بررسی اثر سطوح مختلف کود نیتروژن و علف‌کش پندی‌متالین بر تراکم و وزن خشک علف‌های هرز و عملکرد دانه کنجد (Sesamum indicum L.)

Introduction: Sesame (Sesamum indicum L.) is an annual and thermophilic plant, drought resistant, and short day crop. Sesame has great importance forproviding the needed oil in Iran. In the agro-ecosystems, weeds are considered as unwanted plantsneeding to be controlled. The competition between weeds and sesame for moisture, light and nutrients over the growing season reducesthe quality and quantity of crop grain yield. Although the sesame is a tall and powerful crop, it is a sensitive crop to weed competition. Since the weeds are well adapted to environmental conditions, they are successful in competitionwith crops. Chemical control seems not to be the unique and best way to manage the weeds and it reduces the sustainability of agro-ecosystems. The main approach for sustainable weed management in an integrated weed management program is to decreaseherbicides usage through increasing crop competitiveness with weeds. In this regard, fertilizer management is one of the most important crop management strategies. Among the nutrients necessary for plants, nitrogen is the most crucial nutrient required forplant growth and competition. Therefore, its application management plays a key role in decliningweed interference with crops thereby reducing the herbicide utilization. Hence, the objective of this study was to determine the effects of nitrogen levels and control methods on competition between weeds and sesame.
Materials and Methods: In order to evaluate the effect of different levels of nitrogen fertilizer and controlmethods on density and dry weight of weeds and grain yield of sesame (Sesamum indicum L.), a two-year field study was conducted as a factorial experiment in randomized complete block design with four replications during 2012 and 2013 growing seasons in Agriculture Research Center of Jiroft. The treatments included controlmethods at five levels (pendimethalin herbicide application at 0, 2, 3, and 4 lit ha-1 and weeding) as the first factor and nitrogen fertilizer rates at three levels (56, 80 and 104 kg ha-1), as the second factor. The nitrogen fertilizer rates (urea N, 46%) corresponded to 70, 100 and 130% of the optimum nitrogen requirement. 80 kg ha-1 net nitrogen was considered as the optimum rate needed for sesame production. Fertilizerwas placedbeside the plants and banded beneath the surface in three stages (pre-planting, pre-flowering and pre- capsule forming). The herbicide was applied using a back pack sprayer equipped with a flood jet nozzle with 2 bar pressure and 350 lit ha-1 output volume of the sprayer. Weed species each were oven-dried at 75°C for 48 h and weighed for measuring dry weight. Also, the grain yield of sesame was measured at the end of the growing season. The data were statistically analyzed using combined variance of analysis and the differences among mean values of the treatments were compared by LSD test (p≤0.05) using SAS 9.4 software.
Results and Discussion: Evaluation of weed flora indicated that 5 weed species including Egyptian crowfoot grass (Dactyloctenium aegyptiumL.), prostrate pigweed (Amaranthus blitoides S.Wats), digera (Digera muricata (L.) Mart.), purple nutsedge (Cyperus rotundus L.), and barnyard grass (Echinochloa colonum L.) were dominant species, with high evenness in the experimental plots. The results showed that the weed control method and nitrogen levels had a significant effect (p≤0.01) on density and dry weight of all weeds. Further, the weed control method had a significant effect (p≤0.01) on grain yield of sesame in the first and second year.The influences of nitrogen levels and interaction effects were, however, not statistically significant on grain yield,whereas the effect of nitrogen levels on density and dry weight of weeds was different.Under non-weed control conditions, nitrogen fertilizer increased the density and dry matter of weeds, and under weed control conditions, nitrogen fertilizer decreased the density and dry matter of weeds. After hand weeding, the application of pendimethalin at 4 lit ha-1 in addition tousing104 and 80 kg ha-1 of nitrogen was the best treatment for reducing the density and biomass of weeds. Furthermore, the application of pendimethalin at 4 lit ha-1 resulted in the greatest seed yield.
Conclusions: Using agronomic methods to improve crop competitiveness against weeds can be effective in reducing the contribution of herbicides into weed management programs. Moreover, these approaches would be considered as effective step towards sustainable weed management. It seems that combining nitrogen application with weed control improves the efficacy of weed control through improving competitive ability of crops. Therefore, it is necessary to consider the application of nitrogen fertilizer as an effective component in the integrated weeds control management in sesame production system.