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Conservation agriculture in Southern Africa:Advances in knowledgeChristian Thierfelder1*, Leonard Rusinamhodzi1, Amos R. Ngwira2, Walter Mupangwa1,Isaiah Nyagumbo1, Girma T. Kassie1,3 and Jill E. Cairns1

1International Maize and Wheat Improvement Centre (CIMMYT), P.O. Box MP163, Mount Pleasant, Harare,Zimbabwe.2Department of International Environment and Development Studies, Noragric, University of Life Sciences, Aas,Norway.3International Centre for Agricultural Research in the Dry Areas (ICARDA), Addis Ababa, Ethiopia.*Corresponding author: [email protected]

Accepted 19 December 2013 Review Article

AbstractThe increasing demand for food from limited available land, in light of declining soil fertility and future threats of climatevariability and change have increased the need for more sustainable cropmanagement systems. Conservation agriculture(CA) is based on the three principles of minimum soil disturbance, surface crop residue retention and crop rotations, andis one of the available options. In Southern Africa, CA has been intensively promoted for more than a decade to combatdeclining soil fertility and to stabilize crop yields. The objective of this review is to summarize recent advances inknowledge about the benefits of CA and highlight constraints to its widespread adoption within Southern Africa.Research results from Southern Africa showed that CA generally increased water infiltration, reduced soil erosion andrun-off, thereby increasing available soil moisture and deeper drainage. Physical, chemical and biological soil parameterswere also improved under CA in the medium to long term. CA increased crop productivity and also reduced on-farmlabor, especially when direct seeding techniques and herbicides were used. As with other cropping systems, CA hasconstraints at both the field and farm level. Challenges to adoption in Southern Africa include the retention of sufficientcrop residues, crop rotations, weed control, pest and diseases, farmer perception and economic limitations, includingpoorly developed markets. It was concluded that CA is not a ‘one-size-fits-all’ solution and often needs significantadaptation and flexibility when implementing it across farming systems. However, CAmay potentially reduce future soilfertility decline, the effects of seasonal dry-spells and may have a large impact on food security and farmers’ livelihoods ifthe challenges can be overcome.

Key words: conservation agriculture, mulching, no-tillage, rotation, sustainable intensification, Southern Africa

Introduction

The increasing demand for food from limited landresources requires new sustainable ways of food pro-duction. Short-term needs of farmers must be balancedwith long-term environmental sustainability. The solu-tions must be viable at the field, farm and communitylevels, and beyond. They need to take into account thecomplexity of the systems in which farmers operate1,2 andthe environmental implications their actions might have3.Farming systems in Southern Africa are constrained by

numerous factors, including inherently infertile sandysoils in some areas4,5, drought6 and limited access to, anduse of, mineral and organic fertilizers7–9. Average annual

fertilizer use in sub-Saharan Africa is below 10kgha−1 ofNPK fertilizer7,10, significantly smaller than in Asia andLatin America11. Farmers own relatively small landholdings; e.g., in Malawi the average household landsize is <1.2ha12, and maize is often grown in monoculturewith few legumes or other crops in rotation13,14. Marketsfor both agricultural inputs and outputs are not well de-veloped, leading to high input and low output prices15.Since the 1920s, the moldboard plow has been used as a

land preparation tool in Southern Africa in an attempt toreduce labor requirements, bury weeds, mobilize nutrientsand prepare a fine soil tilth16,17. Intensive use of the plow isreported to have increased chemical, physical andbiological soil degradation18,19, and increased soil erosion

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© Cambridge University Press 2014

and run-off20,21, soil crusting and sealing22, reducedaggregate stability13, limited water infiltration and lessavailable soil moisture23,24 have been documented. Inmanual systems since the 1930s in Malawi, smallholderfarmers have been encouraged to plant crops on ridges,rather than on the flat or in mounds. Crop ridges areconstructed using a broad-bladed hoe and, in the nextseason, the ridge is split and reformed in the previousfurrow. There is a growing body of evidence that thecontinued use of a hoe to the same depth results in theformation of a compacted horizon immediately belowthe ridge25,26. The intensive cultivation of soils has ledto losses in soil carbon and reductions in biologicalactivity13,27. However, there are also conflicting resultsfrom the region28 and elsewhere29 that show no causalrelationship between intensive tillage and carbon loss.Limited retention of surface crop residues adds to thedecline in soil carbon in conventional systems30.Furthermore, accelerated soil erosion in tilled systemsdecreases the amount of important plant nutrients (e.g. N,P and K) through the loss of arable top-soil31.Recently there has been increased demand for

‘greener’ solutions to tackle the soil fertility decline inSouthern Africa32. The way to achieve this is uncertainand often contested33. Different approaches have beenproposed to increase the productivity of land, rangingfrom simple technology interventions, such as increasedfertilizer use and/or improved seed varieties, to moresystems-oriented solutions, such as agroforestry, perma-culture, integrated crop-livestock systems or conservationagriculture (CA)34–38. Breeding improved varieties isoften proposed as a direct route to increasing cropproductivity and closing the yield gaps in the environ-ments of smallholder farmers in Southern Africa39,40.However, it is rather unlikely that one single interventionsuch as seed or fertilizer will solve all of the smallholderfarmers’ challenges and thus combinations of synergistictechnologies are required.CA, a cropmanagement system of three basic principles

applied in a mutually reinforcing manner, has beenproposed as one of the options to alleviate soil fertilitydecline36. CA is based on a combination of: (1) minimumsoil disturbance, i.e., no soil inversion with the plow orhoe; (2) surface crop residue retention as mulch with livingor dead plants; and (3) crop rotations and associationsof different crop species over time38,41. Commonly usedterms in CA and conventional tillage systems aredescribed in Table 1. CA is widely adaptable and theapplication of its principles may vary among individualfarmers depending on the site and the farmer’s circum-stances36,42.Over the past 40 years, the application of CA within

large-scale commercial farming systems has greatlyexpanded in Latin America, the United States andAustralia43–48. Based on these successes there was amajor push by the international donor community in thelate 1990s to also adapt this cropping system for

smallholder farmers in Southern Africa49. Nevertheless,concerns were raised about the widespread promotion ofCA in Southern Africa without adequate (research)evidence that CA increases yields and livelihoods offarmers and improves soil quality indicators50,51. Gilleret al.51 argued that farmers can only apply CA in certain‘niches’ under very specific circumstances, which was con-tested by many scholars, with opinions summarized onthis website52. Related studies17 questioned the suitabilityof CA for the majority of farming systems in SouthernAfrica and stated that an ‘across-the-board recommenda-tion of conservation agriculture’ would be whollymisplaced32,53, and this sparked further debate54,55.One of the main arguments against promoting CA in

