Threshold level (TL) is a simple concept that integrates biological and economical factors, which it is intended to make profitable the results of a decision process for weed control. The objective of this study was to determine TL for arrowleaf sida (Sida rhombifolia) infestations, calculated on the basis of a single year, that could justify economic application of control measure in soybeans. Field experiments were carried out in Passo Fundo and Eldorado do Sul, RS - Brazil. Treatments consisted of arrowleaf sida densities and soybean seeding times in relation to vegetation cover desiccation (3, 7, and 11; and 20, 24, and 28 days after). TL values varied from 2 to 50 plants m-2, whereas optimum TL ranged from 1 to 6 plants m-2. It was observed different responses for crop-weed interference relations with regard to the different environments evaluated. Delaying soybean seeding in relation to vegetation cover desiccation increases arrowleaf sida interference severity in soybeans. It was noted that seeds produced by non controlled weed plants, at low TL densities, make difficult the control decision process based on this criterion.
Sida rhombifolia; weed management; optimum threshold level
