sustainable agricultural development in sub-saharan africa: the case for a paradigm shift in land...
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Sustainable agricultural development in sub-SaharanAfrica: the case for a paradigm shift in land husbandry
J . W. Gowing & M. Palmer
School of Agriculture, Food and Rural Development, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
Abstract
In order to tackle poverty and hunger in sub-Saharan Africa (SSA) there is a strong case for a focus
of effort on improving rainfed agricultural systems. The challenge is to deliver a transformation of
agricultural productivity in such systems without adverse impacts on environmental goods and ser-
vices. We examine the growing advocacy of ‘conservation agriculture’ (CA) as the desired approach
and assess the evidence to support the assertion that it can deliver sustainable agricultural develop-
ment in SSA. We examine in particular the evidence which derives from experience with ‘zero tillage
sustainable agriculture’ in Brazil. We ask the question, is there a case for a paradigm shift in land hus-
bandry? The case for a paradigm shift hangs on the premise that conventional practice promotes land
degradation, while adoption of CA practice delivers a range of benefits through promoting soil ecosys-
tem health. The guiding principle is to promote biological tillage through minimizing mechanical soil
disturbance and maintaining permanent organic soil cover. We examine evidence of benefits in the
context of the wider debate on low-external-input technology. We conclude that CA does not over-
come constraints on low-external-input systems and will deliver the productivity gains that are
required to achieve food security and poverty targets only if farmers have access to fertilizers and her-
bicides. We conclude also that widespread adoption of the new paradigm amongst millions of small
farmers in order to achieve the ‘doubly green revolution’ in SSA is subject to the familiar constraints
of knowledge transfer and success will depend upon creating innovation networks. Further, we con-
clude that amongst small-scale farmers partial adoption will be the norm and it is not clear that this
will deliver soil health benefits claimed for full adoption of the new paradigm.
Keywords: Africa, conservation agriculture, land husbandry, sustainable development
Introduction
Fifty years ago there were fewer than half as many people as
there are today. They were not as wealthy and the pressure
they inflicted on the environment was lower. The widely held
view then was that global development would necessarily
involve increased exploitation of environmental resources, in
particular, land and water. Since then population growth
and dietary change have driven up demand for food and
other agricultural products and in the next 50 years, global
food and feed crop demand is expected to double. The con-
sensus view has changed; now there is greater recognition of
conflict between development and environmental protection
objectives and many commentators are asking: is there
enough land and water to deliver food security and sustain-
able development over the next 50 years? (Greenland et al.,
1998; Rosegrant et al., 2002; Brown, 2004; CAWMA, 2007).
Choices about management of land and water resources
will determine to a large extent whether we reach the inter-
linked multiple objectives of economic and social develop-
ment as articulated in the Millennium Development Goals
(MDGs).1 The challenge to the world community on poverty
and hunger in particular is encapsulated in two ambitious
targets:
Correspondence: J. W. Gowing. E-mail: [email protected]
Received July 2007; accepted after revision September 2007
1Goal 1, eradicate extreme poverty and hunger; Goal 2, achieve uni-
versal primary education; Goal 3, promote gender equality and
empower women; Goal 4, reduce child mortality; Goal 5, improve
maternal health; Goal 6, combat HIV ⁄AIDS, malaria and other dis-
eases; Goal 7, ensure environmental sustainability; Goal 8, develop a
global partnership for development.
Soil Use and Management, March 2008, 24, 92–99 doi: 10.1111/j.1475-2743.2007.00137.x
92 ª 2007 The Authors. Journal compilation ª 2007 British Society of Soil Science
• To halve between 1990 and 2015, the proportion of people
whose income is less than one dollar a day.
• To halve between 1990 and 2015, the proportion of people
who suffer from hunger.
Recent analysis by the World Bank and ILO has shown
that relative to other economic sectors, increasing agricul-
tural productivity offers the best prospect of tackling poverty
and hunger (Majid, 2004; Rosegrant et al., 2007). In order to
achieve the MDGs there is a strong case for a focus of effort
on improving productivity of low yield rainfed agricultural
systems (CAWMA, 2007). The challenge therefore is to
develop technologies and practices that deliver improved
agricultural productivity in such systems without adverse
impacts on environmental goods and services.
