Field and greenhouse bioassay experiments were conducted to evaluate the effects of mesotrione soil residues on injury and yield of soybean, green bean, pea, cucumber, sugar beet and lettuce. There was a significant difference of mesotrione carryover between studies which can be explained by differences in soil pH and moisture. The conventional and the simulated field residue carryover studies successfully measured mesotrione persistence and rotational crop sensitivity. The simulated residue carryover study provided a more rigorous test of rotational crop sensitivity to mesotrione residues than the conventional residue carryover study, especially at higher doses for the more sensitive crops. The greenhouse bioassay was a simple and sensitive tool in detecting small amounts of herbicides present in the soil. Dose-response curves developed for sugar beet and green bean indicate similarities between results from the greenhouse and from the field mesotrione residue carryover study. Under similar environmental and soil conditions to those observed in these studies and using similar dose applications, sugar beet, green bean and cucumber injury and yield reductions are likely when these crops are grown in soils containing mesotrione residues.

Weed management programs in corn typically include herbicides applied both preemergence (PRE) and postemergence (POST) for season-long weed control. Mesotrione is a new triketone herbicide registered for PRE and POST control of broadleaf weeds in corn. Triketone herbicides function through inhibition of the enzyme p-hydroxyphenylpyruvate dioxygenase. Mesotrione applied PRE did not adequately control common lambsquarters (Chenopodium album L.), smooth pigweed (Amaranthus hybridus L.), common ragweed (Ambrosia artemisiifolia L.), or morningglory species (Ipomoea spp.) in conventional tillage corn, but control of these weeds was generally improved in no-till corn. Mesotrione combinations with acetochlor did not always improve control of broadleaf weeds, but increased control of smooth pigweed and giant foxtail (Setaria faberi Herrm.). POST applications of mesotrione at 105 g ai/ha controlled most annual broadleaf weeds except common ragweed, but did not control giant foxtail. The addition of atrazine at 280 g ai/ha to mesotrione, however, improved control of common ragweed. Tank-mixtures of glyphosate, imazethapyr, or imazethapyr plus imazapyr with mesotrione improved control of giant foxtail in herbicide-resistant corn. Corn injury was usually low from PRE and POST mesotrione applications in non-genetically modified corn, however, greater injury occurred in glyphosate-resistant varieties. Corn treated with mesotrione combinations generally yielded similar to corn treated with commercial standards. Mesotrione applied POST also suppressed the perennial weeds horsenettle (Solanum carolinense L.) and Canada thistle [Cirsium arvense (L.) Scop.]. Additions of atrazine increased the rate of plant tissue necrosis on these perennial weeds as compared to the slower bleaching symptoms associated with mesotrione applied alone. In general, Canada thistle plants were more susceptible to mesotrione in the rosette stage of growth than when bolting. Absorption, translocation, and metabolism of 14C mesotrione in Canada thistle was generally low. However, higher levels of absorption and translocation and lower root metabolism of mesotrione in rosette stage plants compared to bolting stage plants may explain why Canada thistle is more susceptible to mesotrione in the rosette stage of growth. The changes in symptomology and increased control from mesotrione plus atrazine tank-mixtures is likely due to the interrelationship between the modes of action of atrazine and mesotrione. / Ph. D.

Mesotrione is the first triketone herbicide registered for use in turfgrass. Triketones prevent carotenoid biosynthesis by inhibiting the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD). Although mesotrione controls many species of grass and broadleaf weeds, it is best know for selective control of perennial grasses like creeping bentgrass (Agrostis stolonifera L.). Field trials conducted at Virginia Tech and Blacksburg Country Club determined that several programs that integrate herbicide treatment and turf seeding effectively transitioned creeping bentgrass contaminated golf roughs back to a tall fescue [Schedonorus phoenix (Scop.) Holub] monoculture. However, mature weeds require multiple mesotrione applications for effective control. This requirement is a major limitation to mesotrione's competitiveness in turfgrass markets. Several greenhouse and laboratory studies were conducted to evaluate scenarios where mesotrione rates were titrated and applied daily to mimic ascending, descending, and intervallic time-release patterns. These patterns were applied following an initial treatment to foliage or soil to mimic a potential sprayable or granular time-release formulation. These scenarios effectively controlled five targeted weed species equivalent to the standard of two broadcast sprays, regardless of initial application placement or release pattern. However, both time-release treatments and the standard injured tall fescue based on leaf counts, plant weights, and visual phytotoxicity ratings. Additional growth chamber studies found that changes in relative humidity from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity with a concomitant decrease in photochemical efficiency when mesotrione was applied to foliage of smooth crabgrass (Digitaria ischaemum (Schreb.) Schreb. ex Muhl.). Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil, but in older leaves when applied only to foliage. Laboratory studies were conducted to evaluate interspecific differences in 14C mesotrione absorption and translocation between two plant species when applied to foliage or roots. Annual bluegrass (Poa annua L.) absorbed 2- to 4-fold more radioactivity than Kentucky bluegrass (Poa pratensis L.). Both species absorbed less radioactivity through roots than through foliage and root absorbed radioactivity was more often exuded into Hoagland's solution while foliar absorbed radioactivity was often found in other foliage. / Ph. D.