SouthernAfrica is that CAwas successfully applied withinlarge-scale, mechanized commercial farming systems inthe Americas and Australia, and little was known aboutits potential to increase productivity and reduce soildegradation within the smallholder farming systems ofSouthern Africa, where small and fragmented landhold-ings are common and the farming systems are generallymore complex56,57.Like all agricultural technologies, CA cannot be

considered as a panacea to all farmers’ problems underall circumstances, and hence the ‘one-size-fits-all’ argu-ment can hardly be defended. Nonetheless, there isincreasing empirical research evidence on the merits anddemerits of farm-level application of CA in SouthernAfrica. This review summarizes and synthesizes theimmediate farm-level impact of CA in the region, focusingmainly on publications based on multi-year on-stationand on-farm trials. The paper is structured as follows: thesection ‘Effects of CA on Smallholder Cropping Systems’synthesizes the effects of CA on smallholder croppingsystems in Southern Africa and the section ‘Constraints toAdoption of CA Systems in Southern Africa’ presents adetailed analysis of the biophysical and socio-economicchallenges that hinder the adoption of CA in the region.Finally, the brief conclusion focuses on the key lessonslearned from the empirical evidence synthesized.

Effects of CA on Smallholder CroppingSystems

Initial research from 1988 to 2002 largely focused onthe effects of CA on soil quality, such as the effects ofselected CA and non-CA systems on soil erosion, carbon,weeds and water dynamics20,58–60. These studies fromZimbabwe highlighted that reduced tillage and mulchcover reduced erosion and increased soil moisture, whichled to overall greater yields, especially in dry years. Theresults also showed that timing of planting and otheroperations was more important than the type of tillagesystem employed, particularly in the sub-humid parts ofthe country61,62.

2 C. Thierfelder et al.

Effects of CA on infiltration and soil water

Rainfall distribution patterns during the growing seasonin Southern Africa are characterized by mid-season dryspells63,64. Thus, higher infiltration rates under CA andsurface crop residue retention have the potential to buffergrowing crops against intermittent periods of droughtstress. The effect of CA on water infiltration and soilmoisture in Southern Africa has already been reported in

detail by Thierfelder and Wall23. No-tillage and residueretention increase infiltration rates, an effect whichappears almost immediately when the soil is coveredwith mulch. Infiltration measurements at HendersonResearch Station, Zimbabwe and Monze FarmerTraining Centre, Zambia with a mini-rainfall simulatorshowed clearly that CA treatments were able to maintainhigher infiltration rates compared with conventionallyplowed treatments without residue retention across sites

Table 1. Commonly used terms in conservation agriculture and conventional crop management systems of Southern Africa.

Agricultural practice Description

Conservation agriculture practicesBasin planting (also

called pot-holing andconservation farming)

A technology, developed by the Zimbabwean commercial farmer Brian Oldrieve, based on manuallydug planting basins of different sizes (15cm×15cm×15cm in Zimbabwe and 15cm×30cm×25cmin Zambia). Basins are constructed during the winter season thereby spreading labor requirementsduring the off-season.

Ripping (also called rip-lineseeding, MR and tineripping)

Ripping and rip-line seeding practice developed in Southern Africa based on seeding with an animaltraction chisel-tine opener (theMagoye ripper) that is mounted on a plow beam. Rip-lines are createdin April (Zambia) or at the onset of the cropping season in November (Zimbabwe). Sometimesopening wings are used on the ripper attachment (mainly for sandy soils). The implement creates afurrow of approximately 10–15cm width and 10cm depth.

Direct seeding (also calledseeding with a dibble stick,pointed stick, jab-planterand animal tractiondirect seeding)

There are currently three direct seeding systems used in Southern Africa. Seeding with a dibble stick(a pointed stick) where farmers make two holes and place seed and fertilizer. A more mechanizedversion is the jab-planter (matraca) that supplies seed and fertilizer in planting holes created by theimplement. Finally there are animal traction direct seeding systems using Brazilian and locallyproduced direct planters, where the implement creates a rip-line, supplies seed and fertilizer andcovers the line in one operation.

No-till tied ridging This is generally not considered a CA seeding system due to considerable soil movement during landpreparation. In this system, farmers create ridges and furrows using hand-held hoes. Ridges are closedevery 80–100cm across the ridge direction to conserve rainfall water. The systems has proved to be anefficient water conserving technology but cannot be classified as CA in the strict sense.

Residue retention(also called mulching)

Describes the retention of crop residues [living or dead plant material such as maize stalks, leaves ormowed green manure cover crops (GMCCs)] in the form of surface mulch. Different to someconventional systems, it is not incorporated with the plow and should lead to good groundcover (30%or more) for erosion control and reduced evaporation.

Intercropping A crop-association practice where the main crop (commonly maize in Southern Africa) is interplantedwith other crops. Grain legumes (e.g. cowpea, pigeonpea, common beans, groundnuts) are the mostprevalent intercropping species in Southern Africa but GMCCs (such as velvet beans, lablab and fishbean) are also used.

Rotation Rotation is the repetitive sequence of crops in the same place in a defined order. Farmers in SouthernAfrica generally practice rotations between maize and leguminous crops (cowpeas, soybeans,groundnuts, common beans), green manure crops and non-leguminous cash crops (cotton, sunflowerand cassava).

GMCC Green manure cover crops are crop species planted under CA for the purpose of groundcover andfertility improvement. Common species used for GMCC include velvet beans, lablab and sunnhemp.

Conventional agriculture practicesCMP Conventional moldboard ploughing (CMP) is normally performed using a single-row moldboard plow

to a shallow depth (10–15cm). Plowing completely inverts the soil and prepares a clean seedbed withfine tilth, depending on the soil type. Farmers plow their land either during the off-season (July–August) or at the onset of the cropping season in November, once the soil is soft after the first rains.

Tillage Often synonymously used with moldboard plowing but can also be done with implements like diskharrows or hand hoes.

