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Policy Analysis
Resource-Conserving Agriculture
Increases Yields in Developing
Countries
J. N. PRETTY,*,† A. D. NOBLE,‡
D. BOSSIO,§J. DIXON,|R. E. HINE,†
F. W. T. PENNING DE VRIES,⊥AND
J. I. L. MORISON†
Department of Biological Sciences and Centre for
Environment and Society, University of Essex, Wivenhoe Park,
Colchester CO4 3SQ, U.K., International Water Management
Institute (IWMI), P.O. Box 1025, Kasetsart University,
Bangkok 10903, Thailand, International Water Management
Institute (IWMI), P.O. Box 2075, Colombo, Sri Lanka, Impact
Targeting and Assessment Program, CIMMYT, Apdo. Postal
6-641, 06600 Mexico, Mexico, and International Project Office
for Monsoon Asia Integrated Regional Study, Institute for
Atmospheric Sciences, Chinese Academy of Sciences,
P.O. Box 9804, Beijing, China
Despite great recent progress, hunger and poverty remain
widespread and agriculturally driven environmental
damage is widely prevalent. The idea of agricultural
sustainability centers on the need to develop technologies
and practices that do not have adverse effects on
environmental goods and services, and that lead to
improvements in food productivity. Here we show the
extent to which 286 recent interventions in 57 poor countries
covering 37 M ha (3% of the cultivated area in developing
countries) have increased productivity on 12.6 M farms
while improving the supply of critical environmental services.
The average crop yield increase was 79% (geometric
mean 64%). All crops showed water use efficiency gains,
with the highest improvement in rainfed crops. Potential
carbon sequestered amounted to an average of 0.35tCha
-1
y-1. If a quarter of the total area under these farming
systems adopted sustainability enhancing practices, we
estimate global sequestration could be 0.1 Gt C y-1. Of projects
with pesticide data, 77% resulted in a decline in pesticide
use by 71% while yields grew by 42%. Although it is
uncertain whether these approaches can meet future
food needs, there are grounds for cautious optimism,
particularly as poor farm households benefit more from
their adoption.
Introduction
What is the best way to increase agricultural productivity in
developing countries that still, despite efforts over several
decades, have some 800 million people short of food? The
question is controversial, with widely varying positions about
the types of inputs and technologies likely to be effective
(1-4). Great technological progress in the past half century
has not been reflected in major reductions in hunger and
poverty in developing countries.
However, many novel initiatives have emerged that are
demonstrating that agriculture in poor countries can be
greatly improved. Here we evaluate how farmers in 286
projects in 57 countries have improved food crop productivity
since the early to mid 1990s, and at the same time increased
both water use efficiency and carbon sequestration, and
reduced pesticide use. These initiatives also offer the
prospects of resource conserving agriculture both reducing
adverse effects on the environment and contributing to
important environmental goods and services (e.g., climate
change mitigation).
In the past 40 years, per capita world food production has
grown by 17%, with average per capita food consumption in
2003 of 2780 kcal day-1(5), where a majority of the chronically
hungry are small farmers who produce much of what they
eat. Yet consumption in 33 poor countries is still less than
2200 kcal day-1. Food demand will both grow and shift in the
coming decades, as (i) population growth increases absolute
demand for food; (ii) economic growth increases people’s
purchasing power; (iii) growing urbanization encourages
people to adopt new diets; and (iv) climate change threatens
both land and water resources.
Increased food supply is a necessary though not sufficient
condition for eliminating hunger and poverty. What is
important is who produces the food, has access to the
technology and knowledge to produce it, and has the
purchasing power to acquire it. The great success of
industrialized agriculture in recent decades has masked
significant negative externalities, with environmental and
health problems increasingly well-documented and costed,
including in Ecuador, China, Germany, the Philippines, U.K.
and United States (6-11). There are also growing concerns
that such systems may not reduce food poverty. Poor farmers
need low-cost and readily available technologies and prac-
tices to increase local food production and to raise their
income. At the same time, land and water degradation is
increasingly posing a threat to food security and the
livelihoods of rural people who often live on degradation-
prone lands (12).
