Rice ( Oryza sativa L.) is a principal source of food for more than half of the world population, especially in South and Southeast Asia and Latin America. Elsewhere, it represents a high‐value commodity crop. Change in the method of crop establishment from traditional manual transplanting of seedlings to direct‐seeding has occurred in many Asian countries in the last two decades in response to rising production costs, especially for labor and water. Direct‐seeding of rice (DSR) may involve sowing pregerminated seed onto a puddled soil surface (wet‐seeding) or into shallow standing water (water‐seeding), or dry seed into a prepared seedbed (dry‐seeding). In Europe, Australia, and the United States, direct‐seeding is highly mechanized. The risk of crop yield loss due to competition from weeds by all seeding methods is higher than for transplanted rice because of the absence of the size differential between the crop and weeds and the suppressive effect of standing water on weed growth at crop establishment. Of 1800 species reported as weeds of rice, those of the Cyperaceae and Poaceae are predominant. The adoption of direct‐seeding has resulted in a change in the relative abundance of weed species in rice crops. In particular, Echinochloa spp., Ischaemum rugosum, Cyperus difformis , and Fimbristylis miliacea are widely adapted to conditions of DSR. Species exhibit variability in germination and establishment response to the water regime postsowing, which is a major factor in interspecifically selecting constituents of the weed flora. The relatively rapid emergence of “weedy” (red) rice, rice phenotypically similar to cultivars but exhibiting undesirable agronomic traits, has been observed in several Asian countries practicing DSR, and this poses a severe threat to the sustainability of the production system. Stale seedbeds, tillage practices for land leveling, choice of competitive rice cultivars, mechanical weeders, herbicides, and associated water management are component technologies essential to the control of weeds in DSR. Herbicides in particular are an important tool of weed management, but hand weeding is either partially or extensively practiced in countries of Asia, Africa, and Latin America. Though yet to be globally commercialized, transgenic rice varieties engineered for herbicide resistance are a potential means of weed control. The release of herbicide‐resistant rice for red rice control in the United States has indicated the need to critically examine mitigation methods for the control of gene flow. Integrating preventive and interventional methods of weed control remains essential in managing weed communities in DSR, both to prohibit the evolution of herbicide resistance and to maximize the relative contributions of individual components where herbicides are not widely used. There remains a need to further develop understanding of the mechanisms and dynamics of rice weed competition and of the community dynamics of weed populations in DSR to underpin sustainable weed management practices.

Weed Management in Direct‐Seeded Rice

Weed seed banks reflect past weed populations and management practices and are the source of weed infestations to come. The factors affecting weed seed germination, however, are often poorly understood. Depleting the soil seed bank and influencing germination patterns are common goals of enduring cultural weed management practices. Greater understanding of the factors influencing the germination of weed seeds could facilitate the development of more effective cultural weed management practices through either suppressing germination or encouraging germination at times when seedlings can be readily controlled. Such cultural methods may contribute to overcoming problems such as feral crops (e.g., weedy rice), crop volunteers, and the evolution of herbicide resistance in weeds that have, in some systems, increased to a point where the lack of sustainable practices is a threat to productivity. Weed seed germination is commonly influenced by light exposure, soil moisture, burial depth through tillage, the use of mulches, fire for land clearance, and flooding of the soil. Harnessing these factors to influence germination can serve as major entry points for improved weed management. Diverse crop production systems provide a wide range of examples to illustrate how recent advances in the understanding of the responses of weed seed germination can be used to develop new and sustainable cultural management of weeds. Crop management practices, such as adopting no-till crops or delaying tillage, that increase weed seed exposure to predators (ants, beetles, etc.) could be incorporated into integrated weed management programs. Retention of crop residue on the soil surface under no-till systems can suppress weed seedling emergence, delay the time of emergence, and allow the crop to gain an advantage over weeds, and reduce the need for control. Rotation of tillage or crop establishment system could also be adopted to deflect the “trajectories” of likely weed population shifts. In rice, flooding after herbicide application or hand weeding can largely prevent the growth of weeds and reduce the need for further interventions.

