Rodenburg J.,Africa Rice Center |
Bastiaans L.,Wageningen University
Weed Research | Year: 2011
Parasitic weeds of the genus Striga cause high yield losses in cereal crops across Africa. Host-plant defence against Striga spp. can be an effective control strategy. It ideally consists of resistance, to reduce infection, complemented with tolerance, to mitigate the effects of infection. As resistance against Striga spp. can both minimise yield losses and reduce future infestation levels in infested fields, current breeding efforts are mainly directed towards this trait. Because it is nearly impossible to screen for tolerance on highly resistant genetic lines, tolerance is often neglected. Here, we argue reconsidering the role of tolerance, as recent findings regarding the physiological expression of tolerance offer a promising track for identifying the genetic background of tolerance. Identification of quantitative trait loci for tolerance would facilitate the inclusion of this trait in adapted cultivars with high levels of resistance, where its main role would be to function as a safety net in case the genetically highly variable parasite populations overcome host-plant resistance. Because Striga spp. are mainly prevalent in subsistence farming systems, we consider this an important addition and it is for this reason that we make a plea for a more prominent role of tolerance in present-day integrated management of this weed. © 2011 The Authors. Weed Research © 2011 European Weed Research Society.
Saito K.,Africa Rice Center
Field Crops Research | Year: 2010
Improving rice (Oryza spp.) competitiveness against weeds would provide a low-cost and safe tool for an integrated weed management strategy. This paper addresses the underlying causes of tradeoff between yield without weed competition and weed competitiveness. Rice yield or weed biomass under weedy conditions are used as indicators of weed competitiveness. For this analysis, a common database was compiled from the results of 45 concurrent field trials comparing the performance of four to 64 genotypes in weed-free and weedy conditions in Asia, West Africa, North America and Latin America. Cyperus spp., Echinochloa spp. and Eleusine indica were the most frequent dominant weed species, being found in 9, 20 and 7 trials, respectively. Mean relative yield reduction [(yield under weed-free conditions - yield under weedy conditions)/yield under weed-free conditions] across genotypes tested for each trial was defined as weed pressure level. Mean yield without weed competition across genotypes ranged from 1.8 to 11.6 t ha -1 with mean relative yield reduction from almost 0 to 91%. Correlations for rice yield between weed-free and weedy conditions were generally positive, and significant in 27 trials. The correlations were related to weed pressure level and dominant weed species, but not to ecosystem (upland or lowland) or yield level under weed-free conditions. Relative yield reduction had a more pronounced effect than dominant weed species. Correlation decreased as weed pressure level increased, and became negative when relative yield reduction exceeded 80%, suggesting that different morpho-physiological mechanisms are responsible for high yields under weed-free conditions or severe weed competition. Correlations between rice yield under weed-free conditions and weed biomass varied, giving 17 and 19 for positive and negative ones, respectively. These correlations were related neither to weed pressure level, ecosystem and yield level under weed-free conditions, nor to dominant weed species. These results indicate that correlations between rice yields under weed-free and weedy conditions can be strongly affected by weed pressure level, and, unless severe weed competition occurs, there appears to be no tradeoff between them. Association of morpho-physiological mechanisms with weed competitiveness under conditions differing in weed pressure levels and dominant weed species deserves further investigation. © 2010 Elsevier B.V. All rights reserved.
Demont M.,Africa Rice Center |
Demont M.,International Rice Research Institute
Global Food Security | Year: 2013
Economic development in poor countries is often hampered by urban bias. Partly as a result of historical urban bias, African countries have become heavily dependent on food imports with concomitant risks for food security as witnessed during the 2008 food crisis. African governments now recognize that they should reverse urban bias by investing in agriculture in order to decrease food import dependency. However, they typically focus primarily on supply-shifting investments that may be insufficient to render domestically produced food competitive, particularly in import-biased food markets. We review the national rice development investment strategies of 19 African countries and argue that in order to reverse urban bias in African rice markets, more resources will need to be allocated to value-adding and demand-lifting investments. © 2013 Elsevier B.V.
Demont M.,Africa Rice Center |
Demont M.,International Rice Research Institute |
Stein A.J.,International Food Policy Research Institute
New Biotechnology | Year: 2013
Unlike the other major crops, no genetically modified (GM) varieties of rice have been commercialized at a large scale. Within the next 2-3 years new transgenic rice varieties could be ready for regulatory approval and subsequent commercialization, though. Given the importance of rice as staple crop for many of the world's poorest people, this will have implications for the alleviation of poverty, hunger and malnutrition. Thus, policy-makers need to be aware of the potential benefits of GM rice. We provide an overview of the literature and discuss the evidence on expected agronomic and consumer benefits of genetically engineered rice. We find that while GM rice with improved agronomic traits could deliver benefits similar to already commercialized biotechnology crops, expected benefits of consumer traits could be higher by an order of magnitude. By aggregating the expected annual benefits, we estimate the global value of GM rice to be US$64 billion per year. This is only an indicative value, as more GM varieties will become available in future. Nevertheless, such a figure can help guide policy-makers when deciding on the approval or funding of biotechnology crops and it may also raise awareness among consumers about what is at stake for their societies. © 2013 Elsevier B.V..
Saito K.,Africa Rice Center
Field Crops Research | Year: 2014
In West Africa, upland rice (Oryza spp.) is mainly grown in low-input systems by resource-poor farmers. Weeds are one of the major constraints to rice production. Ideal rice varieties are high-yielding with strong weed-suppressive ability (WSA). WSA is the ability to suppress weed growth and reduce weed seed production and is determined by assessing weed biomass under weedy conditions. Development of such varieties requires simple screening protocols based on highly heritable traits, which can be evaluated in small plots under weed-free conditions. Then, in later stages of the breeding program, selected breeding lines could be evaluated with substitute weeds instead of natural weeds, as natural weed growth is heterogeneous. To develop such a protocol, I evaluated agronomic traits of 10 diverse rice varieties grown in plots with different numbers of rows (unbordered 1-row and 2-row, and self-bordered 4-row, in which the middle 2-rows were sampled) without weed competition and evaluated weed biomass of these varieties under competition with rice and cowpea as substitute weeds. I then examined whether agronomic traits in unbordered plots can predict yield and weed biomass in self-bordered plots. Broad-sense heritabilities of agronomic traits measured in weed-free conditions and weed biomass under weedy conditions were estimated. Among agronomic traits in unbordered plots, yield was positively correlated with yield in self-bordered plots (r= 0.88 and 0.96 in 1-row and 2-row plots, respectively, P< 0.01). Weed biomass estimated in competition with two rice varieties was more heritable than weed biomass in competition with cowpea. In 1-row plots, growth vigor at 63 days after sowing (DAS), total aboveground biomass at harvest, and yield were related to weed biomass in competition with cowpea and rice variety Aus 257. Growth vigor at 42 and 63 DAS was a reliable estimator for total aboveground biomass and number of panicles at harvest, as well as integrated information on height and number of stems (height multiplied by number of stems). I suggest that growth vigor at 42-63 DAS in unbordered, 1-row plots appears to be a useful selection criterion for developing high-yielding varieties with superior WSA, and WSA of selected varieties can be validated using rice varieties as substitute weeds. Future research is needed to validate this protocol with breeding populations, a wide range of genetic materials, and other important weed species in West Africa before it can be implemented in breeding programs. © 2014 Elsevier B.V.