Weed Officer, Plant Protection Service, FAO, Rome
Weeds are at present the major biotic constraint to increased rice production worldwide. The importance of their control has been emphasized in the past by various authors (De Datta and Baltazar, 1996; Labrada,1996; Ze-Pu Zhang, 1996). Chemical weed control has increased significantly over the past ten years. This is due to labour shortages, particularly in Southeast Asian countries, leading to an increased shift from transplanted rice to direct-seeding, with a subsequent increase in herbicide use. As has been stated in the past (Labrada, 1996), experience shows that, although herbicide use has increased productivity, there are several weed problems that remain unsolved by the use of the herbicides commonly applied in rice cultivation.
Of several weeds affecting rice production, two major weed complexes have been identified which require an integrated approach for their control. They are the complex formed by various Echinochloa species and the red/weedy rice complex. In order to better understand both problems, FAO organized a global workshop on Red/Weedy Rice Control, held in Varadero, Cuba in 1999, and a regional one-day workshop for Asian countries on Echinochloa Control, held in Beijing, China in 2001. The two events had the following objectives:
Discussion of the status of the problem in affected countries.
Update on newly developed methods for the control of the weeds (both Echinochloa and red/weedy rice).
Discussion and analysis regarding transgenic rice cultivars resistant to herbicides (HRC), particularly in the red/weedy rice workshop.
Definition of follow-up activities.
This paper highlights the main findings, conclusions and recommendations of both workshops. From these events it was once again clear that an integrated weed management approach is urgently required in order to improve rice production. Various preventative, cultural, physical and chemical methods should be rationally integrated to achieve the desired weed control.
One of the major constraints to the production of rice in direct-seeded areas is the incidence of red rice, which is widespread all over the world. Wild rice, Oryza barthi and O. longistaminata, are among the most important weeds in West Africa and Sahel. Oryza sativa L. var. sylvatica is one of the main weeds in direct-seeded rice in the Mediterranean, while in the United States of America, Latin America and the Caribbean its presence currently hinders the harvesting process and reduces the quality of the produce obtained. Crop losses due to red rice incidence may be as high as 60 percent where there is heavy field infestation. The situation in various countries is as follows:
Brazil: Red rice is a common weed in nearly all rice production areas in Brazil. In Rio Grande do Sul, the largest rice-producing state in Brazil, red rice infestation is extremely critical. The main factor contributing to red rice dissemination is the use of rice seed contaminated with red rice seeds. Fields infested with red rice are not ploughed immediately after harvest. Red rice seeds buried deep in the soil last longer than the seeds near the soil surface. Minimum tillage systems are used in many areas with severe red rice problems. In Rio Grande do Sul, about 250 000 ha are cropped every rice season using low or minimum tillage. Cultural practices for red rice control include the use of the stale seed bed technique, water-seeded rice with pregerminated seeds and crop rotation (Noldin and Corbucci, 1999).
Colombia: Red rice is a problem in some rice areas in the country. Each panicle of red rice may cause a loss of 18.6 kg of rice/ha. Crop rotation of rice with sorghum, soybean or Crotalaria has proved useful for a significant reduction in the stand of red rice in heavily infested fields. Fallow with animal grazing has also proved useful for red rice control (Rodríguez Carroza, 1999).
Costa Rica: There are two species of rice crop contaminants in the country: Oryza latifolia, commonly known as Arrozon; and Oryza sativa, which has a red pericarp. Although a small quantity of red rice seed is sufficient to contaminate rice grains, it has been found that stands of red rice of no more than 10 plants/m² do not affect rice yields. Above that level, each additional red rice plant/m² may cause a yield reduction of 90 kg/ha. The main control practices are: i) crop rotation with leguminous crops, such as beans, which should be implemented for more than two years in heavily infested areas; ii) use of rice seed free of red rice contaminants and cleaning of machinery used in infested areas; and iii) stale bed preparation and a combination of puddling and flooding to prevent germination of red rice (Castro Espitia, 1999).
