Environmental impacts and constraints associated with the production of major food crops in Sub ...

28 Pages·2015·2.92 MB·English

FoodSec.(2015)7:795–822
DOI10.1007/s12571-015-0478-1
ORIGINALPAPER
Environmental impacts and constraints associated
with the production of major food crops in Sub-Saharan
Africa and South Asia
TravisW.Reynolds1&StephenR.Waddington2&C.LeighAnderson3&
AlexanderChew3&ZoeTrue3&AlisonCullen3
Received:5February2015/Accepted:7June2015/Publishedonline:7July2015
#TheAuthor(s)2015.ThisarticleispublishedwithopenaccessatSpringerlink.com
Abstract Manyenvironmentalfactorsconstrain the produc- biodiversity, loss of food crop genetic diversity and climate
tion of major food crops in Sub-Saharan Africa and South change. Those during crop production include soil nutrient
Asia.Atthesametime,thesefoodproductionsystemsthem- depletion,waterdepletion,soilandwatercontamination,and
selveshavearangeofnegativeimpactsontheenvironment. pestresistance/outbreaksandtheemergenceofnewpestsand
Inthispaperwereviewthepublishedliteratureandassessthe diseases.Post-harvestenvironmentalinteractionsrelatetothe
depthofrecentresearch(since2000)oncropxenvironment effects of crop residue disposal, as well as crop storage and
interactionsforrice,maize,sorghum/millets,sweetpotato/yam processing.Wefindthedepthofrecentpublicationsonenvi-
andcassavainthesetworegions.Wesummarizecurrentun- ronmental impacts is very uneven across crops and regions.
derstandingsoftheenvironmentalimpactsofcropproduction MostinformationisavailableforriceinSouthAsiaandmaize
systemspriortocropproduction,duringproductionandpost- in Sub-Saharan Africa where these crops are widely grown
production, and emphasize how those initial environmental and have large environmental impacts, often relating to soil
impacts become new and more severe environmental con- nutrient and water management. Relatively few new studies
straintstocropyields.Pre-productionenvironmentalinterac- have been reported for sorghum/millets, sweetpotato/yam or
tions relate to agricultural expansion or intensification, and cassava, despite their importance for food security on large
include soil degradation and erosion, the loss of wild areasofmarginalfarmlandinSub-SaharanAfrica–however,
there is mounting evidence that even these low-input crops,
once thought to be environmentally benign, are contributing
* TravisW.Reynolds to cycles of environmental degradation that threaten current
[email protected] and future food production. A concluding overview of the
emerging range of published good practices for smallholder
StephenR.Waddington
[email protected] farmershighlightsmanyopportunitiestobettermanagecropx
environment interactions and reduce environmental impacts
C.LeighAnderson
[email protected] fromthesecropsindevelopingcountries.
AlexanderChew
[email protected] Keywords Sustainableagriculture .Publicationanalysis .
Goodmanagementpractices .Rice .Maize .
ZoeTrue
[email protected] Sorghum/millets .Sweetpotato/yam .Cassava
AlisonCullen
[email protected]
1 EnvironmentalStudiesProgram,ColbyCollege, Introduction
Waterville,ME04901,USA
2 ApartadoPostal4-205,ColoniaChapultepec, Awide range of naturally occurring biotic and abiotic con-
Cuernavaca,MorelosCP62451,Mexico straints, including poor soils, water scarcity, crop pests/dis-
3 DanielJ.EvansSchoolofPublicAffairs,UniversityofWashington, eases/weeds,andunsuitabletemperatures,arewell-knownto
Seattle,WA98195,USA reduce the productivity of food crops, leading to low
796 T.W.Reynoldsetal.
efficienciesofinputuse,suppressedcropoutput,andultimate- established goodpracticestoovercomeconstraints and miti-
ly reduced food security (e.g., Strange and Scott 2005; gateimpacts.
Gregory et al. 2005; Lal2009 ; Waddington et al. 2010;
Knox et al. 2012). But there has also been growing concern
thatfarmingpracticesthemselves,bothinextensivefoodcrop Materialsandmethods
productionsystems(foundwidelyinSub-SaharanAfrica)and
intensifyingsystems (commoninSouth Asia)areexacerbat- Systematicreviewofcropxenvironmentinteractions
ingbioticandabioticconstraintsonfoodproductionthrough
negative impacts on the environment (Poppy et al. 2014; Drawingontheacademicliteratureandthefieldexpertiseof
Dogliotti et al. 2014; Chartres and Noble 2015). Common crop scientists we reviewed how environmental factors con-
examples include environmental degradation through strain the production of important food crops (rice, maize,
agriculture-related deforestation, soil erosion, nutrient sorghum/millets, sweetpotato/yam, and cassava) in Sub-
mining, water depletion, soil/water/air pollution, biodiver- Saharan Africa and South Asia and how, in turn, their
sity loss, and climate change; all of which threaten the croppingimpactsontheenvironment.1
long-term viability of agriculture and agro-ecosystems As an organizing principle we adopted a value chain ap-
(Cassman et al. 2003; Keating et al. 2010; Phalan et al. proach (Gómez et al. 2011), highlighting key types of envi-
2011; Pretty et al. 2011; Tilman et al. 2011; Chartres and ronmentalconstraintsandimpactsatthreestagesofthefood
Noble 2015). cropvaluechain:
This paper reviews the current body of knowledge on a
broadsuiteofcropxenvironmentinteractions,includingboth & Pre-production, including site/field selection, land clear-
theconstraintsoncropsimposedbytheenvironmentandthe ing,soiltilling,andotherlandpreparationforplanting;
impactsofcropproductionsystemsonthe environment.We & Production,includingnaturalandsyntheticinputsforcrop
focusonkeyfoodcropsinsmallholderproductionsystemsin production(nutrients,water,agro-chemicals)andthecon-
Sub-Saharan Africa and South Asia, including rice, maize, sequences of nutrient and water management and pest
sorghum/millets, sweetpotato/yam,andcassava.We summa- controlstrategies;and
rizeenvironmentalconstraintsoncropyields(includingpoor & Post-production, such as crop residues and other waste
soils, water scarcity, crop pests and disease) and impacts of disposal, and waste and/or pollution attributable to crop
crop production on the environment (such as soil erosion, transport,processingandstorage.
