The Conyza genus includes nearly 150 species, comprising closely related weedy species. Proper identification of Conyza spp. is essential to develop effective strategies for their management. The overlap of traits, species varieties, and the putative occurrence of hybridization hampers the identification of Conyza spp. and its management in agricultural and natural environments. Herein, we assessed five DNA barcodes and 32 morphological traits to classify Conyza spp. and survey their dispersion in soybean fields [ Glycine max (L.) Merr.] in Brazil in 2019, 2020, and 2021. The Conyza accessions included two species, hairy fleabane [ Conyza bonariensis (L.) Cronquist) and Sumatran fleabane [ Conyza sumatrensis (Retz.) E. Walker], and each species comprised two varieties. The ITS and rps16 - trnQ gene regions showed the ability to distinguish between the two Conyza species, while the matK , rbcL , and trnF - trnF gene regions were not polymorphic. Out of 32 morphological traits, phyllary color, involucre shape, capitulescence type, and inflorescence type were the most polymorphic and even reliable for taxonomic purposes. The combination of ITS or ITS + rps16 - trnQ regions and the four morphological markers was able to discriminate 91% of the plants, except those of C. bonariensis var. angustifolia . These results support the taxonomic resolution between C. bonariensis and C. sumatrensis and are useful for other Conyza spp. and other closely related weedy species worldwide. Conyza sumatrensis was detected in 94% of soybean fields across macroregions and seasons in Brazil, while C. bonariensis was sparsely dispersed, mainly in the southern macroregion (MRS 1).

Herbicide-resistant Conyza spp. are a threat to many crops. These widespread weeds are closely related species and often cooccur. To characterize the origins of their resistance and the mechanisms underlying their spread, we assessed the genomic variation in glyphosate-resistant Conyza spp. in Brazil. Twenty populations were sampled from soybean fields across four macroregions (MRSs). A genotyping-by-sequencing study resulted in 2,998 single-nucleotide polymorphisms (SNPs) obtained for C. bonariensis (L.) and the closely related C. sumatrensis (Retz) E. Walker. Higher genomic diversity (π) and heterozygosity (HO/HE) and lower inbreeding coefficient (FIS) values were detected in populations of Conyza spp. from MRS 1 (southern) than in those from other MRSs. Strong genomic structure clustered individuals into three groups (FST = 0.22; p value = 0.000) associated with the MRSs. Thus, resistance to glyphosate originated from independent selection in different MRSs across Brazil. Our dataset supports the occurrence of intraspecific gene flow in Brazil and identified individuals of C. bonariensis that did not group within species. These findings suggest that allelic introgressions within and among species have impacted the evolution and spread of resistance to glyphosate in Conyza spp. We discuss how to mitigate new resistance cases, particularly for the released stacked traits herbicide tolerance in soybeans.

ABSTRACT: Spermacoce latifolia, S. verticillata, and Richardia brasiliensis (family Rubiaceae, tribe Spermacoceae) are glyphosate-tolerant weeds in the soybean producing areas of Brazil. The weed shifts to glyphosate-tolerant weeds across soybean-producing areas has shown the need for adoption of practices that conserve the efficacy of glyphosate. This study evaluated the effect of single- and double-knockdown herbicide applications on the control of S. latifolia, S. verticillata, and R. brasiliensis prior to soybean sowing. Trials were designed as a randomized block and treatments were arranged as a factorial. Factor A was three systemic herbicide treatments 10 days before sowing (DBS), while Factor B was three contact herbicide treatments applied 0 DBS (“sow and apply”). The single- and double-knockdown applications were followed by post-emergence applications of glyphosate or 2,4-D + glyphosate when the crop reached three leaves. The efficacy of control of each weed species was visually evaluated 14 days after sowing (DAS), as well as 0 and 28 days after post-emergence application (DAA). The double-knockdown applications generally provided higher levels of control for the target weeds of the Rubiaceae family than the single-knockdown applications. Glyphosate + 2,4-D, glyphosate + 2,4-D + diclosulam or glyphosate + [halauxifen-methyl + diclosulam] followed by glufosinate or paraquat achieved at least 90% of control of S. latifolia and R. brasiliensis, but did not control S. verticillata (<80%). Glufosinate at 457 g a.i. ha-1 was equivalent to paraquat at 400 g a.i. ha-1 as a contact herbicide in the double-knockdown applications, especially on the first assessment dates.

