To study the competitive effects of REDROOT PIGWEED (Amaranthus retroflexus) and lambsquarter (Chenopodium album) on potato, an additive experiment was conducted in the spring of 2004 in split-split plot based on randomized complete block design with 4 replications at the Seed Potato Production Station of RAN in Firouzkooh. Treatments were included 2 weed species in main plots (Amaranthus retrot1exus and Chenopodium album), weed density in sub plots (2,66, 5.32 and 10.64 plants per m") and relative time of weed emergence in sub-sub plots (8 and 4 days prior to potato and the same time with potato emergence). Potato, cultivar Agria, was planted in constant density 53000 plants per hectare by row space of 75 cm. To interpret the competitive effects, the Cousens's regression model was used. The results showed that both species of weeds significantly reduced tuber yield. 2, 4 and 8 lambsquarter per meter of row reduced tuber yield 12.8, 22.3 and 27.2 percent, respectively. These values for REDROOT PIGWEED increased to 18, 29.4 and 39.6 percent. Parameters i and a estimated by the model were higher for REDROOT PIGWEED. Parameter "i" for three levels of lambsquarter and REDROOT PIGWEED emergence time were 10.05, 7.95, 4.48 and 19.44, 13.22, 8.86 respectively. Parameter "a" for lambsquarter was estimated 56.99, 48.7 and 31.16. This parameter for REDROOT PIGWEED was 61.92, 52.88 and 48.72. Regarding to these estimation, sooner weed emergence will result more tuber yield. Weeds did not affect mean tuber number per plant significantly. But, mean tuber weight was affect significantly. According to the results of this investigation mean tuber weight is the main factor for potato yield loss.

In order to study the influence of REDROOT PIGWEED interference with sunflower, Allstar RM hybrid, an experiment was carried out at research station of Islamic Azad University of Tabriz in 2004, with three different densities of 5, 15 and 25 plants per meter of row and three interference times of emerging with sunflower, 15 and 30 days after sunflower emergence in a distance of 15 cm from both side of the sunflower rows. The experiment was established as a factorial based on randomized complete block design. Analysis of variance indicated that stem height at the ripening stage, antodium diameter, seed number in each antodium, hollow seed percentage, 1000 kernels weight, grain yield, seed oil percentage and oil yield were affected by density and interference time of REDROOT PIGWEED. Interaction of density interference time on stem height at the ripening stage, 1000 seeds weight, grain yield, seed oil percentage and oil yield were significant. The lowest and highest hollow seed percentage obtained from density of 5 plants per meter of row at 30 days after sunflower emergence and full season interference of 25 plants per meter of row, respectively. The highest seed number in each anthodium was obtained in the second level of density. The highest 1000 seeds weight and grain yield observed from density of 5 plants m-1 of row and interference time of 30 days after sunflower emergence. Seed oil percentage in all treatments except density of 5 plants per meter of row and interference time of 30 days after sunflower emergence had significant difference in comparison with the control. It seems that, weed plant number has higher effect than weed interference time on seed oil percentage. This research showed that in studied cultivar, with the short height, lower leaf area and lower competition ability with REDROOT PIGWEED, yield loss is affected more by density than interference time.

In order to study the effect of different REDROOT PIGWEED (Amaranthus retroflexus) density on yield, yield components of soybean cultivar and determining competitive cultivar, a field experiment was carried out at Mazandaran agricultural research station 2006. Experiment was set up at a Randomized Complete Block Design with five rates of PIGWEED (0, 4, 8, 12 and 16 plant per square meter) and three cultivars of soybean (Telare, Sari and Sahare) in factorial arrangement with four replications. For calculating pod loss, number of seed per pod, 100 seed weight and yield; three parameter Cousense equation were employed. The results showed that, there were an inverse relationship between PIGWEED density and number of pod, seed per pod and 100 seed weight.The number of pod and 100 seed weight of grain decreased with increasing of the weed density, but the rate of pod number and 100 seed weight decrease in low PIGWEED density were more, than high density of PIGWEED. For this reason the relation between PIGWEED density with pod number and 100 seed weight was not linear, because in higher densities of PIGWEED by initiating intra-specific competition, the rate of competition reduced between weed and soybean, but between seed per pod and PIGWEED density there was a decreasing linear relation. The rate of number of pod per plant and weight of 100 seed decrease reduced with increasing the weed density. The soybean yield loss in 16 PIGWEED plant density in Sari, Telare and Sahare were 46.33%, 57.4% and 58.31% relative to the weed control, respectively. In conclusion the competition ability of Sari was more than other cultivars, and the amount of yield loss was lesser than Telar and Sahar cultivar.