waterdepletion,pollutionandpestresistance)atthreestages
ofthecropvaluechain(pre-production,duringcropproduc- Peer-reviewed published literature was obtained through
tion, and post-production). Constraints and impacts are then searches of the Scopus academic database, through supple-
reviewedseparatelyforeachcropandregion,usingpublica- mental searches of published and grey literature in Google
tionanalysistoassesstherelativeseverityofcropxenviron- Scholar, and from a range of institutional sources including
mentinteractionsandthequantityofrecentresearchoncrop internationalagriculturalresearchcenters,U.N.agenciesand
environmentalimpactsasreflectedinthepublishedliterature theWorldBank.
since 2000. Future areas of potentially high value study are Additionally, using publication analysis (John and
suggestedgiventheimportanceofthecrop,theintensityofthe Fielding 2014) we undertook a semi-quantitative assess-
crop x environment relationship, and the depth of what we ment of the severity of crop x environment interactions
currentlyknowanddonotknowintheliteraturetodate.We andtheamountofpublishedresearch,usingresultsfroma
concludewithanoverviewofgoodpracticesfromthelitera- systematic Scopus search of literature published since
tureandfromexpertexperienceforovercomingenvironmen- 2000 covering a wide range of categories of environmen-
tal constraints and minimizing negative environmental im- tal constraints and environmental impacts. Good practices
pactsinsmallholdercropproductionacrossregionsandfarm- to manage constraints and reduce the impacts of these
ingsystems. crop systems on the environment were also systematically
Bysynthesizingtheavailableevidenceacrosstheseimpor- assessed and summarized.
tant crops, and emphasizing the feedback loops inherent in
agro-ecologicalsystems,weseektoprovideaframeworkfor
stimulatingacross-cropdiscussionsandinformeddebatesona 1ThisresearchbeganasaseriesofAgriculture-Environmentbriefson
rangeofcropxenvironmentinteractionsinagriculturaldevel- importantfoodcropsinSub-SaharanAfricaandSouthAsia,produced
opmentinitiatives.Thisworkcanhelpresearchplanners,pol- bytheEvansSchoolPolicyAnalysisandResearchGroup(EPAR)for
the Agricultural Development Group at the Bill and Melinda Gates
icymakersandfundingagentshaveabetterunderstandingof
Foundation. They covered rice (Brief No. 208), maize (No. 218),
environmental constraints and impacts associated with food
sorghum/millets(No.213),sweetpotato/yam(No.225),andcassava
crop production practices, and a better appreciation of (No.228).
CropxenvironmentinteractionsinAfricaandSouthAsia 797
Knowledgeassessmentofcropxenvironmentinteractions from the detailed crop-based reviews of literature: land
degradation,wildbiodiversityloss,agro-biodiversityloss,
Thereisalargebutvariablebodyofknowledgeinthepeer- water depletion, water pollution, soil nutrient depletion,
reviewed literature about the many biotic and abiotic con- soilpollution,pestresistance,methane(CH )greenhouse
4
straintsoncropyields,andanincreasingamountknownabout gas (GHG) emissions, nitrous oxide (N O) GHG emis-
2
how agriculture affects the environment. We evaluated the sions, air pollution (largely relating to burning), storage
relative importance of crop x environment interactions by chemicalcontamination,andpost-harvestlosses.
assessing(i)thefrequencywithwhichanenvironmentalcon-
strainttocropproduction,orenvironmentalimpactfromcrop Weclassifiedtheseverityofcropenvironmentalimpactsas:
production,ismentionedinthepeer-reviewedliterature,and
(ii)whetheritischaracterizedinthatliteratureasminor,mod- 0. No mentions of the environmental impact in published
erateorsevere.Thisaccountingdependsonthestockofliter- literatureorexpertaccountsonthecrop
ature,sowealsoassessedtheamountofresearch(numberof 1. Rarelymentionedoraminorimpact
publishedpapers)ontheseenvironmentaltopicsforeachcrop 2. Sometimesmentionedasamoderateimpact
in each region. This helped us to identify apparent gaps in 3. Consistentlymentionedasamoderateimpact
researchoncropxenvironmentinteractions. 4. Sometimesmentionedasasevereimpact
Threecriteriawereusedtosummarizetherecentevidence 5. Consistently mentioned in published literature or expert
currentlyavailableinpeer-reviewedscholarship: accountsonthecropasasevereimpact.