The objective of this work was to evaluate the distribution of imidazolinone-resistant (IMI-R) red rice ( Oryza sativa ) populations, the frequency of alleles conferring resistance to IMI, and the adoption of agronomic practices applied to red rice control, across growing seasons and production regions of the state of Rio Grande do Sul (RS), Brazil. In the experiment, 1,008 red rice populations were screened for resistance to IMI, 760 IMI-R red rice plants were genotyped for the acetolactate synthase (ALS) alleles conferring resistance to IMI, and 40 'Clearfield' rice growers were surveyed. IMI-R red rice populations were widespread throughout RS since the 2006/2007 growing season, with a higher initial frequency in the Depressao Central and Fronteira Sul production regions. The occurrence of IMI-R red rice ranged from 1.6 to 3.5 years after 'Clearfield' rice release. Gly654Glu was the most frequent ALS mutation in IMI-R red rice populations, which shows a gene flow from the most used 'Clearfield' rice cultivars to the red rice plants. Crop rotation systems and certified seed were used by only 30% of the surveyed growers of 'Clearfield' rice, with lower percentages in the production regions where IMI-R red rice appeared faster.

The objective of this work was to evaluate the distribution of imidazolinone-resistant (IMI-R) red rice (Oryza sativa) populations. the frequency of alleles conferring resistance to IMI, and the adoption of agronomic practices applied to red rice control, across growing seasons and production regions of the state of Rio Grande do SW (RS), Brazil. In the experiment, 1,008 red rice populations were screened for resistance to IMI, 760 IMI-R red rice plants were genotyped for the acetolactate synthase (ALS) alleles conferring resistance to IMI, and 40 'Clearfield rice growers were surveyed. IMI-R red rice populations were widespread throughout RS since the 2006/2007 growing season, with a higher initial frequency in the Depressao Central and Fronteira Sul production regions. The occurrence of IMI-R red rice ranged from 1.6 to 3.5 years after 'Clearfield' rice release. Gly654Glu was the most frequent ALS mutation in IMI-R red rice populations, which shows a gene flow from the most used 'Clearfield' rice cultivars to the red rice plants. Crop rotation systems and certified seed were used by only 30% of the surveyed growers of 'Clearfield' rice, with lower percentages in the production regions where IMI-R red rice appeared faster.

Glyphosate-resistant (GR) and glyphosate-tolerant weeds cause considerable yield losses and represent a growing threat to soybean production systems. Despite the relevance of this topic, few studies have evaluated the dispersal of these species in Brazil. The objective of this study was to evaluate the dispersal and frequency of known GR and glyphosate-tolerant weeds in soybean-producing microregions. A total of 2,481 interviews were conducted in different regions of Brazil. The interviews were stratified among 20 edaphoclimatic microregions (ECRs) to cover all of the country's soybean-producing regions. A minimum number of interviews was estimated to generate a margin of error of <= 10% within the ECRs and <= 5% in the country. The values of the farmers' responses were extrapolated to the total soybean production area of each ECR and the country as a whole, and the absolute values of each response were normalized as percentage values. The dispersal and management data demonstrate a loss of efficiency of glyphosate-resistance technology. Species that are naturally tolerant to glyphosate such as goosegrass, Commelina spp., and Ipomoea spp. had a greater presence in the ECRs, as did the resistant biotypes, particularly Conyza spp. and sourgrass, due to the large area cultivated with GR soybean, where glyphosate has been used with high frequency.

Six trials were conducted during 2014/15 and 2015/16 growing seasons in Brazil to determine the effect of 2,4-D formulations and spray nozzles on 2,4-D spray drift under conventional field conditions. An experimental 2,4-D choline formulation with Colex-D ® Technology (GF-3073) and a 2,4-D dimethylamine (DMA) formulation were applied with either XR and AIXR flat-fan spray nozzles. Each plot was 30 m wide by 24 m long (720 m 2 ) with 60 glyphosate-resistant soybean rows spaced 50 cm apart and also 35 potted tomato plants distributed on a grid across the plot 5-m apart. Applications were performed one meter away from the plot edge perpendicular to the soybean rows when wind direction was parallel to the rows with less than 30 degrees of angle deviation. Spray drift treatments were applied in 100 L ha −1 with tractor sprayers at 276 kPa equipped with a 7-m wide boom at 50 cm above the canopy of the soybean plant, operating at 6.8 km h −1 . The distance from the plot edge to the farthest plant with 2,4-D symptoms was assessed for every four soybean rows at 10 and 20 days after treatment (DAT) and potted tomatoes at 10 DAT. GF-3073 reduced the distance of the farthest injured plant with 2,4-D symptoms compared to the 2,4-D DMA formulation regardless of the spray nozzle, assessment date and sensitive species. GF-3073 applied through the AIXR nozzle reduced the relative drift affected area to the standard by 68% at 10 DAT and 67% at 20 DAT for soybean and 60% at 10 DAT for potted tomatoes.

