Use of phytotoxic rice crop residues for weed management

There is a general perception among Cambodian rice (Oryza sativa) farmers that, after harvesting, rice crop residues that are incorporated into the field benefit the growth of the subsequent rice crop. However, the effect of this action upon weed establishment and growth has not yet been considered. A series of pot and field trials were conducted to determine whether such action could inhibit weed establishment and/or growth. The pot studies first evaluated the response of the test plant (rice line ST‐3) and three weed species, barnyardgrass (Echinochloa crus‐galli), small umbrella sedge (Cyperus difformis), and water primrose (Ludwigia octovalves), to the residue of 16 rice lines and the field trials were later conducted to evaluate the response of the same test plants to the residue of seven putatively allelopathic rice lines and one non‐allelopathic rice line. The residue of all the studied rice lines, depending on how long they had been incorporated into the soil, reduced the establishment and growth of all three weed species, as well as the rice crop. However, if the residue's incorporation was delayed by 2 weeks or only a proportion of the residue was incorporated, the rice crop could withstand the growth‐inhibiting effect, while the inhibition of the establishment and growth of the three weed species was retained. These responses of rice and the weeds to rice crop residues might provide a basis for a weed management strategy, particularly in the resource‐poor rice‐production systems of Cambodia.     

Potential allelopathic rice lines for weed management in Cambodian rice production

In Cambodia, the planting of rice lines with a competitive and/or allelopathic ability would be a very useful way to supplement weed management in the rain‐fed, low‐input production systems. The present study examines a wide range of rice germplasm, mainly from Cambodia, and uses a series of bioassay techniques to identify those that might have a weed growth‐suppressing, allelopathic trait. A laboratory bioassay study that involved 359 rice lines showed that there were 15 that could significantly reduce the growth of awnless barnyard grass seedlings. In a second laboratory bioassay, involving the best 96 rice lines that were identified in the first study, 14 were shown to suppress the shoot growth of awnless barnyard grass, 11 could suppress the shoot growth of barnyard grass, six could suppress the shoot growth of small umbrella sedge, four could suppress the shoot growth of two‐leaf fimbristylis, four could suppress the shoot growth of water primrose, and three could suppress the shoot growth of gooseweed. Of the 13 rice lines that were able to suppress the growth of at least two weed species, there were three lines that could suppress the growth of three weed species, one line that could suppress the growth of four weed species, and one line that could suppress the growth of five weed species. In a third soil‐based, pot bioassay that studied the 18 best lines coming from the second laboratory bioassay, all showed a significant weed growth‐suppressive ability. A linear regression analysis showed that there was no correlation between their weed growth‐suppressive ability and their physical seedling size, supporting the idea that the growth suppression was allelopathic in nature and not a physical competition effect. In summary, the results indicate that an allelopathic trait does exist in some Cambodian rice lines and that this trait is effective in the growth suppression of a number of major rice weeds.     

Alfalfa, rice by-products and their incorporation for weed control in rice

Two alfalfa varieties, namely Rasen and Yuba , and rice by-products of Koshihikari ( Oryza sativa L. cv. Japonica ), including hull and bran were used for weed control in rice. Rasen and Yuba were sequentially evaluated to have the strongest allelopathic potential among eight common alfalfa varieties in Japan. Rasen , Yuba and rice by-products in a laboratory experiment exhibited suppression of emergence of lettuce ( Lactuca sativa L.), barnyardgrass ( Echinochloa oryzicola Vasing.), and monochoria ( Monochoria vaginalis Presl var. plantaginea Solms-Laub.). In a paddy field experiment, at 15 days after application, a dose of 1 ton ha⁻¹ of alfalfa, rice by-products and the incorporated alfalfa-rice by-products significantly inhibited weed growth and reduced weed species, except for rice bran. However, at 40 days after application, the greatest weed control was sequentially recorded with Yuba (95.2%), Rasen (90.5%) and the incorporated Rasen -hull (88.3%). Rice hull exhibited stronger (51.7%) weed control than the negligible weed control of rice bran (25.1%). Inhibition of both Rasen and Yuba on weed emergence after application became more vigorous, whereas those of rice by-products were gradually devitalized. Rasen had a maximum increase of rice yield (80.6%) compared to the control and it was slightly higher (9.6%) than the herbicide treatment. Findings showed that Rasen was the most promising material for weed control among those studied. Results denoted that weed control capability of allelopathic materials had an impetuous effect on rice growth and yield.     