The idea of agricultural sustainability centers on food
production that makes the best use of nature’s goods and
services while not damaging these assets. Many different
terms have come to be used to imply greater sustainability
in some agricultural systems over prevailing ones (both pre-
industrial and industrialized) (13). Agricultural sustainability
in all cases, however, emphasizes the potential benefits that
arise from making the best use of both good genotypes of
crops and animals and their ecological management. Agri-
cultural sustainability does not, therefore, mean ruling out
any technologies or practices on ideological grounds (e.g.,
genetically modified crop, organic practice)sprovided they
improve productivity for farmers, and do not harm the
environment (12-16).
In this research, we concentrate on projects that have
made use of a variety of packages of resource-conserving
technologies and practices. These include the following: (1)
Integrated pest management, which uses ecosystem resilience
and diversity for pest, disease, and weed control, and seeks
only to use pesticides when other options are ineffective. (2)
Integrated nutrient management, which seeks both to balance
* Corresponding author e-mail: jpretty@essex.ac.uk; tel: +44-
1206-873323; fax: +44-1206-873416.
†University of Essex.
‡IWMI, Kasetsart University.
§IWMI, Colombo, Sri Lanka.
|Impact Targeting and Assessment Program, CIMMYT.
⊥Chinese Academy of Sciences.
1114 9ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 4, 2006 10.1021/es051670d CCC: $33.50 2006 American Chemical Society
Published on Web 12/21/2005
the need to fix nitrogen within farm systems with the need
to import inorganic and organic sources of nutrients, and to
reduce nutrient losses through erosion control. (3) Conser-
vation tillage, which reduces the amount of tillage, sometimes
to zero, so that soil can be conserved and available moisture
used more efficiently. (4) Agroforestry, which incorporates
multifunctional trees into agricultural systems, and collective
management of nearby forest resources. (5) Aquaculture,
which incorporates fish, shrimps, and other aquatic resources
into farm systems, such as into irrigated rice fields and fish
ponds, and so leads to increases in protein production. (6)
Water harvesting in dryland areas, which can mean formerly
abandoned and degraded lands can be cultivated, and
additional crops can be grown on small patches of irrigated
land owing to better rainwater retention. (7) Livestock
integration into farming systems, such as dairy cattle and
poultry, including using zero-grazing.
Here we show the extent to which recent successful
interventions focusing on agricultural sustainability (some-
times called bright spots (17)) have increased total food crop
productivity in developing regions. Our questions are as
follows: (i) To what extent can farmers increase per hectare
and per farm food production by using low-cost and locally
available technologies and inputs? (ii) What impacts do such
methods have on environmental goods and services (in
particular using the water use efficiency, carbon sequestra-
tion, and pesticide use as proxies to indicate changes in
adverse effects on the environment)?
Methodology
We used both questionnaires and published reports by
projects to assess adoption of sustainable agriculture and
changes over time. As in earlier research (18), data were
triangulated from several sources, and cross-checked by
external reviewers and regional experts. This study involves
analysis of projects sampled once in time (n)218) and those
sampled twice over a 4 year period to assess temporal changes
(n)68). Not all proposed cases were accepted for the dataset,
and rejections were based on a strict set of criteria (18). As
this was a purposive sample of “best practice” initiatives, the
findings are not representative of all farms in developing
countries.
We used a novel typology of farming systems developed
by FAO for the World Bank to classify these projects (19) into
8 broad categories based on the following social, economic,
and biophysical criteria: (i) the available natural resource
base, including water, land, grazing areas, and forest; climate
and altitude; landscape, including slope; farm size, tenure,
and organizations; and access to services including markets;
and (ii) the dominant patterns of farm activities and
household livelihoods, including field crops, livestock, trees,
aquaculture, hunting and gathering, processing, and off-farm
activities; and the main technologies used, which determine
the intensity of production and integration of crops, livestock
and other activities.
Table 1 contains a summary of the global land area and
population located in these eight major farm system cat-
egories. On average, these sustain 2.28 people per cultivated
hectare of land (range 0.5-5.5). A total of 72 farming
subsystems have been identified across the developing
regions, some of which comprised similar systems occurring
on different continents (e.g., wetland rice systems in East
Asia/Pacific and in South Asia). A summary of all these
systems and their locations is contained in the Supporting
Information. In our study, system categories 2-5 are well-
represented, with 40-95 projects in each. System categories
1, 6, and 8 have 15-20 projects each, and category 7 has only
two.
