There are two species of cultivated rice in the world—Oryza sativa L. from Asia and Oryza glaberrima Steud. from Africa. The former was domesticated from the wild progenitor Oryza rufipogon Griff. and the latter from the African wild rice species Oryza barthii A. Shiv. The first known center of rice cultivation in China generated the O. sativa subspecies japonica. The indica subspecies arose from the second center of domestication in the Ganges River plains of India. Variants of domesticated lines and the continuous hybridization between cultivated varieties and the wild progenitor(s) resulted in weedy rice types. Some weedy types resemble the wild ancestor, but the majority of weedy rices today bear close resemblance to cultivated rice. Weedy rice accompanies rice culture and has increased in occurrence with the global shift in rice establishment from transplanting to direct-seeded or dry-drill-seeded rice. Weedy rice (Oryza spp.) is the most difficult weed to control in rice, causing as much as 90% yield loss or abandonment of severely infested fields. The gene flow continuum between cultivar and weedy rice or wild relative, crop de-domestication, and regionalized adaptation have resulted in a myriad of weedy rice types. The complex lineage of weedy rice has resulted in confusion of weedy rice nomenclature. Two names are generally used for weedy rice—Oryza sativa L. and Oryza sativa f. spontanea. Genomic data show that O. sativa L. applies to weedy rice populations derived from cultivated O. sativa, whereas O. sativa f. spontanea applies only to weedy types that primarily descended from O. rufipogon. Neither of these names applies to African weedy rice, which is of African wild rice or O. glaberrima lineage. Therefore, unless the lineage of the weedy population in question is known, the proper name to use is the generalized name Oryza spp.

Weedy rice (Oryza sativa L.) is among the most problematic weeds in rice (Oryza sativa L.) production. The commercialization of herbicide-resistant (HR) rice nearly two decades ago provided an effective tool to manage weedy rice; however, resistance evolution and volunteer HR hybrid rice kept weedy rice at the forefront of rice weed control needs. This research aimed to assess the prevalence and severity of weedy rice infestations, identify production practices that may have contributed to an increase in weedy rice, and determine control strategies that may still be effective on weedy rice across Arkansas and adjacent U.S. Midsouth locales. Two questionnaires, one for rice growers and consultants and one for County Extension agents (CEAs), were distributed through email and physical copies in 2020. Thirty-three respondents returned the rice grower (25) and consultant (8) survey, representing 26 and 7 counties in Arkansas and the Missouri Bootheel area, respectively, as well as four parishes in northeast Louisiana. Eighteen respondents returned the CEA survey. Respondents ranked weedy rice the third most problematic weed in rice, behind Echinochloa spp. and Cyperus spp. The most common infestation levels reported in 78% of fields was less than 12 m^–2. Crop rotation (64% growers/consultants, 50% CEAs) and HR rice technology (27% growers/consultants, 50% CEAs) were the top two most-effective methods for weedy rice management, respectively. Tillage and crop rotation practices significantly influenced weedy rice infestation. Rice–soybean [Glycine max (L.) Merr.] rotation had the lowest weedy rice infestation compared with rice monoculture and other crop rotation practices. Crop rotation was not practiced on 26% of reported fields, primarily due to poor drainage. The imidazolinone (IMI)-resistant rice technology was still effective (>70% control) in 60% of fields, but quizalofop-resistant rice is needed to control IMI-resistant weedy rice. Overall, weedy rice remains a challenging weed in rice production.

Rice (Oryza sativa L.) is the primary staple crop in Taiwan, and it can be grown twice a year. The prevalent subspecies grown in Taiwan is Japonica, and a transplanting system is used for rice production. Although the transplanting system is known for efficient weed control at the seedling stage, weedy red rice (WRR, O. sativa f. spontanea) infestation is progressively being reported. Fieldwork and previous studies have suggested that WRR infestation in Taiwan is probably related to growers' operating practices and their perception of WRR. However, no data are available for a detailed investigation. The present study aimed to collect data on rice growers' backgrounds, farming practices, and perceptions of WRR to quantify and characterize the patterns of farming operations for rice growers in Taiwan and to investigate factors contributing to WRR infestation. We collected 408 questionnaires completed by rice growers from 17 counties covering all rice production regions in Taiwan. The growers' median age was 51 to 60 yr, and 75% of respondents had paddies from 0.25 to 2.75 ha in size, which corresponded with nationwide data for farmers' backgrounds. In general, growers applied similar farming practices for both cropping seasons. Most respondents did not notice WRR infestation or consider it to be a problem: only 9.8% noticed a moderate to severe infestation of WRR in their fields. The major perceived causes of WRR infestation was seed impurity (55.1%) or cultivar degeneration (18.6%). Correlation analysis and farming patterns estimated with a nonnegative matrix factorization algorithm showed that WRR contamination rate was due to the use of dry or wet tillage. The present study provides the first quantitative and qualitative evidence of rice production practices and growers' perceptions of WRR infestation in Taiwan.

