Response of dominant Weedsin wheat field to herbicide application rate.
Chemical weed control seems indispensable and has proved efficient in controlling weed . In Iran, herbicide usage accounts for 41% of the total pesticide consumption. Indiscriminate use of herbicides for weed control during the past few decades has resulted in serious ecological and environmental problems, such as resistance and shifts in weed populations , and greater environmental and health hazards .
Providing a weed-free environment from the time of planting to canopy closure is important for strengthening the native groundcover's competitive ability against weed invasions. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed . Quality losses, due to contamination of cereal samples by wild oats, can be substantial, resulting in rejection for seed and milling. Success of a herbicide application is dependent upon weed species, the timeliness and thoroughness of application, conditions at the time of application, herbicide rate and crop management after the application. Application of herbicides in proper dose would reduce off-target movement of herbicide, and maximize weed control .The efficacy of any herbicide depends predominately on the dose used  and in many instances the same is also decisive for its selectivity. Registered herbicide doses are set to achieve upper limits of weed control under varying compositions, densities, weed growth stages and environmental conditions, and there may be an overestimation of the dose required to get adequate control . To ensure satisfactory weed control, even under unfavorable regimes of crop production factors, manufacturers often recommended higher than necessary doses of an herbicide. However, it is not always necessarily to apply full herbicide dose  and there can flexibility regarding herbicide rates depending on the weed spectrum, densities, their growth stage and environmental conditions . In a study to compare the efficacy of Apyrus[R], Total[R] as the two sulfonylurea herbicides on wild barley control in relation with growth stages of wild barley, Total[R] herbicide had better efficacy to control wild barley in comparison with Apyrus[R] so that by using Total in the second node and 2-4 leaf+second node density decreased to 0 (score 1). The application of herbicide Apyrus[R] in the second node and 2-4 leaf +second node stage of wild barley were low in quantity of wild barley efficient density and dry weight (score 5), respectively. Total[R] efficacy was high in 2-4 leaf stage compared to Apyrus[R] with decreasing 90.78% and 95.55% density and dry weight of wild barley, respectively . Dose-response studies are an important tool in weed science. The use of such studies has become especially prevalent following the widespread development of herbicide resistant weeds .The present study was designed to determine the effect of reduced doses of newly introduced post-emergence herbicides on the weeds control under the semi-arid conditions of Tabriz, Iran.
MATERIALS AND METHODS
The experiment was conducted at the Research Station of the Islamic Azad University, Tabriz, Iran in 2012.Tabriz (Lat. 38[degrees], 5'; Long. 46[degrees], 17' and elevation 1360 m) is located at the north-west of Iran and the climate is semi-arid. Experimental soil was sandy-loam with EC of 0.80 ds.[m.sup.-1], pH of 7.2 and 0.81% organic matter. Some of weather data in experimental site during crop growth cycle is given in table 1.
The experiment was laid out in a randomized complete block design in field condition with three replicates. Seeds of wheat (Triticum aestivum cv. Alvand) used for this study were obtained from Karaj Seed Improvement Institute, Iran. Moisture content of the seed was 10%. The cultivar under study was a medium-repining variety with growth period of 115-120 days. Plot sizes was 5 m x 4 m.
In all the plots two cultivations with a tractor-drawn cultivator along with manuring 12 t [ha.sup.-1] followed by a planking were given to achieve desirable soil structure. Then fields were plotted in the early spring before sowing. Seedswerehand sown on 5th April in rows 15 cm apart at 4-5 cm depth. Based on soil analysis fertilizers P and K were applied basally at the rate of 100 and 50 kg [ha.sup.-1] respectively. Nitrogen was applied at the rate of 130 kg [ha.sup.-1], of which 50% was applied basally and the rest at booting stage. Fiveirrigations were given to all treatments until 20 days after flowering. Herbicides including Apyrus[R], Total[R] and Shualieh[R] were used to control emerged broad leaf and narrow leaf weeds. These herbicides have been previously tested safe on well established wheat plants. The herbicides were applied before stem elongation of wheat in early spring with concentrations of 19.95 g ai, 15.96 g ai, 11.97 g ai [ha.sup.-1] for Apyrus[R]; 36 g ai, 28.8 g ai, 21.6 g ai [ha.sup.- 1] for Total[R] and 24 g ai, 19.2 g ai, 14.4 g ai [ha.sup.-1] for Shualieh[R]. Any pesticide used to control insects. The seeds harvested separately for each plot between 3-8August when spikes were fully ripened at the approximate moisture of 20 %.
At harvesting, agronomic traits and yield components was examined following standard procedures.
Data were statistically analysed using the software MSTAT-C. Analysis of variance was used to test the significance of variance sources, while LSD test (P=0.05) was used to compare the differences among treatment means.