Mesotrione is under evaluation for registration in turfgrass for weed control, but often requires repeat treatments. Previous research in agricultural crops indicates tank mixtures with mesotrione may improve weed control. Three field trials were conducted in 2005 and 2006 in Blacksburg, VA on smooth crabgrass in perennial ryegrass and tall fescue. Data indicate mesotrione applied in combination with bentazon, bromoxynil, or carfentrazone, controlled smooth crabgrass better than any of these herbicides applied alone at all sites. Adding mesotrione to MSMA and quinclorac improved smooth crabgrass on of three sites. Sequential mesotrione applications improved long term weed control. / Master of Science

Doctor of Philosophy / Department of Agronomy / Johanna A. Dille / This dissertation has two complementary components: educational, in a survey of students in Weed Science and their perception of the Kansas State University Agronomy Learning farm, and research, regarding interactions between mesotrione and sulfonylurea herbicides. The Learning Farm serves as a resource where undergraduate students at KSU can develop agronomic skills through hands-on field site experiences and investigations. Students’ perceptions of experiential learning activities in the development of problem-solving and critical thinking skills were studied as a result of the Learning Farm. Activities included: undergraduate students in Weed Science (AGRON 330) developing a weed management recommendation, and Undergraduate Research Assistants (URAs) conducting weed science research projects at the Learning Farm. Students stated that experiential learning activities increased their critical thinking skills, required effective time management, and presented concepts that could be used in other situations. Pre- and post-project evaluation questionnaires showed that URAs had an increased interest in agronomy, weed science, and research following the completion of their project. For the research project, field and greenhouse studies were conducted from 2003 to 2006 to evaluate the efficacy of various sulfonylurea herbicides when applied with mesotrione or mesotrione + atrazine. Research demonstrated that the addition of mesotrione to sulfonylurea herbicides decreased efficacy of sulfonylurea herbicides on green foxtail, yellow foxtail, and shattercane. The addition of atrazine to the tank mix, or increased mesotrione rates, resulted in additional decrease in sulfonylurea herbicide efficacy on shattercane and foxtail species. Additional studies were performed to determine if absorption, translocation, or metabolism was the basis for the reduction in sulfonylurea herbicide efficacy when mixed with mesotrione or mesotrione + atrazine. Results indicated that the cause of antagonistic interaction between mesotrione and sulfonylurea herbicides in green and yellow foxtail was reduced absorption and translocation of the sulfonylurea herbicides. Producers who choose to apply mesotrione and sulfonylurea herbicides to corn should apply the herbicides sequentially to achieve maximum control of weedy grass species.