ABSTRACT - Grain sorghum (Sorghum bicolor) is one cereal crop that faces huge problems with weed interference mostly because the lack of selective herbicides. This study aimed to assess the efficacy and safety of herbicide alternatives for weed control in grain sorghum as well as the selectivity of atrazine + s-metolachlor to different hybrids. Three field trials were designed as a randomized complete block with four replications. All experiments were conducted in Jardinópolis-SP and Mogi Mirim-SP during the 2015/16 growing season. Two trials included acetochlor, flumioxazin, fluroxypyr, mesotrione and s-metolachlor, applied in pre or post-emergence, in association or not with atrazine. A third trial was carried out with rates of the premix containing atrazine + s-metolachlor applied to the following hybrids: 1G100, 1G220, 1G230, 1G244, 1G282, 50A10, 50A40, 50A50 and 50A70. The pre‑emergence herbicides that exhibited satisfactory efficacy of weed control and selectivity to sorghum crop were flumioxazin, atrazine + mesotrione and atrazine + s-metolachlor. For post‑emergence, atrazine, atrazine + acetochlor, atrazine + s-metolachlor and atrazine + fluroxypyr were the best treatments for both efficacy and selectivity. The application of atrazine + s-metolachlor at the evaluated rates was considered selective to the nine hybrids assessed.Keywords: acetochlor, fluroxypyr, mesotrione, herbicide tolerance, weed control. MANEJO QUÍMICO DE PLANTAS DANINHAS EM SORGO GRANÍFERO E SELETIVIDADE DE ATRAZINE + S-METOLACHLOR PARA DIFERENTES HÍBRIDOS RESUMO – O sorgo granífero (Sorghum bicolor) é um dos cereais de verão que mais enfrenta problemas com plantas daninhas em razão da interferência destas espécies e carência de herbicidas para controlá-las. O objetivo deste estudo foi avaliar a eficácia e segurança de herbicidas alternativos no controle de plantas daninhas em sorgo granífero, assim como a seletividade de atrazine + s-metolachlor para diferentes híbridos. Três experimentos foram realizados em campo com delineamento de blocos ao acaso e quatro repetições, sendo conduzidos em Jardinópolis-SP e/ou Mogi Mirim-SP, ao longo da safra 2015/16. Em dois experimentos, acetochlor, flumioxazin, fluroxypyr, mesotrione e s-metolachlor foram avaliados em pré e/ou pós-emergência da cultura, em associação ou não (isolados) com atrazine. O terceiro experimento foi realizado com doses crescentes de atrazine + s-metolachlor e os híbridos de sorgo granífero 1G100, 1G220, 1G230, 1G244, 1G282, 50A10, 50A40, 50A50 e 50A70. Os tratamentos com controle satisfatório de plantas daninhas e seletividade à cultura, em pré-emergência, foram flumioxazin, atrazine + mesotrione e atrazine + s-metolachlor. Em pós-emergência, eles foram atrazine, atrazine + acetochlor, atrazine + s-metolachlor e atrazine + fluroxipyr. A aplicação de atrazine + s-metolachlor nas doses testadas foi seletiva para os nove híbridos avaliados.Palavras-chave: acetochlor, fluroxypyr, mesotrione, tolerância a herbicidas, controle de plantas daninhas. metolachlor at the evaluated rates was selective to the nine hybrids assessed.