Monocropping In contrast to rotation and intercropping systems, monocropping is the repeated planting of the samecrop or crops in the same place year after year. This system is often practiced in rural areas ofSouthern Africa with cereals (maize and sorghum) or cassava. In some areas maize is continuouslygrown for 3–4 seasons and then rotated for one season with either groundnuts or cowpeas.

Note: seeding systems are described in Johansen et al.103 and Sims et al.147.

3Conservation agriculture in Southern Africa

(Fig. 1). This increase in infiltration rate was mainly dueto an increase in biological activity, reduction in soilsurface disturbance and the continuity of macropores13.Similarly, studies conducted under the semi-arid condi-tions of Zimbabwe showed that over time CA practicesimproved hydraulic properties (unsaturated hydraulicconductivity and capillary sorptivity) of a clay loamsoil65. However, studies also showed that the effect of CAon water infiltration was dependent on soil type, with thepotential negative effect of waterlogging on granitic sandysoils, which have a tendency to accumulate too muchwater23,30.Water balance studies over a 5-year period (1994/5–

1998/9) on clay soils at Hatcliffe, Zimbabwe byNyagumbo59, allowed water losses through runoff andevapotranspiration to be compared between a CA system

in the form of mulch ripping (MR) and conventionalmoldboard plowing (CMP) (Fig. 2). Using an improvedsimple water balance technique derived in situ over fiveseasons (1994/5–1998/9), only 26% of the total rainfallunder CMP was contributed to groundwater rechargecompared with 50% under MR. Average runoff ofseasonal rainfall was also reduced under MR, with only1% of rainfall lost due to run off compared with 20%under CMP. Although the difference between seasonalevapotranspiration under CMP and MR was small (51%compared to 46%) soil moisture storage within the top45cm of soil was significantly greater under MRcompared with CMP59 (Fig. 2). Field water balancemodeling studies in South Africa showed that no-tillsystems (rip-line seeding systems without mulch retention)reduced surface runoff by 28% and increased deepdrainage by 19% on a sandy clay loam soil compared toCMP66.Studies on soil moisture by Thierfelder and Wall23

confirmed that CA treatments on an Arenosol atHenderson, Zimbabwe and a Lixisol at Monze, Zambia,had more available soil moisture than conventionallyplowed treatments. The results of these studies showed

Figure 1. Infiltration rates measured with a mini-rainfallsimulator at Henderson Research Station (a) and the MonzeFarmer Training Centre (b) after 8 and 7 years of treatment,January 2012 (Thierfelder, 2012, unpublished data).

Run-off ET Drainage Delta-S

Figure 2. Effects of conventional moldboard plowing andmulch ripping on different water balance components over fiveseasons on a red fersiallitic clay soil at Hatcliffe, Zimbabwe.Note: delta-S, change in profile water storage down to 140cmdepth; ET, actual evapotranspiration (adapted from59).


that CA techniques increase soil water balance attributeswhen compared with conventional plowing. FurthermoreCA systems often resulted in higher water productivitycompared to conventional plow-based tillage systems, byup to ±10kgha−1mm−1, depending on seasonal rainfallpatterns59. Only one study has been conducted on deepdrainage and leaching on a granitic sandy soil, usinglysimeters, at Domboshawa Training Centre, Zimbabwe.The results suggest no-till tied ridging system resulted in21% more deep drainage and consequent nitrate leachingthan CMP60, which could potentially have negative effectson plant growth.In summary, CA generally increases water infiltration

and improves available soil moisture. This can potentiallyreduce the negative effects of in-season dry spells, reducerun-off and provide more available water for plantgrowth. Nevertheless, there are also findings that waterinfiltration in CA systems is dependent on soil type, withthe potential negative effect of waterlogging on graniticsandy soils23,30.

Effects of CA on physical and biological soilquality attributes

Studies on physical and biological properties in Zambiashowed more stable soil aggregates (41–45%) in directlyseeded CA systems compared with the conventionalsystem (24%)13 (Fig. 3). This stability promoted betterwater infiltration and soil water storage in the directlyseeded CA systems compared with the conventionalpractice. Stable soil aggregates protect soil organic carbon(SOC) and improve soil structure under CA or no-tillsystems on soils with 22% clay and 7% silt67. InZimbabwe, the positive impact of CA on soil aggregate

stability was observed on both light and heavy texturedsoils under semi-arid and sub-humid conditions30.In the semi-arid southern regions of Zimbabwe, bulk

density of a clay loam soil decreased over time in CAsystems (rip-line seeding systems and planting basins)compared with the conventional practice68. Soil bulkdensity is a function of soil texture, SOC content andaggregate stability69. In a long-term tillage experiment onboth red clay and sandy soils under sub-humid climateconditions, Nyagumbo59 observed significantly lower topsoil bulk densities under MR compared with CMP onboth sandy soils and clay soils. In sandy soils, significantdifferences were apparent only in the top 0–10cm andgreater bulk densities were recorded under conventionallyplowed plots. On clay soils, significantly lower bulkdensities in MR compared to conventionally plowed plotswere observed at depths of 10–40cm. The higher bulkdensity of the top soil under plowed uncovered fields wasattributed to faster disintegration of soil aggregates by theimpact of raindrops, which resulted in surface compac-tion, particularly in sandy soils with poor soil structure70.These results suggested that physical benefits are asso-ciated with reduced tillage management, although soiltype plays a critical role. Tillage is often promoted as away of loosening up the soil before planting. The resultsdo, however, suggest that this is only a temporarymeasureas most soils quickly lose their stability after tillage, slumpand lead to even greater compaction43,71.There is general agreement that no-tillage and residue

retention increases infiltration and reduces surface run-offleading to lower erosion rates23. A 4-year study underrainfed conditions in Ethiopia72, comparing CA and aconventionally plowed control (CP) showed that CAreduced soil erosion and run-off. In Zimbabwe, an 8-year

Figure 3. Effects of conservation agriculture and rotation on aggregate stability, Monze Farmer Training Centre, Zambia, April200913.