Extent of Agricultural Sustainability and Impacts on
Yields. Table 2 contains a summary of the location and extent
of the 286 agricultural sustainability projects across the eight
categories of farming systems in 57 countries. In all, some
12.6 million (M) farmers on 37 M ha were engaged in
transitions toward agricultural sustainability in these 286
projects. This is just over 3% of the total cultivated area shown
in Table 1. The largest number of farmers was in wetland
rice-based systems, mainly in Asia (category 2), and the largest
area was in dualistic mixed systems, mainly in southern Latin
America (category 6).
We were able to show that agricultural sustainability is
spreading to more farmers and hectares. In the 68 randomly
re-sampled projects from the original study, there was a 56%
increase over the 4 years in the number of farmers (from 5.3
to 8.3 M), and 45% in the number of hectares (from 12.6 to
18.3 M). These resurveyed projects comprised 60% of the
farmers and 44% of the hectares in the original sample of 208
projects (18). In the earlier study, we reported that 89 projects
for which there was reliable yield data showed increases in
per hectare food production.
For the 360 reliable yield comparisons from 198 projects
that we now have, the mean relative increase was 79% across
the very wide variety of systems and crop types (see Table
B in the Supporting Information for full details of changes
in each farming system category). However, there was a wide
spread in results (Figure 1). While 25% of projects reported
relative yields >2.0, (i.e., 100% increase), half of all the projects
had yield increases of between 18% and 100%. The geometric
mean is a better indicator of the average for such data with
a positive skew, but this still shows a 64% increase in yield.
However, the average hides large and statistically significant
differences among the main crops (Figures 2 and 3). In nearly
all cases there was an increase in yield with the project. Only
in rice were there 3 reports where yields decreased, and the
increase in rice was the lowest (mean )1.35), although it
TABLE 1. Summary of FAO-World Bank Farming System Categories in Developing Regions and Number of Project Entries for This
Studya
FAO farm system category
number of
subsystems
land area
(M ha)
cultivated area
(M ha)
agricultural
population (M)
agricultural
population per
cultivated hectare
no. project
entries for
each category
1. smallholder irrigated 1 219 15 30 2.0 16
2. wetland rice 3 330 155 860 5.5 55
3. smallholder rainfed humid 11 2013 160 400 2.5 95
4. smallholder rainfed highland 10 842 150 520 3.5 40
5. smallholder rainfed dry/cold 19 3478 231 490 2.1 43
6. dualistic mixed 16 3116 414 190 0.5 20
7. coastal artisanal 4 70 11 60 5.5 2
8. urban-based and kitchen garden 6 na na 40 na 15
total 72 10068 1136 2590 2.28 286
a
From Dixon and Gulliver (
19
); na )not available.
VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 91115
constituted a third of all the crop data. Cotton showed a
similarly small mean yield increase.
The mean (2.84) and spread was largest in cassava and
sweet potato crops, although the sample is small. Soybean
and groundnut showed mean increases of about 50%. Maize,
millet and sorghum, potatoes, and the other legumes group
(beans, pigeon peas, cowpea, chickpea) all showed mean
yield increases of >100%, significantly higher than those for
cotton, rice, and groundnut (P<0.05). For most of the main
field crops that are well represented in the survey, those with
low yields before intervention often showed larger relative
improvements, either because of growth limiting environ-
ments, or perhaps reduced investment in developing these
crops, although potato showed large increases across the
range (Figure 4).
Though many technologies and practices were used in
these projects, three types of technical improvement are likely
to have played substantial roles in food production in-
creases: (i) more efficient water use in both dryland and
irrigated farming; (ii) improvements in organic matter
accumulation in soils and carbon sequestration; and (iii) pest,
weed, and disease control emphasizing in-field biodiversity
and reduced pesticide (insecticide, herbicide, and fungicide)
use.
Impacts on Farm Water Use Efficiency. Widespread
appreciation of the “global water crisis” recognizes that
scarcity of clean water is affecting food production and
conservation of ecosystems. By 2025 it is predicted that most
developing countries will face either physical or economic
water scarcity (20). Water diverted from rivers increased 6-fold
between 1900 and 1995 (21), far outpacing population growth.
