharvestedfromthelakesforuseasfertilisers,andseaweedshavebeenusedwhereevertheyarewashedashoreforapplicationtosoils.

Ruminantsingeneral,iftheygrazenoncultivatablelandharvestconsiderableNandPandthisthencanfinditswayintocropsviamanureandthereforepotentiallyavoiddownstreamproblems.

Themajorconstrainttotheestablishmentofintegratedfarmingsystemsisthelevelofmanagementthatmustbeexerted

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bysmallfarmer.Thiscanoftenbebeyondhispresentlydevelopedskills.Anexceptiontothisstatementhowever,wasseeninVietnamandBangladeshwherethecollectionofallanimalandhumanwastesintopondsandthesubsequentgrowth of duckweeds has proven to be relatively free of problems and is very skilfully managed by a number ofcooperatingfarmers.However, if thesetropicalsystemshadtobemanagedforproductionofqualitywateraswellasfeedforducksorfishagreaterdegreeofcontroloftheduckweedgrowthwouldneedtobeexerted.

INTEGRATED FARMING (THEORETICAL CONSIDERATIONS)

Oneofthemajorreasonsforthedevelopmentoflivestockproductionincroppingsystemsindevelopingcountrieswastoutilisecropresiduesefficiently,thus,eliminatingwasteandoptimisingtheuseofthetotalbiomassproducedwithinthesmallfarmsystem.Thesmallfarmerhasoftenarequirementfordraughtpowerwithanimalproducts(milkandmeat)assecondaryconsiderations.

Integratedcropandlivestockproductionsystemscanbehighlyefficientpotentiallycropresiduesareusedaslivestockfeedthewasteproducts(e.g.faecesandurine)arefedintoabiogasdigestorandtheeffluentusedtofertilisepondsforaquaticplant/algaeproduction,with fish farmingas the terminalactivity.Thesesystemsareworthwhilepursuingasameans of providing nutrients/fuel for the family, minimising fuel combustion and reducing environmental pollution(Preston,1990).

The array of integrated strategies that could be developed is large. They all have as a central core a basic flow ofnutrients through a number of systems. At each of these steps research can be brought to bear to optimise thepartitioningoftheavailablebiomassintofood,fuelandresidues(seeFigure13).Theenvironmentalattributesofsuchsystems are themethane emissions into the atmosphere and fuel (fossil fuel and fire wood) use areminimised. Inadditiontheefficientandalso

Figure 13: Flow diagram showing the potential recycling of feed and faeces biomass from crop residues in anintegrated farm.

total harnessing of the energy from high producing crops reduce the land areas required per unit of product (seePreston,1990).

Acompletediscussionofthesesystemsisbeyondthescopeofthisdocumentbuttwoexamplesare:

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Theuseofaquaticplants/algaesgrownonbiodigestoreffluentforproteinproductionforpigs,poultry,ruminants,rabbitsandhorsesparticularlyinthehumidtropicsandThefarmingofduckweedsonbiogasdigestoreffluentstofeedfish.

Figure 14: An example of an integrated farming system based on sugar cane and forage trees fractionated toprovide feed for pigs and poultry (the juice and tree leaves), sheep (the cane tops and tree leaves), fuel for thefamily (bagasse and firewood) and litter for sheep and earthworms (bagasse), with recycling of excreta throughbiodigestors to provide fuel (biogas) and fertiliser (the effluent) for water plants in ponds and for the crops(Preston, 1990)

INTEGRATED SYSTEMS INVOLVING DUCKWEED PONDS

ThepublicationofabookletbySkillicornetal.(1993)onduckweedaquaculturebasedontheexperiencesofaprojectinBangladeshoperatedbyPRISMstimulatedconsiderable interest theuseofduckweed.. InCuba,major researchhasbeendevelopedasa resultof the leadworkof thePRISMgroup. It isof significance thatboth thesecountrieshaverelativelyhighpricedfuelandinBangladeshdraughtanimalpower isstill themainfarmpower,whereas inCuba,thefarmers have reverted to draught animal powermainly because of the increased price of gasoline. Neither country,however,hasbeenabletoefficientlyestablishintegratedfarms.Inbothcountriestheobjectivebehindtheresearchonduckweedhasbeentoprovidefoodforcarpand/ortilapiaproductionwithsomespinoffforpig/poultryproduction.