ABSTRACT: The DAS-444Ø6-6 soybean expresses AAD-12, 2mEPSPS and PAT enzymes to provide, respectively, tolerance to 2,4-D, glyphosate and ammonium glufosinate. This event is also associated with DAS-81419-2 expressing Cry1Ac and Cry1F proteins to provide protection against a broad spectrum of soybean lepidopteran insect species. Successful adoption by growers of these technologies relies on the traits performing in the diverse environments of Brazil, especially under adverse weather conditions. The objective of this study was to characterize the tolerance of DAS 444Ø6-6 and DAS-444Ø6-6 x DAS-81419-2 soybeans to 2,4-D and glyphosate in the Cerrado region of Brazil. Eight trials across four sites were conducted in the 2015/2016 summer rainy season using a randomized complete block design, with a factorial arrangement of treatments. Factor A was 2,4-D choline salt (456 g a.e. L-1) + glyphosate DMA salt (480 g a.e. L-1) at 975 + 1,025 and 1,950 + 2,050 g a.e. ha-1, respectively and ready-mix of (2,4-D choline salt 195 g a.e. L-1 + glyphosate DMA salt 205 g a.e. L-1) at 2,000 (975 + 1,025) and 4,000 (1,950 + 2,050) g a.e. ha-1. Factor B was application timing at V3, V6 and R2 soybean growth stages. No visible symptoms of leaf chlorosis, epinasty, or growth inhibition were observed. Slight leaf droop was observed at 1 day after treatment (DAT), but it was null at 7 DAT. Less than 6% and 13% of leaf necrosis or injury were observed at 7 DAT, respectively, at 975 + 1,025 and 1,950 + 2,050 g a.e. ha-1 of 2,4-D choline salt + glyphosate DMA salt, regardless of applied as a ready-mix or not, but it was always 2% or less at 28 DAT. Crop grain yield was not influenced by herbicide treatments, application timing and their interaction, regardless of the soybean event and experimental site. DAS-444Ø6-6 and DAS-444Ø6-6 x DAS-81419-2 soybeans showed high tolerance to 2,4-D choline salt and glyphosate DMA in the Cerrado region of Brazil.

O controle da soja voluntária é primordial, principalmente pela presença de inóculos da ferrugem asiática nestas plantas. Com a introdução da soja tolerante ao 2,4-D (DAS-444Ø6-6 - Enlist E3TM), estudos são necessários para confirmar seu controle químico. O objetivo deste trabalho foi avaliar a eficácia no controle da soja voluntária DAS-444Ø6-6, utilizando herbicidas alternativos ao 2,4-D. O experimento foi realizado em condições de campo, no município de Cascavel, utilizando o delineamento em blocos ao acaso, com três repetições. Os tratamentos avaliados foram: halauxifen-methyl + pyroxsulam (5+18 g i.a. ha-1), halauxifen-methyl + diclosulam (5 + 25 g i.a. ha-1) e halauxifen-methyl + 2,4-D (5 + 783 g i.a. ha-1), o tratamento padrão paraquat (400 g i.a. ha-1) e uma testemunha sem aplicação de herbicidas. As aplicações foram realizadas em pós emergência da soja no estádio fenológico R2. Os resultados mostraram que aos 21 dias após aplicação todos os tratamentos apresentaram controle acima de 90% no controle da soja voluntária. As novas misturas formuladas contendo o herbicida halauxifen-methyl, foram eficientes no controle da soja voluntária DAS-444Ø6-6, em estádio fenológico avançado (R2).

satisfied with the control achieved with this herbicide. The objectives were to evaluate in regional scale the susceptibility of Echinochloa populations to cyhalofop-butyl, and quantify the effect of the weed phenology on its efficacy of control. For this, three trials were carried out under greenhouse conditions with a fully random design, using Echinochloa populations collected in rice fields in the southern region of Brazil. In two trials, the susceptibility level of 156 (2012/13 growth season) and 103 (2013/14 growth season) populations were evaluated with the application of cyhalofop-butyl at 360g ha-1. In other trial, in which treatments were arranged in a bi-factorial design (A = 6 x B = 5), it was evaluated six cyhalofop-butyl rates and five phenological stages of E. crus-galli populations. Echinochloa populations had showed differential susceptibility to cyhalofop-butyl, especially in the 2013/14 growth season, where 20 out of the 103 populations had control lower than 90%. The efficacy of this herbicide was inversely proportional to the phenological stage, and the application timing delay contributed directly to the decrease of susceptibility to the herbicide. Cyhalofop-butyl is an effective alternative to control imidazolinone-resistant Echinochloa populations, as long as the application timing occurs in the early phenological stages (2 to 4 leaves).

