The adoption of integrated weed management requires knowledge of the impact of weed populational density on weed seed production as a factor to consider when implementing the economic threshold approach. Thus, the objective of this research was to evaluate the effects of weed densities and soybean seeding timing after plant cover desiccation on seed production by beggarticks (Bidens spp.) and arrowleaf sida (Sida rhombifolia). Four experiments were carried out in two locations. (1998/99, Passo Fundo - location 1 and 1999/00, Eldorado do Sul - location 2). The treatments consisted of beggarticks and arrowleaf densities, and soybean-seeding times in relation to plant cover desiccation date. In the beggartick trials soybean seeding was performed 3, 7, and 11 days after desiccation - DAD - of plant cover in both locations. In the arrowleaf sida trials, soybean seeding occurred 3, 7, and 11 DAD or 20, 24, and 28 DAD for locations 1 and 2, respectively. Average seed productions of 5.9 and 101.2 thousand m-2 for beggarticks and 1.4 and 28.2 thousand m-2 for arrowleaf sida in locations 1 and 2, were obtained respectively. It was verified that the effect of weed density on seed production depends on soybean emergence timing in relation to weed emergence, and that the sooner soybean is established in relation to plant cover desiccation, the lower the number of seeds produced by the weeds. The high number of seeds produced by beggarticks and arrowleaf sida, even in low densities, may determine that weed density can be lower for the economic threshold than the one established through the interference of these weeds in the soybean crop.
economic threshold; interference; Bidens spp.; Sida rhombifolia