Asystem,thatappearsreadytobeputstraightintofarmpracticearisesfromtheworkofDr.PrestonandhiscolleaguesinVietnam.Itincorporatesaduckweedproductionsystemintoaricefarmormarketgarden.Itdependsonatypical1to0.5hafarmwithonemilkinganimal,acalfandabullockforworkandwithadditionalsmallruminants(goatsorsheep)orrabbitsandwherepossibleapig.

ThepigisacrucialanimalinmuchofthecroppingsystemofsmallfarmersinVietnamparticularlytomaintainriceyields.They are produced on the byproducts of the household and from small inputs of other feeds. The faeces of theseanimalshasbeenthemechanismbywhichsoilfertilityhasbeenmaintainedoverthecenturiesinmuchoftheMekongDelta.However, the systemsare decreasing becauseof the importation of high technology pig production basedonEuropeantechnologyandthiscouldbehighlydetrimentaltosmallfarmriceproductioninthefuture.

GROWING DUCKWEED IN AN INTEGRATED SYSTEM

Thenutrientrequirementsforhighratesofduckweedgrowthhasbeendiscussed.Inpractice,however,standardsandrequirementsonlyprovideabasisforthe"adviser"togiverecommendationtothefarmer.Mostscientistsaredistractedby the establishment of "nutrient requirements". The application of precise levels of nutrients into any system isproblematicparticularlywherethesearetobemetfromthefarmresources.However,nutrientrequirementsestimatedinresearch laboratoriesmaypossiblyemphasiseacritical limitation induckweedproduction throughmineralanalysisofthewater.

Essentially, duckweeds must grow on the effluent from plant and animal production. Often the effluent from plantproduction (drainage) is too low in nutrients for highgrowth ratesof duckweed.Anexception to this occurs in someareaswhere high levels of fertiliser are applied to crops under irrigation. In Pakistan close to Faisalabad duckweed

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growthindrainageditchescansometimesbesogreatastocreatemajorproblemsinwaterpumpblockage.Similarlytherunofffromcottonproductionoftenprovidesagoodmediumforduckweedgrowth.Ontheotherhandeffluentfromintensive animal production almost always is a too concentrated source ofminerals particularly ammonia and needsdilutionwithotherwastewatersources.

Scientists can measure ammoniaN and phosphorous in effluent waters and then make recommendations for theappropriatedilution toprovidepondwater forduckweed farming.On theotherhandchemicalanalysisofwater isnotfeasible or affordable for large numbers of resource poor farmers that may be involved. However, some practicalrecommendationsbasedonsimpleresearchat thefarmlevelcanbegiventoassist farmerstoestablishaduckweedfarm.

OTHER BENEFITS FROM INTEGRATING DUCKWEEDINTO CROP FARMING

Aquaculture of duckweed has been largely promoted as an opportunity crop for use as animal or fish feed. This isparticularly appropriatewherepondshavebecomeunusable for otherpurposesbecauseof pollution.Thisemanateslargelyfromfertiliserrunofforfromwashoutofanimal/humanmanure.SuchpondsareabundantincountriessuchasBangladesh,wherethereareanestimated1.3millionponds(averagesize0.11ha)covering147,000ha.Only46%ofthepondscontain fish.Thepondshavemultipleuse,bathing (washing) irrigationand livestockwateringwhich interfereswithfishculturebutwhichcouldmakethemusefulforduckweedproduction.ArecentWorldBankreviewsuggestedthatabout40,000haofpondscouldbebrought intoduckweedproductionand that ifyieldsvaried from,420tonDM/ha/ythen160,000to800,000tonofduckweedcouldbeavailabletopoultryfarmers.Thepotentialvalueofthiscanbeseenfromthefactthatthehigherquantityofduckweedrepresentstwicetheavailabilityof'homegrown'feedconcentratesinBangladesh.

Themajorproblemsofdevelopingsuchasystemofconcentrateproductionareassociatedwithmultipleownershipofponds, lack of credit and the unavailability of extension services. A major constraint is the lack of a marketingmechanisms including quality control, that can effectively allow a duckweed meal to compete with imported proteinmealsparticularlythosefromEuropeandderivedfromanimaloffal.

