CLIMATE CHANGE AND AGRICULTURE PAPER Challenges for weed management in African rice systems in a changing climate J. RODENBURG 1*, H. MEINKE 2��� AND D. E. JOHNSON 3 1 Africa Rice Center (AfricaRice), East and Southern Africa Rice Program (ESARP), P.O. Box 33581, Dar es Salaam, Tanzania 2 Department of Plant Sciences, Centre for Crop Systems Analysis (CSA), Wageningen University, Wageningen, The Netherlands 3 Crop and Environmental Sciences Division, International Rice Research Institute (IRRI), DAPO Box 7777, Metro Manila, Philippines (Revised MS received 4 November 2010 Accepted 23 November 2010 First published online 25 February 2011) SUMMARY Global changes including increases in temperature, atmospheric greenhouse gases, soil degradation and competition for land and water resources, will have multiple impacts on rice production systems in Africa. These changes will affect weed communities, and management approaches must be adapted to take this into account. Higher temperatures and limited water availability will generally advantage C4 over C3 plants (e.g. rice). Conversely, elevated carbon dioxide (CO2) levels will improve the competitiveness of rice relative to C4 weeds, which comprise many of the problem weeds of rice. Increased atmospheric CO2 levels may also improve tolerance of rice against parasitic weeds, while prevalence of parasitic species may be amplified by soil degradation and more frequent droughts or floods. Elevated CO2 levels tend to promote growth below-ground relative to above-ground, particularly in perennial (C3) species. This may render mechanical control of weeds within a cropping season less effective or even counterproductive. Increased CO2 levels, rainfall and temperature may also reduce the effectiveness of chemical control, while the implementation of adaptation technologies, such as water-saving irrigation regimes, will have negative consequences for rice���weed competition. Rain-fed production systems are prevalent throughout Africa and these are likely to be most vulnerable to direct effects of climate change (e.g. higher temperatures and changes in rainfall patterns). Effective weed management strategies in these environments could encompass off-season tillage, the use of well-adapted cultivars (i.e. those with drought and heat tolerance, high weed competitiveness and parasitic weed resistance or tolerance) and rotations, intercropping or short, off-season fallows with weed-suppressive legumes including those that suppress parasitic weeds. In irrigated, non-flooded rice systems, weeds are expected to become more serious. Specifically, perennial rhizomatous C3 weeds and species adapted to hydromorphic conditions are expected to increase in prevalence. By implementing an integrated weed management strategy primarily targeted at weed prevention, dependency on flood water, herbicides and mechanical control can be lessened. Off-season deep tillage, stale seed bed techniques, use of clean seeds and irrigation water, competitive cultivars, timely transplanting at optimum spacing and judicious fertilizer timings are suitable candidate components for such a strategy. Integrated, novel approaches must be developed to assist farmers in coping with the challenges of weed control in the future. * To whom all correspondence should be addressed. Email: j.rodenburg@cgiar.org ��� Current address: Tasmanian Institute of Agricultural Research (TIAR), University of Tasmania, Hobart, Tasmania, Australia. Journal of Agricultural Science (2011), 149, 427���435. �� Cambridge University Press 2011 doi:10.1017/S0021859611000207 427

INTRODUCTION Rice is an increasingly important commodity in Africa (Balasubramanian et al. 2007) in 2007, rice production reached 23��5 Mt (FAO 2009). Trends show production is rapidly increasing and African production now rivals that of Latin America (Meinke et al. 2009a). Five main rice agro-ecosystems are dis- tinguished in sub-Saharan Africa (SSA) based on hydrology and topography: (1) rain-fed uplands and hydromorphic slopes (0��39 of total area under rice), (2) rain-fed lowlands in valley bottoms and flood- plains (0��33), (3) irrigated lowlands (deltas and flood- plains) and highlands (0��19), (4) deep-water basins along major rivers and (5) mangrove-swamps in lagoons and deltas (4 and 5 combined=0��09) (Balasubramanian et al. 2007 FAO 2009). A review of rice yield losses due to uncontrolled weed growth reported losses in the range of 28���74% in transplanted lowland rice, 28���89% in direct-seeded lowland rice and 48���100% in upland ecosystems (Rodenburg & Johnson 2009). Improving weed con- trol in