Evaluation of rice allelopathy in hydroponics

The inhibitory activity of water extracts from the shoots and roots of three rice cultivars, Taichung native 1 (TN1) and IAC165 (both allelopathic rice) and AUS196 (non-allelopathic rice), grown in hydroponics was evaluated. The release of germination inhibitors by allelopathic rice plants into hydroponic solution was also determined with freshly collected solution and XAD-4 resin desorbate. The degree of the inhibition was quantified in terms of root growth in Echinochloa colona, Echinochloa crus-galli, Echinochloa crus-galli var. oryzicola, Triantema portulacastrum and Lactuca sativa. The allelopathic activity of rice was species specific, and depended on source and concentration. Root length of all test species was inhibited by the different concentrations of shoot extract of allelopathic and non-allelopathic rice. However, of the three cultivars, TN1 showed higher inhibition than IAC165 and AUS196 in all test species. Water extracts of shoots and roots significantly inhibited root growth in E. crus-galli but the shoot extract gave a greater inhibitory effect on E. crus-galli than the root extract. Root exudate of TN1 inhibited root elongation of E. crus-galli from 2 weeks after transplanting (WAT) and the inhibition continued for 4 WAT. The results confirmed the previous finding of a laboratory bioassay that the TN1 had allelopathic activity and produced allelochemicals that inhibit growth of some weed species.     

Quantification of momilactone B in rice hulls and the phytotoxic potential of rice extracts on the seed germination of Alisma plantago‐aquatica

The occurrence of herbicide‐resistant weeds has boosted interest in the use of crop allelopathy as a potential alternative to herbicides for weed control in rice (Oryza sativa). The phytotoxic compounds that are released by rice could help to enhance its competitive ability and improve weed management. This study aimed to screen rice genotypes for phytotoxic activity, quantify the amount of momilactone B in various rice tissues, and identify the potential parental lines for quantitative trait locus analysis. Therefore, a total of 41 cultivars from germplasm collections was evaluated for their effects. Significant differences were found among the rice cultivars in their ability to reduce the germination, root growth, and root dry weight accumulation of Alisma plantago‐aquatica. The leaf extract was the most inhibitory to germination. Out of the five cultivars that were tested, momilactone B was detected in four of them: Marateli, Kizilirmak, Karadeniz, and Kiziltan. Karadeniz and Kiziltan were identified as the rice cultivars with a high momilactone B content in the tissues and therefore they could be used in breeding programs to enhance the phytotoxic potential of rice. The development of a rice cultivar with proven allelopathic characteristics could provide an environmentally friendly and low‐cost approach for the control of A. plantago‐aquatica.     

农艺措施和除草剂用量对化感稻产量及杂草生物量的影响

为实现水稻生产减施除草剂的目标,将化感材料与农艺措施相结合取得对稻田杂草有效控制的效果,田间采用再裂区试验法研究了化感稻3号和N两优201两个水稻化感材料和非化感材料五山丝苗在不同水分管理、移栽密度和除草剂不同用量下,对水稻产量和田间杂草生物量的影响。结果表明:无论早季稻田还是晚季稻田,施用常规除草剂量的小区杂草生物量均为0,施用常规剂量50%除草剂且仅在孕穗期晒田的小区和常规密度小区有少量杂草发生。不施除草剂的水稻材料小区杂草生物量依次为化感稻3号N两优201五山丝苗。移栽后75 d,不施除草剂的小区相同水稻材料孕穗期不晒田比晒田处理、高密度比常规密度处理的杂草生物量小。早季稻田不晒田和高密度提高了水稻的产量,晚季稻田则相反。化感材料化感稻3号和N两优201在田间的抑草效果较五山丝苗要好。在水稻生产中,水稻化感材料结合农艺措施,可以实现水稻不减产,田间杂草得到有效控制,除草剂量减50%的目标。     

