Weed Management in Wet-Seeded Rice in Tropical Asia

Keith Moody
Agronomy-Plant Physiology-Agroecology Division
International Rice Research Institute
Los Baños, Laguna, Philippines, 1992-12-01

Abstracts in Other Languages: 中文, 日本語, 한국어

Introduction

Wet seedling (sowing pregerminated seed onto puddled soil) reduces substantially the amount of labor needed for growing a rice crop. It eliminates the use of seedlings and related operations such as seedling nursery preparation, care of seedlings, pulling, bundling, transporting, and transplanting (Serrano 1975).

For the past decade, of increasing labor costs have resulted in a general shift in rice production systems from transplanted rice in Malaysia, Thailand and the Philippines and from deepwater rice in Vietnam to direct-seeded rice, of which wet seeding has been the main method of crop establishment. This has been accompanied by an increase in weed problems and a shift in the dominant species to grassy weeds. In Vietnam, there has been a shift in the herbicides used from 2,4-D to oxadiazon, pretilachlor + fenclorim, fenoxaprop-ethyl, and pyrazosulfuron.

A multitude of prerequisites, including level land, effective weed control, efficient water management, and timely water supply in relation to crop water demand, need to be met to ensure a successful wet-seeded rice crop (Wah et al. 1982). In most countries, insufficient attention is given to the important operation of weeding. In broadcast seeded fields in the Philippines, Malaysia, Thailand, and Vietnam, it is not uncommon to see fields foul with weeds, mainly grasses. When weed control in rice is neglected, there is a decrease in yield because of weeds, even if other means of increasing production, including application of fertilizers, are practiced.

Land Preparation

Poor land preparation can result in poor stand establishment in wet-seeded rice. Developing rice seedlings can be killed or greatly retarded in their growth when ponding of water occurs (Moody 1977b), due to lack of oxygen and accumulation of toxic concentrations of applied herbicides (Moody 1984). Also, weeds invade the vacant spaces where the rice does not grow (Mabbayad and Moody 1985) resulting in yield losses due to competition (Moody 1986).

A poorly prepared field does not provide a suitable medium for optimum plant growth. If the field is not levelled, the seedlings cannot establish quickly in the low spots and weeds will grow abundantly in the high spots. These conditions will result in stunted plants with low tiller production. Farmers often dig shallow drainage ditches in their fields to drain excess water that remains after puddling or when rain falls soon after seeding to ensure good stand development (Moody 1984, Takashima 1984). Good field drainage and good water control are essential for wet-seeded rice establishment (Moody and Cordova 1985) and for reduction of herbicide phytotoxicity (Moody 1984).

Tillage serves only as a temporary means of weed control because the soil contains many ungerminated weed seeds. Plowing may bury weed seeds at a depth that prevents germination but may also expose other, once deeply buried seeds to conditions inducive to germination (Stoskopf 1985).

Besides forming a hard pan and preparing a slurry into which seedlings or pregerminated seed can be sown, puddling buries the weeds in the lower layers of mud, where they decompose by anaerobic action to form ammonium compounds which are retained much better than nitrate in the soil and can be used directly by the crop (Wrigley 1969).

A well-levelled field without numerous depressions or elevations is essential for good weed control. Fields with many low and high spots result in water depths that are too deep for the rice in the low spots and too shallow for weed control in the high spots (Eastin 1981).

Seeding Rate

Seeding rates used in wet-seeded rice are generally high to help control weeds (Moody 1977a), to compensate for damage by rats and birds (Mabbayad and Obordo 1970), to partially overcome the adverse effects of herbicides (Moody 1984), and to compensate for poor stand establishment (Moody 1986).

Low plant density and the presence of gaps encourages the growth of weeds. Such a stand will, in many cultivars, result in less uniform ripening and poor grain quality. On the other hand, too thick a stand should be avoided because it tends to increase lodging, prevents the full benefit of nitrogen application (Anon 1986) and increases the chances of rat damage (Castin and Moody 1989).

Moody (1977b) reported that there was a significant decrease in weed weight as the seeding rate increased from 50 to 250 kg/ha (Table 1). Thus, high seeding rate can compensate partly for poor weed control. Castin and Moody (1989) reported that there was an increase in grain yield in the untreated check plots but not in the weeded plots as a result of an increased seeding rate. Increase in panicle number as seed rate increased was offset by a decrease in panicle length and grain weight per panicle (Bhattacharjee 1978).

Guyer and Quadranti (1985) noted that higher seeding rates would be beneficial if no weed control is planned or only partial weed control is expected. However, it is not necessary to use high seeding rates to suppress weeds in wet-seeded rice if a herbicide that is effective in controlling weeds is used (Castin and Moody 1989).

