Effect of Crop Residue and Nitrogen Management on Weeds Dynamics and Growth in Cotton (Gossypium hirsutum L.) Farm

Conservation agriculture is an appropriate strategy for maintaining and improving agricultural resources which increases crop production and stability and also provides environmental protection. This attitude contributes to the conservation of natural resources (soil, water, and air) and it is one of the most effective ways to overcome the drought crisis, water management and compensation of soil organic matter in arid and semi-arid regions. Practicing the conservation agricultural systems, which requires an effective usage of previous crop residues and reduced tillage methods, is an irrefutable necessity for arid regions like South Khorasan with low soil organic matter. The addition of crop residues to the soil in conservation agricultural systems, however, might immobilize nitrogen as an important nutritional element affecting plant growth. The transition from traditional to conservation agricultural systems, nevertheless, would affect different constituents of agroecosystems including weeds dynamics and growth, which would eventually affect the crop production in these systems. Given that South Khorasan farmers still use traditional methods of cultivation and they do not return the previous crop residues to the soil in their farming systems, the aim of this study was to investigate the effect of crop residue management and nitrogen on dynamics and growth of weeds of cotton farm. In order to investigate the effect of crop residue management and nitrogen on the growth and dynamics of weeds of cotton farm, an experiment was carried out as split factorial design based on RCBD with three replications at the research field of Faculty of Agriculture, University of Birjand in 2013. In this experiment, the main plot was two tillage methods, including conventional tillage (moldboard plowing) and reduced tillage (disking) and the subplot consisted of a factorial combination of two nitrogen levels (50 and 150 kg ha-1) and five barley residue levels (0, 77, 154, 231 and 308 g m-2) which have been randomly distributed as a factorial in subplots. Weed samplings were performed at four stages including 30, 60, 90 days after planting and also at harvest time and after each sampling, weed density, dry matter and leaf area were measured. Results showed that plow type, residual amount levels and nitrogen fertilizer rate had significant effects on measured traits of weed species including density and dry matter of weeds as well as their leaf area. Increasing the residue amount significantly reduced weeds growth traits. The greatest density, dry matter and leaf area of weeds were observed with disk plow, while mouldboard plowing significantly reduced these traits. A lower disturbance of soil in conservation agricultural systems, which is the case with our study where disking was applied, often results in most weed seeds to accumulate on the upper soil layers and eventually might ends up with a higher weed density. The greater rate of nitrogen resulted in higher density, dry matter and leaf area of weeds. The stimulatory influence of nitrogen on weed emergence has been previously substantiated. Moreover, the interaction effects of studied factors were significant on weeds growth traits. The interaction effects of plow type by residue amount showed that the lowest weed densities were observed with mouldboard plow under all residue amount and the using greatest residue amount (308 g m-2) resulted in the lowest weed density under both tillage regimes. The control treatment (no residue) interestingly showed a lower amount of weed density compared with residue amounts of 77 and 154 g m-2. Weed control by crop residues is probably due to different factors like prevention of light penetration to the soil surface, exuding allelopathic substances, influencing soil nitrate content and moderating soil temperatures and improving crop growth. Our study, however, showed that lower amounts of residues on the soil surface cannot provide enough inhibitory effects to suppress weeds and might even stimulate weed emergence through maintaining more moisture under the residue layer at hot air of early summer. The results of this study revealed that although employment of conservation tillage systems is a suitable method to achieve sustainable agriculture, more extensive research studies are needed on the effects of these systems on weed density and their competition with crop plants. Thus, cover crops and conservation tillage systems can be used as a promising solution for the development of sustainable agriculture and protecting the health of ecosystems. Due to widespread cultivations of wheat and barley in our country, it seems that incorporating the cereal crop residues into the soil might inhibit weeds growth in cropping systems. Our study also showed that nitrogen management is very important strategy in regard to weeds growth in these systems.