Breeding spring wheat for improved allelopathic potential

It is becoming apparent that allelopathy plays a significant role in the competitive ability of cereals against weeds; barley, wheat and rice cultivars with high allelopathic activity have been identified. However, direct breeding efforts to improve allelopathy have to date only been reported in rice. In this study, a breeding programme in spring wheat ( Triticum aestivum ) was evaluated to determine the efficiency of selection of allelopathy and the ability of breeding lines to suppress weeds in the field. The material used originated from a cross between a Swedish cultivar with low allelopathic activity and a Tunisian cultivar with high allelopathic activity. Allelopathic activity was measured as growth inhibition of perennial ryegrass ( Lolium perenne ) roots when grown together with the wheat cultivars on agar. For screening of F₂ populations, a single plant bioassay was used for the first time. In the F₆ and F₇ generations, three breeding lines with an average improved allelopathy of 20% and one line with an unimproved allelopathy activity, but with the same phenotype as the high allelopathic lines, were tested together with the low allelopathic Swedish parent. The main result from the field study was a 19% average reduction of weed biomass for the high allelopathic lines, but no significant reduction of the low allelopathic breeding line. Early shoot length and early crop biomass (stages 37–39, Zadoks scale) and straw length of the high allelopathic lines were not significantly different from the Swedish parent. A negative effect was that grain yield was reduced by 9% in the high allelopathic lines. It is suggested that the reduced biomass of weeds in plots planted with the highly allelopathic wheat lines is related to differences in allelopathic activity and not differences in plant growth.     

Performance of Bangladesh indigenous rice in a weed infested field and separation of allelopathy from resource competition

This study aimed to identify the potential allelopathic indigenous rice (Oryza sativa L. ssp. indica) varieties from Bangladesh using a performance study in a weed‐infested field and to assess the extent of allelopathic interference relative to resource competition in a glasshouse experiment. Six varieties – namely, “Boterswar,” “Goria,” “Biron” and “Kartiksail” as the most allelopathic, “Hashikolmi” as weakly allelopathic and “Holoi” as nonallelopathic – were raised following a nonweed control method. The infestation levels of weed species were calculated using Simpson's Diversity Index (SDI), which ranged from 0.2 to 0.56. However, a significant correlation coefficient (0.87, P < 0.001) was obtained from these field data compared with the root inhibition percentage from the laboratory bioassay, and the “Boterswar” variety was the most allelopathic. The interactions between the allelopathic variety “Boterswar,” weakly allelopathic variety “Hashikolmi” and Echinochloa oryzicola via a target (rice)‐adjacent (E. oryzicola) cogrowth culture were determined in a hydroponic arrangement. The relative competitive intensity (RCI) and the relative neighbor effect (RNE) values showed that the crop–weed interaction was facilitation for “Boterswar” and competition for “Hashikolmi” and E. oryzicola in rice/E. oryzicola cogrowth cultures. The allelopathic effects of “Boterswar” were much higher than the resource competition in rice/E. oryzicola cogrowth cultures. The converse was observed for “Hashikolmi.” Moreover, the mineral content of E. oryzicola was severely affected by “Boterswar”/E. oryzicola cogrowth cultures’ exudate solution. Therefore, the allelopathic potential of “Boterswar” variety might be useful for developing the weed‐suppressing capacity of rice, which will likely have a significant influence on paddy weed control.     

