Chapter 3

Crop-Environment Interactions in Sub-Saharan Africa

By Nicholas J. Papanastassiou

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Table of Contents:
Research Highlights
Executive Summary
Historical Context
Key Research Institutions
Works Cited

Research Highlights

  • This chapter examines the resource requirements and impacts of major crop systems in Sub-Saharan Africa (SSA), and the mitigation and adaptation strategies used to address them through and extensive literature review
  • Pre-harvest constraints such as poor soil fertility throughout SSA cause yield reductions between 50% and 75% in maize, legumes, and yams
  • Disease infection during production causes up to 78-80% yield losses in maize and legumes, and impacts up to 100% of sweet potatoes
  • Post-harvest storage losses throughout the continent, often caused by pests and pathogens, can be up to 30% of harvest in maize and 50% of dry matter in legumes and yams
  • The judicious use of agrochemicals, increased use of genetically improved crop varieties, and proper storage methods can substantially improve yields and reduce losses across all three crops
  • Opportunities for improvement in the agricultural productivity throughout SSA include: the education of farmers on proper implementation of best practices, the development of markets for necessary farming inputs, and research in areas where there are knowledge gaps

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Executive Summary

Over 200 million people throughout Africa are chronically hungry or malnourished. Low crop yields throughout the continent are a key contributing factor: cereal yields in Africa are 40% below that of the developing world average. This low productivity is caused in part by environmental constraints. Overcoming constraints and ameliorating negative environmental effects associated with crop production is a critical step in Africa’s development.The research question addressed by this chapter is: What are the critical resource requirements and impacts of major crop systems in Sub-Saharan Africa, and what known mitigation and adaptation strategies are available to address them? Based upon an in-depth literature review of peer-reviewed articles, papers, and reports from major agricultural research organizations and scientists, this chapter presents an overview of crop-environment interactions for maize, legumes, and sweet potatoes/yams.

Crops in Africa face poor soil fertility as a common pre-harvest constraint. This has caused yield reductions of up to 48% in maize in Tanzania, over 50% for yams in Nigeria, and losses of up to 75% in legumes. Many farmers, especially those growing legumes and yams, do not have access to fertilizers to overcome this. Many crops also face disease infection during production: downy mildew causes up to 80% yield losses in maize, leaf spot causes up to 60% yield losses in legumes, and the sweet potato feathery mottle virus infects up to 100% of sweet potatoes. Finally, all of these crops face poor post-harvest storage conditions. During storage, farmers in Kenya routinely lose 15-30% of their maize harvest, and legumes and yams can both lose 50% or more of their nutritional and economic value due to spoilage, pests or pathogens.

Given these findings, investments by able stakeholders should be directed at increasing the use of agricultural best practices in as highlighted in this chapter, such as judicious agrochemical use and improved storage methods. Furthermore, international institutions have a role in addressing current and future food insecurity in Africa by increasing research on potential best practices where there are research gaps, educating farmers on how to correctly implement best practices, and developing markets for agricultural inputs so that farmers have access to consistent and reliable supplies. With a coordinated, focused, and informed effort, stakeholders from farmers to governments could dramatically improve the state of agricultural production throughout Africa.

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Over 218 million people in Africa live in situations of chronic hunger and malnutrition, and this number could increase beyond 600 million by 2050, given Africa’s projected rates of population growth (FAO, 2009). Food security on the continent needs to increase, and one solution is through increased agricultural productivity. In a region such as Africa, where cereal yields are two-and-a-half times below that of the developing world average, this solution could provide dramatic benefits (FAO, 2009). At the same time, many rural households have an intimate relationship with the environment, meaning that farmers must take care to mitigate the environmental impacts of crop production and the methods used to raise productivity. As such, the question becomes: What are the critical resource requirements and impacts of major crops in Sub-Saharan Africa, and What known mitigation and adaptation strategies are available to address them?

The crops that this research addresses include maize, legumes, and sweet potatoes/yams. The crop production system is the primary unit of analysis. According to the International Rice Research Institute, a cropping system is defined as “[comprising] all cropping patterns grown on the farm and their interaction with farm resources, other household enterprises and the physical, biological, technological and sociological factors or environments” (IRRI, 1978). The main components of the system that this research examines include the pre-production, production, and post-production stages of each crop and the environmental constraints and impacts involved at each stage. In Africa, the 33 million small farms totaling less than two acres comprise 80% of all farms on the continent (FAO, 2009). Therefore, the majority of these crop systems take place in a smallholder context, as opposed to the large, industrialized systems of many Western nations that take advantage of large economies of scale.

This research uses an in-depth literature review of recent scientific studies and publications to answer its question. Sources include articles found on scholar databases such as Scopus and Web of Science, publications from international research institutions such as the Consultative Group on International Agricultural Research (CGIAR) centers, and interviews with experts in the specific crop systems studied. The main CGIAR centers are the International Food Policy Research Institute (IFPRI), the International Maize and Wheat Improvement Center (CIMMYT), and the International Potato Center (CIP).
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Historical Context

Many crops throughout Africa have experienced significant gains in production since 1961. Maize increased from 17.5 million tonnes in 1962 to over 63.5 million tonnes in 2010, an almost four-fold increase. Yam production increased even more, from 7.5 million tonnes in 1962 to 45.5 million tonnes in 2009, a six-fold increase. Sweet potato production increased almost five-fold, although production remains fairly low at 15.2 million tonnes compared to maize and yams. The production of pulses, a subset of legumes, increased by only about 50% and at less than 1 million tonnes, is the least produced category of crops (FAOSTAT, 2012).

This increased production has caused increased environmental degradation as a result of poor practices used by farmers. Their use of monocropping and lack of fallow cropping have resulted in soils leached of nutrients. Increases in the harvested land area for agriculture (Figure 2) have caused cropland conversion from grasslands and forests. This has led to increased levels of erosion, that itself leads to further nutrient leaching when heavy rains come. Increased irrigation has led to falling water tables (Killebrew & Wolff, 2010). These impacts have contributed to the fluctuating or stagnant yields shown in Figure 3.

Maize, sweet potatoes/yams, and legumes are extremely important to subsistence farmers in Africa, as well as to the African economy as a whole. Small-scale farmers are responsible for about 90% of Africa’s total agricultural production, and agriculture accounts for between 30-40% of Africa’s gross domestic product. Investments in agriculture contribute to growth by increasing farm wages, while at the same time decreasing food prices regionally; the associated real income effects are powerful in reducing poverty (IFPRI, 2009).

