This study aimed to assess the interference and threshold level (TL) of Sida rhombifolia, the arrowleaf sida, competing with different soybean cultivars. The treatments comprised different soybean cultivars (NS 6909, NA 5909, DM 5958, Brasmax ELITE, Brasmax LANÇA, and SYN 13561) and densities of arrowleaf sida plants per square meter (m-2) (0, 2, 3, 4, 9, 15, 16, 23, 22, and 58; 0, 2, 3, 3, 6, 6, 10, 11, 18, and 47; 0, 3, 4, 7, 8, 10, 11, 13, 15, and 24; 0, 1, 4, 6, 12, 18, 19, 31, 44, and 50; 0, 4, 5, 6, 9, 13, 17, 20, 20, and 47; 0, 2, 3, 5, 9, 11, 15, 18, 29, and 30, respectively) for each cultivar. Cultivars NS 6909, NA 5909, and Brasmax Lança were more competitive than DM 5958, Brasmax Elite, and SYN 13561. The TL values varied from 0.55 to 0.95 plants m-2 for cultivars NS 6909, NA 5909, and Brasmax Lança, which exhibited greater competitiveness with arrowleaf sida. The lowest values of TL varied from 0.26 to 0.61 plants m-2 for cultivars DM 5958, Brasmax Elite, and SYN 13561, which had less competitiveness with weed. Highlights The rectangular hyperbola model adequately estimates the grain yield losses of soybean cultivars. The density of arrowleaf sida plants was the variable that best fitted the rectangular hyperbola model.  Soybean cultivars NS 6909 IPRO, NA 5909 RG, and Brasmax Lança IPRO are the most competitive in the presence of arrowleaf sida. The levels of economic damage decrease with the increase in grain yield, in the soybean price, with the reduction in the cost of controlling arrowleaf sida and the herbicide efficiency.

The arrow-leaf sida (Sida rhombifolia), in addition to competing with soybeans, which causes productivity losses, is also host to the whitefly, which transmits viruses. The aim was to evaluate the competitive ability of soybean cultivars in the presence of an arrow-leaf sida biotype, with different proportions of plants in the association. The experiments were conducted in a completely randomized design, with four replications. Tested competitors included arrow-leaf sida with the four soybean cultivars in proportions crop:weed of 100:0; 75:25; 50:50; 25:75; and 0:100. Competitiveness analysis was carried out using diagrams applied to replacement series experiments in addition to the relative competitiveness indices. At 50 days after the emergence of the species, the variables height, diameter, leaf area, and dry mass of shoots were evaluated. Competition occurred between soybean cultivars in the presence of arrow-leaf sida, regardless of the plant proportion, causing reductions in all variables evaluated. Interspecific competition causes greater damage to al variables studied than intraspecific competition. There was competition for the same resources between soybean and arrow-leaf sida.

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