The Green Revolution brought substantial productivity
increases over the last quarter of the 20th century through
developing and promoting packages of external inputs
(seed + fertilizer + pesticide). It has been argued that a sec-
ond transformation of agriculture is now required that is
‘doubly green’ (Conway, 1997) in that it should both protect
the environment and boost output. The discussion presented
here should be seen in the context of this debate with a par-
ticular focus on land husbandry.
A general critique of the notion of a doubly green revo-
lution (Blackman, 2000) identified obstacles to the desired
transformation which will entail convincing millions of
farmers to adopt new practices. These obstacles include a
policy environment which favours input-intensive agriculture
and the fact that alternative environmentally friendly tech-
nologies involve high set-up costs. Set against this gloomy
prognosis, there is a body of empirical evidence which sug-
gests that widespread adoption and innovation of alterna-
tive sustainable technologies can and does occur. We will
examine in particular the evidence which derives from expe-
rience with ‘zero tillage (ZT) sustainable agriculture’ in
Brazil.
The target for our discussion will be sub-Saharan Africa
(SSA) where essentially the same system is being actively
promoted by FAO as ‘conservation agriculture’ (Interna-
tional Institute for Rural Reconstruction & African Conser-
vation Tillage Network, 2005). We focus on SSA because it
was largely untouched by the original Green Revolution
and is lagging behind other regions in progress towards
achieving the MDGs. At the midway point between their
adoption in 2000 and the 2015 target date SSA is not on
track to achieve any of the MDGs (United Nations, 2007).
The number of extremely poor people has levelled off, but
does not show significant decline. The number of food inse-
cure people continues to rise. Faced with high population
growth, in order to achieve food security, the current rate
of increase in food production must be doubled. These are
very substantial challenges and we need to consider the
compatibility of short- to medium-term aims with a long-
term sustainability agenda.
What is conservation agriculture?
Conservation agriculture (CA) is described by FAO (http://
www.fao.org/ag/ca) as a concept for resource-saving agricul-
tural crop production which is based on enhancing natural
biological processes above and below ground. CA is charac-
terized by:
• Minimum mechanical soil disturbance
• Permanent organic soil cover
• Diversified crop rotations
This represents a new paradigm in land husbandry which
embraces but is not limited to adoption of conservation till-
age. CA aims to maintain a permanent or semi-permanent
organic soil cover which can be either a growing crop or
dead mulch. Its function is to protect the soil physically from
sun, rain and wind and to provide a substrate for the soil
biota. Mechanical tillage incorporates and buries biomass
and at the same time disturbs the natural soil biological pro-
cesses. Therefore, minimum tillage and direct seeding are
important elements of the concept which derives from experi-
ence in Brazil with so-called ‘ZT sustainable agriculture’.
Zero tillage is an important component of CA but farmers
who have adopted ZT are not necessarily practising CA.
Conservation tillage embraces ZT within a wider set of prac-
tices that aim to leave crop residues on the surface. Thus ZT
is related to and may be a transition step towards CA, but it
is not the same thing. Direct seeding involves seeding ⁄plant-ing without preparing a seedbed and is also an important
component of CA, but the term also applies to the use of
mechanical implements which combine primary and second-
ary tillage with seeding in a single operation. Finally, it
should be noted that CA is not the same as organic farming
(ecological agriculture) although both aim to promote natu-
ral soil processes. An important difference is that CA does
not preclude the use of chemical inputs, in particular fertiliz-
ers and herbicides, but they are not allowed in organic
systems.
The global extent of CA is reviewed by Derpsch (2005).
There has been widespread adoption in large-scale mecha-
nized farming systems in drylands of the United States and
Australia, but for our context the most notable success story
for CA is its widespread adoption in Brazil. Bolliger et al.