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Key Research Institutions

The main institutions that conduct research on crops in developing countries are the CGIAR centers, which are 15 research organizations that collaborate on agricultural research. Those especially relevant to this study are IFPRI, CIMMYT, and CIP. Regionally, the International Institute of Tropical Agriculture (IITA) has an important presence in Africa. Nationally, state-based research organizations, such as the Zambian Agriculture Research Institute (ZARI), are essential for crop development.

These key research institutions, summarized in Table 1, have similar goals and missions, often focused on increasing farm productivity and developing technical innovations:

  • The International Food Policy Research Institute (IFPRI) does not focus on any one or two crops, but instead conducts general food policy research in a global context. Their focus is, however, on developing countries, and they are instrumental in creating national policies and strategies for sustainably meeting food needs in their target countries (IFPRI, 2012).
  • The International Maize and Wheat Improvement Center (CIMMYT) focuses on sustainably increasing the productivity of maize and wheat systems, primarily through the use of biotechnology, traditional agronomy, and agricultural extensions (CIMMYT, 2012). It specifically played a large role in the development of maize hybrids in Malawi. It was regionally present in the late 1970s, but increased its investment in the region dramatically after the establishment of a research station in Zimbabwe in 1985 (Smale et al., 2010).
  • The International Potato Center (CIP), a root-and-tuber research-for-development institution, combines rigorous research with technical innovations. Their program, the Sweetpotato for Profit and Health Initiative, has a goal of reaching 10 million households across 17 sub-Saharan countries in the next ten years. They focus on sweetpotatoes as a means of increasing vitamin A in the diets of African children (CIP, 2012).
  • The International Institute of Tropical Agriculture (IITA) works to find solutions for hunger, malnutrition, and poverty through collaboration with partners to enhance crop quality and productivity (IITA, 2012). The IITA played a role in maize development in West Africa in the 1970s, when it developed open-pollinated varieties that combined both high yields and resistance to rust and blight.
  • The Zambian Agriculture Research Institute (ZARI)’s mission is to provide cost-effective, quality support to farmers in Zambia, using technology to improve crop and soil quality in the country (ZARI, 2012).

Lastly, though not a research institution in the same sense as the organizations above, the Bill and Melinda Gates Foundation (BMGF) additionally supports agricultural research in developing countries. The Foundation’s goal is to reduce hunger and poverty for millions of farm families in Sub-Saharan Africa, with investments typically focused on small, rural farmers with the intent of helping them grow and sell more food. The Gates Foundation Agricultural Development initiative is the largest division of the BMGF Global Development Program (BMGF, 2012).

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This literature review uses scholarly papers from databases and search engines including Google Scholar and Scopus, as well as the following websites: CIMMYT, African Development Bank, World Bank, FAOSTAT, UNEP, Millennium Ecosystem Assessment and IPCC. Searches used combinations of the following terms: maize, legumes, sweet potatoes, yams, developing world, Sub-Saharan Africa, cultivation, soil fertility, constraints, land, pollution, small-holder, environment, environmental impacts, biotic, drought, climate change, natural resources, yield gap, pollution, and storage, among others. The methodology also includes searching for sources that are identified as central works and examining relevant lists of works cited.

Specifically, the focus of this research is on maize, legumes, and sweet potatoes/yams. While millet and sorghum, wheat, and rice are all critically important crops in Africa, they are not included in the scope of this research. The three crops this paper examines provide a good representation of the environmental constraints and impacts of crop production, as well as the current adaptation strategies and potential best practices to raise yields while limiting the environmental impacts.

The three reviews below highlight crop-environment interactions for each crop at three stages of the crop value chain: pre-production (e.g. land clearing), production (e.g. soil, water, and input use), and post-production (e.g. waste disposal, transport and storage). At each stage this research emphasizes environmental constraints on production (e.g. poor soil quality, water scarcity, crop pests, etc.) and also environmental impacts of crop production (e.g. soil erosion, water depletion, pest resistance, etc.). Adaptation strategies refer to currently practiced management techniques used by farmers to overcome the relevant environmental constraints. Best practices refer to key recommended approaches for overcoming environmental constraints and minimizing environmental impacts in different crop production systems.

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Crop 1: Maize and the Environment

Maize is the staple crop for smallholder production and consumption in many parts of Africa. In Lesotho and Zambia, where maize is typically grown on small parcels without the use of purchased inputs, maize accounts for more than 50% of calories consumed daily by the average household (FAOSTAT, 2010).

Briefly, the key environmental constraints to maize production in SSA include:

  • Pre-production
    • Land availability & site suitability
  • Production
    • Poor soil fertility
    • Drought
  • Post-production
    • Crop pests

Key adaptation strategies to these constraints include:

  • Pre-production
    • Intensification
    • Adoption of new seed varieties
  • Production
    • Soil fertility management, including biological (legumes) and chemical (fertilizer) interventions
    • Development and dissemination of drought-resistant varieties
  • Post-production
    • Improved storage (metal silos)
Maize Production Systems:

In Africa, maize is primarily grown by smallholder farmers, typically in monocrop systems in rain-fed areas. Average yields throughout Africa in 2010 were 2,300 kg/ha (Figure 4) (FAOSTAT, 2010). Although the area of maize has continued to increase worldwide, the grain yield per unit land area has leveled and potentially begun to decrease (Waddington et al., 2006).

Pre-production of Maize

Land Constraints: Maize production throughout Africa increased from 17.5 million tonnes in 1962 to over 63.5 million tonnes in 2010, an almost four-fold increase. Much of this increase in production comes from a doubling in the area harvested for maize in Africa from 15.5M ha in 1961 to 30.9 M ha in 2010 (FAOSTAT, 2012): in East Africa, the area harvested for maize grew by 151%.

Adaptations: Common adaptations by farmers to land constraints are converting non-agricultural land to land used to produce maize, and raising yields on current cropland. As much as 55% of new agricultural land in Africa was developed though deforestation between 1975 and 2000, while the other 45% was converted from non-forest natural vegetation (Brink and Eva, 2009). Farmers who intensify and increase fertilizer usage, use fallows and intercropping methods, and transition to higher-yielding varieties of seeds can increase yields in the face of this constraint.

