The use of chemicals for weed control offers the greatest possibilities for relieving the physical effort which, in the past, has been necessary for this tedious chore. Chemicals function on the basis that certain chemicals are capable of killing plants and that they kill some kinds of plants without significant injury to others. As a group, these phytotoxic chemicals are called 'Herbicides'. The use of chemicals for weed control has been practiced largely since 1944 following the discovery of the phytotoxic phenoxy group of chemicals. Today there are more than 180 different chemicals available for use as herbicides. They offer an almost bewildering array of trademark products of which there are thousands each varying from the other as to the active ingredients, concentration and/or formulation of the constituents. Chemical control of weeds has paved the way to mechanisation and modern farm systems with large increases in productivity. As a result, today a farm worker 'feeds' 120 people worldwide compared with less than 20 in 1940.

Advantages of chemical weed control

Herbicides can be applied prior to the emergence of the crop, they can permit closer crop row spacing, they lower labour involvement, they minimise crop root damage, and they can be applied over a wide range of weather conditions and at a very fast rate.

Disadvantages of chemical weed control

Herbicides are costly to produce and to apply (e.g. incorporation), and they may provide inadequate levels of weed control or cause crop damage. Herbicides need sophisticated equipment for application, they tend to change weeds towards resistant types and may pollute soil, water and air with their residues.

Types of herbicides

Inorganic salts

This group has a long history but they have largely been replaced by modern organic herbicides. Sodium chlorate still remains a substantially used chemical for total weed control in non-cropping areas.

Organic herbicides

For an orderly study, organic herbicides may be grouped on the basis of one or more common characteristics such as chemistry, biological effect, application or use. Most text books group herbicides based on their chemical similarity. A number of chemicals will comprise a group or herbicide family. These families are listed below.

Phenoxy-carboxylic acids (e.g. MCPA, 2,4-D)
Substituted benzoic acids (e.g. Dicamba)
Picolinic acids (e.g. Picloram)
Benzonitriles (e.g. Bromoxynil)
Sulphonylureas (e.g. Chlorsulfuron)
Dinitroanilines (e.g. Trifluralin)
Carbamates (e.g. Propham, Asulam)
Bipyrazolum compounds (e.g. Difenzoquat)
Diphenyl ethers (e.g. Diclofop)
Substituted cyclohexanes (e.g. Sethoxydim)
Acid amides (e.g. Propanil)
Triazinones (e.g. Metribuzin)
Dintrophenols (e.g. Dinoseb)
Bipyridilium compounds (e.g. Paraquat)
Organophosphates (e.g. Glyphosate)
Imidazolinones (e.g. Imazaquin, Imazapyr)
Glufosinates (e.g. Glufosinate)