To evaluate the REDROOT PIGWEED (Amaranthus retroflexus) reaction with a view to biological yield and seed production at interference with sunflower, a factorial experiment was conducted with sunflower cultivars (Azarghol, Hysun and Allstar), REDROOT PIGWEED densities (8.3, 25 and 41.7 plants m-2) and relative times of weed emergence (emerged with sunflower; 15 and 30 days after sunflower emergence) during 2006-7 in Tabriz. Results showed that, REDROOT PIGWEED biomass reduced with decreasing of density and delaying in emergence time. Averaged sunflower cultivars, seed production in REDROOT PIGWEED in treatment of 25 weeds m-2 at 15 days after emergence with sunflower was 1.7 times greater than full season interference of 8.3 weeds m-2. Averaged weed densities, with each week delay in REDROOT PIGWEED emergence at between first-second and second-third levels of weed interference, weed seed number per unit area ranged from 23 to 15×103, 55 to 23×103 and 45 to 62×103 in sunflower cultivars, respectively. Each day delay in weed emergence between second-third levels of interference time could decrease seed number per weed plant in Allstar greater than Azarghol and Hysun cultivars. Decrease of weed density and delaying in interference time caused a significant reduction on weed seed production index at interference with all cultivars.

The presence of weed in soybean is a serious threat to crop yield. Little research has addressed multiple weed species interference in soybean. Field experiments were conducted at the Research Farm of University of Tehran, Karaj, Iran, during the growing season of 2007 and 2008 to determine the influence of density of single and multispecies populations of common cocklebur and REDROOT PIGWEED on soybean yield and competitive abilities of weeds. Common cocklebur and REDROOT PIGWEED seeds were sown concurrently with soybean and established at selected density within 15 cm on either side of soybean row. The experimental design was factorial based on randomized complete block design with three replications. Results showed that soybean grain yield was significantly affected by weed competition. Soybean yield loss due to competition was 86% and 70% in first and second experiment, respectively. Soybean yield was affected more by common cocklebur than by REDROOT PIGWEED. Common cocklebur at a density of 2.3 (in 2007) and 6.25 (in 2008) plants per m-1 of row reduced soybean yield by 50%. In contrast, same reduction in soybean yield from REDROOT PIGWEED was obtained at density of 5.9 and 12.1 in 2007 and 2008, respectively. Competitive effect of common cocklebur was equivalent with 2.37 and 1.91 plant of REDROOT PIGWEED density in 2007 and 2008, respectively.

To study the competitive effects of REDROOT PIGWEED (Amaranthus retroflexus) and lambsquarter (Chenopodium album) on potato growth indices, an additive experiment was conducted in the spring of 2004 and 2005 in split-split plot based on randomized complete block design with 4 replications at the Seed Potato Production Station of RAN in Firouzkooh. Treatments were included 2 weed species in main plots (Amaranthus retroflexus and Chenopodium album), weed density in sub plots (2, 4 and 8 plants per meter of row) and relative time of weed emergence in sub-sub plots (8 and 4 days prior to potato and the same time with potato emergence in 2004 and the same time with potato, 2 and 4 weeks after potato in 2005). Results showed that both weed species reduced height, LAI, CGR and RGR. REDROOT PIGWEED caused more reduction in potato growth indices than lambsquarter.

The knowledge of the relationship between seedling emergence time and the prevailing environmental condition such as temperature is useful to timely application of herbicides. Two years of study was conducted to investigate seedling emergence of Xanthium strumarium and Amaranthus retroflexus in maize in two contrasting environments of Karaj and Tonkabon. Thermal time was used for predicting cumulative weed emergence. The Gompertz model was found more likely to predict weed emergence patterns for different sites and years. X. stramurium started its emergence with receiving 200 GDD. It required 500 GDD to reach its maximum emergence. The emergence of X. stramorium continued through the season up to 900 GDD. For A. retroflexus, emergence started at 400 GDD and with receiving a GDD of 600 reached its maximum level. A. retroflexus showed a relatively whole season emergence and continued its emergence up to 1200 GDD. Therefore, choosing the right time for herbicide spraying is accompanied with complexities. Predicting the start and the duration of seedling emergence in fields could optimize weed control timing.

In order to evaluate the effect of four herbicides (Ethalfluralin, Trifluralin, Pendimethalin and Metribuzin) of potato fields in IRAN for control of Lambsquarter (Chenopodium album) and REDROOT PIGWEED (Amaranthus retroflexus), an experiment was conducted at the green house of Mohaghegh Ardabili University in 2008. A dose-response study was conducted based on completely randomized design with four replications. The potato cultivar was Agria. Herbicides were used in six doses including recommended dose, higher and lower doses. All herbicides were sprayed pre-emergence. Results showed that ranking of these herbicides for weed control were Ethalfluralin, Metribuzin, Pendimethalin and Trifluralin, respectively. Also ranking of herbicides for potato safety were Metribuzin, Trifluralin, Pendimethalin and Ethalfluralin respectively.