ABSTRACT Biotic and abiotic stresses, either individually or in combination, may affect soybean growth and development negatively, besides competitiveness with weeds. This study aimed at evaluating the effect of heat stress and plant recovery on crop–weed interaction by analyzing growth variables. Experiments were set up as replacement series of soybean with slender amaranth. Treatments followed a factorial scheme, which was composed of heat stress [high temperature (38/22 ± 2 °C) against the control (28/22 ± 2 ºC)] and of proportions of soybean and slender amaranth, i.e., 100:0 (pure stand of soybean), 50:50 and 0:100 (pure stand of slender amaranth). After being submitted to high temperatures, plants were analyzed in terms of their recovery from heat stress. Variables under evaluation were height, leaf area and shoot dry weight. Results led to the conclusion that the weed shows more competitiveness than soybean, regardless of the heat stress. Interspecific competition for soybean and intraspecific competition for slender amaranth influence their height, leaf area and shoot dry weight negatively, while high temperature affects slender amaranth positively.

O uso intenso de herbicidas com o mesmo mecanismo de ação na cultura do arroz (Oryza sativa L.) tem selecionado biótipos resistentes, como, por exemplo, o arroz-vermelho (planta daninha) resistente aos herbicidas imidazolinonas. Por essa razão, este trabalho foi desenvolvido com o objetivo de avaliar a resistência de arroz-vermelho ao herbicida imazapyr + imazapic, na região sul do Rio Grande do Sul, e o controle do biótipo resistente de arroz-vermelho com os herbicidas alternativos clethodim e glyphosate. Foram realizados três experimentos, em delineamento casualizado, arranjados em esquema fatorial. No primeiro experimento, o fator A testou biótipos de arroz-vermelho [ORYSA 184 (resistente) e ORYSA 188 (susceptível)], o fator B comparou herbicidas (imazapyr + imazapic, clethodim e glyphosate) e o fator C avaliou doses dos herbicidas (0; 0,5; 1; 2; 4; 8; 16 e 32 vezes a dose recomendada). No segundo, compararam-se biótipos de arroz-vermelho e doses do herbicida imazapyr + imazapic (0; 0,5; 1; 2; 4; 8; 16; 32 e 64 vezes a dose recomendada). No terceiro, testaram-se biótipos de arroz-vermelho e doses diferentes do herbicida imazapyr + imazapic para cada biótipo. O biótipo ORYSA 184 é resistente ao imazapyr + imazapic, quando aplicada a dose máxima de registro e estádio indicado. Os herbicidas clethodim e glyphosate, detentores de mecanismos de ação alternativos, controlam o biótipo resistente ORYSA 184 de arroz-vermelho.

The intenseuse of herbicides with the same action mechanism in rice (Oryza sativa L.) has selected resistant biotypes, such as red rice (weed) resistant to imidazolinone herbicides. Therefore, the objective of this work was to evaluate the resistance of red rice to the imazapyr + imazapic herbicide in the southern region of Rio Grande of Sul, Brazil, and the control of the red rice resistant biotype with the alternative herbicides, clethodim and glyphosate. Three experiments were carried out in a completely randomized design, arranged in a factorial design. In the first experiment, the factor A tested biotypes of red rice [ORYSA 184 (resistant) and ORYSA 188 (susceptible)], factor B compared herbicides (imazapyr + imazapic, clethodim and glyphosate) and factor C evaluated dosages of herbicide (0, 0.5, 1, 2, 4, 8, 16 and 32 times the recommended dosage). The second experiment compared biotypes of red rice and dosages of the imazapyr + imazapicherbicide (0, 0.5, 1, 2, 4, 8, 16, 32 and 64 times the recommended dosages). The third experiment tested biotypes of red rice and different doses of the imazapyr + imazapic for each biotype. The biotype ORYSA 184 is resistant to imazapic + imazapyr when applied at the maximum indicated record and stadium dose. The herbicides glyphosate and clethodim, holders of alternative mechanisms of action, control the red rice resistant biotype ORYSA 184.