Cuba: In some heavily infested rice fields, stands of red rice may be up to 240 plants/m². Thirty-nine biotypes of red rice widely distributed throughout the country have been identified. Red rice tends to affect short- rather than long-cycle rice cultivars. A key control measure is the production of certified clean rice seeds in areas completely free of red rice. The rotation of heavily or medium-infested rice areas with Sesbania rostrata is also widely practised, as well as grazing in medium and low-infested areas. Other procedures are stale bed preparation with the use of glyphosate, and puddling combined with alternate flooding and draining (De la Osa and Rivero, 1999).
France: Weeds are the main constraint to rice production in the country and of the major weed species, red rice can cause a 50 percent reduction in crop yields, particularly in areas of monocropping. A weed survey conducted recently showed that 33% of the 263 rice fields evaluated were normally treated with herbicide preplanting treatment, 7% were hand-weeded, 5% mechanically weeded preplanting and 15% were commonly rotated with upland crops; only 19 fields showed no red rice infestation. Red rice was not present in areas rotated with upland crops and areas with high rice seed densities (Mouret, 1999).
Guyana: Red rice affects areas with poor water management. Light red rice infestation is found in 46 percent of 129 452 ha (1998) while 15 and 5 percent show moderate or high infestation, respectively. The main control method is prevention, consisting of the production of certified rice seed. Land preparation is carried out by ploughing and harrowing several times, a practice which eliminates various red rice flushes. This can be done easily when weather conditions are favourable. Water management is another effective measure. Puddling is usually implemented for red rice control. Another very common procedure in fields with low infestation of red rice is roguing (when red rice plants are tall) in order to avoid seed-setting and the increase of red rice seed build-up in soil (Small, 1999).
Italy: Red rice in rice fields has been known to occur in the country since the 19th century. Experimental results showed that minimum tillage and good soil moisture conditions favoured red rice emergence, creating the best conditions for the application of herbicide post-emergence control, while ploughing and flooding had a remarkable effect on weed germination. Harrowing or shallow ploughing were significantly less effective than chemical treatment with dalapon for the control of red rice seedlings. It has also been found that the herbicides, cycloxydim (0.95 kg a.i./ha) and clethodim (0.28 kg a.i./ha), are as effective as dalapon (Ferrero and Vidotto, 1999).
Mexico: Infestations of red rice in rice fields in the country are due to: the use of contaminated seed; irrigation water; and livestock moving from infested land to non-infested rice areas. The main weed control method is manual removal. Another option is to irrigate the field before planting to provoke early emergence of red rice, for subsequent control, either manually or using glyphosate (Torres Martínez, 1999).
Nicaragua: Red rice is present in all rice-producing areas of the country with infestations of up to 89 panicles/m². In Altamira, red rice and Oryza latifolia are present in 10 and 40 percent, respectively, of a rice area of over 2 400 ha. There is no specific method for controlling red rice: normally farmers either apply glyphosate preplanting over the weed or simply rogue the plants (Sarría Fletes, 1999).
Portugal: Red rice is one of the most important weeds in the rice-producing areas in the country. Although various methods are usually implemented for red rice control, such as use of clean rice seeds, crop rotation and water management, chemical control plays an important part in the elimination of red rice. Cycloxydim (0.2 kg a.i./ha), applied preplanting after red rice emergence, and fluazifop-p-butil (0.25 kg a.i./ha), applied post-emergence (3-4 leaf stage of red rice), combined with drainage before application and flooding three days after, are among the most effective herbicide treatments (Martins da Silva et al., 1999).
Senegal: The major red rice species is Oryza barthii A. Chev. Red rice biotypes may also come from cultivated Oryza glaberrima Steud. Red rice infestation is mainly due to the use of non-clean rice seed. There are no data concerning the actual rice area infested by red rice, but some observations indicate that almost 55 percent of a rice area of about 40 600 ha is infested by the weed. Major infestation of this weed is found in direct-seeded areas. At present, no specific methods are implemented to control red rice (Diallo, 1999).
Spain: It has been found that red rice is an important weed in rice as well as other biotypes of wild rice that do not have a red pericarp. The main control methods are: use of clean rice seed; hand-pulling of red rice plants; and the application of glyphosate or sulfosate over the red rice plants with an applicator rope (Gomez Barreda et al., 1999).