Environmental Impacts: Runoff, erosion, and soil degradation are negative impacts of agricultural expansion and intensification. Land runoff from agriculture, together with atmospheric deposition, accounts for about 90% of the phosphorous and 94% of the nitrogen input in Lake Victoria, Africa’s largest lake by area (Odada et al., 2004). In South African streams, fertilizer runoff contributes about 50% of nitrogen and phosphorous pollution (Nkwonta and Ochieng, 2009).

Erosion and lost vegetative cover have caused the depletion of nutrients and organic matter in African soils and have contributed to stagnation in the growth of maize yields to around 1 ton per hectare from the 1980s to the early 2000s. Degraded land in Ethiopia has caused productivity losses that range from an estimated 2% to 6.8% of agricultural GDP (Yesuf et al., 2005).

Best Practices: The judicious use of fertilizers, fallow cropping, and intercropping with leguminous species are best practices for readying land to grow maize. The Ethiopian Environmental Protection Authority (EPA) has identified broadcast, row, side placement, perforated, and liquid methods of fertilizer application as the most appropriate practices for soil fertility management (Federal, 2004). These policies are all geared towards raising soil nutrient levels while minimizing costly fertilizer purchases.

Recycling crop residues also benefit the soil; however, in many Sub-Saharan African smallholder farming systems, villagers often completely remove crop residues from fields for fuel or animal feed, and often do not apply any manure (in part because it is also a source of fuel) (Admasu, 2009).

Seed Constraints: Maize production relies heavily on the use of hybrid seeds to raise yields, which are more responsive to inputs, and often more resistant to drought and pests. However, farmers must replace these seeds every year, as their hybrid vigor declines on an annual basis. Slow turnover of maize is particularly an issue in southern and eastern Africa – in Ethiopia for example the average age of varieties planted was 14 years in 1998. In Kenya in 2010, 48% of maize area was devoted to planting a seed variety that was derived from one released in 1986 (Smale et al., 2011) indicating that slow turnover is a persistent and ongoing issue. Yields in Kenya from 1986 to 2010 decreased by 21% (FAOSTAT, 2010).

Adaptions: The most direct solution to seed-related constraints is to increase the adoption among farmers of new seed varieties. Adoption rates vary widely throughout Africa, from 60% in West and Central Africa, to 96% in Zimbabwe. Often access to markets, high seed and input prices, and uncertainty of future supply limit the adoption of these seeds. Many international research organizations are currently conducting research on how to improve hybrid maize varieties in Africa (CIMMYT, 2012; FAO, 2009; Smale et al., 2011).

Environmental Impacts: There are not any quantified environmental impacts from using new seed varieties that emerge from the literature review. However, increasing yields on current land grown for maize through the use of new seed varieties could reduce the pressure for agricultural expansion, as well as the needs for fertilizer, pesticide, and fungicide use.

Best Practices: The most direct solution is to increase the development and adoption among farmers of new seed varieties.

Production of Maize

Poor soil fertility: Other than drought, soil fertility is the greatest constraint to maize production in Africa. Of the acreage that produces the majority of South Africa’s maize, 25% is susceptible to wind erosion, and more than 5M acres are seriously acidified (Goldblatt, 2010). In Tanzania, the effects of soil erosion have reduced maize yields by 15-48% (Lal et al., 2003). Diminished organic matter content and vital nutrients produce unfavorable growing conditions, and poor soil quality constrains maize varieties that are highly responsive to fertilizer inputs. Depleted African soils have caused maize yields to stagnate since the 1980s.

Adaptions: Maize accounts for 40% of fertilizer use in sub-Saharan Africa (Smale et al., 2011). However, many smallholder farmers do not use any fertilizers at all. Throughout Africa, the average dose is only 17 kg/ha, compared to 100 kg/ha and 270 kg/ha in developing and developed countries, respectively. More than 50% of applied nitrogen fertilizers are not assimilated by plants (Foulkes et al., 2009); however, field experiments in the Aludeka and Emuhaya regions of Western Kenya increased maize yields by 1.1 and 1.3 ton/ha, respectively, with full fertilization rates of 100 kg/ha of nitrogen, phosphorous, and potassium (Tittonell et al., 2008).

Additionally, researchers in Malawi that performed experimental intercropping with legumes, while simultaneously reducing chemical input use to half-fertilizer rates, found that they can sustain previous yields and reduce yield variability from 22% to 13% (Snapp et al., 2010). In Zambia, maize yields under F. albida, a leguminous tree, reached 4 tonnes per hectare compared with 1.3 tonnes per hectare outside the canopy (Gilbert, 2012). Additionally, average returns to labor are five times higher across maize systems intercropped with macuna, groundnut, pigeonpea, tephrosiaa, and soyabean (Sauer and Tchale, 2009). Organic fertilizers, such as crop residue and manure, also increase maize yields. However, in Ethiopia and other parts of Africa, crop residues and manures are exhaustively removed from the fields in order to provide feed for animals and fuel for households (Admasu, 2009).

Environmental Impacts: Excessive and improper fertilizer use can have many negative environmental impacts. Soil organic carbon levels have been reduced from repeated inorganic fertilizers being used by farmers. This reduces soil fertility, deteriorates soil structure, increases erosion and nutrient runoff, and reduces the biomass cover. (Admasu, 2009) Additionally, it can cause algal blooms on surface waters and high nitrate concentrations in drinking water, possibly leading to certain cancers (Federal, 2004).

Conversely, the impacts of organic soil fertility management strategies are overall positive in terms of adding nutrients and matter back into the soil. Field experiments with intercropping Macuna, a legume, with maize increased clay, organic matter, total N, and available P by 14-8, 27-25, 50-43, and 70-83%, respectively, as compared with the  control group (Shave et al., 2012). Maize-cowpea systems are present in Zimbabwe, and maize-pidgeon pea systems are widely practiced by farmers in Malawi, eastern Zambia, and Mozambique. Compared to non-intercropped maize, maize-cowpea system crop yields in Madziwa, Zimbabwe were 22% higher in experimental studies in 2011 (Thierfelde et al., 2012).

