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Definition: herbicide from Philip's Encyclopedia

Pesticide used to kill weeds and other unwanted plants. Selective herbicides kill the weeds growing with crops, leaving the crops unharmed; non-selective herbicides, such as paraquat, kill all the vegetation. There are concerns over their toxicity to humans and their persistence in the environment.

Summary Article: Herbicide
From The Pesticide Encyclopedia

A compound, either organic or inorganic, used to destroy unwanted vegetation like weeds, grasses, woody plants, and sometimes even crop plants, is known as a herbicide or a weedicide. The word herbicide has been derived from the Latin word ‘herba’ (meaning ‘plant’) and ‘caedere’ (meaning ‘to kill’). The term weedicide is no longer used, since the chemical/natural agents can kill non-weed plants as well (as in non-selective herbicides).

Until the beginning of the 1920s inorganic herbicides such as sodium chlorate (NaCl), ammonium sulfate, arsenic and boron compounds and ash and factory wastes were used. At that time, cultural methods for weed control like burning, flooding and increasing salinity were common. Since then, many specific organic herbicides have been introduced. Herbicides work by inhibiting various enzymes that disturb critical biochemical processes in target plants. Some modes of action include inhibition of acetolactate synthase, acetyl-CoA carboxylase, carotenoid biosyntheses, amino acid synthesis and photosynthesis at photosystem I or II.

Herbicides may be applied to the soil or on the foliage. They may be applied:

  • before weeds are visible (pre-emergence stage); or

  • at specific stages after emergence (post-emergence) either before crops are planted (pre-plant stage) or after crops have been planted (post-plant stage).

Some chemical classes of herbicides include phenylpyrazole, triazines, thiadiazole, bipyridylium, amides, urea, uracid and aryloxyphenoxypropionates. Some examples of herbicide active ingredients include 2,4-D, MCPA, MCPB, diquat, paraquat, pyrazoxyfen, refenacet, butylate, molinate, dicamba, picloram and cinmethylin.

While most herbicide usage is a reaction to the nuisance of weeds, some chemicals are applied for agricultural convenience. For example some crops like cotton are sprayed with chemicals to kill outwardly growing leaves, thereby making harvesting easy. Such herbicides are described as desiccants and defoliants. On other occasions, herbicides are applied in minute quantities to help regulate growth (e.g. to increase yields, hasten flowering, delay ripening or increase leaf drop). These chemicals are called plant growth regulators (PGRs).

Repeated use of a few components of the same chemical group can cause the development of resistance in weeds. Many ryegrass varieties, for instance, have developed resistance to diuron, chlorosulfuron, atrazine and diclofop. In India, the most serious case of herbicide resistance and later cross-resistance has been reported in littleseed canarygrass (Phalaris minor) in wheat. Resistance management is therefore an important feature of herbicide application.

Herbicides may be of two major types: (i) selective herbicides such as 2,4-D, phenols, carbamates and urea derivatives, permitting elimination of weeds without injury to crops; and (ii) non-selective herbicides comprising soil sterilants (sodium compounds) and silvicides (ammonium sulfamate), which can kill woody plants and trees.

Weeds are plants and they are often closely related to crop plants, having similar morphology/architecture and physiology (cellular and vascular organization and processes). Many weeds even belong to the same genus as the crop plants and resemble them so closely that they can hardly be distinguished. Hence controlling weeds in crop fields (i.e. killing undesirable plants, weeds, among desirable plants, the crop), is a huge challenge for weed scientists. Proper technical know-how and sensitivity in herbicide usage is a prerequisite for successful adoption of herbicide technology.

© K. Paranjape, V. Gowariker, V.N. Krishnamurthy and S. Gowariker 2015. All rights reserved.

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