A field experiment was conducted from June to December during 2010/11 crop season at HARC to study the effect of herbicides rates on weed dynamics and yield of wheat (Triticumaestivum L.) variety "HAR 604" in randomized complete block design with three replications. the herbicides rates: clodinafop-propargyl(0.065,0.080, 0.105kgha-1) and isoproturon (1.00, 1.25, 1.50kgha-1), hand weeding at tillering and weedy checkwere used. The crop was infested with AvenafatuaL. and PhalarisparadoxaL. among grass weeds andCayluseaabyssinicaMeisn,C. trigynaL., Chenopodium album L., Corrigoialacapensis Wild, Guizotiascabra(Vis) Chiov, Oxalis latifoliaHBK, PolygonumnepalenseL., RaphanusraphanistrumL., SpergulaarvensisL. and Tagetesminuta L. among broadleaved weeds. Hand weeding followed isoproturon at 1.50 kg ha-1 significantly reduced density and dry weight of weeds. Among herbicides, isoproturon provided better control of broadleaved and total weeds, whereas; clodinafoppropargyl proved better than isoproturon in controlling grass weeds. Hand weeding and hoeing at tillering resulted in lowest weed dry weight. Highest grain yield (2289.4 kg ha-1) in was recorded in hand weeding followed by isoproturon at 1.5kg ha-1 (2177.3 kg ha-1). The highest straw yield was recorded in hand weeding followed by isoproturon 1.50 kg ha-1 , and harvest index was also maximum with hand weeding. Maximum N-uptake was also recorded in these treatments. Post emergence herbicides and /or hand weeding and hoeing at tillering can further enhance the weed suppressive effect of the crop.

Abstract:The global climate is changing; along with measuring temperature and CO 2 level changes that are considered major drivers of climate change, there is also increasing attention being given to its impact on agricultural production systems (including weeds). Climate conditions exert a significant influence on the spread, population dynamics, life cycle duration, infestation pressure and the overall occurrence of the majority of agricultural pests. Weeds are among the agricultural pest that can be influenced by climate change. It is expected that climate change will bring about a shift in the floral composition of several ecosystems at higher latitudes and altitudes, as changes in temperature and humidity will be reflected on flowering, fruiting and seed dormancy. Changes in atmospheric CO 2 levels, rainfall, temperature and other growing conditions will affect weed species 'distribution and their competitiveness within a weed population and within crop. Any factor which increases environmental stress on crops may make them more vulnerable to attack by insects and plant pathogens and less competitive with weeds. Many of these weeds reproduce by vegetative means and recent evidence indicates that as a group, these weeds may show a strong response to recent increases in atmospheric CO 2 . Changing and increment of temperature is one main characteristics of climate change which may affect existing plants (weeds shift) and allow some other plants (weeds) to replace native and will be expand in to new areas which is not existed before. Even under drought condition some weeds produce allele-chemical that made weeds to thrive well and compete with crop. An increase in root: stem, with increases in the growth of roots or rhizomes, particularly of perennial weeds, may make it harder to control some weeds that regrow from root fragments left after mechanical tillage. The direct impacts of climate change will be either on the biology of the biological control agent and/or on the ability of the host plant to resist, tolerate or compensate for the presence of the herbivore or plant pathogen. Increased temperature would be expected to increase the rate of life cycles of both the biological control agents and the weeds. Increased water stress will affect the host plant's development, and through this, the development of biological control agents, so they might be less effective in drier situations.

), however, non-significant difference existed among them in Guder, whereas the highest was observed in weedy check (170.93, 382.13 gm -2 ) in Guder and Ambo, respectively. Moreover, those treatments also significantly increased the yield and yield component of maize in both locations.

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