(2006) in a comprehensive review of what they describe as
the ‘zero-till revolution’ report that the system has spread
from less than 1000 ha in 1973 ⁄ 4 to cover 22 million ha by
2003 ⁄ 4 and now represents 45% of the total cultivated area
in Brazil. There is also evidence of CA adoption among
small-scale farmers in the rice–wheat farming system of the
Indo-Gangetic plains in Asia (Hobbs & Gupta, 2002). Lal
(2007) concludes that adoption is practically nil amongst
resource-poor small-scale farmers in SSA, but there is evi-
dence of pockets of adoption; Ekboir et al. (2002) reported
100 000 adopters in Ghana, and Baudron (2005) reported a
10% adoption rate amongst smallholders in Zambia.
The case for a paradigm shift in land husbandry 93
ª 2007 The Authors. Journal compilation ª 2007 British Society of Soil Science, Soil Use and Management, 24, 92–99
Why does CA represent a new paradigm?
Conservation agriculture represents a significantly different
approach to land husbandry because it involves adoption of
biological tillage principles. Conventional agriculture involves
mechanical tillage which aims to incorporate crop residues,
control weeds, alleviate soil compaction and prepare a seed-
bed. However, this often has detrimental effects on the soil
leading to loss of soil organic matter (SOM), reduced fertil-
ity, reduced rainfall infiltration and increased erosion. Con-
servation tillage practices were developed to target in
particular the issue of erosion, through reducing tillage effort
and retaining at least 30% soil cover by mulching with crop
residues. This in itself involves a change of practice on the
part of the farmer, but adoption of CA requires a more radi-
cal shift in thinking – a new paradigm.
Conservation agriculture starts from the principle that soil
disturbance through mechanical tillage including hand
power, animal power and tractor power is detrimental. Soil
degradation is seen as a problem characterized by loss of
organic matter content and soil biota, collapse of porosity,
soil capping and compaction. Porous and well-structured soil
conditions are seen as critical to maximizing crop productiv-
ity (Shaxson & Barber, 2003). Loss of soil porosity results in
(i) unacceptable run-off of rainwater leading to early onset
of drought and accelerated erosion, (ii) hindrances to opti-
mum functioning of plant roots and (iii) restrictions to effec-
tive functioning of soil biota in nutrient cycling. The primary
cause is regarded as mechanical tillage. The guiding principle
therefore is to minimize soil disturbance and to rely on bio-
logical tillage within a healthy soil ecosystem.
Soil life plays a major role in many natural processes that
determine nutrient and water availability for agricultural
productivity. A healthy soil ecosystem (Bot & Benites, 2001,
2005) will:
• Decompose organic matter into humus;
• Retain nitrogen and other plant nutrients;
• Increase soil aggregate stability;
• Increase soil porosity;
• Protect roots from diseases and parasites;
• Make available nutrients to the plant;
• Produce hormones that help plants grow.
The living part of SOM includes a wide variety of micro-
organisms such as bacteria, viruses, fungi, protozoa and
algae. It also includes plant roots, insects and earthworms.
Micro-organisms, earthworms and insects help break down
crop residues and manures by ingesting them and mixing
them with the minerals in the soil, and in the process recy-
cling energy and plant nutrients. Plant roots, fungal hyphae
and sticky exudates from earthworms and micro-organisms
bind soil particles into water-stable aggregates, reducing ero-
sion and increasing porosity (Tisdall & Oades, 1982). Shal-
low-dwelling earthworms create numerous channels
throughout the topsoil, which increases overall porosity,
while large vertical channels created by deep-burrowing
earthworms improve soil structure in subsoil. The required
shift in thinking therefore is to manage land in a way that
promotes soil ecosystem health.
What is the evidence-base for promoting CA?
The direct benefits of CA adoption in Brazil are said to
include sustainable high yield levels, reduced soil erosion and
reduced costs (lower net inputs of fertilizer, fuel and labour).