Suriname: Ten percent of rice fields are infested by red rice. Every 1 percent of red rice infestation reduces rice yield by 6 percent. The most common practice for controlling red rice is sowing under water. Following seed bed preparation the field is not drained. A water layer of 10 to 30 cm above the soil surface is maintained, depending on the evenness of the field. Pregerminated seed (24 hours soaking in water and 24 hours incubation period) is sown in the flooded field. The flood is maintained until grain ripening, with intermittent draining periods. Water availability and good levelling (the latter a quite common practice) are required for optimum control results (Khodabaks, 1999).
Venezuela: Red rice was probably introduced into Venezuela with rice seed from the United States in 1948. By 1988, red rice was already considered one of the major weeds in lowland and irrigated rice in Venezuela. It is considered to be the third weed in terms of economic importance after Echinochloa colona and Ischaemun rugosum. The main control methods of red rice are: i) use of seed certified as free of red rice (the main source of red rice infestation is the use of contaminated rice seed); ii) puddling during land preparation; and iii) occasional use of some herbicides - replanting treatments with glyphosate or paraquat; preplanting with soil-acting compounds, such as pretilaclor, molinate, oxadiazon or oxyfluorfen; herbicide applied over the water layer (oxadiazon, oxyfluorfen or molinate) (Ortiz, 1999).
Viet Nam: Weedy rice infestation is highest under dry rice seeding followed by wet seeding and zero-tillage seeding. The most severe infestation is observed in summer-autumn. The typical characteristics of weedy rice, compared with popular modern rice varieties in Viet Nam, are short duration, tall plants, weak culm, small seeds, easy shattering and red pericarp. The average yield loss due to weedy rice ranges from 15 to 17 percent. Methods of weedy rice control in Viet Nam are: i) use of clean seeds; and ii) good land preparation and water management, sowing in lines, broadcast seeding and rotating rice with upland crops. The most effective herbicides against weedy rice control are thiobencarb, molinate, oxadiazon and oxadiargyl (Chin Duong Van et al., 1999).
The main conclusions drawn from the above presentations are:
1. There is no simple method for the control of weedy/red rice. Only through the integrated control approach can weedy/red rice infestation be effectively reduced.
2. The main sources of weedy/red rice infestation are: rice seeds contaminated with weed seeds, and weedy/red rice seed bank in soil. Therefore, any control measure should be aimed at the reduction of infestation from these sources.
3. In some countries the presence of weedy/red rice seeds is tolerated in rice seeds. However, experience in the control of this weed in countries that use advanced technologies shows that not even one single weedy/red rice seed should be tolerated in rice seeds.
4. The reproduction of basic and foundation seed should be carried out in areas which are totally free of weedy/red rice infestation. Certified rice seeds should be free of weedy/red rice seeds.
5. To date, the most efficient control measures have been those based on the combination of wet soil preparation (to bring about the emergence of weedy/red rice seeds), followed by the application of herbicide (e.g. glyphosate) over the weed stand and before rice seeding, and water management before and after seeding.
6. Under upland and irrigation conditions, it is advisable to implement, whenever possible, minimum tillage or zero tillage combined with the use of non-selective herbicides. This practice is cheap and sustainable for rice farmers.
7. The use of post-emergence herbicides in the process of land preparation needs improvement. It is necessary to find other chemical alternatives in order to avoid repeated use of the same chemical. This also applies to post-harvest application in rice areas.
8. Within the context of integrated management, it is necessary to conduct regular studies of the behaviour of available rice cultivars in terms of: their ability to compete with weedy/red rice; life-cycle; and tolerance to submersion during flooding.
The situation in Asia and Oceania with regard to Echinochloa spp. in rice is as follows:
Australia: The country produces 1.2 million tonnes (Mt) of rice annually from 150 000 ha of rice fields and is a net rice exporter. Australian control strategies of Echinochloa spp. and other weeds are best described as an integrated weed management system, principally based upon herbicides augmented by other methods, such as fallow management, laser-aided land preparation, judicious fertilizer application and water management with early inundation. One crop of rice is grown annually and alternated with other agricultural activities. Rice is often alternated with grazing (principally of sheep), cotton, maize or fallow. Weed control between rice-growing periods is an important component of Echinochloa control. Early deep flooding of paddies is used to reduce germination of Echinochloa at the 1-3 leaf stage. This is often in combination with the addition of undiluted herbicide to irrigation water. Strict control of weeds in irrigation channels is also used to reduce seed input to paddies (Hetherington, 2001).