Increasing demand for freshwater now threatens the integrity
of many aquatic ecosystems, and their associated environ-
mental services (22). As agriculture accounts for 70% of
current water withdrawals from rivers, improving the pro-
ductivity of water use in agriculture is a growing challenge.
The potential for increasing food production while
maintaining water-related ecosystem services rests on ca-
pacity to increase water productivity (WP), i.e., by realizing
more kg of food per unit of water. Sustainable agricultural
practices may do this by (i) removing limitations on
productivity by enhancing soil fertility; (ii) reducing soil
evaporation through conservation tillage; (iii) using more
water-efficient varieties; (iv) reducing water losses to un-
recoverable sinks; (v) boosting productivity by supplemental
irrigation in rainfed systems; and (vi) inducing microclimatic
changes to reduce crop water requirements (23). We cal-
culated changes in WP for field crops in 144 projects from
the data set (Table 3) based on reported crop yields and
average potential evapotranspiration (ETp), for each project
location during the relevant growing season. Actual evapo-
TABLE 2. Summary of Adoption and Impact of Agricultural Sustainability Technologies and Practices on 286 Projects in 57
Countriesa
FAO farm system category
number of
farmers adopting
number of hectares under
sustainable agriculture
average % increase
in crop yields
1. smallholder irrigated 177,287 357,940 129.8 ((21.5)
2. wetland rice 8,711,236 7,007,564 22.3 ((2.8)
3. smallholder rainfed humid 1,704,958 1,081,071 102.2 ((9.0)
4. smallholder rainfed highland 401,699 725,535 107.3 ((14.7)
5. smallholder rainfed dry/cold 604,804 737,896 99.2 ((12.5)
6. dualistic mixed 537,311 26,846,750 76.5 ((12.6)
7. coastal artisanal 220,000 160,000 62.0 ((20.0)
8. urban-based and kitchen garden 207,479 36,147 146.0 ((32.9)
all projects 12,564,774 36,952,903 79.2 ((4.5)
a
Yield data from 360 crop project combinations; reported as % increase (thus a 100% increase is a doubling of yields). Standard errors are
given in brackets.
FIGURE 1. Histogram of change in crop yield after or with project,
compared to before or without project (
n
)360, mean )1.79, SD
0.91, median )1.50, geometric mean )1.64).
FIGURE 2. Box and whisker plot of change in crop yield after or
with project, compared to before or without project. Bold lines
within boxes indicate median value, box limits indicate interquartile
range (i.e., 50% of values lie within the box), whiskers indicate
highest and lowest, excluding outliers (O, 1.5-3×box length
distance away from edge of box) or extremes (*, >3×box length).
“Other” group consists of sugar cane (
n
)2), quinoa (1), oats (2).
1116 9ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 4, 2006
transpiration (ETa) was assumed to equal 80% of ETp, and
ETa to remain a constant at different levels of productivity.
WP gains were high in rainfed systems, and moderate in
irrigated systems, and were in agreement with other studies
reporting ranges of WP (23). The very large increase for the
vegetables and fruits is probably an overestimate as we did
not adjust ETp for new crops or lengthened cropping periods.
Variability was high due to the wide variety of practices
represented in the dataset, but do indicate that gains in WP
are possible through adoption of sustainable farming tech-
nologies in a variety of crops and farm systems. Our results,
and others (24-25), demonstrate that the greatest op-
portunity for improvement in water productivity is in rainfed
agriculture. Better farm management, including supple-
mental irrigation and fertility management can significantly
reduce uncertainty, and thus avoid chronic low productivity
and crop failure that are characteristic of many rainfed
systems.
Impacts on Carbon Sequestration. The 1997 Kyoto
Protocol to the UN Framework Convention on Climate
Change established an international policy context for the
reduction of carbon emissions and increases in carbon sinks
to address the global challenge of anthropogenic interference
with the climate system. It is clear that both emission
reductions and sink growth will be necessary for mitigation
of current climate change trends (26-28). Carbon sequestra-
tion is defined as the capture and secure storage of carbon
that would otherwise be emitted to or remain in the
atmosphere (29).