They are widely reported and provide a reasonable explana-
tion of widespread adoption, but it has been hard to find evi-
dence to substantiate them in accessible peer-reviewed
literature. However, Bolliger et al. (2006) recently published
a detailed review of the evidence accumulated over several
decades on CA innovation in Brazil.
Empirical evidence of the benefits of CA from SSA is lim-
ited. Lal (1998) reports long-term trials in Nigeria with ZT
rather than CA systems, although these were research station
experiments rather than on-farm trials. He found that ZT
slightly outperformed conventional tillage in most seasons,
but soil chemical and physical quality declined under contin-
uous maize cultivation regardless of the tillage system. Rock-
strom et al. (2007) reports CA trials in Ethiopia, Kenya,
Tanzania and Zimbabwe that have generated yield improve-
ments in the range of 20–120% in smallholder rainfed agri-
culture. However, it should be noted that the so-called CA
systems adopted in these trials failed to maintain permanent
organic soil cover (J. Rockstrom, personal communication)
and also included limited use of chemical fertilizer.
In the case of rice–wheat systems in South Asia, Hobbs
(2007) provides a better substantiated range of benefits,
which include reduced production costs (fuel and labour),
increased yield and reduced pest and disease problems. He
reports that CA leads to significantly improved physical and
chemical properties of soil and that biotic diversity in the soil
is increased. This supports the argument that CA creates a
more healthy soil and enables more efficient use of natural
resources.
Documented evidence on the issue of soil health is more
widely available (though not from SSA). CA practices have
been shown to produce higher surface SOM content (Roldan
et al., 2003; Alvear et al., 2005; Diekow et al., 2005; Madari
et al., 2005). Bolliger et al. (2006) report average-enhanced
SOM accumulation rates of 0.4–1.7 t ha)1 year)1 under ZT
systems in comparison with conventional tillage. Increased
ground cover is associated with an increase in biodiversity
both above ground and in the soil (Kendall et al., 1995; Jai-
pal et al., 2002). Reduced tillage has been shown to increase
soil fauna (Karlen et al., 1994; Buckerfield & Webster, 1996;
Clapperton, 2003; Birkas et al., 2004; Rodriguez et al., 2006).
However, there is also evidence that these effects may be
exaggerated by increased stratification in low ⁄no till systems.
Peigne et al. (2007) reports that conservation tillage leads to
94 J. W. Gowing & M. Palmer
ª 2007 The Authors. Journal compilation ª 2007 British Society of Soil Science, Soil Use and Management, 24, 92–99
increased soil organic carbon (SOC) at 0–5 cm depth but
there is no significant increase in the overall mass of SOC or
soil microbial biomass in the whole topsoil layer (0–30 cm).
There is still some controversy as to the true effects of ZT
on SOC, and in particular claims about carbon sequestration
potential. Farage et al. (2007) modelled dryland farming sys-
tems in Nigeria and Sudan and showed rates of carbon
sequestration in the range 0.08–0.17 t ha)1 year)1. However,
this depends upon the input of organic matter, which is
acknowledged to be a serious constraint in SSA because of
competing demands on crop residues as fodder and animal
manure as fuel (Lal, 2004). Ball et al. (1998) concluded that
additional carbon fixation by storage of SOM and oxidation
of atmospheric methane was very limited under reduced till-
age and likely to be only a short-term effect. Problems of
reporting arise firstly due to increased SOM stratification
under ZT, and failure to take sufficient notice of differences
in depth distribution with tilled systems. Secondly, it is diffi-
cult to separate the effects of reduced tillage from those of
increased biomass production and retention under ZT sys-
tems, and hence the desirability of ZT as opposed to increas-
ing productivity of conventional systems through fertilizer
use increase and greater biomass return is difficult to evalu-
ate. Ringius (2002) considers the opportunities and chal-
lenges for soil carbon sequestration in Africa and points out
that under the Clean Development Mechanism of the Kyoto
protocol, forest planting would be eligible for credit and
therefore attract investment from foreign governments
towards their own emission reduction obligation, but soil
carbon sequestration was not included in the scheme. For
SOC restoration to be considered as a carbon sink and hence
attract investment in a similar way to forest management,
further research is required to assess the genuine potential
and cost justification.