Made available in DSpace on 2017-07-21T19:59:48Z (GMT). No. of bitstreams: 1 Karin Cristina.pdf: 1779954 bytes, checksum: fa7540fc1601cdb177d071d632782f2e (MD5) Previous issue date: 2009-10-05 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Chemical fertilizers and pesticides have become an important part of modern agriculture. The use of these chemical brought great benefits but also created new problems, such as disposal after use, persistence over time and accumulation in the environment, resulting significant bad consequences to public health and adverse impact on ecosystems. Maize culture deserves attention because the large amount of herbicides used. Mesotrione is a new selective herbicide developed for use in maize culture. Due it has not been used for a long time, there is little information available on the ecotoxicological risk. Only two mesotrione-degrading strains, from soil and water, both the genus Bacillus, were isolated and identified. The aim of this study was to isolate and characterize endophytic and epiphytic bacterial strains collected from maize leaves, able to biodegrade the herbicide mesotrione. Samples were collected in maize planting variety Penta without treatment with mesotrione and maize planting variety DKB 234 treated with mesotrione, at Fazenda Escola Capão da Onça, Ponta Grossa, PR. The microorganisms were isolated and cultured in mineral medium supplemented with mesotrione as sole carbon source. Of the twenty samples were isolated 347 tolerant and/or mesotrione-degrading strains. These were selected in increasing concentrations of the herbicide and had their degradation capability evalueted by VIS spectrophotometry. Ten strains were able to grow in the presence of high concentrations of mesotrione. Of these, 9 strains (7 epiphytic and 2 endophytic) were isolated from maize variety Penta without treatment with mesotrione and only 1 epiphytic strain was isolated from maize variety DKB 234 treated with mesotrione. Five strains had the ability to degrade the herbicide confirmed. One strain was identified as Acinetobacter baumanii and other as Micrococcus luteus. Just M. luteus is an endophytic strain, the others are maize epiphytics. Four mesotrione-degrading strains were isolated from maize untreated with mesotrione, only one strain of Gram-negative bacilli non fermenter was isolated from maize treated with the herbicide. The growth in high concentration of the strains that didn’t have the mesotrione degradation capability detected by spectrophotometry indicates the possibility of these strains degrade this herbicide when in association with other microorganisms consortia in the environment. The real quantitative capability of mesotrione degradation by the strains recorded by spectrophotometry may be underestimated due to the accumulation of degradation products which have the same wavelength as the mesotrione. This study reports the first isolation and characterization of mesotrione degrading strains endophytic and epiphytic from maize. / Os adubos químicos e pesticidas tornaram-se parte integral da agricultura moderna. O emprego destas substâncias químicas trouxe grandes vantagens, mas também criou novos problemas, como a eliminação após o uso, a persistência ao longo do tempo e o acúmulo no ambiente, acarretando conseqüências adversas consideráveis à saúde pública e um impacto negativo nos ecossistemas. A cultura do milho merece atenção pela elevada quantidade de herbicidas usados. O mesotrione é um novo herbicida seletivo utilizado na cultura do milho. Devido sua introdução recente, há pouca informação referente ao risco ecotoxicológico do mesotrione e de seus produtos de degradação. Apenas duas linhagens bacterianas do gênero Bacillus, degradadoras do mesotrione, provenientes do solo e da água, já foram isoladas e identificadas. O objetivo deste estudo foi isolar e caracterizar linhagens bacterianas endofíticas e epifíticas, isoladas das folhas de milho, com capacidade de biodegradar o herbicida mesotrione. As coletas foram realizadas em plantação de milho variedade Penta sem tratamento com o herbicida mesotrione e em plantação de milho variedade DKB 234 com tratamento com o herbicida, na Fazenda Escola Capão da Onça, Ponta Grossa, PR. Os microrganismos foram isolados e cultivados em meio mineral com mesotrione como única fonte de carbono. Das vinte coletas foram isoladas 347 linhagens bacterianas tolerantes e/ou degradadoras. Estas foram selecionadas em concentrações crescentes do herbicida e tiveram sua capacidade de degradação investigada por espectrofotometria visível. Dez linhagens capazes de crescer na presença de altas concentrações do herbicida foram avaliadas. Destas, 9 linhagens (7 epifíticas e 2 endofíticas) foram isoladas de milho variedade Penta sem tratamento com o herbicida mesotrione e 1 foi isolada de milho variedade DKB 234 com tratamento com mesotrione. Cinco linhagens tiveram a capacidade de degradar o herbicida comprovada. Uma linhagem foi identificada como Acinetobacter baumannii e outra como Micrococcus luteus. Somente Micrococcus luteus é uma linhagem endofítica, sendo as demais epifíticas do milho. Quatro linhagens degradadoras foram isoladas de plantação de milho sem tratamento prévio com mesotrione; apenas uma linhagem de bacilo Gram-negativo não fermentador foi isolada de plantação sob tratamento com o herbicida. O crescimento, em alta concentração, das linhagens que não tiveram a capacidade de degradação do mesotrione detectada pela espectrofotometria indica a possibilidade destas linhagens participarem da degradação deste herbicida quando associadas com outros microrganismos em consórcios no ambiente. A capacidade quantitativa real de degradação do mesotrione pelas linhagens investigadas registrada pela espectrofotometria pode estar subestimada devido ao acúmulo dos produtos de degradação que possuem o mesmo comprimento de onda no espectro de absorção que a molécula de mesotrione. Este trabalho é o primeiro relato de isolamento de linhagens bacterianas degradadoras do herbicida mesotrione epifíticas e endofíticas de milho.