One of the actions farmers can take is to increase carbon
sinks in soil organic matter and above-ground biomass. We
calculated the potential annual contributions being made in
these 286 projects to carbon sink increases in soils and trees,
using established carbon audit methods (30) (Table 4). As
the focus is on what sustainable methods can do to increase
quantities of soil and above-ground carbon, we did not take
account of existing stocks of carbon. Soil carbon sequestration
is corrected for climate, as rates are higher in humid
compared with dry zones, and generally higher in temperate
than tropical areas (28-29).
These projects were potentially sequestering 11.4 Mt C
y-1on 37 M ha. If scaled up, assuming that 25% of the areas
under the different farming system categories globally (Table
1) adopted these same sustainability initiatives, this would
result in sequestration of 100 ((4) Mt C y-1The average gain
was0.35tCha
-1y-1, and an average per household gain of
0.91tCy
-1. The per hectare gains vary from 0.15tCha
-1
y-1for smallholder irrigated systems (category 1) to 0.46 t C
ha-1y-1for category 3 systems. For most systems, per
households gains were in the range 0.05-0.5tCy
-1, with the
much larger farms of southern Latin America using zero-
tillage achieving the most at 14.9tCy
-1. Such gains in carbon
may offer new opportunities to households for income
generation under emerging carbon trading schemes.
Impacts on Pesticide Use. Integrated pest management
(IPM) programs are beginning to show how pesticide use
can be reduced and modified without yield penalties in a
variety of farm systems, such as in irrigated rice in Asia (31)
and rainfed maize in Africa (32). In principle, there are four
possible trajectories an agricultural system can take if IPM
is introduced: (i) both pesticide use and yields increase (A);
(ii) pesticide use increases but yields decline (B); (iii) both
pesticide use and yields fall (C); or (iv) pesticide use declines,
but yields increase (D).
The conventional wisdom is that pesticide use and yields
are positively correlated, and so only trajectories moving
into A and C are likely (33-34). A change into sector B would
be against economic rationale, as farmers’ profits would
invariably fall and behavior change. A shift into sector D
would indicate that current pesticide use has negative yield
effects. This could be possible with excessive use of herbicides
or when pesticides cause outbreaks of secondary pests (35).
We analyzed the 62 IPM initiatives in 21 developing countries
in the dataset (Figure 5). The evidence on pesticide use is
derived from data on both the number of sprays per hectare
and the amount of active ingredient per hectare. There is
only one case in sector B reported in recent literature (36),
and so this was not included.
Sector A contains 10 projects where pesticide use in-
creased. These are mainly in zero-tillage and conservation
agriculture systems, where reduced tillage creates benefits
for soil health and reduces off-site pollution and flooding
costs. These systems usually require increased use of
herbicides for weed control (37), though there are examples
of organic zero-tillage systems (38). The 5 cases in sector C
show a 4.2% ((5.0) decline in yields with a 93.3% ((6.7) fall
in pesticide use. Most cases, however, are in category D where
pesticide use declined by 70.8% ((3.9) and yields increased
by 41.6% ((10.5). While pesticide reduction is to be expected,
as farmers substitute pesticides by information, the cause of
yield increases induced by IPM are complex. It is likely that
farmers who receive good quality field training will not only
improve their pest management skills but also become more
efficient in other agronomic and ecological management
practices. They are also likely to invest cash saved from
FIGURE 3. Mean changes in crop yield after or with project,
compared with before or without project. Vertical lines indicate (
SEM. “Other” group consists of sugar cane (
n
)2), quinoa (1), oats
(2).
FIGURE 4. Relationship between relative changes in crop yield
after (or with project) to yield before (or without project). Only field
crops with
n
>9 shown.
VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 91117
pesticides in other inputs such as higher quality seeds and
fertilizers. This analysis indicates considerable potential for
avoiding environmental costs.
Discussion
It is uncertain whether progress toward agricultural sus-
tainability, delivering benefits at the scale occurring in these
projects, will result in enough food to meet the future food
needs in developing countries after continued population
growth, urbanization, and the dietary transition to meat-
rich diets (39). Even the substantial increases reported here
may not be enough. However, more widespread adoption of
these resource conserving technologies, combined with other
innovations in crop and livestock genotypes, would con-
tribute to increased agricultural productivity (1, 16), par-
ticularly as evidence indicates that productivity can grow in
many farming systems as natural, social, and human capital
assets also grow (40). Our findings also show that poor
households benefit substantially.