Does Brazilian experience of CA represent a case
for adopting the new paradigm in SSA?
To answer this question, we can examine CA in the context
of the wider debate on ‘low-external-input technology
(LEIT)’ (Tripp, 2006) or ‘resource-conserving agriculture’
(Pretty et al., 2006). The test is that adoption of the new par-
adigm in SSA should deliver poverty alleviation and food
security benefits.
So-called ‘bright spots’ are examples of interventions
which have successfully reversed the continuing downward
spiral of poverty, and which reveal positive impacts on land
and water resources. Pretty et al. (2006) gathered evidence
from 286 ‘bright spots’ in 57 poor countries. These projects
made use of a variety of packages of resource-conserving
technologies and practices, including:
• Integrated nutrient management
• Conservation tillage
• Agroforestry
• Water harvesting
• Livestock integration
• Integrated pest management
This was a purposive sample of ‘best practice’ which found
that the mean relative yield increase was 79% across a very
wide range of crops and systems. For the farm system cate-
gories most relevant to our discussion (smallholder rainfed
humid, smallholder rainfed highland, smallholder rainfed
dry ⁄ cold) and for maize, millet, sorghum, potatoes and
legumes across all systems, mean yield increase exceeded
100%. But will this be enough? CA is being promoted for
small-scale semi-subsistence farmers in SSA as a resource-
conserving practice, but the evidence suggests that food secu-
rity benefits are open to doubt. Pretty et al. (2006) conclude
that it is uncertain whether progress is sufficient to meet
future food needs in view of continued population growth,
urbanization and dietary transition to meat-rich diets.
A review of the research conducted in recent years on
LEITs by Graves et al. (2004) is also revealing in that it
raises doubts about their sustainability. They ask the ques-
tion, ‘can low-external-input agriculture meet future food
security needs while protecting the environment’? They echo
the concern about the potential productivity of such systems
in which the constraint is shown to be maintenance of soil
fertility and SOM. In any low-input system, the alternative
to mineral fertilizer is ‘biomass transfer’ in the form of ani-
mal manure, or direct input of cut plant material (which
may be composted). The system, in effect, involves ‘nutrient
mining’ from the soil where the biomass is produced. The
quantity of biomass available to small farmers is commonly
insufficient because of limited land and ⁄or limited labour.
They conclude that low-external-input technologies appear to
have limited potential to increase food production for the
target group of resource-poor farmers. We are not aware
that any evidence has been presented which shows that this
constraint in low-input systems can be overcome by adoption
of CA principles.
Turning to the poverty-alleviation objective, a recent anal-
ysis of low-input technological interventions, by Tripp (2006)
paints a negative picture of progress made, and questions
some of the claims made about their potential. He concludes
that, contrary to the belief that low-input techniques have
greatest benefit for resource-poor farmers, patterns of uptake
do not differ significantly from Green Revolution technol-
ogy; it is generally the farmers who are better resourced with
access to markets who invest in any new technology. The
‘small farmers’ reported to have adopted CA in Brazil are
very different from their counterparts in SSA. Smallholders
in Brazil are defined as those farming up to 50 ha (Bolliger
et al., 2006) and land husbandry operations are largely
mechanized, often with animal power. Tripp (2006) also dis-
putes the idea that there is evidence for spontaneous farmer-
to-farmer diffusion of technology, often seen as integral to
the ultimate success of participatory agro-ecological research
The case for a paradigm shift in land husbandry 95
ª 2007 The Authors. Journal compilation ª 2007 British Society of Soil Science, Soil Use and Management, 24, 92–99
methods, arguing that a much more clear definition of
exactly what is needed (in terms of policy, replication, exten-
sion, new organizations, etc.) than a general reference to
‘scaling up’ is required to achieve success on a meaningful
scale. This issue is examined further considering the evidence
from Brazil below.