farmers��� fields was shown to increase rice yields by 15���23%, depending on agro-ecosystem, and it is estimated that weeds may account for annual rice yield losses in SSA of at least 2.2 million tonnes equating to US $1.45 billion (Rodenburg & Johnson 2009). Given that demand for food is projected to rapidly outpace increases in supply (e.g. von Grebmer et al. 2008), effective weed control is a priority in these systems. Important weeds of upland rice include the per- ennials Imperata cylindrica, Cyperus rotundus and Chromolaena odorata, the annuals Digitaria hori- zontalis and Euphorbia heterophylla and the parasitic weeds Striga hermonthica and Striga asiatica (Table 1). In lowland rice, the perennial weeds Oryza long- istaminata and Cyperus spp. and annual weeds Echinochloa spp., Oryza barthii, Ischaemum rugosum, Cyperus difformis, Cyperus iria, Fimbristylis littoralis and Sphenoclea zeylanica cause serious losses. Common weed management practices in rice-based cropping systems include soil tillage, flooding, fallow and crop rotations, clearance by fire, hand- or hoe- weeding and herbicides these practices are often used in combination (Rodenburg & Johnson 2009). Climate change is one of many risk factors affecting rice production and weed management. For the purpose of the present review, ���climate change��� is used in the broad sense, including direct and indirect impacts of climate on the environment and on people. Major global changes include further increases in atmospheric greenhouse gases and likely changes in temperatures (0��2 ��C/decade), soil degradation and competing claims for land and water (IPCC 2007). For Africa, climate trends suggest that variability in rainfall will increase and monsoon regions may be- come drier (Giannini et al. 2008), leading to a 5���8% increase in drought-prone areas in the Sahel and southern Africa by 2080 (IPCC 2007). Equatorial zones of Africa may receive more intense rainfall (Christensen et al. 2007). Spatial distribution of future rainfall, however, remains highly uncertain (Giannini et al. 2008), particularly for the Sahel for which there are a number of conflicting projections (e.g. Cook & Vizy 2006 Hoerling et al. 2006 Biasutti et al. 2008). While many of the aforementioned changes began decades ago, the rates of changes have recently accelerated and impacts are increasingly apparent (Rozenzweig et al. 2008). This adds urgency to the required analyses of adaptation options for farmers and for policy and adaptation measures to be intro- duced (Meinke et al. 2009b). Changes in atmospheric carbon dioxide (CO2), rainfall and temperature will affect weed species��� distribution and prevalence within weed and crop communities. Climate changes may also necessitate adaptation of crop management practices, which in turn affect weed growth and the proliferation of certain species. Environmental con- ditions will impact on effectiveness of weed manage- ment operations such as chemical and mechanical control. Obviously, the magnitude of these effects will largely depend on the extent of local and regional changes to environmental conditions. The present paper discusses (1) the likely effects of projected climate changes on the competitiveness and distribution of major weeds of African rice ecosystems and (2) the consequences of changing climates and changing weed community compositions for weed management in African rice production systems. The objectives are to describe likely climate change effects on weeds in African rice production systems and to identify potentially effective coping strategies for the resource-poor farmers in these systems. CLIMATE CHANGE EFFECTS Direct effects ��� weed competition, abundance and distribution Temperatures, atmospheric CO2 concentrations and rainfall irregularities will increase (IPCC 2007), and this will affect weed species in different ways, depend- ing on their photosynthetic pathways and tolerance to environmental stress. Under drought and high temp- eratures, plants with the C4 carbon fixation pathway have a competitive advantage over plants possessing the more common C3 pathway (e.g. Yin & Struik 2008). This competitive advantage of C4 weeds diminishes or even reverses under conditions of high soil nitrogen or atmospheric CO2 concentrations (e.g. Carter & Peterson 1983 Bazzaz & Carlson 1984). Of the 56 weed species most cited in relevant peer- reviewed literature (Rodenburg & Johnson 2009), 20 species (0��36 of total weed species) are C4 types (Table 1). The C4-type weed species are most reported 428 J. RO D EN B U R G , H. M EI N K E A N D D. E. J OH N S O N