Weedy rice: An expanding problem in direct‐seeded rice in the Philippines

Weedy rice is morphologically similar to cultivated rice (Oryza sativa L.). It has biological characteristics that identify it as a weed. Its important weedy characteristics, that is, early and heavy seed shattering, makes it very difficult to control. Weedy rice has not been reported to be an important weed problem in transplanted, flooded rice. However, the shift to direct‐seeded rice (DSR) due to water issues and high costs of labor has increased reports of weedy rice becoming an expanding important problem in Vietnam, Malaysia, Thailand, and the Philippines. Experts believe that the growing adoption of DSR in Asian countries will result in the rise of weedy rice as one of the top troublesome weeds in rice production. Early and recent surveys in the Philippines have indicated the urgent need to increase awareness of weedy rice among farmers to allow the implementation of a number of effective location‐specific weed management strategies. These surveys and other studies conducted since weedy rice was first reported in 1991 confirmed that weedy rice and grass weed species caused major problems in DSR areas. About 35% of the 4.56 M ha harvested area in the country is planted with DSR. As cultivated and weedy rice are close relatives, it would be very difficult to implement management options very early in crop growth. However, a deeper understanding of the underlying traits of weedy rice can help develop a holistic approach toward effective and economic weed management.     

Effects of duck activities on a weed community under a transplanted rice–duck farming system in southern China

Rice–duck farming is one of the traditional, but current and gradually popularized, ecological farming techniques for paddy rice production in China. In comparison to rice monoculture farming, we investigated a weed community and weed infestation in a transplanted rice–duck farming system and examined the effects of duck activities on the weeds. Three treatments were imposed in the experiment: a rice–duck coculture with duck grazing and disturbance effects, a rice–duck coculture with duck disturbance effects only, and a control (rice monoculture, no duck effects). Rice seedlings were nursed and transplanted in this experiment. The total number of weed species was significantly lower in the treatment with both duck grazing and disturbance and in the treatment with duck disturbance only than in the monoculture rice treatment (control). The Shannon‐Wiener Biodiversity Index of the weed species decreased but the Pielou Evenness Index increased in both treatments with ducks compared to the monoculture control. Sorensen's coefficient of similarity of the weed communities was low between the treatments with ducks and the monoculture control, implying that the weed communities diverged. The total density and biomass of the weed community in both the treatments with ducks were significantly lower compared to the monoculture control. However, compared to the rice monoculture control, the weed biomass was reduced by 98% in the treatment with duck grazing and disturbance and by 84% in the treatment with duck disturbance only, implying that only 14% of the weeds were controlled by duck grazing and that 84% of the weed biomass was inhibited by duck activities in the rice field with ducks. The rice grain yield increased in the treatments with ducks compared to the control. The results suggested that duck activities, like grazing weeds and disturbance of the soil and water, had a significant effect on the weed community structure and that weeds could be well controlled without herbicide application under a rice–duck farming system.     

Development of lowland weed management and weed succession in Japan

Since the introduction of rice production in Japan, lowland areas have been managed for rice production with the purpose of better rice growth, as well as lesser weed infestation. Rice is cropped every year in lowland fields by repeated cultivation of a single crop, with high yields and without soil sickness usually being observed in upland fields. This is probably because the irrigation water supplies various nutrients for healthy rice growth and the drainage washes out and removes harmful factors. However, until recently, the wet or flooded conditions of lowland fields in the Asian monsoon region never have allowed humans to cultivate useful summer crops, except rice or some aquatic plants. Therefore, the management of lowland areas in the Asian monsoon region has been significantly different from European field management, where crop rotation has been the traditional standard practice. Paddy weeds are aquatic plants or hygrophytes that have adapted to lowland fields. Traditionally, tillage and puddling were practiced seasonally in lowland fields on a regular schedule every year. Rice cultivation technology was developed and supported by regional irrigation systems that created stable environments for typical paddy weeds to complete their life cycle. After the introduction of chemical weed control, rice fields became very severe habitats for these paddy weeds, where they could not grow and reproduce without strategies for survival under herbicide exposure. Even so, many of the traditional paddy weeds survived because of their accumulated or uneradicated seed banks, although several aquatic plants were listed as endangered or threatened species. The important weed species changed, sometimes rapidly and sometimes slowly, depending both on their reproductive system and their biological response towards field management and weed control systems. Very recently, the level of perennial weeds, herbicide‐resistant weeds, and weedy rice has increased in paddy fields that are highly dependent on herbicide use. In addition, several hygrophyte species have invaded paddy fields. In order to address these issues, the improvement and application of integrated weed management methods are expected to be critical.     