study showed increased average annual soil losses of5.1 tha−1 under conventional plowing compared with1.0 tha−1 under MR without soil inversion but residueretention20. Recently, Thierfelder et al.73 showed thatlarger increases of cumulative soil erosion occurred onconventionally plowed fields compared to rip-line seededCA treatments with sole maize or maize–legume inter-cropping (Fig. 4).CA has also been associated with increased soil

biological activity. Nhamo74 showed that there wasmore abundance and activity of soil biota under maize-based CA cropping systems than with conventionalpractice, both on an experimental research station andon-farm in Zimbabwe. Under residue-covered fields,termites were more abundant, particularly in the sandysoils. Tillage and removal of residue disturb their habitatsand also limit energy sources for termites, while differentmulches (maize or grass residues) that contain celluloseand crude protein attract termites. Increases in termitenumbers show a clear effect on increased biologicalactivity. This does not necessarily translate into entirelypositive effects (i.e., increased nutrient mobilizationthrough residue decomposition) as crops (especiallycereals) might be attacked by specialized termite species,especially toward harvest when residue cover would havediminished51. Furthermore, studies on two contrastingagro-ecologies in Zimbabwe showed that reduced tillagewith residue cover yielded significantly greater speciesrichness and macrofauna abundance than conventionalsystems, with a significant and positive correlation be-tween residue application rates and species richness75.Although there are conflicting results on the effects of

CA on termites, beetles and centipedes74, increasedearthworm activity has been observed in residue-covered

fields compared to conventionally plowed fields inZambia (Fig. 5)13. Soil texture, moisture and organicmatter have a significant effect on earthworm densities,with decreased density in soils with high clay content andorganic matter in Zimbabwe74. Declining earthwormdensities have been previously attributed to the effect ofsoil inversion by the plow76, the effects of temporaryreduction in soil pores, especially macropores77 and lowersoil moisture on conventional fields without mulchprotection.In summary, CA leads to greater aggregate stability

and, in the long-term, also reduces bulk density. Studieson biological soil fertility are rare and show that CAincreases biological activity which can be positive (moreearthworms) or can be potentially negative if termites area problem at the site.

Effects of CA on soil carbon

In-situ retention of crop residues coupled with no-tillagehas the potential to increase SOC. The conversion of fieldsfrom conventional tillage to no-tillage has been shown toincrease SOC in a sandy loam soil in farmers’ fields inMalawi78 and on sandy soils in both Zimbabwe30,79 andZambia13. However, consistent and sufficient carboninputs are a major determinant of changes in SOC ratherthan tillage27, and lack of sufficient carbon inputs can leadto no, or very slow, increase80.Intercropping legumes and maize increased SOC

under no-till, largely due to increased biomass productioncompared to no-till continuous maize cropping. On asandy soil in Zimbabwe, SOC in the first 30 cm increasedwhen maize was intercropped with pigeonpea comparedto continuous maize under no-till (+12%) and

Figure 4. Effects of two conservation agriculture and one conventionally plowed cropping system on cumulative erosion,Henderson Research Station, Zimbabwe, 2006–201273.


conventional tillage (+34%)73. Furthermore in centralMozambique on similar soil types, Rusinamhodzi et al.81

reported a significant increase in SOC after 5 years ofintercropping (Table 2). Farmers can use a pigeonpea–ratoon crop, reducing the need for tillage while increasingbiomass accumulation, which together have the potentialto significantly increase SOC. These results suggest thatin the long-term CA may be important for maintainingSOC relative to conventional agriculture. However, thegreatest increase might only be achieved when crops areadequately fertilized, sufficient biomass is producedand adequate residue amounts are retained30,80. Withoutsufficient fertilization (e.g., through mineral fertilizer,manure and/or compost) farmers cannot raise theirproductivity to a level that enables them to retain atleast 2.5–3 tha−1 of biomass, which is needed to cover atleast 30% of the soil surface. Rotations and intercroppingsystems might also add to the carbon pool, but theirpotential to increase SOC largely depends on the agro-ecological environment and their overall biomass pro-duction73,81.It is worth noting, that while reduced tillage helps to

reduce decomposition rates, it may also play a negativerole in SOC stratification within the soil30. SOC increasesare expected over time if the amount of crop residuesretained is more than the loss through the oxidationprocess30. However, the importance of CA in the shortterm might be more in the maintenance of SOC ratherthan in its absolute increase. Nyamangara et al.80

generally found no significant increase in soil carbonand phosphorus in their study from Zimbabwe, althoughthere was some accumulation in organic carbon inone agro-ecological region in CA fields relative to

conventionally plowed fields when CA was practiced fora longer period.

Pest and disease dynamics

CA has been associated with increased insect pests anddiseases. However, one of the main principles, croprotation, is the most important strategy available forthe smallholder farmer to break pest and disease lifecycles13,82. The parasitic weed striga (Striga asiatica (L)Kuntze) is associated with large losses in maize pro-duction throughout Southern Africa83. Estimates forgrain yield loss due to striga in farmers’ fields rangebetween 15 and 95%84. The effects of striga have also beenassociated with declining soil fertility85. A study compar-ing CA with herbicides and conventional treatments withmanual weeding only on 54 paired demonstration plots inMalawi86 showed a drastic reduction in striga emergenceunder CA fields (Fig. 6). Rusinamhodzi et al.81 alsoobserved a significant decrease in striga when intercrop-ping was used in a no-till system compared to continuouscropping of maize.No-till, residue retention and increased soil moisture at

the surface can promote the survival of pathogens untilthe next crop is planted87. Predominant insect pests anddiseases of maize in Southern Africa include grey leaf spot(GLS), maize streak virus (MSV), rust, ear rots andstriga88. GLS is caused by the fungal pathogenCercosporazeae-maydis and is a serious foliar disease associated withyield loss in Southern Africa89. The increase of GLS intemperate maize has been linked to reduced tillagepractices90. With crop residue retention, the fungalpathogen responsible for GLS remains in the debris of

Figure 5. Impact of conventional and conservation agriculture on earthworm counts (per m2) in the first 30cm in three consecutiveyears. Monze, Farmer Training Centre, Zambia, 2007–200913.


diseased plants, providing an earlier and more extensivesource of inoculum for the nextmaize crop. This facilitatesearlier infection of the next maize crop and allows moresecondary cycles of the fungus91. Work is currentlyunderway in Southern Africa to determine if there isincreased prevalence of GLS in CA-based systems andwhat can be done with conventional and molecularbreeding to overcome this challenge88. MSV is trans-mitted through leafhoppers (Cicadulina spp.). Little isknown about the effects of CA on leafhopper populationsand MSV infection and further research is required.Ear rots are caused by a range of fungi includingFusarium graminearum, F. verticillioides, Aspergillusflavus and A. parasiticus92. Lipps and Deep93 showedthat reduced tillage decreased the prevalence ofF. graminearum.The incidence of white grubs (larvae of Phyllophaga

ssp. and Heteronychus spp.) has been highlighted as apotential pest under CA51 based on one observation infarmers’ fields in Barue, Mozambique. This was alsoobserved on an on-station, long-term trial at Chitedze,Malawi. However, when maize–legume rotations (i.e.,maize–cowpea rotations) were introduced on these sites,

white grub damage was not observed. Together theseresults suggest that while pests and diseases carried overthrough residues can be a threat to successful implemen-tation of CA systems, appropriate rotations and chemicalcontrol measures can be used to overcome most of thesechallenges for the smallholder farmer.

