The application of herbicides in early broadcast postemergence and band directed of herbicides in sorghum has raised considerably with increasing planting acreage in Brazil. However, these products can cause phytoxicity since they are not completely selective to the crop. Since these herbicides basically block metabolic processes in the plants, one questions what effects these injuries will have on grain yield. Therefore, the objective of this research was to study the phytotoxic effect caused by the application of herbicides on the crop, at the initial phase and at late postemergence in directed spray, as well as on sorghum grain yield. This study was carried out during the growing season of 1994/95 and 1995/96, using the hybrid BR 700, in a complete randomized block design, with 12 treatments and four replications. The treatments consisted of the combination of the following herbicides at several rates: cyanazine + simazine with or without mineral oil, applied at the 4-and-6 leaf growth stages; paraquat + adjuvant and ametryn + mineral oil, applied as directed spray at the 12-leaf growth stage. The control treatments were weed free and weeded plus mechanical defoliation of the first, second and third pair of sorghum leaves at the 12-leaf stage. Leaf area, plant dry weight, leaf chlorophyll content, plant height, stand, weight of panicles and grain yield were also evaluated. Cyanazine + simazine + mineral oil application at the 4-leaf growth stage drastically reduced sorghum stand in both evaluations. Plant development evaluated by leaf area and stem diameter was not much affected. The poorest performances were observed with the weeded treatment and cyanazine + simazine. Stand reduction affected the panicles and grain yield directly. The best treatments for weed control were: paraquat + adjuvant, ametryn + mineral oil, mechanical defoliation and cyanazine + simazine applied at the 6-leaf growth stage. Except for the application of cyanazine + simazine at the 4-leaf growth stage, herbicide application in postemergence on sorghum crop was selective. Even though injuries were caused by the herbicides, they did not cause a reduction in the sorghum grain yield. Good weed control using herbicides allowed the sorghum plants to better express their yield potential.
postemergence; direct spray; Sorghum bicolor
