عنوان مقاله [English]
Introduction: Elimination or reduction of tillage in conservation agricultural systems has led to wide variations in germination, emergence, and growth of weeds and has caused variations in the density and diversity of weeds under such systems. Maintaining crop residues on the soil surface has many potential benefits in agricultural production systems; such as reducing water and wind erosions, increasing the soil organic matter content, improving the soil structure and sowing conditions, as well as a better weed management through allelopathy or physical interference provided by the crop residues. Crop residue and tillage system cause potential changes in the soil temperature and water content, which influence soil density, structure, moisture, as well as soil temperature and nutrients. Crop residues act as mulches and can effect on weed seed germination and seedling emergence. Therefore, the objectives of current study were to evaluate the effects of no-tillage systems on: (i) narrow- and broadleaved weed densities, (ii) crop yield and (iii) water productivity (WP).
Materials and Methods: A two-year field study based on a split plot experiment in a randomized complete block design with four replications was carried out in Zahedshahr, Fars province, Iran (latitude 28˚44΄N, longitude 53˚48΄E, 1180 m altitude) during 2009-10 and 2010-11 growing seasons. The planting practices including moldboard plow, disk and leveler were practiced in conventional tillage plots and crop planting was performed using a drill seeder (made by Kohorasan Co., Iran). The no-tillage plots were directly planted without any soil disturbance and removing wheat residues of the last year using a direct planter machine (model Berteni, Argentina). Furrow irrigation was used at both systems and a water counter (model WD, size DN100) was applied to measure the amount of used water based on m3 ha-1. 1 x 1 m quadrates that were installed at the center of each plot. Crop density, narrow and broadleaved weed densities were recorded at 30 days after the beginning of crop emergence. Wheat grain yield was calculated at harvest time. Comparison of the means was conducted based on protected LSD (PLSD) at 0.05 significant levels.
Results and Discussion: Tillage systems indicated a significant effect on wheat density. The average number of crop seedlings in no-tillage plots was 27 % higher than in the conventional tillage plots. Maintaining crop residues on the soil surface provided a better site for crop germination and emergence. The response to tillage system and year effects varied depending on the weed species. Results showed that the effect of tillage systems, year and their interactions were significant on the density of Lolium temulentum and Sinapis arvensis, while in the case of Phalaris minor and Hordeum spontaneum just the tillage regimen significantly influenced the weed density. L. temulentum and S. arvensis plants showed an almost similar pattern in their response to tillage systems and year effects. The density of these two species significantly decreased under no-tillage system compared with conventional tillage operations. Moreover, their densities in conventional tillage plots were significantly greater in the second year of the experiment than the first year. The lower seedling emergence of P. minor and H. spontaneum under no-tillage circumstances is not surprising, as crop residues prevent from reaching the light on the soil surface and the light requirement for germination of these species has been reported in several studies. The water productivity of the no-tillage plots was greater than of the conventional ones at both two years of the experiment. Preservation of wheat residues on the soil surface decreases soil temperature via shading and causes reduces the evaporation rate from the soil surface. Although there was no significant difference between wheat yields in the two growing seasons under conventional tillage environment, wheat yield in no-tillage plots was significantly greater in the second year of the experiment than that of the first year. Low weed densities and high WP were observed under no-tillage conditions, although the crop yield was greater in conventional tillage plots. It seems that this yield reduction in no-tillage plots is mostly due to greater C/N ratio in no-tillage plots than conventional tillage ones.
Conclusion: According to the results, although wheat yield decreased under no-till system, increased water productivity, weed control and reduced cultivation costs might justify the adoption of no-tillage cropping systems by local farmers. Other principles of conservation agriculture including suitable crop rotation systems and planting cover crops must be incorporated into the no-till cropping system. As the occurrence of autumn rainfall is usual in these dry regions, employment of the stale seedbed might be another promising technique which controls early season weed species in no-tillage systems.