IRANIAN JOURNAL OF WEED SCIENCE
Year:2010 | Volume:6 | Issue:2|Papers Count:8

Field experiment was conducted to study the interaction between imazethapeyr dose and Xanthium strumarium density on soybean yield. Four densities of weed (0, 4, 8 and 12 plant.m-2) × 5 doses of herbicide (0, 25, 50, 75 and 100 g a. i.ha-1) was assigned in a factorial arrange of treatments with four replications. A combined model incorporated standard dose response curve and rectangular hyperbola competition model gave a good description for soybean yield. When no herbicide was applied, soybean yield was decreased by 58, 73 and 80% at densities of 4, 8 and 12 plant.m-2. Where the densities of common cocklebur was low (0, 4 plant. m-2) applying herbicide at half of the recommended dose could save the yield by 90% however; with increasing weed densities to 8 and 12 plant.m-2, 20 and 30% yield loss was caused in soybean yield. No difference was found between herbicide application at 75 and 100 rate of the recommended dose.

In order to evaluate the efficacy of molecular method to detect the resistance of weeds to herbicides, 69 resistant or susceptible grass weed biotypes to clodinafop propargyl herbicide, including Lolium spp. (6 biotype), Avena spp. (30 biotype), and Phalaris spp. (33 biotype) were studied, dCAPS method as a simple method to diagnosis Ile1781Leu amino acid substitution in ACCase enzyme was used. The results of greenhouse experiment for Avena spp., Phalaris spp. andLolium spp. were 90%, 79% and 100% similar to molecular experiment respectively. In general, the similarity of molecular method with current methods was about 85%. Isoleucine (Ile) to leucine (Lue) substitution at position 1781 (Ile /Leu) was the most common amino acid substitution selected with ACCase-inhibiting herbicide. The partial differences (about 15%) between glass-house and molecular methods also can explain by non-target site mechanism (enhancing metabolism) or another amino acid substitution.

In order to investigate the effect of most common paddy selective herbicides on causing growth disorder called dwarfism two experiments were laid out in 2007 in Rice Research Institute of Iran, Rasht. The dose response study was carried out by herbicides thiobencarb "TB", butachlor and oxadiargil and Hashemi rice cultivar in Petri dishes containing agar and Youshida growth medium. Visual evaluation of herbicides toxicity on seedlings showed that TB caused specific growth irregularity including dark greenish leaves, dwarf stature of seedlings, short thick leaves and brown color roots that are known as symptoms of dwarfism. Regression analysis of rice seedlings height, root length and fresh weight by fitting sigmoidal logistic dose response equations showed that the inhibitory effect of thiobencarb on rice growth was significantly more than two other herbicides (P<0.001) as shown by model parameters. The dosage needed for 50% reduction in height (GR50) of rice seedlings was 40 times lesser for TB compared with two other herbicides. The second experiment was arranged in order to evaluate the accuracy the results of first study, as dwarfism symptoms were only observed in Petri dishes treated with TB. The study was carried out in pots containing paddy soils with the precedence of severe dwarfism. Treatments included time of TB application (before and after transplanting), pots water level (saturated and flooded) and dosages (in 6 levels 0, 2, 4, 6, 8 & 10 kg.ai.ha-1) in a split split plot design with factorial arrangement of treatments in 3 replications. Also two other herbicides were included as control. Toxicity of herbicides were evaluated visually based on 0-100 where 0 indicates no effect, and 100 was complete plant death.Results of this experiment also confirmed the results of previous one and symptoms of dwarfism were observed only in pots treated with TB. Data analysis showed that toxicity of TB was affected by dosage, time of application and water level significantly (P<0.0001). According to these results applying the TB in saturated soils was three times more toxic for transplanted rice compared with flooded pots, and there was a direct significant (P<0.001) sigmoidal relation (r2>95%) between dosages and toxicity of TB. Spraying TB after transplanting compared with before transplanting decreased dwarfness intensity significantly. Based on the results of this study for management of TB toxicity in paddy rice it is needed the herbicide to be applied in flooded soils, and after transplanting in minimum recommended doses.

To determine the critical period of weed control of two planting distances on canola yield, a factorial design with 3 replications was conducted using Hayola 401 variety, in paddy field of Rice Research Institute of Iran (RRII) in Rasht, during cropping season of 2006.Planting distances (15 and 30 cm) and hand weedings (12 treatments in two sets) were the two factors of the experiment. The hand weeding treatments were in two sets, the first set was weed free periods including cotyledon stage, 0, 2, 4, 8 leaf stages and flowering stage. The second set of hand weeding treatments was weed infested periods including cotyledon stage, 0, 2, 4, 8 leaf stages and flowering stage. The two sets of hand weeding treatments were fitted to Logistic and Gompertz functions to determine the critical period of weed control in this plant. Results showed that the effect of planting distances of canola, treatments of weeding, among weeded and unweeded treatments, and their interactions were significant on grain yield, biological yields, oil production and harvest index. There were significant differences between weed infested and weed free treatment in oil yield, grain and biological yields in each planting distances. The 30 cm planting distances had significantly higher grain yield (2502.4 kg/h) than 15 cm planting distances (2141 kg/h). The critical period of weed control for 15 cm distance was between 27 and 85 days after planting (DAP) (emergence to 10-leaf stage), and between 35 to 64 DAP (2-6 leaf stages), using 10 and 5% acceptable yield reduction, respectively. In 30 cm planting distances, these periods were between 32 and 95 DAP (2-10 leaf stages) and 50 to 77 DAP (4-8 leaf stages) using 10 and 5% acceptable yield reduction, respectively.