Best Practices: Notably, there is growing evidence that chemical fertilizers and organic soil amendments are not merely substitutes, but also complement one another in important ways. For example, in field experiments the use of optimal combinations of applied fertilizer for monocropped maize in Malawi (approximately 80 kg N/ha and 31 kg P/ha) was associated with yields of 4.2 tonnes/ha. However yields reached 5.7 tonnes/ ha in the same area when researchers applied 69 kg N/ha and 37 kg P/ha in a legume/maize intercrop system – and furthermore the polyculture system resulted in double the phosphorous uptake in maize over optimal fertilizer levels in the monocropped system (Akinifesi et al., 2007). Adding fertilizer to an experimental intercropped system with semiperennial legumes in Malawi led to efficiency gains of greater than 100% (Snapp et al., 2010).

Therefore, fertilizer use in tandem with soil fertility management practices will yield the greatest results. When using fertilizers, farmers should first increase the organic matter and nutrient content of the soil. When they apply inorganic nutrients on soils depleted of soil organic matter, much of the applied nutrient leaches away or is otherwise unavailable to plants (Solomon et al., 2000). It is not until there is a greater than 3% carbon content in the soil that marginal returns to fertilizers become significant (Marenya and Barrett, 2009).

Water Constraints: High on the list of reported farmer constraints is drought, which affects rainfed lowland and rainfed upland production systems that support 48 million rural poor in Asia and produce 16 million tonnes of maize (Gerpacio and Pingali, 2007). In Africa, about 22% of mid-altitude or subtropical maize and 25% of lowland tropical maize growing regions are affected by drought (Cairns et al., 2012).

Despite being drought-constrained, maize is water-efficient relative to other major crops. Maize needs only around 850 liters of water per kilogram of grain production (with 2-4 irrigations) compared with 1,000 l/kg for wheat grain (1-3 irrigations) and over 3,000 l/kg for rice grain (Ali et al., 2008).

Adaptations: There is very little irrigated maize production in Africa. Therefore, for smallholder farmers, the most feasible solution is to adopt seeds with increased drought resistance. For both temperate and tropical regions in Africa, drought-resistant varieties have increased yields by between 73 and 146 kg per ha per year (Campos et al., 2004).

Environmental Impacts: There are not any quantified environmental impacts of using seeds with drought resistance that emerge from the literature. However, reducing losses from drought would reduce the pressure for agricultural expansion.

Best Practices: Investing in research to develop drought-resistant seeds and varieties could have a high return on investment. The Drought Tolerant Maize for Africa initiative by the CIMMYT is working to develop appropriate seeds (CIMMYT, 2012).

Biotic Constraints: Downy mildew can cause yield losses of up to 80% in tropical regions. Turcicum blight can cause yield losses of 15-20% in tropical, mid-altitude regions. Grey Leaf Spot can cause losses of 30% in Africa when infection is present and the maize crop flowers. Leaf damage from Armyworms can reduce yields by 10%. Other pests of significance include stem-borers and the parasitic weed Striga (Pingali and Pandey, 2000).

Adaptations: In Africa, pesticide, herbicide and fungicide use is low. A United Nations Environment Report (2002) found that sub-Saharan Africa accounted for only 5% of global pesticide imports.  For many pests, such as downy mildew and blight, the only economic solutions for smallholders have been the development of resistant varieties (Pingali and Pandey, 2000). Striga resistant strains have been developed by researchers and implemented by farmers in western Kenya, and an econometric analysis shows that a 1% increase in the adoption of this variety would increase maize yields by about 22% (Mignouna et al., 2010)

Environmental Impacts: There are not any quantified environmental impacts of using resistant varieties that emerge from the literature. However, reducing losses from biotic constraints would reduce the pressure for agricultural expansion and would reduce needs for chemical fungicide use.

Best Practices: Intercropping and crop diversification have both been reported to reduce the prevalence of weed and insect pests, suggesting agro-ecologically complex systems may be effective at controlling pests. Two thousand farmers in western Kenya have experienced increased maize yields of 60-70% by adopting maize, grass-strip, and legume intercropping systems that both suppress Striga growth and trap stem-borers (Pretty et al, 2003).

Post-Production of Maize

Post-Harvest Storage Losses: There can be considerable post-harvest losses due to pest infestation, often related to the duration of the grain storage. In Kenya, harvest losses range from 15-30%, the majority of which are from pests (Bett and Nguyo, 2009). Given the often high costs of inputs related to maize production, minimizing post-harvest losses is extremely important for smallholders.

Adaptations: In warmer growing climates where maize is stored for longer periods of time, post-harvest pest control is especially important.

Environmental Impacts: There are not any quantified environmental impacts of storage methods that emerge from the literature review. However, reducing post-harvest losses would reduce pressures for agricultural expansion, as well as reduce needs for fertilizers, pesticides, and fungicides during production.

Best Practices: A critical practice is to ensure that the maize harvest is properly dried and able to be stored safely. Maize should be dried to 13-14% moisture content before being stored (Paudyal et al., 2001). Once dry, the most effective means of storage for maize is in metal silos, which diminish harvest losses to almost zero and can fit inside of a house (Tefera et al., 2011). Both the FAO and CIMMYT have successfully disseminated these silos throughout developing countries.

Crop 2: Legumes and the Environment

Dry beans, peanuts, and cowpeas are the three most important pulses in sub-Saharan Africa, as measured by total production. About 4 million tonnes of dry beans, 5 million tonnes of dry cowpeas, and 9 million tonnes of peanuts were produced throughout the region in 2010 (FAOSTAT, 2012), with most of the production coming from East Africa and West Africa, specifically. Nigeria, which is the largest producer and consumer of cowpeas, accounts for 61% of production in Africa and 58% of production worldwide (FAOSTAT, 2012). Two-thirds of total peanut production comes from West Africa.

Briefly, the key environmental constraints to legume production include:

  • Pre-production
    • Soil nutrient availability
  • Production
    • Diseases (leaf spot and southern bean mosaic virus) and pests (aphids, pod-borers, and weevils)
  • Post-production
    • Storage losses

Key best practices to these constraints include:

  • Pre-production
    • Fertilizer use
  • Production
    • Fungicide use, pesticide use, and improved varieties
  • Post-production
    • Proper storage methods
Legume Production Systems

Legumes in a small holder context are often intercropped with crops such as maize in order to improve yields in the primary crop, and this will likely remain true into the future (Jansa et al., 2011; Naab et al., 2009; Thierfelde et al., 2012). There is a great variety in the yields of beans, from less than 200 kg/ha in Eritrea to over 1,000 kg/ha in Madagascar (Figure 5) (FAOSTAT, 2008). Cowpea is grown under both rainfed conditions and by using irrigation or residual moisture along river or lake flood plains (Dugje et al., 2009). Yields for cowpea of under 500 kg/ha are lowest in Western Africa, where the majority of production occurs, and are highest in mid-Africa at around 750 kg/ha (Included in Figure 6) (FAOSTAT, 2012). Groundnut yields throughout Africa range from 700 kg/ha in East Africa to 1,400 kg/ha in South Africa (Figure 7) (FAOSTAT, 2012).

