Challenges for weed management in African rice systems in a changing climate

Abstract

SUM M ARYGlobal 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 C 4 over C 3 plants (e.g. rice). Conversely, elevated carbon dioxide (CO 2 ) levels will improve the competitiveness of rice relative to C 4 weeds, which comprise many of the problem weeds of rice. Increased atmospheric CO 2 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 CO 2 levels tend to promote growth below-ground relative to above-ground, particularly in perennial (C 3 ) species. This may render mechanical control of weeds within a cropping season less effective or even counterproductive. Increased CO 2 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 C 3 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.

Effects of Water Depth and Seedling Rate on Weed Control and Yield of Late Season Lowland Rice (Oryza sativa L)

“…Bhagat et al (1999) reported that broadleaved weeds produced higher weed biomass than sedges and grasses in flooding regimes, while in saturated condition the opposite result was obtained. Rodenburg et al (2011) reported that in irrigated, non-flooded rice systems, weeds are expected to become more serious specifically perennial rhizomatous weeds and species adapted to hydromorphic conditions are expected to increase in prevalence.…”

Weedy rice in sustainable rice production. A review

“…Planting herbicide-resistant seeds was thought to eliminate weedy rice problems, but the emergence of herbicide-resistance among the weeds generated more severe issues by making weeds more competitive (Bhatia and Mitra 2003;Lu and Snow 2005), and climate change is predicted to worsen weed management in rice cropping (Rodenburg et al 2010;Ziska et al 2012). Appropriate management and control of weedy rice can only be achieved by an integrated approach that includes both basic and advanced methods.…”

Challenges and Opportunities in Weed Management Under a Changing Agricultural Scenario

“…On the contrary, elevated CO 2 levels will improve the competitiveness of C3 crops relative to C4 weeds. Increased atmospheric CO 2 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 [65]. Climate change is expected to promote a proliferation of new weed species and cause shifts in the composition of weed flora, especially in the tropics and subtropics.…”