Effects of CA on productivity

Significant yield benefits under CA in Southern Africaare possible, although they may be site-specific and inresponse to different agro-ecological environments30.Studies have shown that rotation94 as well as appropriatefertilization80 are critical for CA results to become sig-nificant. This was successfully shown in componentomission trials inMalawi,Mozambique and Zimbabwe95.In some environments the benefits of CA started after

1–2 seasons, whereas in other environments the benefitsrequired more seasons for effects to build up 23,94, which isin agreement with a recent meta-analysis96. For example,in the high-rainfall environment of Zidyana EPA,Malawi, characterized by sandy loam soils, substantialmaize yield benefits were obtained after five seasons28.

Table 2. Selected top-soil (0–20cm) properties of fields under different durations of maize–pigeonpea intercropping in Ruaca, centralMozambique (source: 81).

Duration ofintercropping

Bulk density(mgm−3) pH

Organic C(%)

Total N(%)

Available P(mgkg−1)

Exch. K(cmolckg

−1)Exch. Ca

(cmolckg−1)

0* 1.5 5.9 0.2 0.02 0.7 0.1 0.51 1.4 6.0 0.6 0.04 2.8 0.2 2.23 1.4 5.9 1.2 0.08 6.9 0.3 3.65 1.3 6.0 1.4 0.09 8.4 0.3 3.8

* Corresponds to continuous sole maize production.

Figure 6. Incidence of striga (S. asiatica L.) on CA plots (with and without legume intercropping) and conventionally plowed fieldafter 5–8 years of treatment, central and southern Malawi (Thierfelder, 2012, unpublished data).


This lag period between implementation and effects of CAis mainly related to the need to produce sufficient cropresidues and improve degraded soil fertility, applying theright fertilizer and seed, equipment, planting at the righttime and inclusion of a systematic rotation scheme. Inon-farm trials in Monze, Zambia, incremental benefits ofCA systems compared to CP treatments were significantfollowing the third cropping season97 (Figs. 7 and 8),contrary to the suggestion that CA needs a very long timeuntil yield benefits materialize51.Although maize yield benefits in CA systems take 3–5

seasons to occur, with some exceptions in unfavorableenvironments, the trend is not as clear when maize isintercropped or rotated with legumes, which tend torespond less to fertility increases and water conservation.Increased water accumulation in the soil can cause rootrot, thereby reducing the yield of legumes98. Legume yielddata from Malende, Monze (Zambia) from 2007 to 2012showed very variable results between two CA treatmentsand the conventional control97, with significant yielddifferences only after the sixth cropping season (Fig. 9).In low-yielding environments CA has potential to

double the maize yields obtained under conventionaltillage, which was previously shown by Thierfelder andWall30 in a study carried out at Zimuto Communal Area,Zimbabwe. Under semi-arid conditions of southernZimbabwe, CA (planting basin and rip-line seedingsystems) produced 102–142% more cowpea grain com-pared to conventional practice in a drought year68.

However, in a season with above average rainfall, CAand conventional systems produced similar cowpeayields.

Figure 7. Effects of rip-line and direct seeding CA treatments as compared to conventionally plowed control treatments atMalende Agriculture Camp, Monze, Zambia, 2006–2012 (adapted from97).

Figure 8. Effect of two CA treatments and a conventionalplowed control treatment on maize grain yield at MalendeAgriculture Camp, Monze, Zambia, 2006–2012 (adaptedfrom97).


Maize yields under no-till with mulch retention weremarginally better than under conventional tillage in aregional study on long-term trials in Southern Africa94,i.e., closer to the 1:1 line (Fig. 10) but the inclusion ofrotation or intercropping systems led to yields that werecloser to the 1:2 line, implying that in some instances yieldunder CA was almost double that in conventional tillage.The results also highlighted the importance of legumeswithin a rotation and crop diversity. Substantial yieldincreases were observed, and in some cases maize yieldsfollowing legumes were almost double that of continuousmaize under no-till. At Henderson Research Station inZimbabwe, there was a significant increase in maize yieldsplanted after sunnhemp after several years73.CA has generally been reported to increase labor use

efficiency and returns per unit labor compared toconventional agriculture99. For instance, significantlyhigher labor productivity (in kg person-day−1) andreturns to labor (USD person-day−1) for CA wereobserved compared to conventional farming across low,average and high seasonal rainfall levels inZimbabwe99–101. Mazvimavi et al.79 also showed thatfarmers practicing CA increased yields by 10–100%compared with conventional, depending on fertilizerrates and management, experience of the farm householdand seasonal rainfall patterns.Similar results were reported by Ngwira et al.28 in

Malawi, where the cost of producing a kilogram of maizewas shown to be less under CA systems compared to

conventional practices. Although labor input in this studywas small compared with previous studies, they showedthat CA reduced farm labor requirements (Table 3).Related studies in Malawi showed that returns to laborwere more than twice as high under CA using monocrop-ping and intercropping with legumes, both on and offfarm, as under conventional tillage practice using theconventional ridge and furrow system28,102.The use of planting basins, a technology promoted by

most development organizations in Zambia andZimbabwe, based on manually dug planting holes103,spreads out the labor requirement for land preparationand encourages timely planting, thereby reducing thepeak labor load at planting104. Although using basinsrequired more labor for smallholder farmers in Zambia ascompared to the hand hoe and animal traction system105,Haggblade and Tembo106 highlighted that the majoradvantage of using basins was the disaggregation of peaklabor times. However, Umar et al.105 stressed that farmersperceived labor requirements very differently and,whereas basin planting was perceived to be more laborintensive, significant labor reductions were recorded onanimal traction ripping. Results for manual CA systems inMalawi28,102 showed labor savings of about 9 and 19person days ha−1 in land preparation and weeding underCA. Manual CA systems practiced in Malawi involvedirect seeding using a dibble stick, which requiressignificantly less labor than preparing the traditionalridges and furrows manually.