OTHER CONSIDERATIONS

Amajorbenefitofusingduckweedsisemerging.Thereisaccumulatingevidencethatduckweedsreleasecompoundsthat have insecticidal properties particular to the larval stages of mosquitoes. Thus the development of duckweedaquacultureinthewettropicsmayhaveimplicationsformosquitocontrolinruralareaswheremalariaisagainbecomingaseriousproblem.

Eidetal.(1992a)publishedevidencethatanextractofLemnaminorhadinsecticidalactionagainstthemosquitoCulexpipenspipens.Thesameextractcontainedsynomoneswhichalsorepelledovipositionbythefemalemosquito.Wheresublethal doses of synomones were added to water it was found that all larval stages of the mosquitoes weremalformed.Duckweed synomones added towater also repelled the ovipositing of Piophila casei, and effected larvaldevelopmentandreducedsurvival.SimilarlySpodepheralittarolislarvaeweremalformedwhensynomonesfromLemnaminorwereaddedtotheirculturemedium.

IftheinsecticidalpropertiesofLemnaminorandotherduckweedsaresufficienttotrulycontrolmosquitopopulationsitwillhaveanimmenseeffectonhealthofpeopleinareaswheremosquitobornediseasesareendemicandresistanceofthe parasitic stage to drugs has increased. It is also a further inducement to cultivate duckweeds widely for watertreatment (purification)andanimal feed.A furtherpotential is thecommercial cultivationofduckweedasa sourceofinsecticidesinwaterwhereitisdifficulttosprayforcontrolofmosquitolarvaeorwheretheuseofothercontrolmeasureareimpractical.

Other research workers have also associated duckweed presence with reduced (or elimination) of mosquitodevelopment. For example Marten et al. (1996) showed that Anopheles albimanus populations were negativelycorrelatedwiththeamountofcoverofthewaterbyLemna.Therelativecoverofwatersurfacewithduckweedwasalsonegativelycorrelatedwithpopulationsof fishandother insects indicatinghow intricate theassociationsare innaturalecosystems.

Eidetal. (1992b)havemade theobservation that themosquitoCulexpipenspipiensnever colonisedsewagewatercoveredwithduckweedandBellinietal.(1994)observedthatLemnacoveringthesurfaceofricepaddyfieldsstrongly

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effected mosquito populations. However, again other organisms might also have been involved in the control ofmosquitos.

Lemna trisulca appears to produce allelo chemicals that are active against algae (Crombie & Heavers 1994) andMesmar and Abussaud (1991) suggested that extracts of Lemna minor were active in inhibiting the growth ofStaphylococcusaureus.

Theroleofduckweedsinpreventingalgalgrowthcanbebyshading,byperhapstheproductionofalgacidesifthiscanbesatisfactorilyprovenand inadditionbecause they lower thenutrient supply,particularlyPconcentrationseither ineffluentwatersfromsewageplantsorinwaterbodies.

CholerahaslongbeenassociatedwithseasonalcoastalalgalbloomsoffBangladesh.Fluorescentantibodytechniqueshave shown that a viable, non cultivatable form of Vibro cholerae in awide range ofmarine life, including algae. Inunfavourable conditions V. cholerae assumes sporelike forms which as conditions improve reverts to a readilytransmittibleandinfectiousstate.Algalbloomswhichareassociatedwitheutrophicationhavebeenrelatedtothespreadandpersistenceofcholera.Preventionofalgalbloomsmaythereforebeofconsiderablebenefit(seeEpstein,1993).

CONCLUSIONS ON THE POTENTIAL OF DUCK WEED TO PRODUCE CHEMICALS OF IMPORTANCE TO HUMANHEALTH

Although the literature is sparse and not totally convincing on the potential of duckweed to control of mosquitopopulations,theauthorhasheardfarmersinmanycountriesexpressopinionsthatgrowingduckweedonpondscontrolmosquitopopulations.Farmers inagroupofvillages inVietnamthat traditionallyproducedduckweedswereadamantthatmosquitoeswerenotaproblemsolongastheirlagoonswerecoveredwithduckweed.

Researchintotheinsecticidalpropertiesofduckweedisworthyoffollowup.Iftheproductionofnaturalinsecticidescanbepromotedatthesametimeasimprovinghealthconditions,purifyingwaterandprovidinganaturalfoodresourceforanimalproductionitmayhavefarreachingimplications.Anewnaturallyoccurringinsecticideproducedfromduckweedsmay be as revolutionary as the discovery of pyrethrins and perhaps this potential alone may give the necessarygovernment resolve to support integrated farming with a duckweed crop as a major component of such farms incountrieswherethisisappropriate.

