نشریه دانش علفهای هرز
سال یازدهم شماره 1 (بهار و تابستان 1394)

An experiment was conducted to find out effects of different tillage systems on soil weed seedbank density under cornwheat rotation. A split-plot trial based on RCBD with six replications was carried out during two years (2012 and 2013). Main plots included conventional seed bed prepration by moldboard ploughing (CT), minimum tillage (MT) and no-tillage (NT). Each main plot was divided in to two subplots (with herbicide and control). Seed densities were estimated by direct seed extraction in systematic method by an auger with 7cm diameter from 0-20 cm depth from each sampled plot prior to corn planting, just before applying treatments. Sampling was also replicated and continued after corn harvesting, pre and post tillage and finally after wheat harvesting. The seedbank was initially composed of 19 species, mostly annuals. Mean comparison of weed seedbank post wheat harvesting showed Sorghum halepense, Lactuca serriola and Convolvulus arvensis was significantly increased in no till plots, while perennial weed seeds such as Convolvulus arvensis and Malva neglecta were not affected by herbicides. CT, moldboard ploughing increased Avena loduviciana and Hordeum sponteneum seeds significantly by vertical distribution of buried seeds to soil surface.

To evaluate weed management problems in wheat fields during 2011 and 2012, 1300 farms were visited, in 13 provinces with different climatic conditions. 1300 Farms (each province 100 farms) were visited, and information on weed management methods was collected according to a questionnaire form. These forms were later evaluated and analyzed. Changes of the most dominant weed species in the last decade indicate that management actions during this period were not appropriate. The usual method for managing weeds in wheat fields were chemical control and the use of non-chemical methods was not used very common. In more than 20% of cases herbicides were not used at the right time. Most farmers (over 50%) used herbicides clodinafop-propargyl and tribenuron-methyl herbicides and the ability to mix with each other due to their low price, easy accessibility. About 30% of farmers used standard equipment o spray herbicides (tractor air assisted boom and micron air sprayer) but the rest used unconventional equipment (tractor air assisted lance, turbo liner, atomizer and wheelbarrow lance sprayer). The major weak points in wheat management practices included: Unawareness of the extent of damages caused by weeds, delayed weed control, incorrect spraying practices, lack of access to necessary instructions and failure to comply with the proper cropping pattern.

Hordeum spontaneum C.Koch. is known as a significant factor in decreasing wheat yield. Its control by stale seedbed is a step toward achieving sustainable agriculture in wheat. For this purpose, field experiments were performed during 2007-2008 and 2008-2009 in the experimental fields of the Iranian Institute of Plant Protection, Varamin, Iran. In order to contaminate the field with the weed, wheat was planted in association with H. spontaneum, during the first year. After harvesting wheat in the summer, an experiment was conducted with 9 treatments. Treatments included: 1) field plowing after wheat harvesting and corn (Cultivar SC704) planting, irrigating weekly up to corn harvesting, 2) field plowing no corn cultivation, but irriatted up to corn harvesting in treatment 1. Treatments 3, 4, 5, 6, 7, 8, and 9 were not plowed but irrigated only once, each at an indicated date (July 4th, July 19th, August 1st, August 18th, September 10th, September 23th, and October 8th). Results revealed that stale seedbed from the middle of August favored H. spontaneum emergence. Thus its density reduced in wheat up to two thirds and wheat grain yield increased two times. Field plowing after wheat harvesting increased H. spontaneum burial depth and reduced its density in corn and wheat.

Winter weeds have different emergence flashes in field and response to this question whether there is flash emergence after applying herbicide, can be effective in the selection of right time for weed control. Therefore, emergence pattern of the wild oat seedlings was studied for three years from 2011 to 2014 in cereal fields of Fars province. The cumulative emergence pattern of wild oat during 2011-2012 showed that 50% of emergence occurred at growing degree days (GDD) of 115. The emergence percent revealed that 21.09, 45.00, 19.37 and 2.43% seeds respectively at GDD of 99.25, 136.75, 180 and 220 were emergence. The emergence pattern in 2012-2013 crop years also showed that a GDD of 127 was required for 50% emergence. This amount for 2013-2014 crop years was 146 GDD. With regard to the contamination and presence of wild oats in the fields, final results of this study showed that the hypothesis of various flashes after the herbicide application is rejected. All the weed seedlings emerge in in fields during the first month after planting.

