Elsevier

Advances in Agronomy

Volume 93, 2007, Pages 153-255
Advances in Agronomy

Weed Management in Direct‐Seeded Rice

https://doi.org/10.1016/S0065-2113(06)93004-1Get rights and content

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.

Section snippets

INTRODUCTION

Over 1800 plant species have been reported as weeds of rice in South and Southeast Asia (Moody, 1989), and there is an enormous diversity of taxa considered to be weeds of rice (Soerjani et al., 1987). There are two major reasons for this. The first is that rice is grown over a range of agroecosystems, characterized by the presence or absence of water (from dry land to fully flooded land) for all or parts of its growing season, which generates highly diverse weed floras. The second is that, in

OCCURRENCE OF MAJOR WEEDS IN DIFFERENT METHODS OF DIRECT‐SEEDING ACROSS THE WORLD

Globally, weed communities of DSR are floristically diverse because they (1) span temperate and tropical regions, (2) reflect different agroecosystems in a region, (3) may vary in relation to seasonal crop management patterns at the farm level, and (4) may differ because of spatial heterogeneity that is often linked to patterns of flooding and drainage and soil nutrition at the field level (Moody 1995, Mortimer and Johnson, 2005). They can vary further in relation to the efficacy of weed

INTEGRATING WEED MANAGEMENT PRACTICES IN DIRECT‐SEEDED RICE

The traditional practice of puddling soil to kill existing weeds and aid water retention, transplanting rice seedlings into standing water to achieve an optimum stand density, and maintaining standing water to suppress weeds, followed by one or several periods of manual weeding, is a well‐established example of integrated weed management (IWM). It is integrated in the sense that it involves preventive actions followed by precise interventions as a response to the consequences of suites of

FUTURE RESEARCH NEEDS

Weed infestation is a major threat to yield and further expansion of DSR throughout the world. In most developed nations, direct‐seeding is the sole method of rice establishment and is reliant on mechanization and close attention to weed management. While shortage of labor in Asian agriculture is encouraging the adoption of direct‐seeding, so too is the need to improve water productivity (Tuong et al., 2005). “Aerobic” rice (Bouman, 2003) may reduce water consumption by up to 50% (Yang

ACKNOWLEDGMENTS

The authors are grateful to Cropnosis Limited for providing data on herbicide sales; Drs. Y. Singh, M. M. Kyu, H. Pane, and A. Abeysekera for their advice on the distribution of weed species; Professor Robert E. L. Naylor, University of Aberdeen, UK; Drs. N. T. Yaduraju and H. Pathak, Indian Agricultural Research Institute, India for providing critical and constructive comments on the chapter; and Dr. Bill Hardy, Senior editor, IRRI for editing the chapter.

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