FARMING SYSTEMS FOR DUCKWEEDS

Why duckweed?

Anumberofaquaticplantsincludingduckweedshavegreatpotentialfordevelopmentforvariouspurposes.

Essentiallyaquaticplantsmayorwillbegrownindevelopingcountrieswhere:

thereisanunusedareaofstandingwateravailablethatiseitherfreeorisrelativelyinexpensivetopurchase,rentorlease.freshwaterfish/crustaceanproductionisimpractical,notpracticedorthereareotherconstraintstotheirproduction,suchaspollution.thereisaneedtocleanwaterofchemicalsbeforereuseorreleaseintotheaquaticecosystemofrivers/deltasorseas.thereisamarketfortheproductortheproductcanbeintegratedintoasystemofproductionenhancingtheeconomicviabilityofthefarmeitherbeingusedasmulch,fertiliser,feed/foodandperhapsevenfuel.thereisalevyonindustriesindisposingofwatercontaminatedwithchemicals.legislationisenactedinordertocleanupvastareasofpondsorwetlandsthathavebecomeunusablefor,inparticular,fishproduction.duckweedmatsonstandingwaterreducethehealthhazardsfromcleanorpollutedwaterbodies

Candidatesforuse inanyof theseapplications includeduckweeds,Azolla,Pistia,Ecihhorniaandafewlessorknownaquaticplants.

There are major advantages for floating aquatic plants as the water depth is not critical and harvesting does notnecessarilydisturb theunderlyingecosystem in themud.Easeofharvest is importantandAzollaandduckweedarereadilyharvested,buthavethedisadvantageofhavingtobeprotectedfromwindandwatercurrentstoencouragetotal

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coverageoflagoonsandhencemaximumyields.ThereissomeevidenceforasymbioticassociationofLemnaandNfixingbacteria,butonlowNwatersLemnagrowthisslowandtheproductislowinprotein,onhighphosphoruswaterlow in N, Azolla with its association with N fixing bacteria is more appropriately grown. However, Azolla has somegreaterproblemsassociatedwithitscontinuousgrowthascomparedtoLemnaparticularlyfrominsectdamage.AdditionofNfertiliserinaquaticmediamostlyremovesthemajoradvantageofAzolla,thatis,itsabilitytogrowonlowNwater.

RUSTIC METHOD FOR ESTABLISHING DUCKWEED

Asimpleapproachtoestablishingaduckweedpondsystemisoftentheonlywaytostart.

Ifwater emanating fromananimal productionunit is takenasanexampleof how toapproach its use for duckweedproduction. First the water has to be collected in some suitable settling pond. To obtain information on the water'spotential to grow duckweed, the water is serially diluted in small containers with water relatively free of minerals.Duckweed isseeded intoeachwatercontainerand itsrelativegrowthmonitoredbyeye. Itquicklybecomesapparentwhatistheappropriatedilutionandthiscanbefurtherrefinedbysuccessiveharvestingfromthecontainerstodeterminethedilutionatwhichduckweedgrowsatthegreatestrate.Thissystemcanberecommendedwheretheobjectiveistoproduceduckweedandthereisnoconstrainttodisposingoftheeffluentoritisnotrequiredtorecyclethewater.

Ingeneral,itappearsthatinmostsystems,Nquicklybecomesthelimitingnutrientasduckweedmatsgrow.ThesecondpotentiallylimitingnutrientisP.ThusthereissometimesgreatbenefitsinprovidingextraN(asurea)attheendofthegrowthperiodfollowingharvestofhalftheduckweedmat.ThisallowsfurthergrowthofduckweedandfurtherreductionofPcontentintheeffluentwater.WatercanbecleanedeffectivelybygrowingandharvestingduckweedonlywhenthisisawelldesignedsuccessionoffertiliserapplicationsthatrebalanceNPKforgrowthaftereachharvest.Eventuallythemineralsmaybereducedtoacceptablelevels.