& Severityofenvironmentalconstraints reported.We sum- Assessmentsofimpactsweresimilarlymadebytheauthors
marized, for six general categories, the relative signifi- and small panels as described for constraints, though with
cance of various environmental constraints on crop pro- moredifficulty aswhile ‘yield gaps’ and ‘percentagelosses’
duction based on a comprehensive review of published representusefulacceptedmeasuresoftheseverityofenviron-
literatureandconsultationwithcropexperts.Thecatego- mentalconstraintsoncropyields(Waddingtonetal.2010;van
ries include land availability, nutrient constraints, water Ittersumetal.2013),therearenosuchestablishedmethodsfor
constraints, biotic constraints, climate change, and post- evaluating the environmental impacts of crop production.
harvestlosses. Nevertheless,ourcategorizationprovidessomeindicationof
the relative importance of different environmental impacts
We assessed the severity of these categories of environ- both within crops and across different crops and systems as
mentalconstraintsforeachcropona5-pointscaleasfollows: judgedbytheacademicandexpertcommunitiestodate.
The assessment of the severity of crop x environment in-
0. Nomentionsoftheenvironmentalconstraintinpublished teractions also depends on how much we know about the
literatureorexpertaccountsonthecrop issues,i.e.,howmuchresearchhasbeencompletedonapar-
1. Rarelymentionedoraminorconstraint ticularproblem,whichwasassessedbyourthirdcriterion.
2. Sometimesmentionedasamoderateconstraint
3. Consistentlymentionedasamoderateconstraint & Depth of research on crop x environment impacts. We
4. Sometimesmentionedasasevereconstraint conducted a comprehensive series of searches in the
5. Consistently mentioned in published literature or expert Scopusacademicdatabaseforpeer-reviewedarticlespub-
accountsonthecropasasevereconstraint. lished between 2000 and 2014 on the 13 categories of
environmental impact for each crop and region.
Initialassessmentsweremadebytheseniorauthorandthen Appropriate sets of terms were constructed for the
smallpanelsof2–4researchers(theauthors,plus1or2sci- searchesinconsultationwithcropexpertsandthesearch
entistswithcrop-specificexpertise)validatedormodifiedthe information compiled in spreadsheets. Counts of peer-
categorizations.Theresultantcategorizationindicatestherel- reviewedarticlespublished onthevarious aspectsofen-
ative importance, in very broad terms, of different environ- vironmental interactionswere then generated for the dif-
mentalconstraintsoncropyields. ferentcropsandregions.Theseprovidequantitativeinfor-
mation on the degree to which environmental problems
& Severity of environmental impacts reported. Precise esti- havereceivedattentioninrecentscholarlydebates.
mates of crop-specific environmental impacts are rarely
available.Howeverbasedonthepublishedliteratureand Thenumberofenvironmentalstudiesretrievedthroughour
expertopinionsomeassessmentsoftherelativeseverityof Scopus searches is highly uneven across crops, across envi-
differentenvironmentalimpactscouldbemade.Thirteen ronmental impacts, and across regions and continents. The
majorcategoriesofenvironmentalimpactwereidentified quality and depth of studies conducted also varies by crop
798 T.W.Reynoldsetal.
andregion.Inanattempttoensureonlyqualitypaperswere & Loss of wild biodiversity, both off-farm and on-farm:
includedinthecounts(butstillenableunder-researchedcrops Croplandexpansion,croppingintensification and repeat-
suchassweetpotato and cassava tobewellrepresented), we edplantingscannegativelyaffectwildbiodiversitydirect-
eliminated all papers published before 2013 that Scopus re- ly(e.g.,removaloftropicalforests,habitatloss,orpesti-
ported had been cited fewer than two times. We also report cides killing non-target organisms), as well as indirectly
paperscitedonly2–4timesseparatelyfrommoreextensively (bydisruptingbreedingcyclesanddestroyinghabitatsof
citedpapers(with5ormorecitationssincetheirpublication). sensitivespecies)(Phalanetal.2011).
Finally,asthequalityofveryrecentpublicationsmaynotyet & Loss of food crop genetic diversity: Shifts to more-
bereflectedincitationcounts,allpaperspublishedin2013– intensivefarmingsystemsoftenreducethenumberofcrop
2014with0–1citationstodatearereportedindependentlyas species in agro-ecosystems (e.g., by removing trees or
Bnewpublications.^ intercrops from farm fields). Replacement of multiple
locally-adaptedandgeneticallydiversecroplandracesor
varieties with a smaller number of modern varieties also
Cropxenvironmentinteractionsthroughthevalue reduces local and regional agro-biodiversity; in some
chain casesincreasingvulnerabilitytodrought,pestinfestations
and other abiotic or biotic threats (Altieri and Nicholls
Pre-production 2004;Snappetal.2010).
& Climatechangeandairpollution:GHGemissions(such
For all food crops, farming decisions (including the choice asCO ,CH andN O)fromcropfieldstendtoincrease
2 4 2
of crop or variety to plant, the types and amounts of with increased cropping intensity, and when forests/
inputs and their management) are directly shaped by the grasslandsareconvertedtofoodcropping.CO emissions
2
availability and quality of cropland. In areas where land ariseprimarilyfromlandconversion(releasingCstoredin
suitable for crop production remains relatively abundant – forests),soiltillage(releasingsoilC)andburningoffields
such as in much of Sub-Saharan Africa – the dominant andcropresidueswhichreleasesbothGHGsandparticu-
response to land constraints continues to be conversion of late air pollution. Other major GHG sources are more
forests, grasslands and other non-agricultural land to crops. crop-orsystem-specific:CH emissionsareprimarilyas-
4
In South Asia, where land is now relatively scarce, sociatedwithfloodedricefields,andN Oemissionsarise
2
farmers have primarily responded to land constraints fromNfertilizerapplication(Reayetal.2012).