Weed-suppressing rice cultivars – does allelopathy play a role?

A range of 111 rice cultivars was studied for weed-suppressing ability in field experiments with a sown infestation of Echinochloa crus-galli. Cultivars differed significantly in their ability to suppress the growth of E. crus-galli , and the differences were reasonably reproducible over three seasons. The same rice cultivars were tested in a laboratory screening for allelopathic potential, which showed significant differences in the ability to reduce root growth of E. crus-galli . Correlation between the laboratory screening and the field experiments showed that field performance could be described to some extent by E. crus-galli root length reduction in the laboratory. Plant height in the field experiment was correlated with weed biomass 8 weeks after seeding. Even among the most weed-suppressing rice cultivars, however, all heights were represented. None of the measured growth parameters from greenhouse studies could explain the distribution of weed-suppressing rice cultivars. This indicates that allelopathy in combination with competitive ability determines the weed interference outcome of a given rice cultivar.     

Contribution of allelopathic effects to the overall weed suppression by different cover crops

Cover crops can suppress weeds within agricultural fields due to competitive and allelopathic effects. Glasshouse experiments were conducted to evaluate the relative proportions of allelopathic effects to the total weed inhibition. Six different cover crop species were combined with three weed species in the presence or absence of active carbon over a period of four weeks. Active carbon was used as an adsorbent for allelopathic substances in the soil. Our study revealed that the competition between cover crops and weeds shifted, possibly due to the minimisation of allelopathic effects by active carbon in the soil. We assume that the degree of cover crops allelopathic effects on weeds is species‐specific, both on the side of cover crops and on the weed side. The cover crops Raphanus sativus, Fagopyrum esculentum and Avena strigosa showed the highest allelopathic weed suppression with up to 28%. Additionally, Stellaria media turned out to be the most sensitive weed against allelopathic effects induced by all cover crops, except for Linum usitatissimum and Guizotia abyssinica. The knowledge about the contribution of competitive and allelopathic effects by cover crops would help to create cover crop mixtures with high weed suppressive ability.     

Utilization of Bidens pilosa var. radiata (Sch. Bip.) Sherff integrated with water irrigation for paddy weed control and rice yield production

The application of allelopathic plant materials combined with an agricultural production method is a concept that can be used to manage weeds during rice production. This research investigated the effects of the utilization of Bidens pilosa var. radiata integrated with water irrigation for weed control and rice production. B. pilosa and water extract exerted more toxicity against Echinochloa crus‐galli than the extracted residue applied at the same rates, suggesting that weed control was caused by compounds extracted from B. pilosa. The changes in water properties after the application of B. pilosa were explored. Dissolved oxygen was rapidly reduced after application and then slowly increased within 3–5 days after application (DAA), while the pH also decreased at the early stage and then increased back to normal values within 1–3 DAA. Electrical conductivity increased with increasing rates of application, suggesting that allelochemicals were released from B. pilosa. The concept was studied under field conditions. B. pilosa was applied along with irrigation at 7 days after sowing (DAS) on a direct wet seed system. The results showed that application rates of 2, 4 and 6 tons ha^?1 decreased the number of weeds by 52.16, 86.73 and 95.18% at 30 DAA and reduced dried weight of weeds by 17.65, 34.69 and 86.82%, respectively. The rice yield showed an increase of 72.73, 81.03 and 73.66% when compared with the nonweeded crop, while herbicide treatment increased by 84.68%. The findings demonstrate that the application of B. pilosa integrated with water irrigation was able to control weeds in a direct seeded rice production system.     

