evaluation of weed control methods on sugar beet (beta vulgaris l.) yield at different levels of nitrogen

introduction: weed competition is one of the major factors which limit sugar beet production in the world. weed – crop interactions are based on competition for water, nutrients and light and allelopathic effects may also play a small role. in sugar beet weed interference, all these factors are important too, but the light is of prime importance. due to the fact that a lot of weeds can grow above the sugar beet canopy and reduce the amount of photosynthetic radiation reaching the crop, these weeds are stronger competitors compared to smaller weeds. in much sugar beet growing areas dicot weeds of the families chenopodiaceae, asteraceae, brassicaceae and polygonaceae are of major importance. the monocots are less important compared to dicot weeds. competition from uncontrolled annual weeds that emerge within 8 weeks of sowing or within 4 weeks of the crop reaching the two-leaf stage can reduce root yields by 26–100% .weeds that emerge 8 weeks after sowing, and particularly after the sugar beet plants have eight or more leaves, are less likely to affect yield. although tractor hoeing and hand labour are still used in many production areas, herbicides have been the primary method of weed control in sugar beet. the effectiveness of pre-emergence residual herbicides decreases with reductions in rainfall or soil wet content. therefore, less than 10 % of the total sugar beet crop is treated with pre-emergence herbicides. the remaining 90 % depends solely on a selection of post-emergence herbicides to maintain season-long weed control. the major herbicides are phenmedipham, chloridazon, metamitron. mixtures of post-emergence, broad spectrum herbicides have to be applied to control the wide range of weed species in sugar beet crops. materials and methods: to study the effects of weeds control by hand weeding and herbicides combination with two selective herbicides at different levels of nitrogen application on sugar beet yield and quality characteristics, an experiment carried out in torbatejam township as statistical design with split plots in a randomized complete block design with three replications during 2014. experiment treatments included, the main factor involving four levels of different nitrogen application (0, 100. 150 and 200 kg.ha-1), sub factor involving combination of chloridazon + phenmedipham and metamitron + phenmedipham at 5 kg.ha-1 herbicides. four weeks after treatments, sampling of weeds and sugar beet carried out in middle of the plots with 0.5 × 0.5 quadrate. then, samples were dried at oven-dried at 75 °c for 48 h and weighed. at the final harvest, to determine the grade, amino nitrogen, sodium, potassium with betalyzr at sugar sector of agricultural center laboratory, sampling was removed from the middle of each plot. results discussion: the results showed that application of nitrogen fertilizer and herbicide treatments were significantly different from each other at 1% and 5% levels, respectively. based on experiment results, the highest root yield of sugar beet was obtained hand weeding with 200 kg n.ha-1 treatments. in between treatments of weed chemical control, metamitron + phenmedipham herbicides with 200 kg n.ha-1 was showed the highest root yield of sugar beet. high net sugar beet yield also was obtained at complete weed control with 200 kg n.ha-1, and metamitron + phenmedipham herbicides application with 200 kg n.ha-1 treatments. also, the highest net and gross sugars were obtained at without weed control + 0 kg n.ha-1 treatments. conclusion: in conclusion, according to results of this study root yield and net and gross sugar were increased by increasing 200 kg nitrogen per hectare. also, the highest net and gross sugar yield related to the using of hand weeding and combination herbicides of metamitron + phenmedipham and chloridazon + phenmedipham and application of 200 kg.ha-1 nitrogen fertilizer with weed control as well. between weed controls treatments, root yield were increased by hand weeding compared to herbicide application and between herbicide treatments at nitrogen different levels, by using of metamitron + phenmedipham than chloridazon + phenmedipham. between chemical treatments, net and gross sugar yield, shoot dry weight and dry matter yield of sugar beet were more in combination of metamitron + phenmedipham than chloridazon + phenmedipham. while, weeds density and biomass were lesser in herbicide combination of metamitron + phenmedipham compared to chloridazon + phenmedipham. on the other hand, among treatments interaction, the highest root yield and percent of sugar and pure sugar were obtained by 200 kg per hectare nitrogen fertilizer accompanied weeds control and without weeds control with lack of nitrogen application respectively.