Figure 9. Effect of two CA treatments and a conventional plowed control treatment on cowpea grain yield at Malende AgricultureCamp, Monze, Zambia, 2006–2012 (adapted from97).


Other results do, however, highlight that reduced laborrequirements around planting in CA systems can be offsetby increased labor for weeding, particularly in the earlystages of adoption104,107. Nevertheless herbicides can beused to reduce labor requirements under CA if they areavailable and affordable to smallholder farmers108,109.Although it is true that CA often conforms to what

Pampel and van Es110 termed as an ‘environmentallyprofitable practice’ (i.e., good for the environment and

profitable), this is not always the case. Particular location-specific constraints might result in reduced yields, orinstitutional factors may favor alternative practices51,111.Thus, it is necessary to consider site-specific conditions indetermining the financial attractiveness of CA. Evenwhere the financial incentives may appear attractive, aconsideration of non-financial factors (e.g., risk of cropfailure) is required to understand the actual and potentialadoption of CA112.

Figure 10. Effects of conservation agriculture on maize grain yield: (a) yield advantage of no-till plus mulch over conventionaltillage, (b) yield advantage of no-till and residue retention with non-legume rotation over no-till and residue retention withoutrotation, (c) yield advantage of no-till and residue retention with legume rotation over no-till and residue retention withoutrotation and (d) yield advantage of no-till and residue retention with 3-year rotation over no-till and residue retention withoutrotation (source:94).


Constraints to Adoption of CA Systems inSouthern Africa

Although the practice of CA can provide many benefitsfor smallholder farmers in Southern Africa, large-scalespontaneous adoption has been hampered by a number ofconstraints36,51. These constraints include trade-offsbetween residue retention and livestock feed in mixedcrop–livestock systems, the need for crop rotations andintercropping, weeding intensity and control, laborconstraints and lack of access to complementary inputs.Often, significant knowledge gaps and the perception offarmers that tillage is necessary to produce crops aremajor impediments to widespread adoption of CA36.The majority of studies on CA in Southern Africa

highlighted the beneficial effects of CA on selectedparameters. However, these past researches were largelyconducted within experimental stations and did not takeinto account socio-economic constraints in real farmsituations that could hinder the widespread adoption ofCA. Recently, more studies reporting results from on-farm and on-station trials have been published concen-trating on some of the above-mentioned challenges andconstraints28,37,68,73,86,94,96,97,99,101,102,104,113–116.

Trade-offs in mixed crop–livestock systems

Crop and livestock production are closely integratedin mixed smallholder farming systems of SouthernAfrica117–119. Crops supply residues to feed livestockduring the dry season while livestock, in turn, providedraft power andmanure for crop production37,119–121. Thejudicious use of livestock manure could greatly enhancecrop productivity in these nutrient-deficient systems122.For example, a study in the semi-arid Shurugwi region of

Zimbabwe showed that banded application ofmanure in arip and pot-hole systemwithout any residue cover resultedin almost a three-fold yield increase compared to theunfertilized conventional control123.In Southern Africa, livestock also plays a crucial role in

the livelihoods of smallholder farmers as a source ofdraught power, cash income and as a risk buffer124,125.Crop residues are also used for bedding in kraals (animalpaddocks) during the rainy season, construction (fencingand thatching) and as a source of fuel79,126.With the advent of CA systems, Southern Africa

smallholder farmers face the challenge of trade-offsbetween keeping crop residues for application on thefield as mulch and feeding them to livestock37.Contribution of communal rangelands toward livestockfeed requirements is reduced by the poor quality andseasonal variability of forage biomass quantities pro-duced124,127. Innovative ways of enhancing already de-veloped legume, grass and agroforestry species as fodderare therefore required and need to be integrated into CAsystems. This will allow part of the crop residues to beused in CA systems and to feed livestock as fodder duringthe dry season121,125,127. New ways of additional residueproduction through intercropping and relay cropping areother alternatives that should be explored.

Intensification

Rotations in land-constrained farming systems.Although farmers are aware of the benefits of croprotations, socio-economic factors often hinder integratingrotations within their farms128,129. In Malawi, forexample, the government subsidized maize seed andinputs (inorganic fertilizer, agrochemicals) to achievemaize self-sufficiency at the national level, which encour-aged monocropping102. In areas where farm sizes are

Table 3. Farm enterprise budget analysis for conservation agriculture and conventional tillage practices, Malawi, 2005–2011.(source: 28).

Lemu Zidyana

CP CA CAL CP CA CAL

Gross receipts 528.6 881.5 979.7 1047.2 1309.5 1293.7Variable costsInputs 238.5 341.0 353.6 221.7 323.7 346.1Labor days (6h days) 61.7 39.9 49.4 61.7 39.9 49.4Labor costs 159.5 103.2 127.9 155.6 100.7 124.7Sprayer costs 1.7 1.2 1.7 1.2Total variable costs 398.1 445.9 482.8 377.3 426.1 472.1Net returns (US$/ha) 130.5 435.5 497.1 669.9 883.3 821.9Returns to labor (US$/day) 1.8 5.2 4.9 5.4 9.8 7.6

Note: CP, conventional practice; CA, conservation agriculture sole maize; CAL, conservation agriculture maize–legume intercrop.Labor data (in person hours and minutes) was obtained from one on-farm trial per site for each operation (laying crop residues,tillage, herbicide application, planting, fertilizer application, weeding, harvesting, etc.). Price data were based on all applied inputs(seed, herbicides, fertilizers, etc.) from each of the plots during the entire period of the study.