Breeding of commercially acceptable allelopathic rice cultivars in China

BACKGROUND: One promising area of paddy weed control is the potential for exploiting the weed‐suppressing ability of rice. This study was conducted to develop commercially acceptable allelopathic rice cultivars using crosses between allelopathic rice variety PI312777 and commercial Chinese cultivars (N2S, N9S, Huahui354, Peiai64S and Tehuazhan35), and to assess their weed suppression and grain yield in paddy fields in relation to their parents. RESULTS: There was a positive dominance in the crosses Huahui354 × PI312777 and N2S × PI312777 but recessive or negative dominance in N9S × PI312777, Peiai64S × PI312777 and Tehuazhan35 × PI312777. Huahui354 × PI312777 and N2S × PI312777 showed stronger weed suppression than their parents and other crosses. Finally, an F₈ line with an appearance close to Huahui354 and a magnitude of weed suppression close to PI312777 was obtained from Huahui354 × PI312777. This line, named Huagan‐3, was released as a first commercially acceptable allelopathic rice cultivar in China. The grain yield and quality of Huagan‐3 met the commercial standard of the local rice industry. Huagan‐3 greatly suppressed paddy weeds, although suppression was influenced by year‐to‐year variation and plant density. There was no certain yield reduction in Huagan‐3 even under a slight infestation of barnyard grass in paddy fields. CONCLUSION: The successful breeding of Huagan‐3 with high yield and strong weed suppression may be incorporated into present rice production systems to minimise the amount of herbicide used. Copyright © 2011 Society of Chemical Industry     

Weed suppression ability of upland rice under low-input conditions in West Africa

Weeds are a major constraint to upland rice production in west Africa. The objectives of this investigation were to study differences in weed suppression ability among upland rice cultivars and to determine the morphological traits involved. Twelve contrasting cultivars, including West African indigenous Oryza glaberrima (Steudel) lines and traditional and improved O. sativa L., were cultivated under natural weed competition and low-input conditions in Côte d'Ivoire in two seasons. Significant differences between cultivars were observed in weed biomass at 100 days after seeding (DAS) in 1994 and 1995, indicating differences in their competitive ability. Weed biomass was negatively correlated with rice root growth at early growth stages and with rice shoot and root growth at later growth stages. Across cultivars, grain yields with a single hand-weeding were 60% in 1994 and 49% in 1995 of those in plots that were weeded regularly. High grain yields in weedy plots were associated with low weed biomass. IG 10, an O. glaberrima cultivar, was the most competitive against weeds.     

Weed problems and their management in rice fields of Malaysia: An overview

The available literature on weed problems and weed control in rice fields in Malaysia is reviewed and suggestions for future research are included. The problem of weed competition with rice is of great economic importance in the country because it causes a 10–35% reduction in grain yield. About US$4.10 million is spent annually on herbicides for rice alone, and this amounts to approximately 7% of the total expenditure on herbicides. The shift in methods of rice culture from transplanting to direct seeding has magnified the weed problem. The problem of weedy rice is particularly acute in the direct-seeding rice areas. Although a number of sulfonylurea herbicides (e.g. cinosulfuron, metsulfuron, bensulfuron and pyrazosulfuron) have been found to be suitable alternatives to the old herbicide 2,4-D, an integrated weed management program must be developed in order to reduce the problem of herbicide resistance in weeds. Weed control methods that are more friendly to the environment and affordable to farmers must be sought. Weed-competitive and allelopathic rice varieties must be produced. The problem of weeds in rice calls for systematic studies on biotechnological methods of control of weedy rice. Detailed studies on the biology and ecology of notorious rice weeds, particularly Oryza sativa L. (weedy rice), Echinochloa spp., Leptochloa chinensis (L.) Nees, Fimbristylis miliacea (L.) Vahl. and Limnocharis flava (L.) Buch. need to be done in order to formulate successful weed control measures.     

