Changes in weed species composition on a specific site can occur in response to any agricultural production practices. Species that are best adapted to a particular practice are selected for and the species will increase in the population over a period of time. Over the past 50 years, repeated use of herbicides has resulted in the evolution of resistant weed species. Currently, 216 herbicide resistant weed biotypes in 45 countries have been documented. The rapid increase in the number of cases of herbicide resistance in many agricultural areas in the world verifies that the phenomenon is global. The first resistant weed species to triazines was discovered in 1970 in the United States. Presently, at least 60 weed species have been documented with one or more biotypes resistant to triazine herbicides in the world. Fifty-three weed species have evolved resistance to ALS inhibitor herbicides in 14 countries. Nineteen weed species have evolved resistance to ACCase inhibitors in 17 countries. Fourteen weed species have evolved resistance to urea herbicides. There are 26 weed species resistant to bipyridilium herbicides and 17 weed species resistant to synthetic auxins. Mechanisms of resistance to herbicides is attributed to: i) modified photosystem II protein-binding site (triazines and uracils), ii) modified ALS binding site (sulfonylureas and imidazolinones), iii) enhanced detoxification (aryloxyphenoxy propionates and substituted ureas), iv) enhanced detoxification and by sequestration (dinitroanilines). Herbicide resistance has also been observed in developing countries. Some of the species include Phalaris minor resistant to isoproturon (India), P. minor and P. paradoxa resistant to diclofop (Mexico), Echinochloa colona resistant to propanil (Columbia, Costa Rica and Nicaragua), Conyza bonariensis resistant to paraquat (Egypt) and Echinochloa crusgalli resistant to butachlor (China and Egypt). Multiple resistance to more than one class of herbicides with different modes of action have been identified. Chlorotoluron resistance to Alopecurus myosuroides (UK), trifluralin resistance to Eleusine indica (Canada, USA), diclofop-methyl resistance to Lolium rigidum (Australia), metsulfuron resistance to Lactuca serriola (USA) have been noted. The discovery of glyphosate resistant Lolium rigidum in Australia (1996) and Lolium multiflorum in California (1998) has definitely reminded us the continual evolution of herbicide resistant weed species. For the last two decades, a significant progress has been made in the area of herbicide resistance, including mechanism of resistance, community and population ecology, and management tactics for resistant species. The present global widespread of weed resistance to different classes of herbicides has dearly reached the stage where more concerted efforts in research and education is needed. At the same time, we must not forget the ecological implications of these species in production agricultural systems.
