Introduction

The removal of unwanted plant species from crops is mainly accomplished in intensive systems of agriculture by the use of herbicides. The losses in yield due to the competitive effects of weeds have long been recognised, and even today, if weeds are not controlled, very high reductions in yield may occur as shown by the data in Table 2.1. Such high yield losses can easily be prevented by use of appropriate herbicides in the agricultural systems of developed countries, but reductions in yield of 50% or more due to weed infestation are still common in poorer areas of the world. Losses in yield may also be accompanied by reductions in crop quality. For example, cereal and legume grains may be smaller in weed-infested than in weed-free crops, and the grain itself may be contaminated with weed seeds. Unwanted plant species in crops may carry plant pests and diseases, and weeds such as field bindweed (Convolvulus arvensis) and cleavers (Galium aparine) may also hinder harvesting operations.

Vegetation may also be controlled in amenity and other non-crop areas by herbicidal means, although non-chemical procedures of weed control are often employed. Weeds give an unsightly appearance to flower beds and shrub borders, both in domestic gardens and municipal displays. Colonisation by weeds may also result in the deterioration of hard surfaces such as roads, other tarmac areas, pavements and railway tracks as well as presenting a physical hazard to people and modes of transport.