Pre-production of Legumes

Soil Nutrient Availability: The lack of soil nutrients is one of the main reasons for the gap between average bean yields throughout Africa and yields under optimal conditions (Jansa et al., 2011). In Rwanda specifically, average yields are 500 kg/ha, but potential yields are as high as 5,000 kg/ha (Jansa et al., 2011). Optimal growing conditions for beans include light, loamy soils that provide the crop with adequate levels of nitrogen (N) and phosphorous (P). Between 40% and 50% of bean production areas are affected by moderate to severe P deficiency. Soils that do not release enough P to the bean plants during the growing season can cause yield losses of 60-75% (Jansa et al., 2011).

In areas where there is little soil nitrogen, cowpeas cannot fix enough N from the atmosphere to compensate for this deficiency (Dugje et al., 2009). Poor soil fertility also limits the yield potential of groundnuts, as P deficiency is inherent to many soils in West Africa, where groundnuts are primarily grown (Naab et al., 2009).

Adaptations: Most farmers in Northern Ghana do not currently use any external fertilizers for groundnut production (Tsigbey, 2003). Beans are also rarely fertilized in subsidence farming elsewhere in Africa (Jansa et al., 2011). However, legumes are often intercropped with maize, and 40% of all fertilizer use in sub-Saharan Africa is devoted to maize production (Smale et al., 2011).

Environmental Impacts: Beans are dependent on soil N, and respond well to fertilizer inputs. However, they generally only recover less than 50% of applied chemical fertilizers (Jansa et al., 2011), leading to N runoff. The general effects of fertilizer runoff have been well-documented; however, there are not any quantified effects of fertilizer runoff from applications to dry beans in Africa that emerge from the literature.

Phosphorous, when it is applied as fertilizer, is applied in greater quantities for cowpeas and beans than Nitrogen, and thus P runoff is likely to be a greater issue (Dugje et al., 2009, Jansa et al., 2011).

There are also positive soil-related environmental impacts that result from growing legumes. They generally improve the quality of soils in which they are planted: cowpea fixes 240 kg/ha of N and deposits up to 60 – 70 kg/ha in the soil (Singh, 2011).

Best Practices: High levels of P (1,000-2,000 kg/ha) and N (100 kg/ha) are needed to significantly improve bean yields to above 2,000 kg/ha (Jansa et al., 2011). However, these levels are largely unattainable given current input markets and incomes in Africa. Instead, applications of plant residues can be just as effective as inorganic NPK fertilizers at raising yields. Best practices include using plant residues together with low rates (~10 kg/ha) of P fertilizers to achieve the best results for bean production (Jansa et al., 2011).

Starter doses of 15 kg N/ha and 30 kg P/ha are recommended for cowpea production (Dugje et al., 2009). Applying inorganic P to plots in Northern Ghana increased pod yield by 108% compared to farmers’ current practices, and also helps cowpeas fix nitrogen (Naab et al., 2009).

Production of Legumes

Disease: Early leaf spot, late leaf spot, rust, and groundnut rosette disease are the major foliar diseases affecting African groundnuts, and cause low yields throughout the semi-arid regions (Varshney et al., 2009). These diseases can cause yield losses of 50-60% throughout Africa (Naab et al., 2009). In experiments in Northern Ghana, early leaf-spot disease was observed in 90% of farmers’ groundnut plots. At this same location, severe leaf defoliation was recorded at 80% of the studied locations, with poor pod formation among the affected plants. Pod losses from Cercospora amounted to 78% on-farm (Tsigbey, 2003).

The southern bean mosaic virus (SBMV) can cause up to 63% reductions in the yield of cowpeas when the crop is inoculated with the virus under controlled experiments in Nigeria (Taiwo and Akinjogunla, 2005).

Adaptions: Fungicides are not frequently used to combat disease in groundnuts. (Naab et al., 2009) This is also the case for cowpeas and beans. Instead, improved varieties that show pathogen resistance are more frequently used (Baoua et al., 2012; Asiwe, 2009; Tsigbey, 2003), although yield impacts of this practice were not discovered through this literature review.

Environmental Impacts: The growing use of disease-resistant varieties will likely lead to the evolution of pathogens to overcome these resistances (Jansa et al., 2011). However, it will also increase production on the land currently used to produce legumes, preventing the need for agricultural expansion.

Best Practices: Best practices for control of pests in groundnut include fungicide application of Tebuconazole at 0.22 kg/ha. Doing so can increase pod yields for groundnut by 140% (Tsigbey, 2003). Researchers also recommend the continued development of bean, cowpea, and groundnut varieties resistant to diseases (Asiwe, 2009; Jansa et al., 2011).

Pests: Major pests affecting cowpea production in South Africa are aphids, thrips, pod-sucking bugs, and the cowpea weevil (Asiwe, 2009). In the West African Sahel, the pest B. atrolineatus was found in 80-90% of cowpea plants at the time of harvest (Livinus et al., 2012). Major insect pests affecting peanut production include hoppers, millipedes, termites, and white grubs. Termite damage is also prominent during late harvested crop (Varshney et al., 2009).

O.bennigseni was found to cause yield losses in bean plants in Tanzania ranging from 8-31% in the 1980s, and farmers in the country have since reported increasing damage to their plants (Paul, 2007).

Adaptions: A lack of pest control methods among farmers for both beans and groundnuts has been cited by researchers as problematic to preventing pest losses (Naab et al, 2009; Asiwe, 2009). Breeding efforts focused on developing aphids resistance in the 80s led to early maturing cowpeas being introduced in Ghana that significantly raised farmer yields. In 1994, the yield advantage of using these improved varieties was estimated to be between 25% and 46% in Niger and Cameroon. Past breeding efforts have also greatly improved the resistance of beans to pests.

Environmental Impacts: The use of pest-resistant varieties could lead to the evolution of pests to overcome these resistances. However, it will also increase production on the land currently used to produce legumes, preventing the need for agricultural expansion.

