ABSTRACT The knowledge of the adequate period for the beginning of weed management in lettuce crop can increase the efficiency of production, reducing the costs with management. The aim of this study was to determine the pre-interference period (PIP), the critical time for weed removal (CTWR) and the critical period of weed control (CPWC) in three lettuce cultivars, Elisa, Lucy Brown and Solaris. Coexistence and the absence of weeds were evaluated for each cultivar. The design was in randomized blocks with seven treatments (0, 7, 14, 21, 28, 35 and 42 days after transplanting - DAT) of coexistence and the same seven periods of weed absence with four replicates in each experiment. The highest infestation densities were with the species Oxalis latifolia, Coronopus didymus and Amaranthus hybridus. The leaf area and number of leaves per plant were reduced by up to 50% and 66%, respectively, in relation to the longest period of coexistence with weeds and yield reduced by 48% (Elisa), 40% (Lucy Brown) and 33% (Solaris). The cultivar Solaris showed greater tolerance to weed interference. Based on yield, PIP was 1, 3 and 3 DAT and CTWR was 37, 37 and 35 DAT, and CPWC was 2 to 37, 3 to 37 and 3 to 35 for the cultivars Elisa, Lucy Brown and Solaris, respectively.

RESUMO O conhecimento do período adequado para início do manejo de plantas daninhas na cultura da alface pode aumentar a eficiência de produção reduzindo os custos com o manejo. Objetivou-se neste trabalho determinar o período anterior à interferência (PAI), o período total de prevenção da interferência (PTPI) e o período crítico de prevenção da interferência (PCPI) em três cultivares de alface, Elisa, Lucy Brown e Solaris. Foram avaliadas a convivência e a ausência de plantas daninhas para cada cultivar. O delineamento foi em blocos casualizados com sete tratamentos [0, 7, 14, 21, 28, 35 e 42 dias após transplantio (DAT)] de convivência e os mesmos sete períodos de ausência de plantas daninhas com quatro repetições em cada experimento. As maiores densidades de infestação foram com as espécies Oxalis latifolia, Coronopus didymus e Amaranthus hybridus. A área foliar e número de folhas por planta foram reduzidas em até 50% e 66% respectivamente em função do maior período de convivência com as plantas daninhas e produtividade foi reduzida em 48% (Elisa), 40% (Lucy Brown) e 33% (Solaris). A cultivar Solaris apresentou maior tolerância à interferência das plantas daninhas. Com base nos dados de produtividade o PAI foi de 1, 3 e 3 DAT e o PTPI foi de 37, 37 e 35 DAT, sendo o PCPI de 2 a 37, 3 a 37 e 3 a 35 para as cultivares Elisa, Lucy Brown e Solaris, respectivamente.

RESUMO Devido à baixa disponibilidade de ingredientes ativos de herbicidas, o manejo químico de plantas daninhas na cultura da alface é um desafio. Objetivou-se avaliar a sensibilidade da alface à aplicação dos herbicidas flumioxazin e trifluralin, isolados ou em mistura em pré-plantio de mudas de alface cultivar Lucy Brown. Dois experimentos foram realizados. O experimento A recebeu uma operação de revolvimento no solo antes da aplicação dos herbicidas e outra antes do transplantio das mudas; o experimento B recebeu apenas um revolvimento do solo anterior à aplicação do herbicida. O delineamento experimental foi em blocos casualizados em esquema fatorial (3x3) + 1 (2 fatores com 3 níveis e uma testemunha adicional) com 4 repetições. Ambos experimentos receberam os tratamentos herbicidas flumioxazin (120 g ha-1), trifluralin (450 g ha-1) e a associação flumioxazin + trifluralin (120 + 450 g ha-1) aplicados em três épocas distintas antecedendo o transplantio (0, 7 e 14 dias). A produtividade de alface após aplicação de trifluralin (48,4 t ha-1) foi maior do que sob aplicação de flumioxazin (16,8 t ha-1) no experimento A. No experimento B flumioxazin aos 14 DAT reduziu o número de folhas de alface (25,6), todavia não houve diferenças de produtividade. O herbicida trifluralin causou menores níveis de fitointoxicação na alface e foi o mais seletivo dentre os tratamentos quando aplicado nas três épocas nos dois experimentos.

ABSTRACT Due to the low availability of herbicide active ingredients, the chemical weed management in lettuce crop is a challenge. The aim of this study was to evaluate the sensitivity of lettuce to the pre-planting application of the herbicides flumioxazin and trifluralin alone or in tank-mixture in lettuce, Lucy Brown cultivar. Two experiments were conducted. In experiment A, the soil was tilled before the herbicide application and it was tilled again before seedling transplanting. In experiment B, the soil was tilled just once before the herbicide application. The experimental design was in randomized blocks in a factorial scheme (3x3) +1 (2 factors with 3 levels and an additional control) with 4 replicates. We applied flumioxazin (120 g ha-1), trifluralin (450 g ha-1) and flumioxazin + trifluralin (120 + 450 g ha-1) in three different seasons before transplanting (0, 7 and 14 days). Lettuce productivity after trifluralin application (48.4 t ha-1) was higher than after flumioxazin application (16.8 t ha-1) in experiment A. In experiment B, flumioxazin decreased the number of lettuce leaves (25.6) at 14 DAT; no differences in lettuce productivity were verified, though. The herbicide trifluralin caused lower levels of phytotoxicity and was the most selective between the treatments when applied in the three seasons in both experiments.

Abstract Conyza species are important weeds in global agriculture, especially due to their capacity to evolve resistance to multiple herbicide mechanisms of action. We aimed to evaluate the frequency and distribution of resistance to glyphosate and chlorimuron-ethyl in Conyza spp. populations from Brazil. Seed samples were collected from grain production areas across nine Brazilian states over five consecutive years (2014 to 2018). Prior to resistance monitoring trials, dose-response assays were conducted to determine a single dose of glyphosate or chlorimuron-ethyl to discriminate resistant and susceptible populations. Resistance monitoring based on plant responses to the application of discriminatory doses of glyphosate (960 g ha-1) or chlorimuron-ethyl (20 g ha-1). Populations were classified as resistant, moderately resistant, or susceptible to either herbicide. While glyphosate resistance was highly frequent (71.2%) in all the five years, chlorimuron-ethyl resistant populations occurred at 39.8% of the total. The frequency of multiple resistance to both herbicides (35.3%) was proportional to the occurrence of chlorimuron-ethyl resistance (39.6%). Resistance to glyphosate and to chlorimuron-ethyl were found across all states evaluated.