Can Brazilian experience of CA promotion be
replicated in SSA?
We can again learn from the evaluation of evidence from
‘bright spots’. Noble et al. (2006) examined whether there
are key factors that are fundamental to success, and whether
these can be developed into guidelines that would enhance
up-scaling and increase food security and household incomes.
Through a consultative process 10 possible drivers were iden-
tified:
Individually based Drivers: Leadership, aspiration for
change.
Socially based Drivers: Social capital, participatory
approach.
Technically based Drivers: Innovation and appropriate
technologies, quick and tangible benefits, low risk of failure.
Externally based Drivers: Supportive policy, property
rights, market opportunities.
Penning de Vries (2005) examined community-based
‘bright spots’ in African agriculture, but it is individual-based
‘bright spots’ that are most relevant to our discussion of land
husbandry interventions. For these the key factors influenc-
ing adoption were found to be:
• quick and tangible outcomes
• a participatory approach in implementing the technology
• strong leadership by the individual or group adopting the
technology
• supportive policy
• markets
With the exception of the reference to ‘markets’, which is
becoming common in the discourse on agricultural develop-
ment in SSA, there is a remarkable similarity to the list
developed by Hudson (1991) in his assessment of the reasons
for adoption or non-adoption of soil conservation practices.
There are cases of what could be termed spontaneously
driven ‘bright spots’ that grew from within, without incen-
tives or external support. However, in the majority of cases
the development of the documented ‘bright spot’ was contin-
gent on an external priming agent which facilitated progress
through financial and non-financial contributions. We must
examine the implications of this evidence for future expan-
sion and up-scaling of knowledge-intensive innovations that
characterize CA.
Ekboir (2003) cites the importance of ‘innovation net-
works’ to the spread of CA in Brazil, and particularly high-
lights the importance of agrochemical companies as agents
with sufficient coverage and resources to promote developed
technologies. This, he argues, is the key difference between
its widespread adoption in Brazil and its minimal success in
other countries. Evidence of adoption in SSA, e.g. in Ghana
and Zambia, appears to confirm the importance of such sup-
port networks. Ekboir et al. (2002) in their study of CA
adoption in Ghana identified the active markets for agricul-
tural services and the important role of agrochemical dealers.
Baudron (2005), in his study of CA adoption in Zambia,
emphasizes the importance of making input packages avail-
able to farmers.
Conclusion
The case for a paradigm shift in land husbandry hangs on
the premise that conventional practice promotes land degra-
dation while adoption of CA practice delivers a range of
benefits through promoting soil ecosystem health. Does it
therefore deliver the doubly green revolution that will lead to
sustainable agricultural development in SSA?
Accumulated evidence shows that if the conditions are
right then CA does deliver benefits sufficient to promote its
adoption amongst farmers. Substantiated benefits include
reduced production costs, improved soil conditions and
reduced erosion. There has been widespread adoption in
large-scale mechanized farming systems in drylands of the
United States and Australia, but more relevant to our discus-
sion is the evidence which derives from experience with ‘ZT
sustainable agriculture’ in Brazil.
We have examined CA in the context of the wider debate
on LEIT. We have examined the subsidiary propositions
that:
• conservation agriculture can deliver the productivity gains
that are required to achieve the MDG food security target
and that advocacy of CA is compatible with the MDG
poverty target;
• widespread adoption of the new paradigm amongst mil-
lions of small farmers in order to achieve the ‘doubly green
revolution’ in SSA is achievable.
In considering the first proposition, the available evidence
suggests that LEITs can deliver yield improvements in the
range of 20–120% in smallholder rainfed agriculture com-
pared with ‘unimproved’ traditional systems. At the optimis-
tic end of this range, this will deliver doubling of production
compared with the baseline, which equates to a growth rate
of 3% per annum over 25 years. This is an improvement
over past agricultural growth performance in SSA, but is not
sufficient to achieve the MDG food security target. Low-
external-input techniques, in general, have limited potential
to deliver a sustainable increase in food production for the
target group of resource-poor farmers. Such systems depend
upon importing biomass to maintain fertility and, in effect,
involve ‘nutrient mining’ from the soil where the biomass is
produced but the quantity of biomass available to small
farmers is insufficient because of limited land and ⁄or labour.