through a process of intensification, involving multiple
cropping during the year, typically facilitated by the adop- The environmental and productivity-related impacts of
tion of irrigation, mechanization, organic and synthetic fer- land-usedecisions are not onlydirect, but alsosystemicand
tilizers, and pesticides. In both cases – whether expanding cyclicalinnature.Forexample,inadditiontotheintrinsiclost
agricultural production onto new land, or intensifying ag- value of wild biodiversity, impacts stemming from land-use
ricultural production on existing cropland – cropping de- decisions may also inhibit provision of valuable ecosystem
cisions have direct and often significant impacts on land servicessuchaspollinationandpestcontrol,withimplications
cover, soil structure and soil nutrients, as well as implica- forfuturecropproduction(Bommarcoetal.2013).Similarly,
tions for on-farm and off-farm biodiversity (Stevenson climate change, though far less controllable by individual
et al. 2014). farmers,has impacts onboththe globalenvironmentand on
Key environmental impacts from agricultural expansion futurelocalcropproductioninsomespecificregions(Burke
andintensificationbroadlyinclude: etal.2009).Consequently,interventionsdirectedatminimiz-
ingoreliminatingtheenvironmentalimpactsofcroppingpre-
& Land degradation and erosion: Land clearing exposes productioncanhavepositiveimplicationsthroughoutcurrent
landtophysicalandchemicaldegradation,aswellascon- and future crop production cycles and in locations far from
tributing to air pollution. Over-cultivation and tillage of theirorigin.
degraded and marginal lands damages soil structure,
drives soil loss through erosion processes and reduces Production
water retention capacity (e.g., Fowler and Rockström
2001; Hobbs et al. 2008). Loss of vegetative cover also Oncecropshavebeenselectedandplanted,variousenviron-
worsenswindandwatererosiononslopinguplands(Bai mentalfactors(includinginadequateaccesstoanduseofsoil
etal.2008).Landclearingandtillagemayalsohaveen- nutrients, water shortages and drought, and direct damage
vironmentalimpactsintheformoffossilfueluseforma- frompests,weedsanddiseases)cansubstantiallycompromise
chinery, or forage/feed production and GHG emissions productioninbothSub-SaharanAfricaandSouthAsia.Atthe
associatedwithdraftanimals. sametime,commonresponsestotheseproductionconstraints
CropxenvironmentinteractionsinAfricaandSouthAsia 799
such as applying chemical fertilizers, water extraction and and disease-resistant crop varieties have gone hand-in-
irrigation,andapplyingpesticidesandherbicidesoftenthem- hand with the emergence of resistance in some pests,
selves pose significant environmental risks and costs for sometimes resulting in devastating outbreaks (Oerke
crops,wildlifeandhumanpopulations. 2006). In other cases, efforts to address crop production
Keyenvironmentalimpactsfromcropproductionpractices constraints have inadvertently introduced new pest and
include: disease problems – for example, the development and
use of early-maturing varieties of sorghum and millet to
& Soilnutrientdepletion(Bnutrientmining^):Nutrientmin- overcomedroughtconstraintshasexposedgrainstofungi
ingoccurswhencroppedsoilsexperiencenegativenutri- and molds that now devastate harvests in some regions
ent balances, with extraction losses occurring faster than (Haussmannetal.2012).
the replacement of nutrients (Cobo et al. 2010). Effects
maybeespeciallyseverewhenfoodcropsareintegrated As with pre-production decisions, the environmental and
intointensiverepeatedsequencesandrotationswithinad- productivity-related impacts of crop management and input-
equatenutrientmanagement,asiscommoninSouthAsia usepracticesarebothdirectandsystemic.Forexample,while
(Timsina et al. 2010), or when socio-economic circum- syntheticNwilloftenincreasecropproduction,particularlyin
stances and limited technical options prevent adequate themanynutrient-depletedfarmsofSub-SaharanAfrica,the
replenishment of nutrients on depleted soils, as in much efficiency with which crops are able to convert synthetic N
of Sub-Saharan Africa (Vanlauwe et al. 2010; Shiferaw fertilizerstoincreasedproductionhingesupontheavailability
etal.2011). of micronutrients and water (often environmentally deter-
& Soil and watercontamination: Excessive applications of mined). In the absence of these other inputs, the addition of
syntheticnutrientscanaccumulateinandacidifysoils,and large amounts of synthetic N will not be cost-effective for
runoff nutrients may accumulate in rivers and lakes and farmers,andmayfurtherexacerbateotherenvironmentalcon-
leach into groundwater (Fageria2011).Already a severe straints such as soil acidification or contamination of water
problem in large parts of South Asia, nutrient supplieswithimplicationsforcurrentandfuturefarmproduc-
contamination/accumulationiscurrentlyonlyalocalissue tionandlivelihoods.