Increasing public awareness and concern about the impacts of herbicides on the environment, development of herbicide-resistant weeds, and high economic cost of herbicides have increased the need to reduce the amount of herbicides used in agriculture.Prediction of weed emergence timing would help reduce herbicide use through the optimization of the timing of weed control. There are several models that could be used for predicting weed seedling emergence. However, the ability to predict emergence of given species is different between models. For better prediction of emergence we should be able to select a suitable model. Therefore, Xanthium strumarium seedling emergence at two different burial depths from an experiment conducted in 2009-2010, was used to find and develop the best emergence model. The number of X. strumarium seedlings was recorded every three days and then removed from pots. Emergence for each species was expressed as a cumulative percentage of total emergences. Percentage of cumulative emergence values was explained against thermal time (TT) using Logistic, Gompertz and Weibull modified functions. The three models were compared using the Akaike information criterion. The Weibull model gave a better description than other models. Conversely, Logistic model gave the worst fit, with AIC values far higher than Weibull and Gompertz models. Thermal time required for given seedling emergence was affected by burial depth and increased with soil depth. For example, when seeds buried in the 5 cm depth, they required 744 TT for 50% emergence. However, seeds in 2 cm depth had a shorter emergence time-span and required 391-488 TT for 50% emergence.

In this study the efficacy of 2, 4-DB (Butress 42.3%EC) on weed control of alfalfa was compared with commonly used herbicides in Alfalfa. This experiment was conducted in Hamedan and Yazd during 1386-1387 in a randomized complete block design with 3 replications and 11 treatments in established Alfalfa. Treatments were included of bentazone (Bazagran) (SL 48%) at doses of 2, 3, and 4 L/ha, 2, 4-DB (Buterss) (EC 42.3%) at doses of 2.5, 2.8, 3.1, and 3.5 L/ha, imazethapyr (Pursuit) (SL 10%) at doses of 0.4, 0.7, and 1 L/ha and weedy check as treatment.The effect of herbicides on alfalfa biomass and the biomass and weeds biomass and density were studied during 3 harvestings. In Hamedan, all herbicides treatments had no phytotoxocity on alfalfa.Only bentazone caused chlorosis in alfalfa that recovered after 1 to 2 weeks. Butress not only was suitable to control the weeds, especially in field bindweed (Convolvulus arvensis) control. In Yazd, 2, 4-DB specially in its higher doses had partial phytotoxicity on alfalfa, but controlled well Malva, Lactuca and field bindweed. Butress reduced the density and biomass of field bindweed by 100% and 84%, respectively. As a conclusion, 2, 4-DB specially in its higher doses was the best herbicide to control field bindweed.

Acetyl co-enzyme A carboxylase inhibitors are commonly used for post emergence control of grass weeds in broad leaf crops. Some of these herbicides are applied to control grass weeds in wheat and barley fields. Recently some aryloxyphenoxy propionate-resistantPhalaris minor Retz. populations that have developed the resistance to diclofop methyl, fenoxaprop-P butyl and clodinafop propargyl, have been detected in wheat fields of Fars where these graminicides had been applied for several years. Previous biochemical studies revealed that mechanism of resistance in SR3, MR4 and AR biotypes is due to presence of an herbicide-resistant AC Case enzyme in the biotypes. There was not significant difference between susceptibility of AC Case enzyme of other resistant biotypes (FR2, FR3, FR4, FR5, FR6, FR8, MR1, MR2, ER1, ER2 and GR2-1) and susceptible one (ES). In order to study the non-target site based mechanism (s) of resistance to diclofop methyl in two resistant Phalaris minor Retz. biotypes, some experiments were conducted to investigate retention, absorption, translocation and metabolism of diclofop methyl in the resistant (SR3 and FR8) and susceptible (ES) biotypes in Agro – Biochemistry Laboratory of Córdoba University, Spain, during 2008.The amount of herbicide retention and absorption did not differ among resistant and susceptible biotypes. Almost all of the absorbed herbicide remained in the treated leaf and only a little bit of diclofop methyl was trans located to other parts of the treated plants in both resistant and susceptible biotypes. The results of this study showed that there was not difference in retention, absorption, translocation and metabolism of diclofop methyl between FR8 and susceptible one, suggesting unknown mechanism of resistance may be involved in FR8 plants.

Pattern of leaf area and plant dry matter distribution, can play key role in weed-crop competition for achieving the light.A field study was undertaken to aim of investigation of leaf area and dry matter profile of soybean cultivars and weeds.The experimental design was a randomized complete block using a split-plot arrangement with three replications. Main plots were weed infested (natural infestation) and weed free, and subplots were four soybean cultivars (Sahar, Hil, Sari, Telar) and two lines (033, 032). At soybean canopy closure stage, sampling of leaf area and dry matter profile of cultivars and weeds was carried out. Interference with weeds caused different cultivars of soybean shift the maximum layer of leaf area and dry matter to the upper layer of canopy. Distribution of leaf area and dry matter in weeds canopy was related to its species and the growth form.Abutilon theophrasti increased its height and height of branching, and spread its leaves on the soybean canopy. The hill cultivar in weedy plots produced maximum grain yield and had more weed suppressive ability than other ones.