The transition from swidden agriculture to intensive maize (Zea mays L.) monoculture in upland regions of Southeast Asia has led to declining soil fertility problem, which necessitates the use of external nutrient inputs to sustain crop productivity. Conventional intensification via the use of mineral fertilisers, nevertheless, is environmentally and economically unsustainable. The integration of legumes into maize-based cropping systems offers the opportunity to improve soil fertility as well as crop productivity; and it has been proposed as an alternative to conventional intensification in upland farming systems. The main aim of this study was to investigate the feasibility of integrating legumes into maize-based cropping systems as an agroecological intensification option for smallholders in uplands of Southeast Asia. Three study areas representing different types of maize-based cropping systems along a gradient of agricultural intensification were selected for the study. The village of Ban Huai Puk in northern Thailand that practised maize monocropping represented high-external-input intensive farming systems, Pang Daeng Nai village in northern Thailand that practised maize/legume relay cropping represented low-external-input intensive farming systems, and the villages in northern Laos that cultivated maize with a short fallow period represented low-input farming systems. Data were collected from field surveys, a set of researcher- and farmer-managed field trials under rain-fed conditions and one pot experiment under controlled greenhouse conditions. A comparative study of maize monocropping and maize/legume relay cropping showed that both cropping systems had similar economic returns, implying that the low-external-input legume integration approach could represent an alternative pathway to intensification. Ricebean (Vigna umbellata L.) was the most preferred legume species among the others to be integrated with maize due to its high selling price, ease to grow and harvest, stable market demand and ability to improve soil fertility. The results from researcher-managed field trial demonstrated that water availability had a significant effect on crop responses to P fertilisation under P-deficient conditions. A positive maize yield response to P fertiliser application was observed under high water availability but not under low water availability, indicating that maize-ricebean intensification with moderate use of fertiliser-P inputs could be an option for upland smallholders under normal weather conditions but not under drought conditions. The results from the field study carried out in ten villages of Laos in which ricebean was introduced to maize production systems for the first year showed that the initial performance of ricebean was affected by drought and grazing livestock. The results also demonstrated that the top-down Farming Systems Research & Extension intervention approach by the extension agents in promoting the innovation of legume integration into maize-based cropping systems without considering farmers’ needs and local testing of optimum planting practices (i.e. plant density, plant spacing and sowing time) resulted in failure of the innovation. Thai Green was identified as a P-efficient ricebean cultivar due to its ability to produce the highest root hair length, root biomass, total root length and total P content when grown under P-deficient conditions. Introducing legumes as relay crops into maize-based cropping systems can be an agroecological intensification option for the smallholders in upland regions of Southeast Asia. The profitability of growing legumes, labour availability, the financial status of the household and access to extension services were the major factors influencing the decisions of resource-poor maize smallholders in legume adoption. The type of intervention approach by the extension agents needs to be considered to ensure sustained adoption of legume innovation. Since extreme weather events caused by climate change are becoming more common and unpredictable, it is imperative to find ways to reduce the risks that farmers may face upon integrating legumes under variable weather conditions. The assessment of the sustainability and resilience of legume integration showed that agroecological practice of legume integration is more sustainable and resilient than the conventional intensive monocropping of maize. Last but not least, it is important to note that legume integration alone is no panacea to address declining soil fertility problem. Legume integration combined with holistic approaches to managing agricultural soils should be promoted.
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Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, 2017