CHAPTER 5: Duckweed as a source of nutrients for domestic animals

PRELIMINARY

Although farmers,particularly inSouthEastAsiaandprobablyelsewherehaddeveloped theuseofduckweedsasasource of nutrients for livestock, the actual controlled experimentation that has been typically used to develop suchcommercialcropsassoyabeansormaizeforlivestockfeedhasnotbeenundertaken.Thereare,however,anumberofreportsintheliteratureontheuseofduckweedsasfeedsupplementsforfishandlivestock.Thesereportresearchwithdomesticanimals inwhichnormal feedproteinsourceshavebeen replacedbyduckweedmealonan isonitrogenousbasesincompletedietsbasedoncompoundedconcentratediets.

Duckweedsarehighlyvariableintheircomposition.Theygrowslowlyonlownutrientwatersandarehighinfibre,ashandcarbohydratesbutcontainlowcrudeprotein.Incontrastwhengrownonwatershighinammoniaandmineralstheygrow rapidly and have a high protein content associated with a high ash and are often lower in fibre. Becauseduckweedsrespondquicklytotheavailabilityofnutrientstheyoftenhavehighlyvariablelevelsofsomenutrientswhichmakes it difficult to prescribe the amounts needed for livestock and fish over a period of, say, one year. Carefulinterpretation of some studies reported is required when the quality of duckweed given in diets to livestock is notconsistent throughout the study. In terms of domestic animal/fish nutrition, duckweedsmay be used inmany ways.Theseinclude:

AsatotalfeedAsasupplementalsourceof:

proteinphosphorousandothermajormineralstracemineralscolouringpigmentforeggyolk/fleshofchickensvitaminAandtheBgroupfibreinlowfibredietsforpigsandpoultry

Duckweedshavebeen largely researchedas a total feed for fish, including carp and tilapia production, as a proteinsupplementforpigsandpoultry(includingducks)andasfermentableNandmineralsupplementforruminants.

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Theresearchonduckweedsasa feedaresummarisedbelow.Theuncertaintyof theconclusionsandthedifficulty inmakingclearrecommendationslargelypertainstothefactthequalityoftheduckweedusedbyvariousresearchers(i.e.its nutrient densities)were variable.However, as a resource that can be harvested for labour costs alone in naturalconditions,itobviouslyrepresentsavaluableassettotheresourcepoorfarmer.Inmanycountriesitcouldhavealowcostwhereitisgrownonsewageandithastobedisposedoffromtheworksatasubsidisedprice.

It appears to be a resource that ismost conveniently used by the small holder farmer, particularly in an integratedfarmingsystem.Unfortunatelymuchoftheresearchhasattemptedtodemonstratethevalueofduckweedasaproteinsource indiets thataremostcommonlyused in industrialproductionsystems.This isparticularly trueof theresearchwithpoultryandyet itsmajorapplicationprobablyliesinthemoredifficultsituationof increasinganimalproductiononsmallfarms.

CHEMICAL COMPOSITION OF DUCKWEED

Protein and amino acid composition

Thecrudeprotein contentof duckweedsdependmostlyon theNcontentof thewateruponwhich theygrow.Somepublicationshave indicated that therearesomevariations inaminoacidcontentofduckweedproteins.High levelsoflysine have been reported in studies coming from the duckweed research programmes in Bangladesh. However, itappears that the protein component of most aquatic plants including duckweeds have similar amino composition toterrestrial plant proteins in general (Table 7). In this respect the amino acid composition is influenced by themajorenzymeproteininplantthatisribosebisphosphatecarboxylase.ProteinextractedfromLemnaminorwhenfedtoratscompared equallywith the nutritive value of awheat flour diet. This indicates that Lemnameal has a relatively highbiologicalvalueforratgrowth(Dewanji&Matai,1996).Duckweedsthereforearegood

Table 7: Amino acid composition in aquatic plants (g/100g protein) grown on wash water from a pig farm inCuba (unpublished Instituto de Investigaciones Porcinas, Havana) The wash water was collected and aerated toreduce total N (Figueroa, V. personal communication)