small, farmers often dedicate most of their land area to thestaple food crops (maize, sorghum) rather than rotationalcrops such as legumes13, which has serious implicationsfor the nutrition of smallholder farmers. Where the size ofthe land holdings are greater, such as in Zambia, there isgreater potential for crop rotations, although marketsmay limit successful implementation94. The incomegenerated through a range of different crops (includinglegumes) can be less than the production costs.Furthermore, seed companies are often not interested inseed production of most legume crops. These crops areself-pollinating, so farmers can easily recycle rather thanbuy new seed.Intercropping systems—the struggle to produce more

from given land. Intercropping maize and legumes isanother way to achieve food security and cash income forsmallholder farmers, and abundant literature fromSouthern Africa supports this observation81,102,130–133.Although intercropping with legumes is associated withmany benefits, including improving soil structure andfertility and the potential to increase farm income throughselling legume grain, the potential of legumes withinfarming systems of Southern Africa is currently limitedby a plethora of constraints. These include poorly de-velopedmarkets for seed and produce, lack of appropriategermplasm and phosphorus-deficient soils134,135.Intercropping maize and pigeonpea has been shown tobe highly profitable in Southern Africa, with a tripled rateof return compared to maize cropping alone, although thenew spatial arrangement increased labor requirements forweeding by 36%81. In a maize–pigeonpea intercroppingsystem, Ngwira et al.102 also reported higher rates ofreturn compared to maize cropping alone, althoughmaize

yields were lower in this study due to drought stresscompetition between main- and intercrop. The authorsconcluded that intercropping maize and pigeonpea underno-till was an attractive option due to associated maizeyield improvement and economic returns when the pricesof maize and pigeonpea grains are favorable. However,the late maturity of pigeonpea can be a problem in mixedcrop–livestock systems as free-grazing of cattle and goatscan destroy the entire crop before harvest.In contrast, Waddington et al.132 showed intercropping

was an unattractive option compared to growing maize inmonoculture, based on the results of a 12-year study onconventional agriculture in Zimbabwe. This was attrib-uted to the small yield benefits compared to the relativelyhigh investment costs for legume intercropping. In centralMozambique (Fig. 11) the emergence of a market forpigeonpea was a major driver for intercropping, despitedestruction by livestock of the late maturity pigeonpeacrop81. It is important to note that the crop used forintercropping is crucial, and poorly chosen intercrops andthe timing between both crops may have a negative effecton productivity, particularly when the farmer has limitedaccess to already scarce resources. Intercropping maizeand cowpea can potentially reduce maize yields by up to86% as compared with a sole maize stand131 if the timingbetween both crops is poor (e.g., when cowpea is seededtoo early, competition is too high), or can reduce thecowpea yield when cowpea is seeded too late and maize isshading too much. These results suggest that the benefitsof intercropping are not universal but vary acrosslocations, seasons, timing between the main crop andthe intercrop, and the crops used for intercropping. Socio-economic constraints can also have an overriding effect on

Figure 11. The proportion of farmers practicing maize–legume intercropping between 2007 and 2011 in Ruaca and Vunduzicommunities in central Mozambique. Proportions are based on a total of 52 households in Ruaca and 43 households in Vunduzi(adapted from:81).


the widespread uptake of intercropping. Thierfelderet al.73 observed that despite the plot-level benefits ofintercropping as reported in literature, the benefits wereonly measured at the field scale and lacked properintegration at the farm and community level128,129.

Weed incidence when converting to CA

A major challenge to the adoption of CA in smallholderfarming in Southern Africa is related to weeds and theirmanagement51,105,113. The dynamics of weed populationsunder CA are different from conventional tillage prac-tices, including more complex interactions between weedsand crops under CA136,137. In the semi-arid south ofZimbabwe annual and perennial monocotyledonous anddicotyledonous weed species (e.g. Cynodon dactylon L.,Richardia scabra L. and Commelina benghalensis L.) werefound to thrive differently under conventional and no-tillage practices113 and have been reported to be majorproblems when tillage is minimized and residues areretained138. Weed density varied across crops, with thedensity of annual and perennial weed species higher undersorghum compared to cowpea113. However, duringperiods of low rainfall, crop residue retention was foundto be associated with increased mid-to-late season weedgrowth, in part due to improved soil moisture conser-vation in a clay-loam soil (Cambisol) in Zimbabwe113.Mulch initially suppressed annual weeds but perennialrhizomes and their extensive root structure persisted underno-tillage and led to increased late-season weed growths.Weed species found in Southern Africa are affecteddifferently by mulching, and mulch levels of >4 tha−1

suppress a wider spectrum of weeds113.The application of pre-emergence herbicides can limit

weed development under CA, although there are potentialproblems of evolved weed resistance to herbicides139.Mwale140 reported a greater reduction in weed popula-tions and weed seed banks under CA with herbicideapplication compared with conventional tillage systems,particularly during the first 55 days after planting maize.Herbicide use in CA can therefore potentially relievefarmers from weeding operations during the critical timesthat coincide with periods of food insecurity in mosthouseholds. In a study carried out in Zimbabwe, the use ofherbicides in combination with manual weed control wasfound to be beneficial both in time spent on weeding,reduction in weed densities and overall gross margins109.However, labor reductions as such have little benefit tofarmers if the saved labor is not used for additionalincome generation. Discussions with farmers revealedthat the saved labor is used for additional cultivation,dedication of land area to higher-value crops, productionand value addition to farm products (e.g., makingdoughnuts, jam or marmalade and drying cowpea leavesto sell as relish on the market), and off-farm labor, whichin turn has a direct impact on farmers’ livelihoods.Although chemical weed control can be (but does not need

to be) a component in CA systems, the suitability ofpromoting such a system among resource-poor farmershas been questioned due to accessibility and affordabil-ity141, as well as the dangers of handling herbicides102.The use of green manure cover crops such as oats

(Avena sativa L.), grass pasture142 and velvet beans(Mucuna pruriens L.) has been shown to be effectivein managing weeds in maize/cover crop rotations143.However, legumes that do not contribute to the householdfood basket have often been rejected by farmers in thepast, which indicates a constraint to integrate greenmanure cover crops (GMCCs) into the smallholderfarming systems134. There is therefore a need to evaluatethe profitability and sustainability of these croppingsystems over a longer period under different farmers’circumstances.

