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Herbicides used in the Clearfield® rice (Oryza sativa L.) production system have a potential for leaching. This can result in contamination of underground water resources and cause injury to not tolerant crops that are sown in a succession and/or crop rotation. The objective of this study was to determine the leaching potential and the residual activity of the herbicides used in the Clearfield® rice system. The experiment was conducted over a period of two years and consisted of conducting a field test to be followed by two bioassays with a year of difference between their implementation. Initially an experiment was conducted in lowland area where it was planted the cultivar of rice ‘PUITA INTA CL’. Approximately one and two years thereafter, soil samples from each plot were collected at intervals of 5cm to a depth of 30cm (B factor) for the bioassay to evaluate persistence of herbicides. Factor A was composed of mixtures formulated of imazethapyr + imazapic (75 + 25g a.i. L-1), imazapyr + imazapic (525 + 175g a.i. kg-1) in two doses, imazethapyr (100g a.i. L-1) and treatment control without application. Basing on results, it was concluded that the mixtures imazethapyr + imazapic, imazapyr + imazapic and imazethapyr leached into the soil, reaching depths of up to 25cm in lowland soil. Imidazolinone herbicides used today in the irrigated rice Clearfield® system are persistent in soil, and their phytotoxic activity can be observed up to two years after application. RESUMO: Os herbicidas utilizados no sistema Clearfield® de produção de arroz irrigado (Oryza sativa L.) apresentam potencial de lixiviação. Isso pode causar contaminação de mananciais hídricos subterrâneos e fitotoxicidade as culturas não tolerantes semeadas em sucessão e/ou rotação. O objetivo deste trabalho foi determinar o potencial de lixiviação e o efeito residual dos herbicidas utilizados no sistema Clearfield® de arroz irrigado. O experimento foi conduzido durante dois anos e consistiu da realização de dois bioensaio com diferença de um ano entre a implantação dos mesmos. Inicialmente foi instalado o experimento em área de várzea onde foi semeada a cultivar de arroz ‘Puitá INTA CL’ em área de várzea. Aproximadamente um e dois anos após, foram coletadas amostras de solo de cada parcela, em intervalos de 5cm até a profundidade de 30cm (fator B) para a realização do bioensaio a fim de avaliar persistência dos herbicidas. O fator A foi constituído pelas misturas formuladas imazethapyr + imazapic (75 + 25g i.a. L-1), imazapyr + imazapic (525 + 175g i.a. kg-1) em duas doses, imazethapyr (100g i.a. L-1) e um tratamento testemunha sem aplicação. Com os resultados obtidos, foi possível concluir que as misturas imazethapyr + imazapic, imazapyr + imazapic e imazethapyr lixiviam no solo, atingindo profundidades de até 25cm em solo de várzea. Os herbicidas imidazolinonas, hoje utilizados no sistema Clearfiel® de arroz irrigado, são persistentes no solo podendo seus efeitos fitotóxicos serem observados até dois anos após sua aplicação.

Red rice is one of the major troublesome and difficult weeds to control in rice production regions. The introduction of the Clearfield&#174 technology allowed producers to control red rice using rice genotypes tolerant to the imidazolinone herbicides. However, because the consecutive use of this technology red rice biotypes have evolved resistance to imidazolinone herbicides, the rice-soybean rotation has been an alternative used by producers to control this weed. This system allows the use of herbicides with different modes of action to control red rice, such as S-metolachlor. Thus, greenhouse and field experiments were carried out during the 2011 to 2012 and 2012 to 2013 growing seasons to evaluate: 1) sensitivity of imidazolinone-resistant red rice to S-metolachlor; 2) red rice control and soybean tolerance in response to associations of S-metolachlor and glyphosate. In greenhouse, S-metolachlor effectively controlled both susceptible and imidazolinone-resistant red rice in preemergence. In field, preemergence applications of S-metolachlor provided greater red rice control in comparison to S-metolachlor alone in early postemergence. The association of S-metolachlor with glyphosate did not improve red rice control in preemergence application. However, association of S-metolachlor with glyphosate significantly improved red rice control in early postemergence applications. S-metolachlor injury to soybean increased with early postemergence applications. These results indicate that S-metolachlor effectively control imidazolinone-resistant red rice in rice-soybean rotation.