Made available in DSpace on 2017-07-21T19:59:47Z (GMT). No. of bitstreams: 1 Stela Becher.pdf: 1500518 bytes, checksum: a9d6338bf319b6f8a5407fc7f6e00bdb (MD5) Previous issue date: 2009-09-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Mesotrione, 2-(4-Mesyl-2-nitrobenzoyl)-1,3-cyclohexanedione, is a trycetone herbicide chemical class, with high leaching potential, and competitively inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD), that converts tyrosine to plastoquinone and α-tocopherol, and was found in plants, fungus, and bacteria, including Acinetobacter. The aim of this work was to characterize pathways of Brazilian strains of Acinetobacter facing mesotrione. Acinetobacter calcoaceticus and Acinetobacter sp. were selected in up to 15.00 mM (resistance) and 0.03 mM (tolerance) concentrations of mesotrione and tested at 35 mM buffered solution in spectrophotometry until 60 hours, showing to be incapable to degrade mesotrione. Computer bacterial cells measures (ImageJ 1.37v) indicate starvation process, probably due to non use of mesotrione as carbon source, and this characteristic could be consider a marker for herbicide degradation. Mesotrione herbicide activity is major in N, and enhanced by S, and has a strong electron withdrawing. For this reason, we studied antioxidative stress enzymes catalase, superoxide dismutase and glutathione reductase, by SDS-PAGE, non-denaturatig PAGE and spectrophotometry. Acinetobacter calcoaceticus showed a higher response to oxidative stress than Acinetobacter sp., with more soluble proteins and lower number of electrophoresis bands isoforms (12 against 23), because its enzymes were selected for this herbicide and show better adaptation than Acinetobacter sp.. A microbial strategy of herbicide resistance/tolerance and specific oxidative stress raised by mesotrione herbicide could give additional non-degradating options to microorganisms, not necessarily leading to fast herbicide degradation, so determining the herbicide fate in soils and groundwater. / Mesotrione, 2 - (4-mesil-2-nitrobenzoil) -1,3-cicloexanodionas, é um herbicida da classe química tricetona, com alto potencial de lixiviação, inibe competitivamente a 4-hidroxifenilpiruvato dioxigenase (HPPD), que converte a tirosina em plastoquinona e α-tocoferol, e foi encontrado em plantas, fungos e bactérias, incluindo Acinetobacter. O objetivo deste trabalho foi caracterizar modos de adaptação de linhagens brasileiras de Acinetobacter ao mesotrione. Acinetobacter calcoaceticus e Acinetobacter sp. foram selecionadas em até 15.00 mM (resistência) e 0,03 mM (tolerância) de mesotrione e avaliadas para degradação em experimentos de até 60 horas a 35 mM do herbicida em espectrofotometria, mostrando-se incapazes de degradação. Medidas das células bacterianas em computador (ImageJ 1.37V) indicam processo de inanição, provavelmente devido à não utilização de mesotrione como fonte de carbono, e essa característica pode ser considerada como um marcador para a degradação do herbicida. A forte atividade oxidativa do herbicida mesotrione está vinculada ao radical N da molécula, e reforçada pelo radical S. Por este motivo, foi estudado o estresse oxidativo para as enzimas catalase, superóxido dismutase e glutationa redutase, por SDS-PAGE não denaturante, PAGE e espectrofotometria. Acinetobacter calcoaceticus mostrou uma maior resposta ao estresse oxidativo do que Acinetobacter sp., apresentando mais proteínas solúveis e menor número de isoformas de bandas em eletroforese (12 contra 23), provavelmente devido a suas enzimas terem sido selecionadas para este herbicida e mostraram melhor adaptação que Acinetobacter sp.. Estratégias de resistência microbiana ao herbicida e padrões específicos de enzimas anti-oxidativas poderiam dar opções não degradativas aos microorganismos, levando á manutenção de maiores quantidades herbicidas em solos e águas subterrâneas, e por mais tempo.

