Herbicides

0
433

Herbicides are chemicals used to destroy unwanted plants (terrestrial or aquatic) called weeds. Herbicides fall into two broad categories: inorganic (e.g., copper sulfate, sodium chlorate, and sodium arsenite) and organic (e.g., chlorophenoxy compounds, dinitrophenols, bipyridyl compounds, carbamates, and amide herbicides). Historically, inorganic compounds were the first available and the first used. There has been over a long period a continuous effort to develop herbicide compounds that are more selective—that affect weeds, as opposed to desirable plants.

Select Source:
Chemistry: Foundations and Applications
Chemistry: Foundations and Applications
Encyclopedia of Food and Culture
World of Microbiology and Immunology
Encyclopedia of Public Health
The Columbia Encyclopedia, 6th ed.
The Oxford Companion to the Body
Plant Sciences
Plant Sciences
A Dictionary of Plant Sciences
World Encyclopedia
A Dictionary of Ecology
The Oxford Pocket Dictionary of Current English
A Dictionary of Biology
Oxford Dictionary of Rhymes

Herbicides

Herbicides


Herbicides are chemicals used to destroy unwanted plants (terrestrial or aquatic) called weeds. Herbicides fall into two broad categories: inorganic (e.g., copper sulfate, sodium chlorate, and sodium arsenite) and organic (e.g., chlorophenoxy compounds, dinitrophenols, bipyridyl compounds, carbamates, and amide herbicides). Historically, inorganic compounds were the first available and the first used. There has been over a long period a continuous effort to develop herbicide compounds that are more selective—that affect weeds, as opposed to desirable plants.

Historical Developments

The decade 1890 to 1900 saw the introduction of sprays for controlling broad-leaved weeds in cereal crops, and the first efforts by the U.S. Army Corps of Engineers, using sodium arsenite, to control aquatic plants in waterways. In 1925 sodium chlorate (directly applied to soil) was first used for killing weeds. The earliest importation (from France) of sodium nitrocresylate, as the first selective weed killer, was in 1934. The year 1945 witnessed the introduction of organic herbicides and the advent of 2,4-D growth regulator (2,5-dichlorophenoxyacetic acid), subsequently leading to development of 2,4,5-T (2,4,5-trichlorophenoxyacetic acid). During the years 1965 to 1970, U.S. military forces used 2,4,5-T (Silvex) and related materials as defoliants in Vietnam, without knowing that an inevitable by-product of the synthesis of 2,4,5-T was a toxic substance, 2,3,7,8-tetrachlorodibenzodioxin (dioxin). There is still debate over the extent of damaging effects sustained by those airmen, soldiers, and civilians who were exposed to this material. Dioxin was present at a level of about 2 ppm (mg/kg sample) in some of the samples of 2,4,5-T (called Agent Orange), but other samples contained more than 30 ppm of the by-product. Dioxin was eventually found to be highly toxic to guinea pigs (the LD50 value was 1 ppb, or 1 μ g compound/kg of sample), which led to the labeling of dioxin as “the world’s most deadly poison,” an impressive, if inaccurate, title (inaccurate because of a unique sensitivity of guinea pigs and because some natural toxins are known to be more potent).

LEAVE A REPLY

Please enter your comment!
Please enter your name here