96 J. W. Gowing & M. Palmer
ª 2007 The Authors. Journal compilation ª 2007 British Society of Soil Science, Soil Use and Management, 24, 92–99
We see no evidence that CA systems overcome these con-
straints without external inputs.
In considering the second proposition, which concerns
dissemination, there is no evidence that the pattern of uptake
of LEITs differs significantly from Green Revolution technol-
ogy. There are cases of what could be termed as spontane-
ously driven ‘bright spots’ that occurred, without incentives
or external support; however, in the majority of cases the
development of the documented ‘bright spot’ was contingent
on an external priming agent, which facilitated progress
through financial and non-financial contributions. We see no
evidence that CA systems are different in this respect. It is
clear that the key to the widespread adoption of CA in Brazil
has been the success of ‘innovation networks’ and in particu-
lar the presence of agrochemical companies as agents with
sufficient coverage and resources to promote developed tech-
nologies. Where pockets of CA adoption exist in SSA, most
notably in Ghana and Zambia, the same condition applies.
There is a very limited evidence base in SSA, with docu-
mented evidence of the benefits of a paradigm shift coming
mainly from Brazil. Small farmers who have adopted CA in
Brazil appear much better resourced than the target group in
SSA. Most importantly they generally have good access to
markets for inputs and outputs. Nevertheless, it should be
noted that the ‘model’ systems developed by agricultural sci-
entists in Brazil have not proved suited to the circumstances
of small farmers and farmers have frequently adapted tech-
nologies to suit their capabilities. Assessing the impacts of
these ‘imperfect’ systems may well prove the key to sustain-
able intensification of smallholder agriculture in SSA.
A particularly contentious issue has been the reliance on
herbicides, which represent 11–12% of smallholder produc-
tion costs on CA land as opposed to 2–5% under conven-
tional systems. A 17% increase in smallholder herbicide use
is observed with ZT systems in Brazil compared with conven-
tionally cultivated land (Bolliger et al., 2006). It is apparent
that many small-scale farmers in Brazil struggle with weed
control when their access to herbicides is limited and they
often resort to tillage as a solution. The concern about weed
control is echoed by Rockstrom et al. (2007) who suggests
that for resource-poor farm households in SSA the use of
herbicides is not an option. He speculates that the weed
problem ‘will fall below that of the original farming system
after several years’, but evidence from Brazil does not sup-
port this optimism. Further confirmation of the critical nat-
ure of the weed control problem can be obtained from
Peigne et al. (2007) who conclude that in organic farming
systems (i.e. without use of herbicides) ZT tends to increase
weed pressure to a critical level where crop production could
be compromised.
The concern extends beyond the question of weed control
to embrace also the issue of fertilizers. The problem of
securing sufficient biomass for soil cover and for sustaining
adequate fertility will remain a key issue in SSA. The use
of mineral fertilizers, assuming they are available, would be
compatible with a CA system and evidence suggests a prag-
matic approach may be appropriate. A third important
proposition therefore needs to be properly tested in examin-
ing the case for adopting a new paradigm for land hus-
bandry. Given that partial adoption of CA principles
appears to be the norm amongst small-scale, resource-poor
farmers, does it deliver comparable soil health benefits?
What level of mechanical tillage for weed control is permis-
sible? What level of mineral fertilizer input is acceptable?
We agree with the conclusion of Bolliger et al. (2006) that
perhaps the real lessons are to be learnt from the adapta-
tions made by farmers.
Acknowledgements
This paper is based on a presentation made by the authors
at an International Workshop on Land Husbandry which
was convened by the Tropical Agriculture Association and
hosted by University of Newcastle upon Tyne in March
2007. We have benefited from that discussion but the opin-
ions expressed here are our own.
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