Weedy rice (WR) (Oryza spp.) is the most troublesome weed infesting rice paddies in Brazil. Several changes have occurred in this region regarding crop management, especially WR control based on the Clearfield^® (CL) rice production system launched in 2003. This survey's objective was to evaluate the WR infestation status by assessing the producers' perception and the management practices used in southern Brazil after 18 yr of CL use in Brazil. Rice consultants and extension agents distributed a questionnaire to 213 producers in the Rio Grande do Sul (RS) and Santa Catarina (SC) states in the 2018 to 2019 growing season. In RS, most farms are larger than 150 ha, and farmers have adopted the CL system for more than 2 yr and use minimal or conventional tillage, permanent flooding, clomazone PRE tank-mixed with glyphosate at the rice spiking stage, and crop rotation with soybean [Glycine max (L.) Merr.] or pasture. In SC, rice farms are small, averaging from 20 to 30 ha, farmers predominantly plant pre-germinated rice and do not rotate rice with other crops, and roguing is practiced. Comparing both states, the CL system is used in 99.5% and 69.3% of the total surveyed rice areas in RS and SC, respectively. Imidazolinone-resistant WR is present in 68.4% and 26.6% of rice farms in RS and SC, respectively. Rice cultivation in Brazil is currently coexisting with WR with minimal integration of control methods. However, integrated practices can control this weed and are fundamental to the sustainability of systems based on herbicide-resistant rice cultivars.

Weedy rice (Oryza spp.) is one of the most troublesome weeds affecting rice (Oryza sativa L.) production in many countries. Weedy rice control is difficult in rice fields, because the weed and crop are phenotypically and morphologically similar. Weedy rice can be a source of genetic diversity for cultivated rice. Thus, this study aimed to characterize the morphological diversity of weedy rice in southern Brazil. Qualitative and quantitative traits of 249 accessions from eight rice-growing mesoregions in Rio Grande do Sul (RS) and Santa Catarina (SC) states were analyzed. For each accession, 24 morphological descriptors (14 qualitative and 10 quantitative) were evaluated. All 249 accessions from RS and SC are of indica lineage. Considering all the phenotypic traits evaluated, the accessions separated into 14 distinct groups. One of the largest groups consisted of plants that were predominantly tall with green leaves, intermediate shattering, and variable flowering time. Distinct subgroups exist within larger clusters, showing discernible phenotypic diversity within the main clusters. The variability in flowering time was high (77 to 110 d after emergence), indicating high potential for flowering synchrony with rice cultivars and, consequently, gene flow. This indicates the need to remove escapes when planting herbicide-resistant rice. Thus, weedy rice populations in southern Brazil are highly diverse, and this diversity could result in variable response to weed management.

A total of 452 rice farmers from three main granary areas of Muda Agricultural Development Authority (MADA), Kemubu Agricultural Development Authority (KADA), and Integrated Agricultural Development Area Barat Laut Selangor (IADA BLS) were surveyed in 2019. The goal was to determine farmers' knowledge of and management practices for weedy rice (Oryza spp.) as well as the adoption level of Clearfield^® rice technology (CRT) in Malaysia. Most farmers (74%) were adept at recognizing weedy rice. The majority of farmers (77%) perceived transplanting and water seeding rice systems as the best options to manage weedy rice, while only 10% of the farmers adopted CRT. The low level of adoption of this technology was due to several constraints, including the high cost of the CRT package and occurrence of imidazolinone (IMI)-resistant weedy rice in their farms. Farmers from MADA and IADA BLS reported the occurrence of IMI weedy rice in their farms for more than nine planting seasons, whereas those from KADA reported having resistant weedy rice for five to six planting seasons. The main factor contributing to the evolution of IMI-resistant weedy rice was ignorance about the technology and deliberate disregard of stewardship guidelines. The survey revealed that there is a need to increase awareness about CRT through training and educational programs for proper adoption of this technology.

