Determining the Optimum Concentration of Pretilachlor and Sunrice plus Herbicides for Weed control in Rice

In order to determine the optimal rates of Pertilachlor and Sunrice­-plus herbicides for weed control of rice (Oryza sativa Var.Tarom Hashemi), a factorial experiment was conducted with a randomized complete block design with four replications in Rice Research Farm of Faculty of Agriculture, University of Guilan, Rasht, northern Iran. The first factor was weed control treatment which consisted of lower than, equal, and more than recommended levels of Pertilachlor (i.e. 0.5, 0.75, 1 kg a.i ha-1) and Sun­rice plus (0.45, 0.9, 1.35 kg a.i ha-1) herbicides, along with unweeded and twice hand-weeded controls. The second factor consisted of two levels of nitrogen fertilizer (N) application (i.e. 0 and 90 kg ha-1 of N). Barnyardgrass (Echinochloa crus-galli L.) and Purple nutsedge (Cyperus rotandus L.) were the most dominant weeds of the experimental plots. The highest number of filled panicles per plant, number of grains per panicle, and grain yield were found in recommended rate of Pertilachlor, Sun­rice plus and more than recommended rate of Sun­rice plus due to adequate weed control (>70%) at the presence of 90 kg ha-1 of N. The lowest number of grains per panicle, number of panicles per plant and grain yield were observed in unweeded treatment, at both levels of nitrogen fertilizer application. It was concluded that application of N is effective in increasing grain yield of the examined rice genotype only if weeds are effectively controlled by herbicides. Hence, application of N in the unweeded conditions does not lead to an increase in grain yield of rice, because the consumption of N by weeds decreased the effect of nitrogen on rice grain production.