in Sub-Saharan Africa (where fertilizer underuse is pre-
dominant)butwillgrowassystemsintensify.Overuseof
synthetic N is also a major source of global GHG emis- Post-production
sions (associated with fertilizer manufacture and use)
(Reay et al. 2012). Meanwhile overuse or improper use Noteworthy crop- or system-specific environmental impacts
ofpesticidesandotheragrochemicalsinintensifyingsys- inpost-productionincludetheintroductionofenvironmental
temsmaythreatenhumanhealth(viapoisoning)andfur- or human health toxins in crop storage (either from storage
ther contaminate soil and water, in addition to being an chemicals,orfromcontaminantsofbitoicoriginsuchasafla-
inefficient use of scarce farm resources (Oluwole and toxin (Gnonlonfin et al. 2013), as well as the emission of
Cheke2009;Gupta2012). GHGs from the burning of crop residues (Andreae and
& Water depletion: Drought and water shortages represent Merlet 2001; Lal 2005; Smith et al. 2008). Burning harms
significant constraints to yields and reduce viable localairqualityandcontributestorespiratoryailments,aswell
cropping areas (de Fraiture et al. 2010; Li et al. 2011), as depleting soil organic C that could otherwise be used to
with climate change effects predicted to increase both stabilize soil structure, maintain soil fertility and raise the
the severity of droughts and the area of cropland that is water-holdingcapacityofsoils.
drought-prone (Gregory et al. 2005). Efficient irrigation Cerealcropssuchasmaizealsosuffersignificantlossesin
technologies can address water constraints to a degree, traditional storage from various pests and diseases (Tefera
buttheshortageanddepletionofsurfacewater(especially 2012), while inadequate harvest, storage and processing
in Sub-Saharan Africa where irrigation is poorly devel- methods are major problems leading to high rates of post-
oped) and groundwater resources (mainly in South Asia harvestspoilageamongrootandtubercropssuchascassava
wheremoreirrigationsystemsalreadyexist)aregrowing andsweetpotato(Lebot2009).Thislostproductionequatesto
problems(Alietal.2009;Wadaetal.2010). notonlywastedeffortbyfarmers,butalsowastedlandclear-
& Outbreaks, pest resistance, and new pests and diseases: ing(inextensivecroppingsystems)andwastedagro-chemical
Pests and diseases are frequent constraints and can be application(inmoreintensivesystems)fortheproductionof
sufficientlydevastatingforsomecropsthattheyseverely food that will never be eaten. In other words, post-harvest
restrictcropping,asisthecasewithviraldiseasesofcas- losses of crops carry the burden of all resources consumed
savainpartsofEastAfrica(Leggetal.2006,2014;Beed in producing the harvest that is lost. Reducing post-harvest
2014). Application of pesticides and shifts towards pest- losses from poor processing or storage pests thus both
800 T.W.Reynoldsetal.
a Articles on crop x environment interactions since 2000 b Articles on crop x environment interactions since 2000
by stage in the crop value chain by crop and region
250 Pre-Production South Asia
Production 1200 Sub-Saharan Africa
s200 Post-Production
e
cl
rti150 900
a
of
er 100 600
b
m
u
N 50 300
0 0
Rice Maize Sorghum &Sweetpotato Cassava
Millets & Yam
Fig.1 a–bNumberofresearcharticlespublishedbetween2000and2014across13environmentalcategoriesatthreestagesofthevaluechain(1a)and
bycropandregion(1b)formajorfoodcropsinSub-SaharanAfricaandSouthAsia
increasesfoodavailabilityandreducestheper-unitweightor Wittetal.2007),withtheshortageoflandalsoamajorissuein
per-unitareaenvironmentalimpactofagivencropharvest. South Asia (Fig. 2d). Especially in South Asia, agricultural
intensification (involving the adoption of modern irrigation,
fertilizers, improved seeds, and pesticides) has contributedto
Currentunderstandingofcropxenvironment dramatic gains in rice yields since the 1960s (Dawe et al.
interactionsbycropandregion 2010). However, increasing evidence suggests that intensive
ricesystems,ifnotproperlymanaged,cancauseseriousenvi-
Whilethemanyenvironmentally-determinedbioticandabiot- ronmental harm by reducing soil fertility, polluting soil and
ic constraints have long been a mainstay of the agricultural water, depleting groundwater, using large amounts of fossil
researchanddevelopmentliterature,therehasbeenarelative- fuels for water pumping, and contributing to climate
ly recent increase in the number of studies examining the change(IRRI2004).Manyoftheseissuesareespeciallyacute
environmental and human health implications of food forhigh-yieldintensiveirrigatedwinterseasonrice,whichhas
cropping systems (Fig. 1a). We identified 3694articlespub- becomeveryimportantinpartsofSouthAsiainrecentdecades
lishedsince2000thataddresscropxenvironmentinteractions (Alietal.2009;Barkeretal.2010;Timsinaetal.2010,2011).
insomeformforatleastoneofourfivefocuscropsinthetwo Anadditionalenvironmentalimpactuniquetofloodedrice
regions(Fig.1b). systems is an increase in insect-borne disease: flooded rice
The following sections summarize the current status of fieldshavebeenassociatedwithanincreaseinmalariatrans-
publishedfindingsoncropxenvironmentinteractionsbycrop mission among farmers, workers, and communities adjacent
andbyregion. to flooded rice-producing areas in both Africa and Asia
(Larsonetal.2010).
