مجله پژوهش علف های هرز
سال چهارم شماره 2 (پاییز و زمستان 1391)

To evaluate the competition effect of cotton planted in different row spacings on growth, leaf area and its vertical distribution in canopy of velvetleaf an experiment was conducted in a completely randomized block design with split plot arrangement of treatments with three replications at Experimental Station of Gorgan University of Agricultural Sciences and Natural Resources during 2011 growing season. Result showed that maximum velvetleaf leaf area index with 6.1197, was obtained in plot of 12 plant /m-2 density and 80 cm cotton row spacing. 50% of maximum velvetleaf leaf area index was achieved after 1229, 1067.1 and 1219.8 growth degree day in 20, 40 and 80 cm of cotton row spacing, respectively. According to result of vertical distribution of leaf area study, maximum cotton leaf area in UNR, was observed in 5 plant.m-2 weed density and the most of this leaf area, with 27586.4 cm-2, was distributed at 75 cm cotton height. Maximum velvetleaf leaf area was observed in cotton planted in ultra narrow row (UNR) and 12 plant /m-2 weed density; the maximum amount of leaf area in this condition with 20870.34 cm-2 was distributed in 175 cm velvetleaf height. In contrast, in conventional row (CR) system the maximum velvetleaf leaf area was observed in 12 weed plant /m-2, and the maximum leaf area in this condition, with 34351.18 cm-2, was distributed at 225 cm velvetleaf height. In CR system, maximum cotton leaf area was obtained in no weed plant /m-2 and maximum produced leaf area in this condition, with 21658.5 cm-2, was distributed at 100 cm plant height.

Field experiments were conducted to evaluate the competitive ability of some traditional and new wheat (Triticum aestivum) cultivars with feral rye (Secale cereale L.) in Dargaz. Experimental factors were four wheat cultivars (Sayson, Alvand, Chamran and Sepahan) and plant density of feral rye at five levels of 0, 20, 40, 60 and 80 plants m-2. The new cultivar of Sepahan had lowest initial slope of yield loss (parameter I) and maximum yield loss (parameter A) in biological and grain yield traits. Alvand cultivar had maximum grain yield and number of fertile spikes. Low slope of harvest index loss and initial slope of increasing of feral rye seed blending percentage (parameter s) obtained in both Alvand and Sepahan cultivars. Maximum reduction, due to feral rye interference, resulted in Chamran cultivar that also had maximum population growth rate of weed and minimum economic threshold (ET) among cultivars. Generally, Sayson and Chamran, due to lower competitive ability and more weed seed production than the other two cultivars, were not suitable for cultivation in rye contaminated lands. Also, late maturity and coincidence of sensitive reproductive stages with region warm winds, lead to unsuitability of Sayson cultivation. Generally, Alvand cultivar due to better control of weed population, higher qualitative threshold (QT) and grain yield was provided all benefits of short-term and high-term in the fields infested by rye.

A field experiment was conducted in 2011 to determine the effect of different cover crop planting data on weed control, accumulation and remobilization of photosynthetic material in corn. The cover crops included which either planted simultaneous with corn or soybean, fenugreek and cowpea 21 days after corn planting. The results showed that the second date of cowpea planting and first date of soybean reduced dry weight of weed (97% and 93%), respectively. The highest grain yield of corn was related to weed control (11853.3 kg.h-1) and second date of cowpea planting (11447.4 kg.h-1), and the lowest one was showed in sole corn with weed interference (5246.1 kg.h-1). In addition, sole corn treatment with weed control (65.8 gr.plant-1) has the highest and first date of cowpea planting and sole corn without weed control have the lowest dry matter remobilization (17.29 and 6.28 gr.plant-1 respectively). The highest remobilization efficiency was 19.57% and 19.07% and remobilization contribution was 31.17% and 31.28% in sole corn with weed control and second date of cowpea planting, respectively.

A field experiment was conducted to determine the critical period of weed control (CPWC) of commercial potato fields in randomized complete block design with four replications at Ardebil. Trial including twelve treatments consisted of six initial weed-free periods in which plots were kept free of weeds for 0, 10, 20, 30, 40 and 50 days after crop emergence (DAE), and then weeds were allowed to grow until harvest and six initial weed-infested periods in which, weeds were allowed to grow for 0, 10, 20, 30, 40 and 50 DAE, after which the plots were kept free of weeds until harvest. Results showed that the beginning of CPWC was 472 and 593 growth degrees days (GDD) equates 10 and 18 DAE at 5 and 10% Acceptable Yield Loss (AYL) respectively. The end of the CPWC was 1513 and 1261 GDD equates 69 and 57 DAE at 5 and 10% AYL respectively. Practical implications of this study are that post-emergence herbicides or other weed control methods should be used in Ardebil to eliminate weeds from 18 days post-crop emergence up to 57 days.