But improving agricultural sustainability alone will not
solve all food poverty problems. The challenge is to find ways
to improve all farmers’ access to productive technologies
and practices that are also resource conserving. The critical
priority is now international, national, and local policy and
institutional reforms (41) designed to benefit both food
security and income growth at national and households levels,
while improving the supply of critical technologies that
improve the supply of environmental goods and services.
Acknowledgments
We are grateful to all project staff and scientists who made
data available on projects, to earlier comments and sug-
gestions from researchers involved in the IWMI Bright Spots
research program, to Noel Aloysius for input for some of the
research, to David Tilman for comments on an earlier
manuscript, and to two referees for their helpful comments.
The research was funded by the U.K. Department for
International Development. The views expressed in this paper
are those of the authors and do not necessarily reflect the
policies of their organizations.
Supporting Information Available
Table A1 containing full details of the classification system
developed by FAO (Dixon and Gulliver, ref 19) for farming
systems. This separates farming systems into 8 types (ir-
rigated; wetland rice based; smallholder rainfed humid;
smallholder rainfed highland; smallholder rainfed dry/cold;
dualistic; coastal artisanal fishing; urban-based) for six regions
of the world (Sub-Saharan Africa; Middle East and North
Africa; Europe and Central Asia; South Asia; East Asia and
Pacific; Latin America and Caribbean). Table A2 summarizing
the location of the 286 projects in this study in these farming
systems types, and giving the impact of agricultural sus-
tainability in each farming system. Part C containing profiles
TABLE 3. Summary of Changes in Water Productivity by Major Crop Type Arising from Adoption of Sustainable Agricultural
Technologies and Practices in 144 Projectsa
crop
water productivity
before intervention
(kg food m-3water ETa)
water productivity
after intervention
(kg food m-3water ETa)
water productivity
gain
(kg food m-3water ETa) % increase in WP
irrigated
rice (
n
)18) 1.03 ((0.22) 1.19 ((0.12) 0.16 ((0.04) 15.5%
cotton (
n
)8) 0.17 ((0.04) 0.22 ((0.05) 0.05 ((0.02) 29.4%
rainfed
cereals (
n
)80) 0.47 ((0.06) 0.80 ((0.09) 0.33 ((0.05) 70.2%
legumes (
n
)19) 0.43 ((0.07) 0.87 ((0.16) 0.44 ((0.11) 102.3%
roots and tubers (
n
)14) 2.79 ((0.73) 5.79 ((1.08) 3.00 ((0.65) 107.5%
urban and kitchen gardens
vegetables and fruits (
n
)5)
0.83 ((0.29) 2.96 ((0.97) 2.13 ((0.71) 256.6%
a
Standard errors in brackets.
TABLE 4. Summary of Potential Carbon Sequestered in Soils and Above-Ground Biomass in the 286 Projectsa
FAO farm system category
carbon sequestered
per hectare
(tCha
-1y-1)
total
carbon sequestered
(MtCy
-1)
carbon sequestered
per household
(tCy
-1)
1. smallholder irrigated 0.15 ((0.012) 0.011 0.06
2. wetland rice 0.34 ((0.035) 2.53 0.29
3. smallholder rainfed humid 0.46 ((0.034) 0.34 0.20
4. smallholder rainfed highland 0.36 ((0.022) 0.23 0.56
5. smallholder rainfed dry/cold 0.26 ((0.035) 0.20 0.32
6. dualistic mixed 0.32 ((0.023) 8.03 14.95
7. coastal artisanal 0.20 ((0.001) 0.032 0.15
8. urban-based and kitchen garden 0.24 ((0.061) 0.015 0.07
total 0.35 ((0.016) 11.38 0.91
a
Standard errors in brackets.
FIGURE 5. Changes in pesticide use and yields in 62 projects (A,
n
)10; C,
n
)5; D,
n
)47).
1118 9ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 4, 2006
of 47 of the 286 projects (11 in Latin America, 17 in Africa,
and 19 in Asia) as examples of how the technologies were
adopted and their environmental and social outcomes. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Received for review August 23, 2005. Revised manuscript
received November 17, 2005. Accepted November 18, 2005.
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