Labor constraints

The availability and costs of labor play a pivotal role inthe likelihood of farmers to adopt a new technology.Certain forms of CA practices (e.g., the basin systems,based on manually dug planting basin) can be more laborintensive than conventional agriculture if compared toanimal traction moldboard plowing, particularly duringthe initial stages of adoption115, which can be a hugeconstraint to thewidespread adoption on large farm areas.When CA includes planting basins and manual weeding,high labor demand is largely associated with the diggingof planting basins and weeding practices99,107.The labor intensity of manual CA systems is, however,

expected to decline, especially on loam and clay soils asfarmers gain experience in using the technology. Once theinitial digging in undisturbed soil is done the soil becomessofter in the basins in the following seasons101,106. Thisdoes not, however, apply to the sandy soils, as basinscannot be maintained throughout the winter period.In summary, labor constraints are one of the major im-pediments to widespread adoption of CA, which hasbeen shown in recent studies by both ICRISAT andFoundations of Farming, which reported that farmers inZimbabwe tended to have only 0.5ha or less of their farmsunder CA from land sizes of 4–6ha116.

Lack of access to complementary farm inputsand an enabling environment

The effect of the intertwined challenges discussed above iscompounded by lack of access to complementary farminputs, such as fertilizer, herbicides and pesticides, thatcould offset some of the adversities. Lack of access toinputs in Southern Africa are due to low purchasingpower, distorted or missing markets and lack of socialinfrastructure8,144.Perceived economic constraints to the adoption of CA

are likely to vary between countries as well as marketenvironments as they are determined by the distance tomarkets, infrastructure and an overall enabling


environment. To discover these relationships a QualitativeAssessment Tool for Conservation Agriculture(QAtoCA) was tested in Malawi and Zimbabwe toidentify the major constraints to adoption of CA andto dissect complex interactions within the community,markets and political environment145,146. Focus groupdiscussions in a guided process revealed that the overalllikelihood of adoption for CA in Malawi was 86%compared to only 64% in Zimbabwe (Fig. 12a and b).These results showed that the environment for adoption ofCA at both the village and regional scale is much morepositive in Malawi than Zimbabwe, where constraintssuch as competition for residues between livestock andCA tended to reduce adoption. Malawian farmers usemanual seeding technologies and herbicides for weedcontrol and found CA to be an attractive option becauseof the reduced labor requirements during land prep-aration. Zimbabwean farmers, on the other hand, usemore animal traction for land preparation and found that,with no access to herbicides, manual requirements forbasin preparation and weed control were more labordemanding under CA101,113. Other economic factors inZimbabwewere also amajor deterrent to the CAadoptioncompared to Malawi, where the markets were perceivedto be more favorable (Fig. 12a and b). At the time ofthe study Zimbabwean farmers faced serious problemsaccessing attractive markets for maize, the most commoncrop, that were not governmentally controlled. Insummary, access to complementary farm inputs and anenabling economic and political environment are crucialfor farmers in the decision process to adopt CA. If theadversities are too challenging, the likelihood of farmersadopting the technologies will be slim.

Conclusion

This review highlights recent advances in understandingthe practice of CA in Southern Africa. Results show thatCA has many environmental sustainability benefits,which can be summarized as: increases in infiltration,resulting in greater available soil moisture and a higher

potential for groundwater recharge; reduced soil erosionand run-off; improved soil structure, more biologicalactivity, e.g., through greater macrofauna abundance anddiversity for system resilience; better soil quality. CA wasalso shown to offer direct benefits to farmers, includingincreased crop productivity, which could lead to betteryields and hence increased food security, although there isoften a time delay until yield increases in maize and othercrops become significant. Regional labor productivitystudies show benefits of CA systems over conventionalpractices, although little work has been done so far toclearly understand the risk implications of CA at thefarms and on a larger scale. In cropping systemstraditionally based on manual land preparation, CAoffered an opportunity for labor savings to othereconomic activities through reduced labor on landpreparation, particularly when applied in combinationwith herbicides. However, where animal traction iscommon (e.g., in large parts of Zimbabwe and Zambia),hoe-based CA systems suffered setbacks of increasedlabor compared to moldboard plowing. In similar cir-cumstances, where farmers used only the hoe for landpreparation, CA still offered huge advantages in terms ofenabling farmers to plant timely, as basin preparation wascarried out in winter and reduced labor at peak labortimes.Nevertheless, the review also shows that CA promotion

and adoption is constrained by a number of biophysical,socio-economic and socio-cultural factors, which arevaried and depend on the site and the farmer’s circum-stances. These constraints include challenges of retainingresidues in areas where mixed crop–livestock systems arecommon; application of rotations and sustainable in-tensification strategies due to dysfunctional input andoutput markets; the control of weeds in CA systems; pestsand diseases; institutional, labor and market constraints.Often the mind-set and knowledge of smallholder farmersabout no-tillage and residue retention plays a critical rolein the adoption process of this relatively new way offarming.If sustainable intensification through CA should be

one of the pathways to agricultural development in

Figure 12. An evaluation of the likelihood of CA adoption using the QAtoCA in (a) Nkotakota District, Malawi and (b)Goromonzi District, Zimbabwe in 2011 (adapted from146).


Southern Africa, there is a need to better understandfarmer decision processes, and more research is essentialto overcome, or at least find solutions for, these criticalbiophysical and socio-economic challenges.Generalizing the economic performance of CA under

different circumstances is not appropriate. Site-specificfactors determine technologies that offer the best resultsfor individual farmers. Given the glaring paucity ofempirical evidence about the farm-level impact of CA onlabor productivity and the financial viability of CA inSub-Saharan Africa, it is imperative to suggest a case-by-case analysis of CA impacts on labor productivity overtime and from a society-wide perspective.This review of CA as a crop management system for

smallholder farmers in Southern Africa revealed that it isnot a ‘one-size-fits-all’ solution to all of the farmers’problems, and often there is a need for significant adapt-ation and flexibility in its application. CA is therefore nota fixed recipe but a set of principles that have to beadapted and fine-tuned to the needs of the farmers.However, the benefits of practicing CA can potentiallyreduce future soil fertility decline, the effects of seasonaldry-spells and socio-economic constraints, and may havea large impact on food security and farmers’ livelihood ifthe challenges can be overcome.

Acknowledgements. This work was financially supported by theInternational Fund for Agriculture Research (IFAD), theGerman Government (BMZ), the Climate Change, Agriculturaland Food Security (CCAFS) and MAIZE Consultative Groupon International Agriculture Research (CGIAR) research pro-grams and by the work of numerous farmers, field extensionofficers and researchers in Southern Africa. Their contribution isgratefully acknowledged.

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