Influência do estádio de desenvolvimento de Cenchrus echinatus na supressão imposta por atrazine

Cenchrus echinatus is an important weed in areas cultivated with corn, sorghum and pearl millet in Brazil. Although atrazine is one of the most used herbicides in such crops, not much has been done to determine weed susceptibility as a function of its growth stage at post-emergence applications. This work aimed to evaluate the suppression imposed by the herbicide atrazine applied at post-emergence, during three developmental stages of this weed. The assay was carried out under greenhouse conditions, in pots of 10 dm-3, in a factorial scheme 5 x 3 composed by five rates of atrazine (0; 0.5; 1.5; 2.5 and 4.0 kg ha-1 ), combined with three stages of C. echinatus development during herbicide spraying at post-emergence (first pair of leaves; second pair of leaves and two tillers). Applications performed at later stages of weed development were inefficient to provide control, despite significant reductions of dry biomass, plant height and production of reproductive structures. Aiming at weed control, the best results were found for rates ³ 3.5 kg ha-1 in plants at the stage of first pair of leaves.