The use of legumes in intercropped systems can have positive effects on pest control in the system at large. 2,000 farmers in western Kenya have experienced increased maize yields of 60-70% by adopting maize, grass-strip, and legume intercropping systems that help trap stem-borers (Pretty et al, 2003).

Best Practices: Proper spacing between cowpea plants has been shown to be effective at reducing the infestation of the Macuna pod borer and PSBs, the most yield-limiting pests, in Nigeria. Plant spacings of 1.0-1.5m were found by researchers to be most effective (Asiwe et al., 2005). Insecticide application significantly reduces leaf area loss from 40% to 13%, given an insect abundance of 0.69 O. bennigsenu per plant in on-farm trials in Tanzania (Paul, 2007).

Post-Production of Legumes

Storage losses: The primary constraint to post-harvest production of legumes is pest infestation during storage, and studies from the ‘70s, ‘80s, and ‘90s have quantified these effects. Bruchid beetles cause dry weight losses of 10-40% in dry beans in less than six months of storage, and potentially up to 70% (Jones et al., 2011). Some farmers experience total crop loss from insect infestation within 4-5 months (Jones et al., 2011). Cowpea losses can be up to 50% from certain pests (Keneni et al., 2011). Moulds are also damaging to legumes, and while they are not as serious a problem as pests, they can cause a crop to be completely inedible (Golob, 2009).

Adaptations: Solar disinfection of seeds is often promoted by extension agents, and reduced the percentage of damaged seeds to under 1% in trials in Kenya. Farmers also use techniques such as delayed threshing and admixing with plant oils and other botanicals (Jones et al., 2011).

Environmental Impacts: There are not any quantified environmental impacts of solar disinfecting, delayed threshing, or admixing that emerge from the literature review. However, reducing post-harvest losses would decrease pressures for agricultural expansion.

Best Practices: Best practices include the use of storage bags, properly drying the harvest, and using insecticide dusts. Applying neem seed powder to the harvested crop at 1.5kg/100kg beans was shown to keep grain damage under 15% for 5 months in Northern Tanzania (Jones et al., 2011). Using PICS, a type of storage bag, can reduce cowpea losses to .5% of dry matter (Jones et al., 2011). Pulses should be dried to 13-14% moisture contents, and groundnuts to 7%, in order to prevent mold infection (Keneni, 2011).

Crop 3: Sweet Potatoes/Yams and the Environment

Root and tuber crops (including sweet potato and yams, in addition to cassava and aroids) are the second most cultivated species, after cereals, in tropical countries. Though different species, sweet potato and yams are often grouped together for scientific study because they are vegetatively propagated, produce underground food, and are bulky and perishable (Lebot, 2009). Both are important food sources and are also used for animal feed. East and West Africa account for 93% of African land use for growing sweet potatoes, and East Africa specifically produces 62% of all sweet potatoes grown on the continent. For yams, West Africa accounts for 90% of global land area for production and is responsible for 90% of total global harvest (FAOSTAT, 2012).

Briefly, the key environmental constraints to sweet potato and yam production in SSA include:

  • Pre-production
    • Land suitability
    • Planting material
  • Production
    • Pests (sweet potato weevil and yam nematode) and diseases (SPVD & SPFMD)
  • Post-production
    • Shelf-life

Key adaptation strategies to these constraints include:

  • Pre-production
    • Hand irrigation
    • Plant material treatment
  • Production
    • Use of improved cultivars
    • Pesticide use
  • Post-production
    • Proper post-harvest handling
Sweet Potato and Yam Production Systems

In Africa, sweet potato and yam are primarily grown by female smallholder farmers on polycropped marginal lands across a variety of growing climates (Ewell, 2011). There is a cluster of sweet potato production around Lake Victoria in Eastern Africa (CIP, 2010). Yields for sweet potatoes in East Africa are 5,300 kg/ha (Figure 8), less than one-fourth of what they are in China, the world’s largest producer (FAOSTAT, 2012). Sweet potatoes have a flexible growing season, allowing it to be grown anywhere from three to ten months of the year in some countries (Ewell, 2011). In West Africa, the major farming systems for sweet potato are root crop systems, where livelihoods depend primarily on yams, cassava, legumes, and off-farm work, and cereal-root crop mixed systems, where livelihoods depend primarily on maize, sorghum, millet, cassava, yams, and cattle (Gruneberg et al., 2009).

Pre-production of Sweet Potato and Yam

Land Suitability: Sweet potatoes are often grown by farmers on marginal lands that are acidified and waterlogged (Fuglie, 2007). Where they are grown in areas with a prolonged dry period, they are susceptible to drought. Farmers in Tanzania, Uganda, and Rwanda identified drought as the largest production constraint in a survey (Fuglie, 2007).

Average yam yields in southwestern Nigeria have decreased by more than 50% between 1995 and 2000 because of declines in soil fertility (Agbaje et al., 2005). Intercropping yams with maize or cassava extracts high levels of nitrogen from the soil due to intense competition for nitrogen among the three crops (Agbaje et al., 2005).

Adaptations: Fertilizers and irrigation for sweet potatoes are not commonly used by farmers in SSA, and they have little experience doing so (Oswald et al., 2009). A study in Nigeria found that 88% of farmers do not use fertilizer, and when they do, it is often applied incorrectly (Adewumi and Adebayo, 2008). In Tanzania, farmers (mostly women) commonly hand-water sweet potatoes from lakes, waterholes, and rivers, and spend three hours on average every one to two days doing so. In areas with a moderate dry season, growing the crop in the shade of other plants is common, but this does not work for very long dry seasons (Namanda et al., 2011).

Meanwhile 60% of farmers growing yams in Nigeria used NPK chemical fertilizers that resulted in increased yam yields. The planting of leguminous cover crops by farmers to increase soil fertility has also been reported by researchers (Agbaje et al., 2005).

Environmental Impacts: The general effects of fertilizer runoff have been well-documented; however, there are not any quantified effects of fertilizer runoff from applications to sweet potatoes and yams in Africa that emerge from the literature.

Best Practices: Applied fertilizer and manure can be advantageous to sweet potato production. Applying 5 Mt/Ha of poultry manure can raise yields by 43% (Agbede and Adekiya, 2011). Added nitrogen levels of between 40-80 kg/ha from inorganic fertilizers can raise sweet potato yields in Nigeria to 27.2 Mt/Ha and 24.8 Mt/Ha for white-fleshed and orange-fleshed cultivars, respectively (Okpara et al., 2009). For Nigerian yam production, the production of organic fertilizer from crop residues such as cassava peels, yam peels, maize crops, and animal dung increases yam yields (Agbaje et al., 2005).