ABSTRACT: Cyhalofop-butyl stands out among the herbicides in the control of imidazolinone-resistant Echinochloa species; but, rice farmers are not always satisfied with the control achieved with this herbicide. The objectives were to evaluate in regional scale the susceptibility of Echinochloa populations to cyhalofop-butyl, and quantify the effect of the weed phenology on its efficacy of control. For this, three trials were carried out under greenhouse conditions with a fully random design, using Echinochloa populations collected in rice fields in the southern region of Brazil. In two trials, the susceptibility level of 156 (2012/13 growth season) and 103 (2013/14 growth season) populations were evaluated with the application of cyhalofop-butyl at 360g ha-1. In other trial, in which treatments were arranged in a bi-factorial design (A = 6 x B = 5), it was evaluated six cyhalofop-butyl rates and five phenological stages of E. crus-galli populations. Echinochloa populations had showed differential susceptibility to cyhalofop-butyl, especially in the 2013/14 growth season, where 20 out of the 103 populations had control lower than 90%. The efficacy of this herbicide was inversely proportional to the phenological stage, and the application timing delay contributed directly to the decrease of susceptibility to the herbicide. Cyhalofop-butyl is an effective alternative to control imidazolinone-resistant Echinochloa populations, as long as the application timing occurs in the early phenological stages (2 to 4 leaves).

O objetivo deste trabalho foi avaliar o desempenho do metodo de graus-dia, usando series historicas de temperatura media diaria do ar (Tm) e dados de Tm do ano da safra, para estimar a data de ocorrencia dos principais estadios de desenvolvimento da planta de arroz.

Several studies have expressed concerns about the effects of gene flow from transgenic herbicide-resistant crops to their wild relatives, but no major problems have been observed. This review describes a case study in which what has been feared in transgenics regarding gene flow has actually changed biodiversity and people's lives. Nontransgenic imidazolinone-resistant rice (IMI-rice) cultivars increased the rice grain yield by 50% in southern Brazil. This increase was beneficial for life quality of the farmers and also improved the regional economy. However, weedy rice resistant to imidazolinone herbicides started to evolve three years after the first use of IMI-rice cultivars. Population genetic studies indicate that the herbicide-resistant weedy rice was mainly originated from gene flow from resistant cultivars and distributed by seed migration. The problems related with herbicide-resistant weedy rice increased the production costs of rice that forced farmers to sell or rent their land. Gene flow from cultivated rice to weedy rice has proven to be a large agricultural, economic, and social constraint in the use of herbicide-resistant technologies in rice. This problem must be taken into account for the development of new transgenic or nontransgenic rice technologies.

The sorghum crop in Brazil has expanded substantially.Among the factors that interfere in sorghum yield is the interference imposed by the presence of weeds.The objective of this study was to assess the potential of different herbicide treatments applied in pre-emergence or post-emergence of sorghum in terms of selectivity and weed control.Two experiments were conducted, one for each application modality: experiment 1: pre-emergence; experiment 2: post-emergence.The experimental design was a randomized block design with four replications, in split plots.For experiment 1, the pre-emergence herbicides applied constituted the plots, and the different sorghum hybrids (50A10, 50A40, 50A50, 1G100, 1G233, SS318) constituted the subplots.For experiment 2, the post-emergence herbicides applied constituted the plots, and the different sorghum hybrids (50A10, 50A40, 50A50, 1G100, 1G233, SS318) constituted the subplots.Based on the results obtained, and on the discussion done, it is possible to conclude that herbicides and their respectively doses that had potential for use in sorghum crop in pre-emergence were: atrazine (1000 and 2000), mesotrione (100), tembotrione (75), atrazine + mesotrione (1000+100 and 2000+100) and atrazine + trifluralin (1000+1000 and 2000+1000).Meanwhile in post-emergence the best options were: atrazine (1000 and 2000), mesotrione (50 and 100), bentazon (720), fluroxypyr (100), mesotrione + atrazine (50+1000) and mesotrione + fluroxypyr (50+100).All of those treatments provided less than 25% of plant injury which means less potential to reduce the sorghum grain yield.

ABSTRACTHerbicides mixtures are used in many situations without the adequate knowledge related with the effect on major target weeds. The objective of this study was to evaluate the effects of different herbicides mixtures used in irrigated rice in order to establish the adequate combinations for the prevention and management of herbicide resistance in barnyardgrass (Echinochloa crus-galli). Three experiments were performed at field conditions with all major post-emergent herbicides used in irrigated rice in Brazil. The first experiment was performed with barnyardgrass resistant to imidazolinone herbicides and herbicides applied at label rates. The second and third experiments were performed with barnyardgrass resistant and susceptible to imidazolinone herbicides applied at doses of 50 or 75% of the label rates. The occurrence of additive, synergistic and antagonistic effects was identified at 18, 18 and 64%, respectively, among the total of 50 different associations of herbicide and rates evaluated. In general, the mixture of ACCase inhibitors with ALS inhibitors, quinclorac, clomazone + propanil or thiobencarb resulted in antagonism. Sinergic mixtures were found in clomazone with propanil + thiobencarb, profoxydim with cyhalofop-butyl or clomazone, and quinclorac with imazapyr + imazapic, bispyribac-sodium or cyhalofop-butyl. The mixtures of quinclorac with profoxydim were antagonic. Rice grain yield varied according to the efficiency of weed control. Seveveral mixtures were effective for imidazolinone resistant barnyardgrass control.

