RESUMO As condições ambientais podem influenciar na metabolização de herbicidas e interferir na evolução da resistência de plantas daninhas. Objetivou-se avaliar o efeito de regimes térmicos no controle de capim-arroz (Echinochloa crus-galli) suscetível e resistente a herbicidas do grupo químico imidazolinonas. O fator A (biótipos) foi constituído de um biótipo suscetível e um resistente a imazethapyr; o fator B (dose) constou de seis doses de imazethapyr entre 0 e 800 g ha-1 para o biótipo resistente e entre 0 e 100 g ha-1 para o biótipo suscetível; o fator C (temperatura) foi composto de quatro regimes térmicos (16/16 ºC, 25/25 ºC, 16/25 ºC e 25/16 ºC) aos quais as plantas foram submetidas antes e após a aplicação do herbicida. Considerando-se o biótipo resistente, os maiores índices de controle foram observados nos regimes térmicos com oscilação de temperatura, principalmente para 25/16 ºC. Na dose de imazethayr recomendada (100 g ha-1), o herbicida proporcionou controle superior a 90 % para 25/16 ºC e menos de 50 % para 16/16 ºC, no biótipo resistente. A massa seca da parte aérea teve redução superior a 80 % para 25/16 ºC e 16/25 ºC e inferior a 30 % para 16/16 ºC. As condições de temperatura em que as plantas são submetidas no período pré e pós-aplicação do herbicida interferem no nível de resistência a imazethapyr em capim-arroz.

ABSTRACT Environmental conditions can influence herbicide metabolism and interfere with the evolution of weed resistance. This study aimed to evaluate the effect of thermal regimes on the control of barnyardgrass (Echinochloa crus-galli) susceptible and resistant to herbicides of the imidazolinones chemical group. Factor A (biotypes) consisted of one biotype that is susceptible and one that is resistant to imazethapyr; factor B (dose) comprised six imazethapyr doses between 0 and 800 g ha-1 for the resistant biotype and between 0 and 100 g ha-1 for the susceptible biotype; factor C (temperature) consisted of four thermal regimes (16/16 ºC, 25/25 ºC, 16/25 ºC and 25/16 ºC) to which the plants were submitted before and after the herbicide application. Concerning the resistant biotype, the highest control levels were observed in the thermal regimes with temperature fuctuation, especially at 25/16 ºC. At the recommended imazethapyr dose (100 g ha-1), the herbicide resulted in a control level over 90 % at 25/16 ºC and less than 50 % at 16/16 ºC, in the resistant biotype. The shoot dry mass was reduced by more than 80 % at 25/16 ºC and 16/25 ºC and less than 30 % at 16/16 ºC. The temperature conditions to which the plants are subjected in the pre- and post-application period interfere with the level of imazethapyr resistance in barnyardgrass.

Abstract Background: The use of pre-emergent herbicides is an important tool to control weeds, however the tolerance of soybean to these herbicides can vary according to the type of soil. Objective: The aim of this study was to evaluate sulfentrazone and diclosulam soybean selectivity in a sandy soil, in order to establish these herbicides as supporting tools in weed control. Methods: The treatments consisted of six doses of sulfentrazone (150 to 400 g ha-1) and diclosulam (25.2 to 75.6 g ha-1), and an untreated control. A field study was repeated in two growing seasons (2013/14 and 2014/15) in a sandy soil. Results: The occurrence of injury depended on the growing season. The first season presented lower rainfall rates during the crop cycle. The recommended dose of sulfentrazone (200 g ha-1) caused 26% and 10% of plant injury at 15 days after the treatment for the first and second season, respectively. For diclosulam, the recommended dose of 35.3 g ha-1 caused 20% and 8% of plant injury, respectively, for the first and second season. However, at the recommended doses, only for diclosulam and in the rainy season there was a reduction in soybean productivity. Conclusions: The herbicide sulfentrazone, although causing visual soybean plant injury, present satisfactory selectivity at recommended doses and can be used as an important tool on weed control on sandy soils. Similar response was observed for diclosulam in 2013/14. However, in a rainfall season diclosulam impacted on soybean yield at the recommended dose for sandy soils, with selectivity depending of the growth season.