Additionally, using improved cultivars suited to local land conditions can yield large production gains. Genetic improvements could increase yields throughout Africa by 3-40% compared to healthy local landraces (Gruneberg et al., 2004).

Planting Material: In tropical environments, such as those found in SSA, vine cuttings are the main form of sweet potato propagation (Fuglie, 2007). This vegetative propagation over time causes an accumulation of viruses in the planting material that can significantly reduce plant vigor and yield. A survey sent to sweet potato scientists in SSA identified this as one of the most important constraints to production (Fuglie, 2007). Most often, planting material is affected by sweet potato virus disease before even being planted (Oswald et al., 2009). This constraint is especially large in areas with dry periods lasting more than 4 months (Low et al., 2009).

Adaptations: To overcome disease burdens and yield losses associated with use of poor planting material, farmers try to select healthier vines to cut and replant for next season. Selecting vines which do not have viral infections, based solely on visual inspection, has enabled farmers in southern Uganda to keep the rate of infection in their crops to below 20% (Thiesen, 2006).

Environmental Impacts: There are not any quantified environmental impacts of using improved cultivars that emerge from the literature. Perhaps most the most direct one is the impact from decreased losses of the primary crops that would otherwise be greater if infected vines for sweet potatoes were used and yields were thus lower. Also, using clean planting material would mean less need for pesticide use.

Best Practices: Using clean planting material can increase yields between 56 and 84% in sub-Saharan Africa (Barker et al., 2009). CIP conservatively puts the yield gains from using healthy planting material at 30-50% throughout Africa (Oswald et al., 2009). In areas that are prone to weevils, vines should be dipped in insecticide prior to replanting.

Production of Sweet Potato and Yam

Disease Infection: Due to the vegetative propagation of sweet potatoes and the use of planting material from the previous season’s harvest, most sweet potato production is genetically homogenous. This makes the crop especially susceptible to viral infections, specifically the sweet potato virus disease (SPVD) and the sweet potato feathery mottle virus (SPFMV). SPFMV was found in 100% of crop samples in a recent experiment in Kenya (Opiyo et al., 2010).

Additionally, SPVD infection rates range from 54-94% in Tanzania, 10-40% in central Uganda, and 83% in Rwanda (Barker et al., 2009). Especially in the humid low and mid-elevation regions of East Africa with short dry seasons, such as the areas around Lake Victoria, there is extremely high SPVD pressure (Gruneberg et al., 2009). The disease that causes the greatest reductions in yam yields is the yam mosaic virus (Agbaje et al., 2005; Amusa et al., 2003).

Adaptations: Cultivars resistant to the sweet potato virus disease (SPVD) are widely grown (Barker et al., 2009). Where improved cultivars are not grown, the use of clean planting material can cut down the rate of infection and improve yields (Oswald et al., 2009; Barker et al., 2009). 53% of Rwandan farmers used no control measures against the SPVD (Low et al., 2009).

Environmental Impacts: There are not any quantified environmental impacts of using improved cultivars that emerge from the literature. Perhaps most the most direct one is the impact from decreased losses of the primary crops that would otherwise be greater if infected vines for sweet potatoes were used and yields were thus lower.

Best Practices: Using improved cultivars to known diseases, combined with clean planting materials, together are the most practiced methods to control diseases.

Pest Infection: The sweet potato weevil is one of the most damaging pests to the crop throughout Africa. Historically the weevil can cause yield losses of up to 73% in Eastern Africa (Smit, 1997), and 60-100% during times of drought (CIP, 2010). The sweet potato weevil was the most frequently cited pest constraining production among agricultural experts in Africa (Fulgie, 2007). The major tuber pest for yams according to Nigerian farmers is the yam nematode. Intercropping with ocra, maize, melon, sorghum, or cassava all increases nematode and pest pressure on yams (Agbaje et al., 2005).

Adaptations: 55% of farmers in northeastern Uganda use pesticides to control weevils and other pests. The next most common technique is uprooting and killing pests by hand (Ebregt et al., 2004). Farmers in Nigeria do not use chemicals to control for pests (Agbaje et al., 2005).

As one alternative adaptation commonly used in Nigeria, the use of leguminous cover crops can control the yam nematodes (Agbaje et al., 2005). Weevil-resistant cultivars have been developed by CIP in Kenya and Uganda but have not been introduced to other areas of Africa (CIP, 2010)

Environmental Impacts: Pesticide use for sweet potato production throughout Africa is fairly low, but can lead to resistance among pests and a decrease in other species. Especially given the cluster of sweet potato production around Lake Victoria, and the reliance of local populations on the lake for water and fish, pesticide runoff could have severe consequences; high levels of POPs have been found in the Nzoia River basin (Twesignye et al., 2011). Leguminous cover crops generally have positive environmental impacts in that they add to the nitrogen content of soils (Agbaje et al., 2005).

Best Practices: Clearing the land, planting another crop, and cleaning the planting materials with insecticides after each harvest helps to break weevil incidence cycles. Integrated Pest Management (IPM) techniques, such as using deep-rooted varieties, rotating crops, and hilling up soil around the base of the plant can reduce damages from sweet potato weevils from 45 to 6% of the crop over a period of 6 years, according to a 2000 CIP study (Low et al., 2009).

To control for nematodes and other pests in yams, researchers recommend that the planting site be tested for the presence of the pathogen prior to planting pests (Amusa et al., 2003). Additionally, crop rotation, the use of nematicides, and dipping seed pieces in Nemacuron can all prevent the presence of pests (Amusa et al., 2003). In Nigeria, Increasing the length of the fallow period has led to decreased infection rates of nematodes in yams (Agbaje et al., 2003).

Post-production of Sweet Potato and Yam

Shelf life: Sweet potato is the world’s seventh most important food crop but its potential to contribute to food security and income is limited in tropical countries by its short shelf-life (Kihurani et al., 2012). Roots are perishable and either rot or become non-marketable after 1-2 weeks (Thiele et al., 2009). Yams are subject to losses of up to 50% of fresh matter during storage, predominately resulting from microbial attacks.

