Herbicide: Encyclopedia II - Herbicide - History

Herbicide - History

Prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds. Mechanical control (including tillage) was also (and still is) used to control weeds.

The first widely used herbicide was 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D. It was developed by a British team during World War II and first saw widespread production and use in the late 1940s. It is easy and inexpensive to manufacture, and kills many broadleaf plants while leaving grasses largely unaffected (high doses of 2,4-D at crucial growth periods can harm grass crops such as maize or cereals). 2,4-D's low cost has led to continued usage today and it remains one of the most commonly used herbicides in the world. Like other acid herbicides, current formulations utilize either an amine salt (usually trimethyl amine) or one of many esters (ester) of the base compound. These are easier to handle than the acid.

2,4-D exhibits relatively poor selectivity, meaning that it causes stress to non-target plants. It is also less effective against some broadleaf weeds, including many vinous plants, and sedges. A herbicide is termed selective if it affects only certain types of plants, and nonselective if it inhibits most any type of plant. Other herbicides have been more recently developed to achieve desired selectivities.

The 1970s saw the introduction of atrazine, which has the dubious distinction of being the herbicide of greatest concern for groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied. Instead it is carried deep into the soil by rainfall causing the aforementioned contamination. Atrazine is said to have high carryover, a very undesirable property for herbicides.

Glyphosate, frequently sold under the brand name Roundup, was introduced in the late 1980s for non-selective weed control. It is now a major herbicide in selective weed control in growing crop plants due to the development of crop plants that are resistant to it. The pairing of the herbicide with the resistant seed contributed to the consolidation of the seed and chemistry industry in the late 1990s.

Many modern chemical herbicides for agriculture are specifically formulated to decompose within a short period after application. This is desirable as it allows crops which may be affected by the herbicide to be grown on the land in future seasons. However, herbicides with low residual activity (ie decompose quickly) often do not provide season-long weed control.

Adapted from the Wikipedia article "History", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki