نشریه دانش علفهای هرز
سال دوم شماره 1 (بهار و تابستان 1385)

To determine the critical period of weed control in safflower (Carthamus tinctorius), two field studies were conducted in 2002 at the Bajgah and Kooshkak Experimental Stations, College of Agriculture, Shiraz University. Each experiment consisted of two series of treatments. In the first series, weeds were kept in place until crop emergence, leaf formation, stem elongation, lateral stem emergence, head emergence, flowering and ripening, and were then removed and the crop kept weed-free for the rest of the season. In the second series, crops were kept weed-free until the above growth stages after which weeds were allowed to grow in the plots for the rest of the season. The beginning of the critical period was defined as the crop stage in which weed interference reduced crop yield by 10%. Similarly, the end of the critical period was defined as the crop stage in which the crop had to be weed free to prevent more than 10% yield loss. The Weibull and MMF models were used to determine the critical period. The critical period of weed control occurred between safflower early stem elongation and flowering stages (144-220 days after planting, DAP) at the Bajgah site and between early stem elongation andearly head emergence stages (135-184 DAP) at Kooshkak. The onset of interference seems to be less variable than its conclusion, indicating that early weed control is necessary to prevent yield loss. Also, with increase in as the weed interference period increased so yield components (number of heads per plant and number of seeds per head) decreased significantly.

During 2002-2003 growing season, two field experiments in Fars and Khuzestan provinces and three greenhouse experiments at Plant Pest and Disease Research Institute, Tehran, were performed to evaluate the resistance of wild oat to aryloxyphenoxy propionate herbicides. Seed populations of wild oat were collected from nine locations in Fars and Khuzestan, in which seven populations were expected to be resistant to fenoxaprop-p-ethyl or clodinafoppropargyl and two of them were considered susceptible to these herbicides. In field experiments, fenoxaprop-p-ethyl at 75 g ai ha-1 was used in Fars and clodinafop-propargyl at 64 g ai ha-1 in Khuzestan to examine the response of wild oat populations treated at three-leaf stage to these herbicides. In greenhouse experiments, the response of 12 wild oat populations collected from three provinces (Fars, Khuzestan and Markazi) to fenoxaprop-p-ethyl, clodinafop-propargyl, and diclofop-methyl at 900 g ai ha-1 were examined in separate experiments. In each experiment, an untreated control of wild oatplants was maintained for each population (except Fars populations). Wild oat density, shoot dry weight, and visual rating of weed control were recorded. The results showed that wild oat populations in Khuzestan have developed resistance to aryloxyphenoxy propionate herbicides. But wild oat populations of Fars and Markazi are still controllable by this group of herbicides. However, there were some differences between the results of field experiments and those of the greenhouse experiments. For example, one of the populations (KR3) obtained from Khuzestan which was resistant to clodinafop-propargyl in field experiments only, showed resistance to all grass herbicides used in greenhouse experiments. In addition, two (KR1 and KR2) of them were resistant to all herbicides in greenhouse experiments only. Among populations collected from Fars, only one FR4 of them was resistant only to fenoxaprop-p-ethyl in greenhouse experiments. This indicated that the level of resistance of weeds is probably influenced by the environmental conditions.

To evaluate the genetic improvement in yield of wheat (Triticum aestivum) bred between 1956 and 1995, a field experiment was carried out in Arak, Iran, during the 2001-2002 growing season. Six wheat cultivars, from different years of release (Omid 1956, Bezostaya 1969, Azadi 1979, Ghods 1989, Alamot 1995 and Alvand 1995) were sown with and without competition from wild oat (Aven fatua). The results showed a trend to increasing yield at later release dates with a rate of 2.2 % yr-1 and 3.8% yr-1 under weed free and weedy conditions, respectively. The greater yield increase obtained in weedy conditions indicates an improvement in the potential competitive abilities of new cultivars. Harvest Index (HI) and biomass were highest in new cultivars. Increased HI could explain 74% of the gain in yield, whereas, the remaining 26% of yield improvement was attributed to increases in biomass. There were no differences betweencultivars in extinction coefficients (K) and Radiation Use Efficiency (RUE). However, RUE was reduced significantly in weedy conditions. RUE increased with the year of release by 0.77% yr-1 and 0.14% yr-1 under weed free and weedy conditions, respectively. Average RUE values of of 1.3 g MJ-1 (in terms of absorbed photosynthetically active radiation, APAR) and M. Beheshtian-Mesgaran et al. 33 0.83 g MJ-1 were estimated in weed free and weedy conditions, respectively. Leaf area index (LAI) was the only component that decreased as year of release increased.

During the 2003-2004 growing season, an experiment was conducted at the Plant Protection Research Institute in Karaj, Iran, to compare the competitive ability (CA) of eight wheat cultivars released during the last 50 years. Each cultivar (Tabasi, Roshan, Karaj2, Azadi, Niknejad, Mahdavi, Shiraz and Pishtaz) was grown in weed-free conditions or infested with rocket (Eruca sativa). The results indicated that grain yield competitive indices differed significantly between the cultivars. An old cultivar, Karaj2, showed a high ability to withstand competition (AWC), high competitive indices (CI) and low grain yield in the weed-free plots (3865 kg ha-1). Cultivar Tabasi, a less competitive cultivar, had the lowest AWC and CI amongst the cultivars. There was a significant negative correlation between AWC and grain yield in the weed-free plots. Also, no significant correlation was observed between CI and AWC. The number of spikes m-2, number of fertile tillers * Corresponding author. R. Deihimfard; deihim@sbu.ac.ir. R. Deihimfard et al. 54 plant-1, number of plants m-2, and number of kernels spike-1 were the most influenced components of yield in weedy conditions. This study showed that local adaptation is important for the Karaj2 cultivar to prevent yield loss (tolerance) in weed-infested conditions, and more suppression ability in the presence of rocket.

Chickpea (Cicer arietinum) is the most important crop sustaining the economy of district Karak, Pakistan. Weeds cause enormous losses to the quantity as well as quality of the crop. A survey was launched during February 2004, to record the phytosociological status of chickpea farms. A total of 21 chickpea growers from seven villages in district Karak, Pakistan, were randomly selected. At each farm, the density of weeds was determined using standard quadrate method. The density data were subjected to analysis to compute Relative density (%), Frequency (%), Relative frequency (%) and Importance value. Asphodelus tenuifolius emerged as the predominant species in the district. The highest importance value of A. tenuifolius weed was recorded (88.6) in Hada followed by Wachobaqas (65.2) and Bogara (64.8). Whereas, the highest (38.6) importance value of Medicago denticulata was observed in Lakki Ghundaki and Ahmad Wala (32.8). Medicago-Cynodon-Asphodelus community was recognized at Ahmad Wala, Medicago-Fumaria- Asphodelus was prevalent in Ambiri Kala, and Medicago-Asphodelus-Cynodon existed at Lakki Ghundaki and Asphodelus-Cynodon-Euphorbia commenced at Wachobagas. At Bagara, Hada and * Correspondence to: G. Hassan, E-mail: hassanpk_2000pk@yahoo.com H. Hassan et al. 70 Sarkhawa the communities predominant were Ashphodelus-Cynodon-Euphorbia, Asphodelus-Cynodon and Asphodelus-Medicago-Cynodon. Asphodelus, Medicago, and Cynodon in the decreasing order were the major weeds in chickpea farms of Karak. Further studies are suggested to confirm the findings and launch an effective management program.

An experiment was conducted on clay soil at Lods Agronomy Research Center, McGill University to study the response of weeds communities and corn plants to nitrogen and herbicide application. The experiment was arranged as split plots design with 6 replications, in which application of herbicide (at 4 levels) and nitrogen (at 3 levels) were respectively the main- and sub-treatments. The herbicide Nicosulfuron (9 g ha-1) mixed with mineral oil (Agral 90, 25% v/v) was used to control grass weeds. The herbicide dicamba (590 g ha-1) was used for broadleaf control. Nicosulfuron mixed with Agral at the above mentioned rates, and dicamba at 295 g ha-1 were used to control both grass and broadleaf weeds. Weedy check also was considered as control. Nitrogen (ammonium nitrate) at 60,120, and 250 kg ha-1 was applied twice; once at planting time and again 54 days after planting. The results indicated that the density of grass weeds was higher than that of broadleaf weeds. However, broadleaf weeds could grow vigorously and by the end of the season they showed strong growth in most weedy check. Nitrogen had a pronounced effect on corn growth and yield. Reducing the nitrogen supply resulted * Correspondence to: M.H. Rashed, E-mail: mhrashed@yahoo.com M. H. Rashed et al. 82 in pale green leaves in the corn. Increasing nitrogen application increased the corn biomass mainly during the rapid vegetative growth stage.

The efficacy of Iran produced chloridazon 50% SC (CSC) was compared with its original formulation, 80% WP (CWP), the formulation which has been registered to use in sugar beet fields in Iran. The present study was conducted in three different provinces of Iran, including Tehran, Khorasan and Khuzestan during 2001. The treatments consisted of the application of CWP at 3.2 and 4 kg ai ha-1 , CSC at 2.5 and 3 kg ai ha-1 , tank mixed application of CWP or CSC at above mentioned rates with desmedipham (DMP) at 0.8 kg ai ha-1, phenmedipham 6% + desmedipham 6% + ethofumisate6% (PDE) at 0.7 kg ai ha-1. All herbicides were applied as post-emergence when sugar beet was at 4-leaf stage. Weedy and weed free checks were also included. In Tehran experiment, application of CWP at 3.2kg ai ha-1 plus DMP or PDE resulted in the best control of Amaranthus retroflexus. In Khorasan , CSC,CWP and CSC + DMP controlled this weed better than other treatments. In the recent experiment, CSC and CWP, alone or mixed with DMP, controlled A. albus significantly. The effect of CSC at 2.5kg ai ha-1+ DMP, CWP at 4 kg ai ha-1+ DMP, and PDE at 0.7 kg ai ha-1 on Chenopodium album was better P. Shimi et al. 98 than that of other treatments. The treatments had no significant effect on Malva sylvestris, compared with weedy check. The best control of Carthamus oxyacantha and Fumaria officinalis was achieved by application of CWP at 3.2 kg ai ha-1, and CSC at 3 kg ai ha-1+ DES. Beta maritima was more efficiently controlled using CSC at 3 kg ai ha-1and CSC at 3 kg ai ha-1+ DES. The results indicated that for the control of broad leaf weeds in sugar beet fields, the new formulation of chloridazon, (SC), was similar to the original formulation (WP)