Doctor of Philosophy / Department of Agronomy / Kassim Al-Khatib / Growth chamber, greenhouse and field experiments using conventional grain sorghum were conducted to 1) evaluate the differential response of grain sorghum hybrids to POST application of mesotrione at various rates and application timings, and 2) determine the physiology of tolerance of grain sorghum hybrids to mesotrione. Sorghum response ranged from susceptible to tolerant. Mesotrione dose-response studies on four sorghum hybrids revealed that injury symptoms were greatest in Pioneer 85G01 and least in Asgrow Seneca. Mesotrione applied EPOST (early POST) injured sorghum more than when applied at MPOST (mid POST) or LPOST (late POST) timings. Observed injury symptoms were not well correlated with grain yield and were transient, thus injury did not reduce sorghum grain yield. Foliar absorption or translocation of mesotrione in tolerant hybrids did not differ with that of susceptible hybrids but metabolism was more rapid in tolerant than in susceptible hybrids. Initial grain sorghum injury was severe and will likely be a major concern to producers. Field and growth chambers studies were conducted on herbicide-resistant grain sorghum to 1) determine the effect of quizalofop rates, application timings, and herbicide tank mixes on acetyl-coenzyme A carboxylase (ACCase)-resistant grain sorghum injury and yield, and 2) determine if herbicide metabolism is an additional mechanism that could explain the resistance of ACCase- and acetolactate synthase (ALS)-resistant grain sorghum. Depending on rate, EPOST application caused the greatest injury while the least injury occurred with LPOST application. Crop injury from quizalofop was more prominent at rates higher than the proposed use rate (62 g ha [superscript]-1) in grain sorghum. Sorghum grain yield was not affected by quizalofop regardless of rates or application timings. Weed control was greater when quizalofop was applied with other herbicides than when applied alone. Herbicide treatments except those that included 2,4-D caused slight to no sorghum injury. Results of the quizalofop metabolism study do not support the involvement of differential metabolism in the observed response of grain sorghum to quizalofop. Rimsulfuron metabolism by ALS-resistant sorghum is more rapid than the susceptible genotypes, thus explaining the observed rapid recovery of grain sorghum plants from rimsulfuron injury in the field.

Doctor of Philosophy / Department of Agronomy / Mithila Jugulam / Palmer amaranth (Amaranthus palmeri) is one of the most aggressive, troublesome and damaging broadleaf weeds in many cropping systems including corn, soybean, cotton, and grain sorghum causing huge yield losses across the USA. As a result of extensive and intensive selection of pre- and -post emergence herbicides, Palmer amaranth has evolved resistance to multiple herbicide modes of action, microtubule-, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-, acetolactate synthase (ALS)-, photosystem II (PS II)-, hydroxyphenylpyruvate dioxygenase (HPPD)- and more recently to protoporphyrinogen oxidase (PPO)-inhibitors. A Palmer amaranth population from Kansas was found resistant to HPPD-, PS II-, and ALS-inhibitors. The overall objective of this research was to investigate the target-site and/or non-target-site resistance mechanisms in Palmer amaranth from KS (KSR) to mesotrione (HPPD-inhibitor), atrazine (PS II-inhibitor), and chlorsulfuron (ALS-inhibitor) relative to known susceptible Palmer amaranth from Mississippi (MSS) and KS (KSS). Whole plant dose-response assays showed high level of resistance in KSR to mesotrione, atrazine and chlorsulfuron. KSR was 10-18, 178-237 and >275 fold more resistant to mesotrione, atrazine, and chlorsulfuron, respectively, compared to MSS and KSS. Metabolism studies using [¹⁴C] labeled mesotrione and atrazine demonstrated non-target-site resistance to both herbicides, particularly, enhanced metabolism of [¹⁴C] mesotrione likely mediated by cytochrome P450 monooxygenases and rapid degradation of [¹⁴C] atrazine by glutathione S-transferases (GSTs). In addition, molecular and biochemical basis of mesotrione resistance was characterized by quantitative PCR (qPCR) and immunoblotting. These results showed 4-12 fold increased levels of the HPPD transcript and positively correlated with the increased HPPD protein. Sequencing of atrazine and chlorsulfuron target genes, psbA and ALS, respectively, showed interesting results. The most common mutation (serine264glycine) associated with atrazine resistance in weeds was not found in KSR. On the other hand, a well-known mutation (proline197serine) associated with chlorsulfuron resistance was found in 30% of KSR, suggesting ~70% of plants might have a non-target-site, possibly P450 mediated metabolism based resistance. Over all, KSR evolved both non-target-site and target-site based mechanisms to mesotrione and chlorsulfuron with only non-target-site based mechanism of resistance to atrazine leaving fewer options for weed control, especially in no-till crop production systems. Such multiple herbicide resistant Palmer amaranth populations are a serious threat to sustainable weed management because metabolism-based resistance may confer resistance to other herbicides and even those that are yet to be discovered. The findings of this research are novel and valuable to recommend appropriate weed management strategies in the region and should include diversified tactics to prevent evolution and spread of multiple herbicide resistance in Palmer amaranth.