Ricecropxenvironmentinteractions
Sub-SaharanAfricaricesystems
Riceisthemostwidelyconsumedfoodcropofthedeveloping
world, and includes two species (Oryza sativa, native to the Most smallholder rice production in Sub-Saharan Africa is
Asian continent, and Oryza glaberrima, native to Africa) rainfed,low-inputandlow-yielduplandrice.Theoverallrice
grown on over 155 million (M) ha worldwide (FAOSTAT arearemainsrelativelymodestinAfrica,thoughrecentgrowth
2013). Our assessment of the severity of categories of rice trendshavebeendramatic:thericeareaharvestedmorethan
environmental constraints and impacts is summarized in doubled between 1982 and 2012, from 4.9 M to 10.8 M ha
Fig.2a–f. (FAO2013).Someofthisexpansionisduetointensification
In both the dryland upland rice systems predominant in made possible by irrigation and the introduction of Asian
Sub-Saharan Africa and the irrigated rice systems of South sativa varieties into lowland and wetland areas of Sub-
Asia, the single most significant environmental constraint to Saharan Africa (this has led to shifts from one to two crops
riceproductioniswater(Fig.2a):riceis2–10timesmorewater peryear,resultingindouble-countingofsomeareasplantedto
intensivethanothermajorcrops(Boumanetal.2007).Other rice)(Larsonetal.2010).Butformostsmallholders,rainfed
commonricesystemconstraintsincludeinadequatesoilnutri- rice-fallowsystemsproducingonecropperyearremaincom-
ents,weeds(innon-floodedsystems),insects,rodentsandas- mon (Dawe etal. 2010).Major environmental constraints in
sortedotherpests(Waddingtonetal.2010;Nortonetal.2010; the region (Fig. 2a) include water constraints, making up as
CropxenvironmentinteractionsinAfricaandSouthAsia 801
muchas10–31%ofthericeyieldgapinSub-SaharanAfrica, Saharan Africa has been linked to dramatic increases in rice
nutrient constraints (15–30 % of the yield gap) and weeds, productivity–in2009only14%ofriceareaintheregionwas
especially where flooding is not an option for weed control irrigated, but this area made up 33 % of total rice produced
(DobermannandFairhurst2000;Waddingtonetal.2010). (Africa Rice Center 2010). Although intensification also en-
tails impacts such as chemical runoff and GHG emissions,
Relativeseverityofenvironmentalimpacts Africa’slargely such impacts have received little empirical attention in the
drylandricesystemsarerelativelyinsignificantcontributorsto publishedscholarshiptodate.
environmentalimpactstypicalofmoreintensivericesystems,
suchaswaterresourcedepletion,CH emissionsorchemical Research on environmental interactions and areas of
4
runoff(Yanetal.2009).Thekeyenvironmentalthreatsfrom debate Overallresearchonenvironmental impacts ofriceis
extensivelow-productivityriceinSub-SaharanAfricatakethe limitedinSub-SaharanAfrica(Fig.2c).Noestimatesofim-
formofdegradationoffragileanderosion-proneuplands(Bai pactssuchaslandconversionorbiodiversitylossattributable
etal.2008),ortheexpansionofnewsativafloodedricepro- to upland rice are available, though some research is under-
duction into ecologically important lowland/wetland ecosys- way (Phalan et al. 2011). Moreover an expanding body of
tems(Rodenburgetal.2014)(Fig.2b).Therelativelyrecent research surrounding the successes and failings of new
introductionofformalirrigationintoriceproductioninSub- NERICA varieties (open-pollinated improved varieties of
a Relative Severity of Rice Constraints (SSA) d Relative Severity of Rice Constraints (SA)
b Relative Severity of Rice Impacts (SSA) e Relative Severity of Rice Impacts (SA)
c Research on Rice-Environment Interactions (SSA) f Research on Rice -Environment Interactions (SA)
>5 citations since 2000 2-4 citations since 2000 New publication (2013-2014)
Fig.2 a–fRelativeseverityofenvironmentalconstraintsandimpacts,andthedepthofenvironmentalresearchforriceinSub-SaharanAfrica(2a–c)and
SouthAsia(2d–f)
802 T.W.Reynoldsetal.
upland rice) is beginning to identify upland rice constraints emissions, with emissions concentrated in the flooded rice
andimpactswithgreaterdetail(seee.g.,Kijimaetal.2011). fieldsofIndiaandChina(Yusufetal.2012).
Research on environmental impacts in new irrigation-based
rice systems in Sub-Saharan Africa lags behind research on Research on environmental interactions and areas of
production, and as there is very limited local information, debate Thereisawealthofresearchontheenvironmental
currentreviewsofSub-SaharanAfricariceenvironmentalim- impacts of rice production in South Asia (Fig. 2f), includ-
pactslargelydrawontheAsianexperiencewithriceintensi- ingrecentreviews(Pandeyetal.2010).Inparticular,water
fication (Larson et al. 2010). Site-specific (and somewhat and soil pollution related to rice has featured in many re-
morecontested)studiesonimprovedsoilandwatermanage- cent publications. Rice pre-production (land clearing) is
ment in Sub-Saharan Africa such as the System of Rice relativelyunder-studiedbutalsofairlylow-impact:asmost
Intensification (SRI), which was originally developed in potential arable land in South Asia has already been con-
Madagascar(seeDobermann2004;Uphoffetal.2008),sug- verted to agriculture, the new biodiversity and habitat im-
gestsignificantopportunitiesforincreasingyields,watereffi- pacts of rice are likely to be small – although the high
ciencyandpestmanagementinSub-SaharanAfrica. pressure for continued blanket cropping of rice means it
is unlikely that some rice land will be returned to more
‘natural’ agro-ecosystems. Meanwhile the effects of rice
SouthAsiaricesystems farming on soils and chemicalrunoff rates vary by system
andbycropsplantedbetweenriceharvests.Thereisgrow-
InSouthAsia,smallholderriceisproducedprincipallyduring ing consensus on the non-sustainability of intensive pre-
themonsoonseasonunderrainfedconditions,butincreasingly monsoon/winter irrigated rice systems in terms of nutrient
alsounderirrigationpre-monsoon.Withmostofthelandsuit- demand, agro-chemical use and water management (e.g.,
able for ricealready under production, the necessarily inten- Ali et al. 2009; Timsina et al. 2010). Perhaps most visible
sive South Asia rice systems face several well-known biotic among these rice-environment interactions is the effect of
andabioticconstraints(Fig.2d).Mostfarmersarereliantupon rice-related water depletion on rice productivity – already
synthetic fertilizers and pesticides (often at high rates) to thehighfinancialcost(associatedwithpumpsandfuel)of
maintainyields.Soilnutrientshortages,particularlyN,Pand irrigatedwinterriceproductionhasencouragedfarmerson
K, seriously limit production (Witt et al. 2007; Waddington theeasternGangeticPlaintoscalebackonthisproduction
etal.2010)whilepestsincludinginsects,rodentsanddisease system. There is also increasing consensus on the signifi-
also significantly reduce rice yields (Singleton 2003; Mejia cant role of irrigated rice in climate change via CH emis-
4
2004).Waterconstraintsmeanwhileareubiquitous,account- sions (Yan et al. 2009; Yusuf et al. 2012).