RESULTS AND DISCUSSION
Five weeks after spraying wheat plants treated with concentrations of 19.95 g ai [ha.sup.-1] for Apyrus[R]; 36 g ai [ha.sup.-1] for Total[R] and 24 g ai [ha.sup.-1]for Shualieh[R] produced 1618, 1658 and 1620 g [m.sup.-2], respectively, but only 1325 and 1210 g [m.sup.-2] biomass were produced in limited dose of 11.97 g ai [ha.sup.-1] for Apyrus[R]and 14.4 g ai [ha.sup.-1]for Shualieh[R]. When wheat plants were sprayed with reduced doses of Total[R] biomass production in crop plants increased 8% and 7.4%, as compared with the over-dose treatment (Table 2). Belles et al.  reported > 85% and consistent control of wild oat with a half-dose of tralkoxydim and Atlantis 3.6 WG (iodo+mesosulfuron). It seems that weeds in lower dose of Apyrus[R] and Shualieh[R] have not completely controlled.
Tillers number per plant was not affected by the treatments, and all plants produced nearly two tillers. When crop plants were sprayed with different concentrations of post-emergence herbicides seed number produced in wheat spikes was 24.7, but only produced 20 and 13seeds per spike in those plants treated with limited dose of Apyrus[R] and Shualieh[R] (Table 2). It can be concluded that Total[R] is more effective than other herbicides in weed controlling especially in limited dose.
In this study, thousand grain weight in wheat follows the same trend asseed number per spike (Figure 1) and all plants improved seeds with 47 g thousand seed weight at interference with weeds, but thousand seed weight in plots with lower dose of Apyrus[R] and Shualieh[R] reduced thousand seed weight up to 23% and 19% compared with mean of other treatments. Madandoust et al.  in a study on selective herbicides and wheat cultivars resulted that there is a positive and significant correlation between thousand seed weight and seed yield of wheat.
Effect of herbicides and applied doses on seed yield of wheat was significant. The highest yield obtained from those plots treated with 19.95 g ai and 15.96 g ai [ha.sup.-1] for Apyrus[R] and 36 g ai and 28.8 g ai [ha.sup.- 1]for Total[R] followed by 24 g ai [ha.sup.-1]for Shualieh[R].
Avena species are highly competitive, resulting in greater reduction of wheat seed yield, i.e., 85% at 320 seedlings per square meter. Rao emphasized on seed yield reductions of 26 to 63% from four bread wheat cultivars at 90 weed seedlings [m.sup.-2]. With decreasing of herbicides application seed yield reduced 60%, 6% and 36% in Apyrus[R], Total[R] and Shualieh[R], respectively, as compared to the full dose treatment (Figure 2). A mix stand of narrow leaf and broad-leaved weeds is reported to cause 48% yield loss of wheat . Numerous herbicide molecules at lower than recommended rates are effective enough to provide satisfactory weed control without sacrificing yields and increasing weed infestation in the following years [7, 15]. Reduced herbicide doses seem to offer a promising tool for decreasing herbicide usage across the globe. Zhang et al.  reviewed use of reduced herbicide doses and concluded that weed control efficiency tends to be lower and more erratic at reduced doses than at recommended doses, although it was commercially acceptable in most cases.
Wheat farmers could be obtain similar yield by reducing application rates of Apyrus[R] from 19.95 g ai to 15.96 g ai [ha.sup.-1] and Total[R] from 36 g ai to 28.8 g ai [ha.sup.-1].
 Kahramanoglu, I., and F.N. Uygur, 2010. The Effects of Reduced Doses and Application Timing of Metribuzin on Redroot Pigweed (Amaranthusretroflexus L.) and Wild Mustard (Sinapisarvensis L.). Turkish Journal of Agriculture and Forestry, 34: 467-474.
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 Esmaili, A., N. Biabani and Z. Orangi, 2011. Comparison of sulfosulfuron Apyrus[R], sulfosulfuron + metsulfuron-methyl Total[R] herbicides on Hordeumspontaneun control and wheat yield. J. Weed Agroecology, 2(1): 33-42.
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Mitra Nazeri Gahkani and Bahram Mirshekari
Department of Agronomy and Plant Breeding, Tabriz Branch, Islamic Azad University, Tabriz, Iran
Received 11 October 2014
Received in revised form 21 November 2014
Accepted 25 December 2014
Available online 16 January 2015
Corresponding Author: Bahram Mirshekari, Department of Agronomy and Plant Breeding, Tabriz Branch, Islamic Azad University, Tabriz, Iran
Table 1: Some of weather data for experimental site during growth period of wheat. Month Temperature Precipitation Relative ([degrees]C) (mm) humidity (%) April 20.3 38.5 62.0 May 27.9 21.0 60.3 June 31.9 3.5 32.7 July 28.1 3.0 30.7 August 16.1 14.0 69.2 Total -- 79.0 -- Table 2: Above-ground biomass and seed per spike in wheat plants as affected by hebicide dose. Treatments Apyrus[R] Total[R] Application rate (g ai[ha.sup.-1]) 19.95 15.96 11.97 36 28.8 21.6 Above-ground biomass 1618 1400 1325 1658 1525 1535 (g [m.sup.-2]) Seed no. per spike 20 25 24 24 27 24 Treatments Shualieh[R] LSD (5%) Application rate (g ai[ha.sup.-1]) 24 19.2 14.4 Above-ground biomass 1620 1200 1210 130.9 (g [m.sup.-2]) Seed no. per spike 13 24 25 3.2
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|Author:||Gahkani, Mitra Nazeri; Mirshekari, Bahram|
|Publication:||Advances in Environmental Biology|
|Date:||Jan 15, 2015|
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