Palavras-chave: espaçamento entre linhas, modelos matemáticos, Oryza sativa, Echinochloa spp.ABSTRACT -High yield losses in irrigated rice occur due to weed interference, as weeds are also adapted to the flooded environment where the crop is cultivated. The objective of this study was to evaluate the degree of interference and determine the economic weed threshold of populations of barnyardgrass in irrigated rice, according to crop arrangement. Thus, an experiment was carried out under field conditions with rice crop, under the conventional system. The treatments consisted of three arrangements of rice plants, one cultivar BRS Pelota (17 and 32 cm, hand sown) and nine populations of the competitor barnyardgrass present in each sowing method. The rectangular hyperbolic nonlinear regression model appropriately estimates rice productivity losses in the presence of barnyardgrass plants. Manually sown rice crop shows greater competitive ability against barnyardgrass in relation to the 17 and 32 cm spacings. The variable leaf area fits better to the hyperbolic model,compared to the variables plant population, shoot dry weight and soil covering. Sowing the crop by hand increases the economic weed threshold, justifying the adoption of measures to control barnyardgrass at higher population levels. Increased grain yield, rice price, herbicide efficiency and control cost reduction decrease the economic weed threshold values, justifying the adoption of control measures in low populations of barnyardgrass.