ing for as much as 23 % of rice crop losses, including in
irrigatedriceandrice-wheatsystems(Lietal.2011). Maizecropxenvironmentinteractions
Relativeseverityofenvironmentalimpacts Farmorepub- Globally, maize is an extremely important food crop. The
lished research is available for environmental impacts of area of maize harvested worldwide increased 69 % be-
SouthAsianriceproductionsystemsascomparedtoAfrican tween 1961 and 2012, from 106 to 179 M ha
rice systems (Fig. 1b). Rice production in South Asia has (FAOSTAT 2013), accompanied by trends towards inten-
many known environmental impacts, with water depletion sified maize production systems. During the 20th and into
and contamination, the evolution of pest resistance and the 21st century, maize has become the principle food
GHG emissions among the most important (Fig. 2e). Many crop produced and consumed by Sub-Saharan Africa
of these environmental impacts are severe and commonly smallholder farm households (Shiferaw et al. 2011) and
foundduetotherelativelychemical-intensiveandirrigation- it is an increasingly important smallholder food and cash
basedproductionpracticestypicalofwidespreadsmallholder crop in South Asia (Joshi et al. 2005). This growth stems
double-crop South Asian rice production. Irrigation is a key from a combination of non-agricultural land converted to
driver of water depletion in South Asia, with 50 % of all maize-based agriculture, existing cropland converted to
irrigation used for rice (Wada et al. 2010). Meanwhile, the higher-yielding maize, and maize intensification through
overuse of synthetic fertilizer and other chemicals has been annual double-harvests from fertilized andirrigated fields.
linked to runoff and even poisonings, partly owing to input Maize production systems and environmental impacts dif-
usebeyondprescribedlevels(Pingali1995;Pengetal.2006). fer among the regions, as we indicate below.
Historically, an overuse of insecticides for rice has also re- Maize production has important environmental conse-
duced populations of pests’ natural enemies, leading to pest quences in both extensive systems (such as habitat loss,
outbreaks (Heongand Schoenly 1998).Finally, ricesystems soildegradationandGHGemissionsfromdeforestationin
areestimatedtoconstitute10%ormoreofglobalannualCH Africa) and intensive systems (via nutrient mining and
4
CropxenvironmentinteractionsinAfricaandSouthAsia 803
a Relative Severity of Maize Constraints (SSA) d Relative Severity of Maize Constraints (SA)
b Relative Severity of Maize Impacts (SSA) e Relative Severity of Maize Impacts (SA)
c Research on Maize-Environment Interactions (SSA) f Research on Maize-Environment Interactions (SA)
>5 citations since 2000 2-4 citations since 2000 New publication (2013-2014)
Fig.3 a–fRelativeseverityofenvironmentalconstraintsandimpacts,andthedepthofenvironmentalresearchformaizeinSub-SaharanAfrica(3a–c)
andSouthAsia(3d–f)
contamination, and pesticide contamination in South Africa(Shiferawetal.2011).Thecropistypicallygrownin
Asia) (Fig. 3b, e). The relatively widespread and rising Africaasaprimaryfoodcrop,inrainfedsmallholderfarming
use of synthetic fertilizers in maize systems is also re- systems, often on marginal soils and/or newly cleared land,
sponsible for the release of GHGs, both during manufac- andwithinadequateornosyntheticinputs.
ture of the fertilizer and in its use (Reay et al. 2012). Soil infertility and nutrient shortages represent the most
severe and widespread constraints to maize yields in Sub-
Sub-SaharanAfricamaizesystems SaharanAfrica(Muelleretal.2012).Droughtisalsoamajor
constraint(Waddingtonetal.1995;Gibbonetal.2007),with
TheareadedicatedtomaizeinSub-SaharanAfricamorethan small changes in rainfall patterns/amounts and temperatures
doubled (15.5 to34.1M ha) from1961to2012(FAOSTAT leadingtoappreciablelossofyield(Lobelletal.2011a).Pests
2013).MaizedominatesinsouthernandeasternAfrica,con- suchasdownymildew,greyleafspot,armyworm,stemborers,
stituting20to50%offoodconsumption.Inrecentdecades, and the parasitic weed Striga spp. also hamper production
maize has also spread in western and central Sub-Saharan (Pingali and Pandey 2000). Pests also damage maize post-
