Adoption of the threshold level (TL) concept for weed management evaluates weed populations to implement control measures only when infestations overcome the TL. The objective of this research was to define TL for mixed infestations of Bidens pilosa and Bidens subalternans in the soybean crop. Field experiments were conducted in Passo Fundo and Eldorado do Sul, RS, Brazil. Treatments consisted of mixed densities of the beggartick species and soybean seeding at 3, 7, and 11 days after cover crop desiccation. TL variable values were obtained from 0.4 to 33 beggartick plants m-2. Different responses were obtained for crop-weed interference relations between the two locations evaluated. Delaying soybean seeding in relation to cover crop desiccation increases beggartick interference in this crop. Increases in yield loss per weed unit, in crop yield potential, harvested product value and herbicide efficiency were found to decrease TL values, making control potentially more economic. On the other hand, higher weed control cost increases TL values. It was also verified that seeds produced by non-controlled weeds below TL could compromise adoption of control decision based on this criterion.
Bidens pilosa; Bidens subalternans; weed management; control decision