SUMMARYThe lack of control of barnyardgrass in flooded rice cultivated with imidazolinone-resistant rice cultivars is challenging the utilization of this system, which is continuously expanding for new rice areas worldwide. The objectives of the present study were to evaluate the frequency, distribution and mechanisms of imidazolinone resistance in barnyardgrass to establish the best practices to control and prevent this problem. The distribution of resistance was evaluated in 624 populations collected in Southern Brazil. The frequency of imidazolinone-herbicide resistance was 0·81, broadly distributed in all sampled regions. Resistance to quinclorac was also found in 0·19 of the populations, but all of the evaluated populations were susceptible to cyhalofop-butyl. Further studies were conducted in six populations. The enhanced metabolism was assessed with the metabolic inhibitors that reversed the resistance to quinclorac from 0·54 to 1·00 in two populations and the resistance to imazethapyr from 0·15 to 0·41 in three populations. The acetolactase synthase (ALS) enzyme activity also indicated the occurrence of altered target site resistance in two populations caused by the ALS gene mutations Trp574Leu and Ser653Asn, which is a novel finding in this species. The herbicide resistance in barnyardgrass in Southern Brazil presented a complex basis of resistance because it is associated with resistance to multiple herbicides due to multiple mechanisms and with multiple mutations of the ALS gene. This indicates that it is necessary to adopt specific measures to prevent and control the evolution of multiple herbicide resistance in this species.

O objetivo deste trabalho foi avaliar os efeitos da utilização de herbicidas residuais, aplicados em pré-emergência do feijoeiro, sobre a redução da infestação de plantas daninhas e o desempenho agronômico da cultura. Foram realizados dois experimentos a campo, sendo que um tinha o cultivar UFT -06 (Grupo Carioca, ciclo curto e porte ereto) e o outro o cultivar IPR Graúna (Grupo Preto, ciclo longo, hábito prostrado). Os tratamentos foram constituídos da aplicação dos herbicidas pendimethalin (1,2 e 1,4 kg ha-1) e trifluralin (1,2 e 1,4 kg ha-1); e, dimethenamid (1,4 e 1,6 kg ha-1) e S-metolachlor (1,4 e 1,6 kg ha-1), e mais testemunha com e sem (TSC) controle. Nas áreas experimentais predominavam infestação de Urochloa plantaginea (papuã). Avaliou-se a densidade e a massa desta espécie daninha, e o número de legumes e a produtividade do feijoeiro. Trifluralin foi o herbicida que mais reduziu a infestação da planta daninha e aumentou o rendimento do feijoeiro, quando comparado aos demais produtos. O cv. de feijão UTF-06 foi mais competitivo com U. plantaginea do que IPR Graúna. Os tratamentos com herbicida reduziram a infestação da infestante, mas isso não resultou em maior produtividade do feijoeiro, quando comparado com o tratamento capinado.

Roguing is a practice used to reduce the seed source of red rice escapes to control in Clearfield-rice areas. However, there is great difficulty in performing it in large and heavily infested rice fields. This objective of this work was to evaluate the effects of the use of imazamox herbicide, applied in different rates and times, on plants of Clearfield-rice and red rice. Four experiments were conducted during the 2007/08 and 2008/09 growing seasons, in completely randomized block design and treatments arranged in factorial design, using three replications per treatment. The treatments had increasing rates of imazamox, application times and rice cultivars. The rice cultivars tested were IRGA 417, IRGA 422 CL, Avaxi CL and Puita INTA CL. The variables evaluated were the number of panicles m(-2), number of grains panicle(-1), spikelet sterility in rice and red rice; and, rice grain yield and its components. The imazamox reduced the seed production of red rice escapes in a simulated situation of commercial Clearfield-rice area. The greater percentage reductions were obtained when this herbicide was applied at final formation of the panicle or panicle exertion of the red rice plant escapes to control. The Puita INTA CL cultivar has high level of resistance to imazamox, independent of rate and application times tested, becoming the only alternative to the use of this practice.