804 T.W.Reynoldsetal.
harvest, with losses in Africa estimated by the World Bank both in the monsoon and with irrigation during the winter
(2011)tobe10–20%ofproduction(Fig.3a). season, in sequence with other crops such as monsoon
rice, to produce feed for sale to expanding poultry indus-
Relative severity of environmental impacts The environ- tries (Joshi et al. 2005). Very recently, maize has also
mental impacts of maize cropping in Sub-Saharan Africa become more significant during the spring (pre-
largely relate to land clearing and degradation, soil erosion, monsoon) season. Thus there are now very large areas
nutrient depletion and biodiversity loss (Fig. 3b). Because with relatively fertile soils and developed irrigation sys-
maize issowidely grown inAfrica, these effects are widely tems planted to intensive maize in South Asia, as well as
seen.Maizeisacommonfirstcropafterslashandburnclear- the more traditional systems (Timsina et al. 2010, 2011).
ingasfarmersvalueitsabilitytoutilizenutrientsreleasedby Constraints to maize in South Asia vary by sub-region.
theburningtoboostyields(Binametal.2004).Deforestation A 2001 survey of farmers in India found post-flowering
destroys wildlife habitat and releases GHGs (Fargione et al. stalk rot to be the most widespread severe constraint
2008),withmaize-relatedclearingcontinuinginseverallarge (Gerpacio and Pingali 2007). Soil nutrient deficiencies
African countries including Nigeria, Ethiopia and Sudan are also a yield barrier (though less severe than in
(Phalan et al. 2013). Efforts to improve soil management Africa), reducing output by up to 14 % (Gibbon et al.
(throughminimaltillage,residueretentionandintercropping) 2007). Losses from drought are relatively modest and oc-
can reduce soil erosion and nutrient losses, but adoption of cur mainly in rainfed upland maize systems (which none-
conservationagriculturetechniquesinSub-SaharanAfricare- theless support some 48 million rural poor). Post-harvest
mainslimited(Bossioetal.2010;Erensteinetal.2012).Other losses are also generally modest, estimated at 2–15 % of
impactssuchasnutrientandchemicalcontaminationareonly production (Fig. 3d).
localizedinmuchofAfrica;inmostareasfertilizersandpes-
ticidesaregenerallyunderused. Relative severity of environmental impacts South Asian
maize is now commonly a high-input crop produced using
Research on environmental interactions and areas of hybrid seed, irrigation, fertilizer (up to 100–200 kg N/ha),
debate There has been a considerable amount of research pesticidesand herbicides (Ali etal. 2009; Joshi etal. 2005).
on the environmental impacts of maize in Sub-Saharan Aswithmanyintensivesystems,repeatedcultivationandthe
Africainrecentyears(Fig.1b),withmostofthepublications overuseofsyntheticinputsmaydegradesoilsandcontaminate
covering soil degradation, nutrient depletion and pests soil and water. Pesticides can destroy beneficial species that
(Fig.3c).Continuousproductionofmaizewithlimitedfertil- manage pests; pesticide poisoning and other human health
izerinputsiscommonlyreportedtocontributetothedepletion impacts have also been reported (Gupta 2012). Herbicide
ofsoilnutrientsandsoilerosioniscommoninmaizecropping use risks killing crops that are often intercropped or rotated
systems,butmaize-specificdataonlanddegradationinSub- with maize, including beneficial legumes (Kanampiu et al.
Saharan Africa are only now emerging (Cobo et al. 2010). 2002). All of this makes soil degradation, water depletion
Moreover, the net effects of maize on land and climate are and pollution, nutrient depletion and chemical pollution key
unclear as higher-yield maize might decrease land clearing impacts(Fig.3e),thoughourreviewfoundlittledataspecific
compared to reliance on lower-yield traditional cereals. to maize for the intensive multi-crop systems increasingly
There is near consensus that the impacts of climate change typicalofSouthAsia.
will be severe for rainfed maize in several large parts of the
continentsuchasinsouthernAfricaandtheSahel(Schlenker Research on environmental interactions and areas of
andLobell2010). debate Concernswithsustainable soiland nutrient manage-
ment have emerged recently in South Asian maize systems
SouthAsiamaizesystems and are receiving more attention in recent scholarship (Ali
etal.2009;Timsinaetal.2010,2011),asareclimaticfactors
Total maize area harvested in South Asia is less than half such as high temperatures. But overall published environ-
that of Sub-Saharan Africa, but the area grew rapidly by mentalresearchspecifictomaizeappearstobeverythinin
92%percentfrom6.0Mhain1961to11.5Mhain2012 SouthAsia(Figs.1band3f).AswithAfrica,thenetenvi-
(FAO 2013). Much of the growth reflects a switch from ronmental impacts of maize in South Asia are also some-
rice, wheat, or dryland cereal crops, and an increase in what ambiguous – in the face of water constraints and
winter and spring (pre-monsoon) maize (Ali et al. 2009; major environmental impacts from intensive rice farming,
Joshi et al. 2005). In India and Nepal, farmers have tradi- maize is seen asa relatively high-yield and water-efficient
tionally grown rainfed upland maize during the monsoon alternative crop for promoting both food production and
season as a supplemental food and income source. resource use efficiency in this region (Ali et al. 2009;
Increasingly, maize is also grown as a high-input crop Timsina et al. 2011).