Azolla Lemna WaterHyacinth

Soybean

Crudeprotein

31.0 28.0 19.0 44.0

Lysine 3.4 3.7 3.1 6.6

Histidine 1.7 1.7 1.4 2.5

Arginine 4.6 5.1 3.7 7.3

Aspartate

Threonine 3.5 4.2 3.3 3.9

Serine

Valine 5.1 5.8 4.5 4.6

Methionine 1.4 1.5 1.2 1.2

Isoleucine 3.8 4.3 3.1 4.5

Leucine 7.1 7.8 5.6 7.7

Tryptophane 3.5 4.2 3.3 3.6

Note:thatLemnaproteinhasalowerlysinecontentconsiderablybelowthatofsoybeanprotein,whichiscontrarytothe

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dataproducedbySkillicornetal.(1993).ItispossiblebuthighlyunlikelythattheessentialaminoacidcontentinLemnaisdependentontheproteincontent.Itisimportanttoemphasisethatitisnottruethatthereisabettercontentoflysinein Lemna as compared to soyabean although extracted protein from Lemnaminor has sufficient lysine tomeet theFAO/WHOReferenceStandardsandNRCrequirementsforchicks(Dewanji,1993)layinghensorpigs(Hanczakowskietal.,1995)sourcesofessentialaminoacidsbutarenotenrichedinanyparticularaminoacidincomparisonwiththeusualproteinsourcesusedinanimalproduction.

DUCKWEED FOR SUPPLEMENTAL NUTRIENTS IN AVIAN DIETS

Poultry egg production

The value of duckweed as a protein supplement to poultrywas recognised some time ago (Lautner&Muller, 1954Muzaffarov,1968Abdulayef,1969).

Truaxetal.(1972)showedthatdriedduckweedwassuperiorinproteinqualityforpoultryascomparedtoalfalfamealandcouldfullysubstituteforalfalfamealat5%ofthetotaldiet.

Interpretationandextrapolation frommuchof theearlierstudiesareconfusingbecause theduckweedwasharvestedfromnaturalsourcesandoftenthematerialcouldhavebeen"old"andlowinproteinbuthighinfibre.ThissituationledHausteinetal.(1990)toreassessthevalueofduckweedasaproteinsupplementforpigsandpoultry.Theyestablishedstudiestoexaminethepotentialtosubstitutenotonlyalfalfameal,butalsofishmealand/orsoyabeanmealwithLemnamealinacompoundedfeed.

The diets used by these researchers at the University of La Molina, Lima, Peru were based on those used in theintensiveeggproductionindustry(Table8).Duckweedwasharvestedfromatertiarysewageeffluentandlagoonrunoffand in general had a medium level of protein (33%CP). BothWolfia and Lemna species were harvested and theirestimated metabolisable energy level was 1,200kcal/kg (in young broilers) and 2,000kcal/kg (in mature cockerels)indicatingapooroveralldigestibilityofduckweedformonogastricanimals(seealsoHanczakowskietal.1995).Itmightbeinferredherethattherewasconsiderableintestinallyindigestiblecarbohydrate(fibre?)present.

Table 8: Composition of diets fed to Topaz layers (Haustein et al. 1990)

Ingredient(%)

Diet

Control Lemna(15%)

Wolfia(15%)

Lemna(25%)

Lemna(40%)

Groundcorn

52 51 51 48 50

WheatMiddlings

19 16 16 * *

Fish Meal(65%CP)

7.5 7.5 7.5 2

SoyabeanMeal (46%CP)

11.0

Duckweed 15 15 25 40

Fishoil 2.5 2.7 2.5 2.5

MineralsandVitamins

+ + + +

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CalculatedME(kcal/g)

2,800 2,800 2,800 2,840 2,800

CrudeProtein(%)

17 17 17.5 17.0

*Approximateasthelevelofwheatmiddlingswasnotstatedbytheauthorsandsmallamountsofothercarbohydrateswereincludedinthediettobalancetheenergy.

Thedietsusedareimportant,becausethesestudiesonlycomparedduckweedproteinwithothersourcesofproteininanotherwisecommercialdietusedforintensiveproductionTable8.TheresultsofreplacingsoyabeanwithamealmadefromLemnaorWolfiaclearly indicated that the latter twoareat leastasgoodassoyabeanasasourceofessentialaminoacidsasthereisvirtuallynodifferencesbetweeneggproductionofbirdsonalldiets(Table9).

Table 9: Performance of Topaz layers fed three isonotrogenous diets based on protein either from soyabean orduckweedafter2weeks(Wolfiadiet)or10weeks(ControlandLemnadiets(Hausteinetal.1990)

D

duckweed

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