The role of seed ecology in improving weed management strategies in the tropics

Weed competition is a major constraint in all the rice production systems in Africa. In addition to the costs of weed control, weeds account for yield losses estimated to be at least 2.2 million tons per year in sub‐Saharan Africa, valued at $1.45 billion, and equating to approximately half the current total imports of rice to this region. Important weeds in upland rice include the perennial species Cyperus rotundus, Imperata cylindrica and Chromolaena odorata, the annual species Euphorbia heterophylla, Digitaria horizontalis, and the parasitic weeds Striga spp. In lowland rice the perennial weeds: Cyperus rotundus, C. esculentus and Oryza longistaminata and annual weeds Sphenoclea zeylanica, Echinochloa spp., Cyperus difformis, C. iria, Fimbristylis littoralis, Ischaemum rugosum, and O. barthii cause serious losses. Common weed management practices in rice‐based cropping systems include soil tillage, clearance by fire, hand‐ or hoe‐weeding, herbicides, flooding, fallow and crop rotations, and these are often used in combination. Labor shortages and lack of access to information, inputs, and credits are widespread constraints for African farmers. To optimize financial, social and environmental costs and benefits, integrated and ecological management approaches are advocated. Locally adapted and affordable combinations of preventive measures and interventions should be targeted. Future weed research should aim to deliver the information and tools for the implementation of these approaches. This requires the generation of knowledge on weed biology and ecology and on the consequences of changes in management and the environment on weed populations. To address the diversity of rice‐based cropping systems in Africa, priorities need to be set and products and information delivered that take full account of local conditions. This will require farmer participatory approaches that are inclusive with respect to resource‐poor farmers and gender.

Weed management in rice‐based cropping systems in Africa

Global distribution of rice weeds – A review

Nikolai Ivanovich Vavilov was an early 20th century Russian plant scientist who was killed by Joseph Stalin in 1943 for his adherence to basic genetic principles. Vavilov is well known within plant breeding and plant evolutionary biology circles, yet the science of Vavilov is just as important to the field of weed science. Specifically, Vavilov proposed that certain weeds adapted to weed control practices to survive in prehistorical agrarian societies. Most would refer to this adaption as crop mimicry, but the term “Vavilovian mimicry” is more apt. Vavilovian mimicry requires three factors: a model—the crop or desirable plant; a mimic—the weed; and an operator—the discriminating agent, possibly human, animal, or machine. In a modern context, it is proposed that weed adaptation to herbicide applications be included as a form of Vavilovian mimicry, with the acknowledgement that the operator is the herbicide. In this context, Vavilovian mimicry is the adaption of the weed mimic to be perceived by the operator as visually, physically, or biochemically indistinguishable from the crop model. This review will cover the history and legacy of Vavilov in a condensed version in the hope that weed scientists will hold this individual in high regard in our future endeavors and begin to acknowledge Vavilov as one of the first scientists to propose that weeds can mimic the attributes of crops.

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Vavilovian Mimicry: Nikolai Vavilov and His Little-Known Impact on Weed Science

Summary Diversity studies of Echinochloa spp. are complicated by problems in taxonomy and species identification, caused by the existence of morphologically intergrading types. Six amplified fragment length polymorphism (AFLP) primer combinations and five microsatellites were used to assess variation in 24 samples morphologically identified as E. crus-galli, E. colona and E. crus-pavonis, from Bangladesh, India, Colombia, Costa Rica, Cote d'Ivoire and Philippines. Out of 909 AFLP bands generated, 775 were polymorphic. Genotype diversity for the microsatellites ranged from 0.28 to 0.72. Similarity matrices were calculated using Jaccard coefficient, and input into cluster and principal coordinates analyses. AFLP and microsatellite results were highly correlated. Echinochloa crus-pavonis and E. crus-galli were intermixed, consistent with the view that E. crus-galli occurs as numerous intergrading races in the four countries (Bangladesh, India, Cote d'Ivoire and Philippines). The E. colona samples clustered as a distinct group. In 15 samples of E. crus-pavonis collected from rice fields in a valley in Cote d'Ivoire (over a 2-km distance), four different genotypes were found in a 4 m × 4 m area. These results suggest that AFLPs and SSRs may be useful not only for discriminating genotypes and studying population structure but also for helping to resolve taxonomic relationships in Echinochloa spp.

D E Johnson

Papers

Genetic diversity in Echinochloa spp. collected from different geographic origins and within rice fields in Côte d'Ivoire