Adaptations: There is little use of pits, clamps, or other storage techniques throughout SSA. Typical adaptations to a short stored life are piecemeal harvesting. With this practice, the most disease-susceptible parts of the potato are eaten first, and the rest of the crop remains stored in the ground for up to six months after the harvest (Low et al., 2009). Strategies focused on improving handling of yam harvests in Nigeria have reduced post-harvest losses (Amusa et al., 2003).

Environmental Impacts: There are not any quantified environmental impacts of storage methods that emerge from the literature. However, reducing post-harvest losses would decrease the pressure for agricultural expansion.

Best Practices: Storage of sweet potatoes in pits in Uganda can prevent rot for up to 4 months (Hall and Devereau, 2000). Although time intensive, drying sweet potato can increase shelf-life to 4-6 months; however, this practice is still susceptible to damage from grain-borers (Thiesen, 2006). Post-harvest losses from storage are relatively lower if clean planting materials are used from the start (Akoroda, 2009).

Treatments of yam tubers with insecticide dust decreased both fungal infections post-harvest as well as physical damages acquired during harvest (Amusa et al., 2003).

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Environmental constraints have large effects on crop production throughout Africa, and can cause harvest losses of up to 100% (Pingali and Pandey, 2000; CIP, 2010). Given that most crop production by small-holders is used for subsistence, these constraints have severe impacts on the food security situation of farming households. However overcoming these constraints, minimizing production losses, and increasing yields and harvests can have multiple payoffs. First, they increase families’ abilities to feed themselves. Second, they reduce the strains on the environment: increasing the amount of food that farmers grow on existing land reduces the need for deforesting land for agricultural expansion, for example, and practices such as intercropping maize with legumes can actually improve soil qualities (Shave et al., 2012; Singh, 2011). A less strained environment, in turn, can further increase long-term, sustainable food production. Increases in both food security and environmental health have the potential to improve the livelihoods of millions of people throughout Africa (BMFG, 2012).

The best practices to overcome these constraints are not always practices that farmers are able to currently implement. Despite poor soil conditions, many farmers do not use fertilizers to grow maize, legumes, or sweet potatoes and yams, because they are either too poor or cannot rely on a consistent supply of fertilizers throughout the growing seasons (Foulkes et al., 2009; Oswald et al., 2009; Jansa et al., 2011). One best practice for growing crops in infertile soils is thus for farmers to adopt the judicious use of fertilizers when possible (Admasu, 2009; Agbede and Adekiya, 2011; Jansa et al., 2011).

Other best practices are more easily implemented. Again considering infertile soils, another best practice is intercropping the main crop with other, secondary crop. This practice can increase the nutrient content and organic matter of soils; intercropping maize with legumes organically adds nitrogen to the system and can substantially raise maize yields (Thierfelde et al., 2012). This method is currently practiced by farmers, and relies less on a costly input or on functioning markets than using fertilizers.

The type and size of constraints also vary across geographic location in addition to across crops. Figures 4, 5, 6, 7, and 8 show yield variability across crops across Africa, which is largely caused by varying microclimates that can either exacerbate or ameliorate constraints. Sweet potatoes, for instance, can experience 100% crop losses from the sweet potato weevil during times of drought (CIP, 2010), and rainfall is considerably affected by geography. Within the crop category of legumes, beans are primarily grown in East Africa, while groundnuts are primarily grown in West Africa because of climactic conditions (FAOSTAT, 2012). Some of this constraint variability is also due to differences in farmer practices.

Much of the research on best practices that has been highlighted in this chapter has come from research institutions performing controlled experiments. As such, farmers on-the-ground throughout Africa do not necessarily know the best production methods, or understand specifically how to implement them. This could pose a challenge for widespread implementation of these policies. The few sweet potato producers in Nigeria who use fertilizers often use them incorrectly (Adewumi and Adebayo, 2008), indicating that simply supplying farmers with inputs will not necessarily be sufficient to improve production outcomes.

Additionally, research for secondary crops, such as legumes and sweet potatoes/yams remains limited compared to research for maize. Yield impact estimates for practices that seek to reduce pest infestation in legumes have not emerged, except for research done in the 1980s and 90s. Other crops, such as yams and sweet potatoes, do not have readily available yield estimates for many adaptation strategies. In order for research institutions, governments, non-profits, and farmers to best direct their efforts to increase yields, they need to know these estimates in order to compare relevant adaptation strategies.

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This chapter’s core policy recommendations are for farmers to implement best practices for agricultural production as described in this research, and for governments and international institutions to research and promote such best practices.

Common best practices that emerged from the literature are the judicious use of inorganic fertilizers, pesticides, and fungicides. These chemicals are effective at overcoming poor soil quality, pest infestation, and disease infection; however, they have negative environmental impacts and as such farmers need to apply them appropriately. Other practices common to the three crops are the use of improved genetic varieties and proper storage methods.

Specific best practices for each crop are:

Maize: Increase the judicious use of nitrogen, phosphorus and potassium (NPK) fertilizers in maize production. Coupled with intercropping, this can raise yields by 500%, as experienced in southern Malawi where yields were raised from .94 tonnes/ha to 5.7 tonnes/ha.

Legumes: Use PICS, a type of storage bag, when storing post-harvest. This practice can reduce cowpea losses to les than 1% of dry matter, down from 50% of harvest losses that often occur in cowpeas.

Sweet potatoes/yams: Use integrated pest management (IPM) techniques. Doing so can reduce damages from sweet potato weevils from 45% to 6%. Judiciously using pesticides could further reduce these damages.

Research institutions and governments should direct investments towards facilitating best practice implementation where farmers are constrained in their ability to do so. Investments could go towards developing input markets to make them readily and consistently available. Additionally, these investments could be used to help subsidize the price of necessary inputs to make them more affordable.

Furthermore, governments and non-profits should help educate farmers on how to correctly implement practices such as intercropping and applying agrochemicals. This dissemination of knowledge would lead to gains in the technical efficiency of farmers.

Finally, research institutions should also direct their efforts to filling existing knowledge gaps in the production of secondary crops. Specifically, they should focus their efforts on quantifying the impacts of the components of each production stage that this paper examined: environmental constraints, adaptation strategies, environmental effects, and best strategies.

To summarize, this chapter recommends three policies:

  • Educate farmers on proper implementation of agricultural best practices;
  • Develop markets for necessary inputs (seeds, agrochemicals, etc.); and
  • Research areas where knowledge gaps remain.

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