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VetBooks.ir Chapter 44

Toxicity of Herbicides

Pawan K. Gupta

INTRODUCTION BACKGROUND
Herbicides, also commonly known as weed killers, are The first discovery in the field of selective weed control
chemical substances used to control unwanted plants. was the introduction of 2,4-dinitro-o-cresol (DNOC) in
They are phytotoxic chemicals used for destroying various France in 1933. This is very toxic to mammals and can
weeds or inhibiting their growth. They have variable cause bilateral cataract in humans. In 1934, phenoxy herbi-
degrees of specificity. The worldwide consumption of her- cides were developed and 2,4-dichlorophenoxyacetic acid
bicides is almost 48% of the total pesticides usage. The (2,4-D) was introduced (Gupta, 2010b). During World War
consumption of herbicides in developing countries is low II, considerable effort was directed toward the development
because weed control is mainly done by hand weeding of effective, broad-spectrum herbicides with a view to both
(Gupta, 2004). Early chemicals used as herbicides include increasing food production and finding potential chemical
sulfuric acid, sodium chlorate, arsenic trioxide, sodium warfare agents (Gupta, 1989). One chemical class of phe-
arsenate, and petroleum oils. Iron and copper sulfate or noxy derivatives including the acids, salts, amines, and
sodium borate were generally difficult to handle and/or esters represents the first commercially available products
toxic, relatively nonspecific, or phytotoxic to the crop as evolving from this research in 1946. Some other herbicides
well as the unwanted plant life if not applied at exactly the used from this class include 4-(2,4-dichlorophenoxy) butyric
proper time (Gupta, 2016a,b). During the last few decades, acid (2,4-DB), 2-(2,4-dichlorophenoxy propionic acid)
the herbicides have represented the most rapidly growing (dichlorprop), 2-(2-methyl-4-chlorophenoxy) propionic acid
section of the pesticide industry due in part to (1) move- (MCPP or mecoprop), and 2-methyl-4-chlorophenoxyacetic
ment into monoculture practices and (2) mechanization of acid (MCPA) (Kennepohl et al., 2010). This class of herbi-
agricultural practices because of increased labor costs. The cides has been in continuous, extensive, and uninterrupted
result has been a plethora of chemically diverse structures use since 1947 and is the most widely used family of herbi-
rivaling the innovative chemistry so as to develop syn- cides. Another chemical class of herbicides deserving partic-
thetic organic herbicides and biopesticides that are quite ular attention is the bipyridyl group, especially paraquat and
selective for specific plants and have low mammalian tox- diquat. Weidel and Russo first described the structure of
icity. The aim is to protect desirable crops and obtain high paraquat in 1882. In 1933, Michaelis and Hill discovered its
yields by selectively eliminating unwanted plant species, redox properties and called the compound methyl viologen.
thereby reducing the competition for nutrients (Gupta, Its herbicidal properties were discovered by ICI in 1955,
2006). and it became commercially available in 1962 (Smith,
Most of the animal/human health problems that 1997; Lock and Wilks, 2010).
0
result from exposure to herbicides are due to their The first urea herbicide, N,N-dimethyl-N -(4-chloro-
improper use or careless disposal of containers (Gupta, phenyl)-urea, was introduced in 1952 by DuPont under
2010a). Very few problems occur when these chemi- the common name of monuron. In subsequent years,
cals are used properly. However, there is increased many more derivatives of this class of compounds have
concern about the effects of herbicides on animal been marketed (Liu, 2010).
health because of runoff from agricultural applications Protopyrinogen oxidase (Protox)-inhibiting herbicides
andentranceintodrinkingwater supply (Gupta, 1986, have been used since the 1960s and currently represent a
1988). relatively large and growing segment of the herbicide

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