V. Sarabi - Department of Agronomy, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran.
A. Ghanbari - Department of Agronomy, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran.
M.H. Rashed Mohassel - Department of Agronomy, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran.
M. Nassiri Mahallati - Department of Agronomy, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran.

Registered dose of herbicides may be higher than rate required for controlling weed species depending on growth stages. In order to study the effect of individual post-emergence application of 2,4-D plus MCPA and three sulfonylurea herbicides at four- to six-true leaf stage of weeds, experiments were conducted in 2011 at the greenhouse of Agricultural Faculty of Ferdowsi University of Mashhad, Iran. Treatments included untreated control and several rates of 2,4-D plus MCPA, foramsulfuron, nicosulfuron and nicosulfuron + rimsulfuron on redroot pigweed, common lambsquarters, common purslane and black nightshade. These herbicides were more effective to control redroot pigweed than other weeds (except nicosulfuron + rimsulfuron), thereupon minimum dose required for a satisfactory efficacy of 90% reduction of redroot pigweed aboveground dry matter (ED90) were 375.26, 23.51 and 63.81 g a.i h-1 of 2,4-D plus MCPA, foramsulfuron and nicosulfuron, respectively. Nicosulfuron and nicosulfuron + rimsulfuron did not control common lambsquarters effectively. Foramsulfuron had the lowest effect on black nightshade and common purslane, so that minimum dose required for a 90% reduction of black nightshade and common purslane aboveground dry matter were 52.42 and 60.26 g a.i h-1, respectively. Thus, these results showed that tank mixtures with other herbicides effective in controlling broadleaf weeds may be required for satisfactory weed control and reduction in sulfonylurea herbicides doses. Keywords: Broadleaf weeds; Dose-response curve; Effective dose; Growth stage; Sulfonylurea herbicides.