China: Echinochloa crus-galli is considered the main rice weed in the country. The dominance of Echinochloa spp. in many rice fields in China is a source of concern to farmers, farm operators, extension workers and policy-makers alike. The country has 33 million ha (Mha) of rice and more than 66 percent of this area is treated with herbicides for weed control. The main herbicides used are MCPA, nitrofen, bensulfuron-methyl, propanil, quinclorac and bentazone. It has been noted that the continuous use of herbicides has led to a shift from susceptible to tolerant weed species (Ze-Pu Zhang, 2001).
Japan: There are 1.76 Mha of rice fields in Japan, with an average yield of about 5.37 t/ha. Herbicide-based control strategies through one-shot herbicide treatments are predominant among Japanese farmers and the sulfonyureas are increasingly popular. Other control methods include deepwater irrigation of fields up to 15 cm and application of calcium cyanamide fertilizer to induce germination of seeds of Echinochloa oryzicola, only for them to be killed by low winter temperatures (Morita, 2001).
Korea: The switch from transplanting to direct-seeded rice culture led to a species shift favouring grassy species with Echinochloa spp. as the dominant representatives. A negligible amount of herbicides were used to control rice weeds in the 1960s, mainly 2,4-D and PCP. While nitrofen was dominant among rice herbicides in the 1970s, the 1980s saw butachlor, pretilachlor and mefenacet dominating over 60 percent of the herbicide market. Rice herbicides in the 1990s and thereafter were dominated by sulfonylureas, with the pyrazosulfuron-ethyl + molinate mixtures forming around 70 percent of the market share. Delayed application of herbicides at 15 days after transplanting (DAT) in rice led to grassy weeds, especially Echinochloa spp. (which display flush germination at 10 DAT), avoiding the direct effect of the treatment (Kim, 2001).
Malaysia: Studies have been carried out on the behaviour of Echinochloa crus-galli and its soil seed bank. The potential of seed bank for augmenting existing populations of E. crus-galli is fairly low, between just 2.32 and 3.95 percent of the total seed rain emerged as seedlings per season. Seed viability varied considerably according to the soil moisture content and this ranged from between 27 and 36 percent under rainfed soil conditions to between 12 and 16 percent in inundated fields. Invariably, seed mortality was higher under inundated field conditions than in rainfed rice fields. Integrated management of E. crus-galli populations below the economic threshold of 1-3 plants/m2 in Malaysian rice fields is necessary, and this can be done by weeding at the critical period of competition between E. crus-galli and rice variety MR84 at 15 to 30 days after sowing, which may result in a yield increase of between 22 and 66 percent. Field inundation at 3 to 6 days after sowing (DAS) also reduced E. crus-galli infestation considerably - but at the expense of low productive crop tillers. Delayed flooding at 15 DAS, coupled with weed-free field conditions at the critical period of competition, enhanced tiller production and improved crop yields (Baki and Azmi, 2001).
Sri Lanka: Although the country has almost reached self-sufficiency in rice, weeds are the major biotic stress and account for nearly 30 to 40 percent of the yield losses. More than 80 percent of rice farmers rely on herbicides for weed control since their introduction in the 1960s, with propanil as a leading choice among herbicides. Of late, herbicides, such as fenoxaprop-p-ethyl, sethoxydim and mefenaset, and herbicide mixtures, such as oxadiazon + propanil, thiobencarb + propanil, butachlor + propanil, quinclorac + propanil or molinate + propanil, are becoming popular among rice farmers as part of their control regimes. In the wet zone, where the majority of rice lands are transplanted, Echinochloa is controlled through water and manual methods in paddy fields. The rapid spread of weed seeds alone with seed paddy is one of the reasons for the increased incidence of the Echinochloa problem in rice. Hence the Department of Agriculture encourages farmers to use weed-free seed paddy (Abeysekera, 2001).
