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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. [ABSTRACT FROM AUTHOR]

Sourgrass (Digitaria insularisL.) and fleabane (Conyzaspp. L.) constitute a major challenge to weed management in soybean [Glycine max(L.) Merr.] production systems in Brazil, especially when both are resistant to glyphosate and present concurrently. The aim of this study was to evaluate the interaction of haloxyfop‐P‐methyl [methyl(R)‐2{4‐{3‐chloro‐5(trifluoromethyl)‐2‐pyridyloxy] phenoxy} propanoate] and 2,4‐D [2,4‐Dichlorophenoxyacetic acid‐N‐methylmethanamine] to control sourgrass at three to four tillers and flowering stages and 12‐ to 15‐cm fleabane at vegetative stage. The experiments were arranged in a randomized complete block design with four replicates and were conducted twice. Treatments were composed of haloxyfop‐P‐methyl and 2,4‐D applied sequentially at 3‐, 6‐, and 12‐d intervals. Treatments also included an untreated check, haloxyfop‐P‐methyl, and 2,4‐D sequentially (without interval) and stand‐alone applications of both herbicides. All treatments across experiments completely controlled fleabane plants, except haloxyfop‐P‐methyl alone. Haloxyfop‐P‐methyl applied ≥6 d before 2,4‐D controlled flowering and three‐ to four‐tiller sourgrass plants at rates of 50 and 100%, respectively. Based on results of this study, haloxyfop‐P‐methyl should be applied ≥6 d before 2,4‐D in a sequential application structure to control fleabane and sourgrass (three or four tillers) when present concurrently.

The aim of this work was to evaluate the competitiveness between soybean cultivars and barnyardgrass, based on morphological and physiological characteristics of species. The experiments were conducted in completely randomized experimental design, with 4 replications. In the first study, for both soybean and barnyardgrass, it was determined the population of plants in which shoot dry matter became constant and independent of the population (16 plants·pot⁕1 or 400 plants·m⁕2). In the second study, 2 experiments were conducted to evaluate the competitiveness of BMX Apolo RR and BMX Potência RR soybean cultivars with barnyardgrass plants, both carried out in replacement series under different proportions of plants·pot⁕1 (100:0; 75:25; 50:50; 25:75 and 0:100) between the crop and the weed. The analysis of the species competitiveness was determined through diagrams applied to replacement series experiments and use of relative competitiveness indexes. At 44 days after the emergence of species, the physiological and morphological parameters of the crop and the weed were evaluated. The BMX Apolo RR and BMX Potência RR soybean cultivars show similar competitiveness when competing with the barnyardgrass; therefore, the ability of one species to interfere on another is equivalent. For plant height, barnyardgrass displays higher competitiveness compared to BMX Apolo RR, with early cycle and short height. The intraspecific competition is more important to barnyardgrass than interspecific competition with soybean cultivars, resulting in negative effects on the morphological and physiological characteristics of species. [ABSTRACT FROM AUTHOR]