Weedy rice (Oryza sativa L.) is a troublesome rice (Oryza sativa L.) weed in Italy and in many other rice areas. The objective of this study was to correlate the O. sativa infestation level in northern Italy, the main European rice-growing area, with agricultural practices adopted by farmers by using data obtained from a farmer survey. In 2018 to 2019, a survey was carried out on 98 rice farms chosen to ensure different sizes, different cultivation practices, and variable degrees of O. sativa infestation. The following information was acquired: farm size; area cultivated with Clearfield^® varieties; the most-adopted agronomic practices (type of tillage, crop rotation, type of sowing, water management, origin of seeds, adoption of stale seedbed, use of imazamox, presence of O. sativa resistant to imazamox); and level of O. sativa infestation: low (≤5 plants m^–2), medium (>5 to 20 plants m^–2), and high (>20 plants m^–2). The data were analyzed through descriptive statistics and ordinal logistic regression to determine which agronomic practices influenced the level of O. sativa infestation. Farm clustering was also determined through two-step cluster analysis. Rice was cultivated as a monocrop and mainly sown in water, using purchased seeds, in plowed fields. More than half of the farms used the stale seedbed practice, and 63% adopted Clearfield^® varieties, while about 45% of the farms reported imazamox-resistant O. sativa. The ordinal logistic regression underlined that use of a stale seedbed was correlated with the infestation level of O. sativa, and the two-step cluster analysis showed that the farms were mainly grouped based on the use of this technique. Most of the farms that used a stale seedbed had higher O. sativa infestation than those that did not use it, meaning that this practice was mainly applied in zones where O. sativa infestations were more serious.

The abundance of weedy rice (Oryza sativa L.) in the soil seedbank was estimated in 2011 in Italian rice (Oryza sativa L.) fields with different histories of imidazolinone-resistant Clearfield^® rice varieties (CL), non-Clearfield^® varieties (NCL), and planting methods. A model was used to predict weedy rice seedbank dynamics over time under different control strategies. Soil samples were taken from 50 rice fields cultivated with CL varieties consecutively for 0, 1, 2, or 3 yr, and weedy rice seedbank data were used in a model considering eight scenarios: (A) rice monoculture with CL and NCL varieties in alternate years; (B) 2 yr of CL, followed by 1 yr with NCL; (C) 3 yr of CL followed by 1 yr of NCL; (D) rice monoculture with only CL; (E) rice monoculture with only NCL; (F) 2 yr of CL followed by 1 yr of rotation with another crop (CR), and then by an additional year with NCL; (G) 1 yr of CR followed by 2 yr of CL, and then by 1 yr of NCL; (H) 2 yr of CR followed by 2 yr of CL. The weedy rice seedbank exceeded 1,000 seeds m^–2 in the surveyed fields with no significant differences between rice planting methods. Highest densities were found in fields cultivated for 1 yr with CL varieties. Simulations indicated that where CL varieties were used in rice monoculture, the susceptible weedy rice seedbank was gradually reduced, producing a depletion after 17, 13, 11, and 9 yr in scenarios A, B, C, and D, respectively. The weedy rice seedbank increased in NCL monoculture (E) and declined significantly in crop rotation scenarios (F, G, H). The simulations indicated that the introduction of crop rotation is crucial for obtaining a relatively fast reduction of weedy rice seedbank and delaying the evolution of herbicide-resistant populations.

