Economic weed threshold (EWT) consists of the number of weeds impacting crop grain yield to justify the cost of their control. The objectives of this work were to evaluate the feasibility of using herbicide experiment results to establish EWT, based on regression between corn grain yield and Brachiaria plantaginea (BRAPL) density assessed on several occasions; and to evaluate crop price and weed control cost impact on EWT. BRAPL densities were determined by applying herbicide treatments allowing several levels of weed emergence and, consequently several levels of weed-crop interference. Weed density was evaluated at 10, 20, 30, 40 and 50 days after crop emergence (DAE) to cover the critical weed-crop competition period. Crop grain yield was determined at crop maturity. The results of the regression between BRAPL density and corn grain yield have adjusted exponential decay curves for assessments performed at 10 and 20 DAE and sigmoidal curves at 30, 40 and 50 DAE. EWT of BRAPL in corn, based on densities assessed at 20 DAE and for 10 t ha-1 of grain yield, ranged from 0.2 and 2.0 plants m-2, depending on crop price and weed control cost. EWT assessed at 50 DAE ranged between 1.5 and 13 plants m-2. This work confirmed that weed densities increase with time, allowing to conclude that EWT can be determined from density data assessed on experiments designed to test residual herbicides. Therefore, the EWT obtained are appropriate for situations requiring post-emergence herbicides in areas treated with residual herbicides. The EWT of BRAPL in corn crop increases with the increase of crop prices and reduction of weed control costs; and grain price increase reduces the relative impact of weed control cost on crop profit.
residual herbicides; research method; interference; competition