The implementation of an Integrated Weed Management (IWM) system is seen by many weed scientists as a means of achieving the goal of reducing herbicides use while still maintaining crop yield. In order to study of the effect of Integrated Weed Management (IWM) emphasized on priming on yield and yield components of corn (Zea mays L.) an experiment was carried out as Randomized Complete Block Design (RCBD) with four replications in Shahrood University of Technology. The treatments included weedy check, herbicide (nicosulfuron 80 g/ha), herbicide (nicosulfuron 40 g/ha), hand weeding at all growth season, hand weeding up to 3 and 6 weeks after emergence, nicosulfuron 40 g/ha +hand weeding up to 6 weeks after emergence, hydropriming, hydropriming+ nicosulfuron 40 g/ha, hydropriming+hand weeding up to 6 weeks after emergence. Results showed that hydropriming+hand weeding at 6 weeks after emergence and hydropriming+ nicosulfuron herbicide 40 g/ha and nicosulfuron herbicide 40 g/ha + hand weeding up to 6 weeks after emergence reduced weed density and weed dry weight as well as nikosolforon herbicide 40 g/ha. There was not significantly difference between IWM treatments, hand weeding during growth season and nicosulfuron 80 g/ha treatments for 1000-seed weight trait. Treatments emphasized on seed priming produced maximum biological yield. There was not significantly difference between hand weeding during growth season treatment, hydropriming+ reduced herbicide and hydropriming+hand weeding up to 6 weeks after emergence and nicosulfuron herbicide 80 g/ha for grain yield trait. Grain yield for weedy check treatment was 39.9 percent less than hand weeding treatment. Generally, IWM emphasized on seed priming reduced weed density and weed dry weight and produced grain yield equal with hand weeding treatment and nicosulfuron herbiside at 80 g/ha treatment.

In order to identify and determine the distribution of weeds in irrigated wheat fields of Kermanshah province, 142 fields in 11 counties were investgated during 2000 to 2008. After weed survey the charsterictics of weed species were evaluated. Then density, frequency, uniformity, mean density, abundance index, Shannon- Wiener diversity index, Simpson index were determined. In irrigated wheat fields of Kermakshah province 91 weed species were identified that 15 species belonged to grass weed and 76 species were belonged to broadleaf weed. Asteraceae, Fabaceae, Poaceae and Brassicaceae were the most important families with 59. 7, 26. 5, 23 and 18. 4 percent of FIV, respectively, were most important plant families. Based on the result of this study Galium tricornatum، Centaurea depressa and Sinapis arvensis were the dominant broadleaved weeds. Hordeum spontaneum and Avena ludoviciana were the important grass weeds. In addition, the most important troubleome preharvest weeds were Glycyrrhiza glabra, Sophora alopecuroides and Convolvulus arvensis in irrigated wheat fields. Shannon- Wiener diversity index showed the highest species diversity were in Kermanshah with 3. 27 and the lowest species diversity in Javanrood and Sarpolezohab were respectively 1. 15 and 2. 28.

A field experiment was conducted in a field located in Shirvan in 2011 to determine the effect of various corn and mungbean intercropping series on yield, yield components and weed biomass. The experimental Treatments were included: sole corn (C), sole mungbean (M), 75% corn+ 25% mungbean (C75M25), 50% corn+ 50% mungbean (C50M50), 25% corn+ 75% mungbean (C25M75), 100% corn+ 25% mungbean (C100M25), 25% corn+ 100% mungbean (C25M100), 100% corn+ 50% mungbean (C100M50), and 50% corn+ 100% mungbean (C50M100). The results indicated that intercropping treatments had significant effect on weed biomass. Additive intercropping leads to reduction of weed biomass when compared with replacement intercropping. The lowest weed dry weight was resulted from C50M100 treatment. This treatment leads to reduction of lambsquarters, pigweed and johnsongrass biomass compared with other treatments. The highest yield of corn (11254.33 kg.ha-1) and mungbean (1905.07 & 1840.17 kg.ha-1) was resulted from C100M25 and M and C25M100 treatments, respectively. The highest land equivalent ratio (LER=1.75) was obtained from C100M50 treatment.

Biotechnology has the potential to bring many benefits to crop production. However, the use of this technology needs to be implemented with caution to avoid that possible risks cause problems to farm production and productivity. So far, the main traits that are commercialized with biotech crops in many parts of the world are herbicide or insect resistance. The fact is that major genes control these characteristics and international companies can easily profit from this technology. Many other traits can have benefits to crop production, including: drought resistance, improved nitrogen use, increased crop yield, increased grain quality, among others. However, most of these traits do not have genes with a universal expression, which increases the liability for companies to develop them. Alternatively, some of these traits are not easily recognized by the farms as worth for the extra technology fee that is charged with the seeds. Therefore, it is reasonable to conclude that these traits should be developed by the public sector in order to become available to the public at no extra cost. It was possible to demonstrate many benefits were attained by the farmers. But, many growers also were aware of the risks involved in the technology and preferred to adopt it with caution. Only a small fraction of the problems are perceived by the farmers, which may explain why they are very slow to avoid the risks and to prevent the problems. The fact that a crop is resistance to a herbicide do not imply that this crop-herbicide can be used as the only strategy for weed control. In reality, it is necessary to use many approaches in order to achieve long term weed control. Among the techniques recommended it is included: crop rotation, rotation of herbicides with different mechanisms of action, integration of methods of weed control, equipment cleaning to minimize the dispersion of resistant and tolerant weed species, among others. In summary, the good and old agronomic practices are still necessary to achieve optimum crop productivity during many years.