Clearfield™ (CL) rice (Oryza sativa L.) is a weedy rice (Oryza spp.; synonym = red rice) control tool that has been used in Brazil since 2003. This system includes the use of an imidazolinone (IMI)-tolerant cultivar and the application of IMI herbicides. In this review article, Brazilian weed scientists evaluate the challenges and lessons learned over 18 yr of CL use. CL system benefits include selective weedy rice control, better crop establishment during the most advantageous period of the year, and more efficient fertilizer use. In Rio Grande do Sul state, the CL system, in conjunction with other improvements, has contributed to rice grain yield gains from 5,500 kg ha–¹ before 2002 to around 8,400 kg ha–¹ currently. In contrast, the main problem that has arisen over this period is the rapid evolution of IMI-resistant weedy rice, caused by gene flow from CL rice cultivars. The off-label use (rate and continuous use) of IMI herbicides has contributed to the evolution of resistance in Echinochloa spp. and other weeds. IMI herbicide carryover has also affected susceptible crops grown after CL rice. Crop rotation with soybean [Glycine max (L.) Merr.] is increasing, ensuring system sustainability. The importance of minimum tillage has also become apparent. Such cultivation includes applying nonselective herbicides before sowing or just before crop emergence (at the spiking stage to eliminate as much weedy rice as possible and other weeds at an early growth stage). It also includes the use of certified seeds free of weedy rice, following label instructions for IMI herbicides, applying the herbicide PRE followed by POST, and complementary weedy rice management practices, such as roguing of surviving weedy rice plants.

Weedy rice (Oryza sativa f. spontanea or O. sativa complex) has become a severe threat to Malaysian rice (Oryza sativa L.) granaries after the direct-seeding method of rice cultivation was introduced in the late 1980s. Since then, researchers have studied the biology and ecology of weedy rice and espoused the evolutionary theory of the origin of Malaysian weedy rice. This review paper aimed to synthesize the body of knowledge about weedy rice and the evolution of herbicide-resistant (HR) weedy rice in Malaysia. The imidazolinone (IMI) herbicide component of the Clearfield^® Production System (CPS) rice package is among the most effective tools for weedy rice control. However, dependence solely on this technology and farmers' ignorance about the appropriate use of IMI herbicides with the CPS rice package have resulted in the evolution of IMI-resistant (IMI-R) weedy rice. This has reduced the efficacy of IMI herbicides on weedy rice, ultimately nullifying the benefit of CPS rice in affected fields. At present, it is assumed that IMI-R weedy rice populations are widely distributed across the rice granaries in Malaysia. Therefore, it is important that integrated management measures be adopted comprehensively by Malaysian rice growers to curb the spread of IMI-R weedy rice problem in Malaysia, especially in fields planted with CPS rice. This review focuses on the biology of Malaysian weedy rice, the history of the establishment of weedy rice in Malaysian rice fields, the impact of HR rice technology on the evolution of IMI-R weedy rice in Malaysia, the distribution of resistant weedy rice populations across Peninsular Malaysia rice granaries, the weedy rice resistance mechanisms, and weedy rice management. The synthesis of all this information is helpful to researchers, policy makers, the private agricultural industry, advisers to farmers, and proactive farmers themselves with the goal of working toward sustainable rice production.

Farming elements other than the crop, when integrated into the system, are supplementary, with multifold uses that include weed and pest management. Elements such as fish and poultry birds are integrated with transplanted wetland rice (Oryza sativa L.) for ensuring farmers' livelihoods and the nutritional security and sustainability of the system. Integrated animal components such as poultry birds and fish also supplement the system with weed control. The role and efficacy of these animal components as tools for managing weedy rice (Oryza sativa L.) were explored, as weedy rice infestation is increasing. This threat of weedy rice is due to scarcity of water resulting from poor water management and improper field leveling. Grass carp (Ctenopharyngodon idella val.) produced the highest reduction of weedy rice biomass, 28% within 24 h under laboratory conditions. Polyculture of C. idella, mrigal (Cirrhinus mrigala Ham.), and silver carp (Hypophthalmichthys molitrix val.) reduced the biomass of weedy rice by 21% within 24 h. In laboratory studies, poultry manure at the highest concentration of 5% reduced the weedy rice seed germination 100% compared with rice seed germination at 91%. This conformed with microplot experiments in which poultry manure at 15.6 g d–¹ resulted in a weed control index (WCI) of 8% in both years. However, poultry manure at 15.6 g d–¹ in combination with herbicide application resulted in the highest control indices of weedy rice: 52% in 2017 and 2018. Integrating fish and poultry with PRE application of oxyfluorfen (0.25 kg ha–¹) resulted in the highest WCI and grain yield in field experiments.