Submitted by Marco Antônio de Ramos Chagas (mchagas@ufv.br) on 2019-01-24T12:02:00Z No. of bitstreams: 1 texto completo.pdf: 2911582 bytes, checksum: 07fd890e71dd9999fc2fe2dcaee36a86 (MD5) / Made available in DSpace on 2019-01-24T12:02:00Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2911582 bytes, checksum: 07fd890e71dd9999fc2fe2dcaee36a86 (MD5) Previous issue date: 2018-09-24 / Uma das opções para evitar a disseminação de plantas daninhas tolerantes e resistentes consiste na utilização de misturas de herbicidas com diferentes mecanismos de ação. O herbicida atrazine, quando aplicado em mistura com óleo mineral, promove controle eficiente de plantas daninhas dicotiledôneas e controle apenas regular das monocotiledôneas que infestam a cultura do milho. Em razão disso, esse herbicida quando recomendado para uso em pós-emergência, deve ser aplicado misturado com outro herbicida. Um dos herbicidas muito utilizado em mistura com o atrazine tem sido o mesotrione. Todavia, quando se misturam duas moléculas químicas que têm atividade biológica, estas podem interagirem promovendo efeitos antagônicos, sinérgicos ou aditivos sobre o organismo alvo. Diante disso, o objetivo desse trabalho foi conhecer o tipo de interação da mistura dos herbicidas atrazine + mesotrione quando aplicados sobre diferentes espécies de plantas daninhas na cultura do milho. Para isso foi avaliada a eficiência dos herbicidas atrazine e mesotrione, em aplicações isoladas e em misturas, sem adição de óleo mineral a calda, em duas doses (dose comercial e 1,3 vezes a dose comercial), no controle da soja-tiguera (Glycine max) e das plantas daninhas Ipomoea grandifolia, Bidens pilosa, Tridax procumbens e Commelina benghalensis. A eficiência de controle dessas espécies variou conforme as doses dos herbicidas e a forma de aplicação, isolado ou em mistura. Constatou-se efeito aditivo da mistura de atrazine + mesotrione no controle das plantas daninhas em ambas as doses analisadas. A aplicação isolada dos herbicidas, independente da dose, promoveu eficiência de controle máximo de 60 % na avaliação feita aos 14 dias após aplicação dos tratamentos. Por outro lado, atrazine + mesotrione na maior dose (2.600 + 124,8 g ha -1 ) controlou cerca de 80% da comunidade infestante aos 28 dias após aplicação. Conclui-se que há um efeito aditivo da mistura de tanque de atrazine e mesotrione no controle das plantas daninhas. Todavia, a aplicação dessa mistura, sem a adição de óleo mineral na calda, não permite controle satisfatório das plantas daninhas avaliadas. / One of the options to avoid the spread of tolerant and resistant weeds is the use of mixtures of herbicides with different mechanisms of action. The herbicide atrazine, applied in mixture with mineral oil, promotes efficient control of dicotyledon weeds and only regular control of monocotyledons. Because of this, it is recommended that the application of this herbicide in post-emergence be carried out in mixture with another herbicide. An herbicide widely used in mixture with atrazine has been mesotrione. However, when mixing chemical molecules that have biological activity, they can interact by promoting antagonistic, synergistic or additive effects on the target organism. Thus, the objective of this work was to know the interaction type of atrazine + mesotrione herbicide mixture when applied on different weed species in the maize crop. For this, the efficiency of atrazine and mesotrione herbicides, applied alone and in mixtures, without addition of mineral oil in the herbicides syrup, in two doses (commercial dose and 1.3 times the commercial dose) were evaluated to control of volunteer soybean (Glycine max) and the weeds Ipomoea grandifolia, Bidens pilosa, Tridax procumbens and Commelina benghalensis. The control efficiency of these plant species varied according to the herbicide’s doses and the application form, alone or in mixture. We observed an additive effect of the atrazine + mesotrione mixture on weed control at both doses. The herbicide application, regardless of the dose, promoted a maximum control efficiency of 60% in the evaluation done at 14 days after application of the treatments. On the other hand, atrazine + mesotrione at the highest dose (2,600 + 124,8 g ha -1 ) controlled about 80% of the weed community at 28 days after application. We concluded that there is an additive effect of atrazine and mesotrione tank mix in weed control. However, the application of this mixture, without the addition of mineral oil in the herbicide syrup, does not allow satisfactory control of the evaluated weeds. / Currículo Lattes não encontrado.