Weeds are usually opportunistic plants with competitive advantages in a wide range of circumstances. Major weeds are those which either through their abundance or persistence or physical properties, hinder or harm social or economic activities, and threaten livelihoods and well-being. In particular, weeds limit food production, by damaging crops; competing with crops for water, light and nutrients; hindering the tending and harvesting of crops; and by their invasion of grazing land. Aquatic weeds add another dimension to the problem: by inhibiting the flow of irrigation water, they too can limit crop production.
An average of more than 10 % of agricultural production is lost world-wide by weed competition. The world sale of herbicides in 1980 was 4.7 billion USA dollars. From one particular study of 5,000 weeds that occur throughout the world, it was concluded that 200 are of concern to agriculture, based upon their geographical distribution and severity. About 30 of these weeds have very wide geographical distribution and may be important in 10 or more world crops. Another 50 species are widely distributed across the world but are more a nuisance than a threat. They are constant companions of man, require much labour to keep them at bay, but are not likely to destroy crops. The weeds of the next group, numbering about 100, are restricted to certain crops or geographical regions. They seem to find ecological barriers, which have not yet been studied. The remaining 20 weeds fall between the three groups and are very difficult to place or classify. For 75% of these weeds, there is almost no background information.