Thailand: The country has more than 10 Mha of rice fields. In direct-seeded rice culture, Echinochloa crus-galli and Leptochloa chinensis represent two problematic weed species, especially in the central and upper north rice granaries. The most important methods for the control of Echinochloa spp. and other weeds are: i) use of weed-free rice seeds; ii) thorough land preparation; iii) water management with fields kept inundated at 5-10 cm following herbicide application; iv) hand-weeding at 30-40 days after transplanting; and v) zero-tillage using glyphosate prior to seeding. Populations of Echinochloa crus-galli resistant to fenoxaprop-p-ethyl, propanil and butachlor were already noted in 1999 (Maneechote, 2001).
Viet Nam: Rice is the most important crop in Viet Nam. Transplanting rice is popular in the north, whereas more than 90 percent of sown area in south Viet Nam is direct seeding. Weedy rice and other species, such as Echinochloa spp., present the major weed problems for rice. The most popular methods of weed control are: i) field inundation at 4 DAS (giving 14.8-30.3% higher yields than with 12 DAS); ii) fields subjected to two rounds of rotovation (higher yields than with one round of rotovation, while zero tillage did not promise better yields for farmers); iii) use of herbicides (>82.2% of Vietnamese farmers), normally supplemented by hand weeding (85% of farmers); and iv) hand weeding, which takes about 150-200 work days/ha and is normally done at 10-15, 25-30 and 40-45 DAS (Chin Duong Van, 2001).
The main conclusions drawn from the above presentations are:
1. Preventative measures, such as enforcement of quarantine laws and regulations and field hygiene, may help to arrest the spread of Echinochloa seeds and propagules from the source of infestation to areas not yet affected.
2. Farmers should be advised and encouraged to use affordable certified rice seeds which are free from Echinochloa seeds and propagules.
3. Farmers should be given advice on proper tillage operations, optimized water resource management and rational use of herbicides for Echinochloa control. Herbicide-based weed control strategies may aggravate the problem of certain weed species instead of alleviating it.
4. Over-reliance on herbicides for Echinochloa control may lead to the incidence of herbicide-resistant biotypes. There are already resistant biotypes in some countries of the region and the problem tends to increase every year.
5. Monitoring of herbicide-resistant Echinochloa populations is necessary for better weed management in areas of rice monocropping where a single herbicide is used repeatedly.
6. Given the increased problems of herbicide resistance, it is recommended that the herbicides most used in rice should be labelled according to their mode of action. This will provide guidance to farmers concerning the herbicides to be used in the case of resistance problems.
7. Farmers should be encouraged, whenever possible, to practise rotation of rice with upland crops in order to help deplete the seed bank of weeds associated with rice. Research emphasis on the prevention of seed-set in Echinochloa spp. should be encouraged.
The conclusions of both workshops make quite clear the importance of preventative measures for better management of weeds, such as red rice and Echinochloa spp. The use of clean seeds is one way of reducing weed infestation in the field, together with regular cleaning of agricultural machinery and implements to avoid further dissemination of weeds. Although tedious, this cleaning is essential if a significant reduction in weed pressure in the fields is to be achieved. Where possible, crop rotation should also be used. Rotation with upland crops not only helps to break the cycle of weeds associated with rice, but it also helps to contain possible problems of herbicide-resistant weed species (Gressel, personal communication, 2001). Land preparation plays an important role in reducing weed infestation. It seems that minimum or zero tillage is effective in areas where rotation with upland crops is possible, while in other areas ploughing and puddling are preferred. In those areas where water is available, e.g. Guyana, southern Viet Nam and Suriname, flooding is an important means for controlling several problematic weeds, including red/weedy rice and Echinochloa spp. The use of herbicides is a necessity in direct-seeded rice where the most important issue is that farmers understand the problem of using the same herbicide repeatedly, particularly in monocropping areas. It is true that resistance can be managed, but for farmers it would be much better to prevent such phenomena. Farmers should get regular advice from extension workers on this issue; otherwise the problem of resistance will become a major constraint to rice production, as are losses caused by pests.
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