In order to evaluate the effect of nitrogen fertilizer and nicosulfuron rimsulfuron herbicide on weed control in grain maizeSC704 hybrid, a field study was conducted as a strip experiment based on randomized complete block design with three replications during 2011 growing season in Research Field of Shiraz University located in Bajgah. The treatments included nitrogen fertilizer rates at four levels (0, 180, 270 and 360 kg ha-1), as main plots A and nicosulfuron rimsulfuron herbicide dosage at four levels (0, one-third, one-half and equal to the recommended dosage175 g ai ha-1) as secondary plot B. Results of interaction analysis showed that maximum weed dry weight was obtained in herbicide free plots with 360 kg ha-1 nitrogen fertilizer; which was significantly different from the recommended herbicide dosage in all fertilization levels. Minimum weeds dry weight was observed in recommended dosage plots with 270 kg ha-1 nitrogen which was not significantly different from 360 kg ha-1 nitrogen. There was no significant difference between 270 and 360 kg N ha-1 under the recommended herbicide dosage (175 g ai ha-1), in terms of maize yield and its components. Therefore, in order to achieve the maximum grain yield and best weed control, it seems that the recommended herbicide dosage and 270 kg N ha-1 would be the best option.

This research was conducted to study the relative fitness of wild mustard biotypes of Khoozestan, Golestan and Kermanshah provinces that are resistant to Acetolactate Synthase (ALS) Inhibitors. Green-house experiments were conducted in two parts. In the first part, plant dry matter, leaf area and plant height change of resistant and susceptible biotypes of each province were compared under no competition with wheat condition. In the second part, growth ability of resistant and susceptible biotypes of each province, growth ability and yield of wheat were studied under competition conditions. According to the results, resistant biotype Ker-2 had significantly higher than susceptible biotype Ker-1, but there was no significant difference between them on the point of final height and dry weight per plant. Resistant biotypes of Golestan and Khoozestan (Kh5 & G3) were lower than susceptible biotypes of these provinces (Kh3 & G9) on the point of mentioned three criteria, significantly under competition with wheat condition. Resistant biotypes of Ker-2 and susceptible biotypes of Ker-1 had no significant difference on the point of ability of competition with wheat. Relative interference index of the two biotypes of Kermanshah were not significantly, different. Finally, difference of growth and yield of wheat were not significant in competition with these two biotypes. According to the results of Golestan and Khoozestan, susceptible biotypes of them (Kh3 & G9) had significantly higher growth and interference index than resistant biotypes (Kh5 & G3) in competition with wheat. Also, wheat had higher grain growth and yield in competition with resistant biotypes rather than susceptible biotypes. Finaly, results showed that among the three resistant biotypes, only Ker-2 had the same competition and growth ability to susceptible biotypes. Therefor it, could disperse rapidly and make high interference due to its resistance to ALS inhibitors.

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.

Wild oat is an economically important annual weed throughout small grains production areas. Timely and more accurate control of wild oat may be developed if there is a better understanding of its emergence patterns. Six wild oat populations, collected from different climate regions, were sown in agricultural reaserch field of University of Tehran located at Karaj, Iran during 2012-2013 growing season. The experimental design was a randomized complete blocks with factorial arrangement of treatments in three replications. Wild oat populations were collected from Ahvaz, Karaj, Kamyaran, Ghazvin, Shiraz and Isfahan. Seedling emergence pattern was surveyed under rain fed and irrigated conditions. Rusults indicated that seedlings emergence were started in 306 growing degrees day (GDD) Less GDD was needed for irrigated (461-482) compared with rain fed conditions (546-665 GDD) to reach 50% emergence. There was no difference of GDD to reach 50% emergence, in Ghazvin and Ahvaz populations under the two moisture conditions. Among wild oat populations, Ahvaz needed maximum growing degree days to reach 50% emergence, receiving 784 and 755 GDD under rainfed and irrigated conditions, respectively. Overall, the results showed that wild oat emergence could be predicted based on growing degree days from soil temperature. This approach can help to provide appropriate time of wild oat control.