Prevention

Planting of clean seed is perhaps the most important weed-management technique to control losses (Stoskopf 1985). Rice seed contaminated with weed seeds may introduce a new species to a given field or add to an existing weed population. Preventing weeds from entering an area may be easier than trying to control them once they have become established. Weeds which mature at the same time as rice are harvested and threshed with the rice resulting in the contamination of rice seed with weed seeds. In the southern United States, many weed seeds are spread almost entirely in rice seed (Smith et al. 1977).

Irrigation water is one of the major means of spread of weed seeds and vegetative propagules. Flowing water moves millions of weed seeds from one place to another. The amount of seed and the type of seed moved depends on the volume and the velocity of the water and the size and the weight of the seed or the vegetative propagules.

Water Management

Water management is a major component of any weed control program, whether a herbicide is used or not. Water depth can be used to control many weeds, but there are a number of species that are relatively unaffected by water depth. Keeping the field flooded after planting will kill some weeds and will slow the growth of others. Mabbayad (1967) reported that weed weight decreased markedly even at a shallow water depth of 2.5 cm (Table 2). In deep water, the most common weed growing was Monochoria vaginalis (Burm. f.) Presl, while at zero water depth (soil saturation) grasses such as Echinochloa spp. and sedges such as Fimbristylis miliacea (L.) Vahl were predominant.

Unavailability of irrigation water and poor water management increase problems with weeds (Smith and Moody 1979). If manual weeding methods are used, more time is spent weeding when water control is poor than when controlled water management is available. Herbicides which give excellent control when applied into water may perform poorly in the absence of standing water.

ARS (1980) reported that in the presence of standing water, lower rates of propanil could be used and these could be applied later in crop growth. For example, to achieve the same degree of control as 1.5 kg ai/ha at 14 days after seeding (DAS) in the presence of standing water, 3.0 kg ai/ha had to be applied at 7 DAS when there was no standing water (Table 3).

Herbicides should not be used instead of good water management. They should be used to supplement good water control. Smith (1967) stated that good water management together with chemical weed control offers an unusual opportunity for conserving moisture and lowering the cost of rice production.

Hand Weeding

Many farmers do not realize that weed control is a limiting factor in crop production. Traditionally, they depend on manual labor to remove weeds. By the time weeds are large enough to be removed by hand damage has been done, yield loss is certain and hand labor cannot undo it. Maximum yields can only be obtained if weeds are controlled early because most damage is done when crop plants and weeds are small.

Weeding can only be done at a time when labor is available, but this may not coincide with the best time to do it to minimize weed competition. Improving weed management by alleviating labor constraints has repercussions for all aspects of crop production, the sustainability of cropping systems and the social conditions of farming families.

Small-scale farmers are particularly affected by the lack of labor for weeding which, to be effective, must take place early in the crop cycle (Anon 1988). If weeding is delayed beyond 20 days after emergence, irreparable damage is done. During early establishment, the weeds make 20-30% of their growth while the crop makes 2-3% of its growth (Moody 1990).

Hand weeding in wet-seeded rice is more time-consuming and not as thorough as in transplanted rice (Moody 1983). The weeders damage the rice as they move through the field, especially during early crop growth, and they fail to remove some of the grassy weeds or they remove rice by mistake, because of the difficulty in distinguishing grassy weeds from rice (Moody and Cordova 1985). In Vietnam, hand weeding is at least five times more expensive than herbicides for weed control in wet-seeded rice.

Interrow Cultivation

Pradhan (1969) reported the successful adoption of direct seeding in puddled rice fields using a seed drill. The practice can replace transplanting without any reduction in yield. Costs are also reduced. The time taken for drilling seeds in a puddled soil using a manually-operated planter was one-third of that required for transplanting.

In India, under lowland conditions, it takes about 200-250 hours/ha to hand weed depending on the weed infestation. In row seeded or transplanted rice where weeds can be controlled by the use of mechanical weeders, it takes about 50-60 hours/ha depending upon weed infestation and soil conditions (Parthasarathi and Negi 1977).

Some farmers in Sri Lanka sow 200-300 kg seed/ha to suppress weeds during the initial growth of wet-seeded rice. When the rice is 10-14 days old, a rotary weeder is passed through the field incorporating strips of rice seedlings into the soil so that the rice that is left appears as a row sown crop. Subsequent rotary weedings are done between the rows thus created. Manikkavasagar (1968) reported that yields obtained using this technique were similar to those obtained with transplanted and row-sown rice, and recommended it in areas where the usual practice is to broadcast seed and to control weeds by hand weeding or to do no weeding at all.