Current status of biological control of paddy weeds in Vietnam

Rice is a staple food in Vietnam and accounts for > 7.7 × 10⁶ cultivated ha, which provide 35.5 × 10⁶ t of rice, of which 4.2 × 10⁶ t were exported in 2004. The enlargement of the cropping area and the enhancement of rice yield have rapidly increased the amount of agrochemicals, including herbicides, in crop production in Vietnam. From 1990–2003, the percentage of herbicides in total pesticides has increased ≈ 10-fold to 30.2%. In addition, the improper use of herbicides caused environmental hazards, unsafe agricultural products, and human health problems. Biological management integrated with traditional weed control techniques might help to reduce the dependence on synthetic herbicides and build eco-friendly, sustainable agricultural production in Vietnam. This paper reviews the efforts in establishing a strategy for biological management of weeds that was conducted in recent years by Vietnamese weed scientists. This has included cropping system management, water and soil management, integrated pest management, and utilization of plant allelopathy as major components of the strategy. Many plants with strong allelopathic potential can be a source for biological weed suppression and soil fertility improvement. The utilization of allelopathic properties in rice might also help to provide new rice cultivars with weed-suppressing characteristics.     

Allelopathic potential of indigenous Bangladeshi rice varieties

A series of experiments was conducted in the laboratory and greenhouse of the Subtropical Field Science Center, University of the Ryukyus, Japan, from April to October 2015 to assess the allelopathic potential of 50 indigenous Bangladeshi rice varieties by using the donor–receiver bioassay, equal compartment agar method (ECAM), plant residue extract method and pot culture method. Lettuce (Lactuca sativa L.), cress (Lepidium sativum L.), radish (Raphanus sativus L.), barnyard grass (Echinochloa crus‐galli L. Beauv.) and jungle rice (Echinochloa colona L.) were used as the test plants. The highest inhibition effect was given by Boterswar, while the stimulating effect was given by Kartikbalam and Panbira in the donor–receiver bioassay and ECAM tests. Boterswar, Goria, Biron and Kartiksail were selected as the highest allelopathic‐potential varieties by the donor–receiver bioassay and ECAM. In the methanol extract test, Boterswar gave the strongest inhibitory effect on both barnyard grass and jungle rice, while Kartiksail gave the highest inhibitory effect on the jungle rice shoot. The growth parameters and total dry matter of barnyard grass in the greenhouse pot experiment were significantly reduced as a result of the application of aqueous extracts of the selected rice varieties, which was similar to the results of the laboratory experiments. The varieties of Boterswar, Goria, Biron and Kartiksail were selected as the most allelopathic among the 50 indigenous Bangladeshi rice varieties. These rice varieties could be used for the isolation and identification of allelochemicals and to further develop new varieties that are tolerant to weeds.     

Weedy rice represents an emerging threat to transplanted rice production systems in Japan

Weedy rice is a conspecific weed of cultivated rice (Oryza sativa L.) and is a primary weed of direct‐seeded rice production systems worldwide. The shift in the method of establishing rice, that is, from transplanting to direct seeding, is considered the main reason for the emergence of weedy rice in Asia and, more recently, Japan. Japan has increasingly adopted directly seeded rice, even though this practice remains limited. In this study, I confirmed the relationship between weedy red rice emergence and the adoption of direct‐seeded rice in Japan. Twenty‐seven areas from eight prefectures infested with weedy rice were selected to confirm the relationship between the rice planting method and the emergence of weedy rice. Fields that were severely infested with weedy rice were selected as survey fields for this study. I found no relationship between the method of planting rice and weedy rice emergence. Changes to herbicide application, duration of rice production and hand weeding also contribute to the emergence of weedy rice in transplanted rice fields in Japan. The results of this study indicate that weedy rice could become a serious problem in Japan, even in transplanted rice fields.