Soybean as bioindicador of residual effect of 2,4-D herbicide in an oxisol from the Brazilian cerrado

Invasive plants, besides competing for space, also compete for water, light, and nutrients against the crop, what may cause drastic production cuts. Chemical control through herbicides is the most widely used method to control these plants. This study aimed to evaluate the persistence of the 2,4-D (dichlorophenoxyacetic acid) herbicide in soybean (Glycine max L.) crops in an Oxisol from the Brazilian Cerrado. The experiment was conducted in a greenhouse using a randomized block design, 5x3 factorial, five application periods (0, 3, 5, 7 and 10 days before sowing), and three herbicide doses (0, 750 and 1500 g e. a. ha-1), in four repetitions. Herbicides were sprayed with the assistance of knapsack sprayer at constant pressure (kept through CO2 compressed). Residual effect was assessed by emergence speed index (ESI), visual plant phytotoxicity, plant height, and shoots and root dry biomass. Persistence of 2,4-D herbicide was short in soybeans planted in an Oxisol. Periods from 0 to 3 days before sowing were the most harmful to soybeans, that is, the closer the spraying was from sowing, higher was its damage to the crop. The 1500 g e. a. ha-1 dose had a higher residual effect.   Key words: Persistence, bioassay, Glycine max, dichlorophenoxyacetic acid.