Anon (1952) noted that it may be profitable to harrow a wet-seeded crop when it is 3-4 weeds old. This presses down both the rice and the weeds into the standing water in the field. After several days the rice plants become erect while the weeds remain under the water and die. This technique is particularly effective in the suppression of F. miliacea. Farmers in Iloilo province, Philippines use a similar technique for the control of sedges such as F. miliacea, Cyperus difformis L. and C. iria L. (Moody 1990).

Herbicides

The success of direct-seeded rice is dependent upon weed control with herbicides (Day 1974). However, herbicides should not be regarded as replacements for other weed control practices but should be used in conjunction with them.

Herbicide use should coincide with the presence of sufficient weeds to warrant treatment, and take place when weeds are most vulnerable. The optimum rate depends on such factors as cultural practices, soil type and environmental conditions. Factors which must be considered when developing a herbicide program are the herbicide itself, weed flora, application method and time, crop tolerance and cost effectiveness (Moody 1992). The use of herbicides assures effective weed control during periods of labor shortage when weeding coincides with other farm work.

Some herbicides and herbicide combinations that have shown promise for weed control in wet-seeded rice are listed in Table 4. Temporary rice injury manifested as leaf chlorosis and inhibition of plant growth frequently occurs, but the rice usually recovers after two or three weeks and produces satisfactory grain yields.

Integrated Weed Management

Herbicide use moves the agroecosystem to low species diversity with new problem weeds appearing, so that there is a need for an ecological approach to weed control instead of relying totally on chemical control methods (Moody 1992).

Mahn and Helmecke (1979) noted that reliance on a single herbicide could result in quantitative changes in the structure of the weed population in as few as five years. As weed population stresses are shifted by herbicides, weeds formerly of secondary importance emerge as primary weed problems. Such problems may be avoided by an integrated system of weed management, possibly rotation of chemicals as well as rotation of crops. Lo (1990) advocated the alternative usage of herbicides with different grass control spectra over seasons to prevent the emergence of tolerant weeds.

Cultural and mechanical control have been the cornerstone of many pest control practices, in agriculture throughout the world. They remain the most widely used control practices in industrial and developing countries, even though many such controls have been eroded by substitution by pesticides.

Until the advent of herbicides, cultural practices through crop rotation and mechanical or hand weeding were virtually the only control mechanisms available against weeds. They remain vitally important but much more serious consideration is needed in establishing how they can be integrated with judicious herbicide use in order to help remove the single most important constraint to crop production (timely availability of labor) in many farming situations in developing countries.

More than for other groups of pests, farmers have many options for the control of weeds. Weed control, whether done consciously or not by farmers, is often achieved by a combination of crop production practices and specific weed management activities. Integrated weed and crop management is not a new concept so, in theory, improved techniques need not be alien to farmers. However, farmers tend to be conservative and reluctant to change traditional practices, especially if they perceive risks.

Hazards of Herbicide Use

Despite the problems and limitations of pesticides, there can be no doubt that they are essential for the maintenance and expansion of world food production, especially in areas favorable for pest development and survival (Gooding 1983). It must never be overlooked, however, that all pesticides are toxic; they must be handled safely so as to reduce or avoid excessive and costly waste, environmental concerns, crop damage, damage to adjacent crops by spray drift, injury to the applicator, excessive contamination and residues and injury to beneficial organisms. The toxicity of different rice herbicides to fish is indicated in Table 4.

Conclusion

There is an increasing trend to replace transplanting of rice by wet seeding. In effect, farmers are substituting capital in the form of seed and herbicides for labor. The results also suggest that the labor cost advantage of wet seeding more than compensates for the increase in herbicide cost, and adoption is increasing even though herbicide costs are rising (Erguiza et al. 1990).

Wet-seeded rice is expected to become increasingly more important in the next decade. The problems of weed control will become more serious. Appropriate weed management technology will be needed to maintain yield stability and reduce the cost of production. Studies will be needed on weed species, population dynamics and control practices by innovative cultural practices in conjunction with biological and chemical weed control.

A dynamic research program is needed to develop the innovative and effective weed management strategies needed to improve crop stability in the tropics. There is also a need for more economical herbicide treatments with reduced handling and environmental hazards without loss of herbicidal efficacy. Worldwide concerns over safe handling of pesticides, environmental issues and sustainable agriculture need to be addressed (Moody 1992).

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Index of Images

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  Table 1 Effect of Seeding Rate on Weight of Weeds Growing in Association with Wet-Seeded Rice

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  Table 2 Effect of Water Depth on Weed Weight<Sup>a</Sup>

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  Table 3 Effect of Water Management and Weed Control Method on Weed Weight and Grain Yield

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  Table 4 Herbicides, and Herbicide Combinations Used for Wet-Seeded Rice, and Their Toxicity to Fish

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