Herbicides

In 1990, a total of about 290 herbicidal chemicals were available for use. Many of these chemicals are used in various types of formulations, each of which is a specific combination of the herbicidal active ingredient, a solvent such as water or kerosene, and various chemicals intended to enhance the efficacy of the herbicide, for example, by increasing the ability of the spray to adhere to foliage, or to spread freely on leaf surfaces. In addition, different companies often manufacture and sell the same formulations under different names, so the number of commercial products is larger than the number of actual formulations.

The United States accounts for about one-third of the global use of pesticides, much more than any other country. In 1989, herbicides accounted for about 61% of the 1,100 million lb (500 million kg) of pesticides used domestically in the United States. During recent years, eight of the ten most commonly used pesticides in the United States have been herbicides. Listed in order of decreasing quantities used, these herbicides are: alachlor (100 million lb [45 million kg] used per year), atrazine (100 million lb [45 million kg]), 2,4-D (53 million lb [24 million kg]), butylate (44 million lb [20 million kg]), metolachlor (44 million lb [20 million kg]), trifluralin (31 million lb [14 million kg]), cynazine (20 million lb [9 million kg]), and metribuzin (13 million lb [6 million kg]). During the mid-1980s, amide herbicides accounted for 30% of the herbicides used in the United States, triazines 22%, carbamates 13%, N-anilines 11%, and phenoxys 5%. These data reflect a large decrease in the usage of phenoxy herbicides, which were used much more commonly prior to the 1980s. For example, during the mid-1970s, about 50-80% of the small-grain acreage in North America was treated with phenoxy herbicides, mostly with 2,4-D. Since the mid 1990s, the use of glyphosate has increased tremendously.

In terms of quantities applied, by far the major usage of herbicides is in agriculture. Intensive systems of cultivation of most major species of annual crops requires the use of herbicides. This is especially true of crops in the grass family. For example, at least 83% of the North American acreage of maize (or corn) cultivation involves treatment with herbicides. In part, herbicide use is important in maize cultivation because of the common use of zero tillage systems. Zero tillage involves direct seeding into unploughed soil, a system that has great benefits by reducing erosion and saving fuel, because tractors are used much less. However, one of the most important agricultural benefits of ploughing is the reduction of weeds that results. Consequently, zero tillage systems would be not be practical if they were not accompanied by the use of herbicides. This is only one example—most of the areas of grain crops cultivated in North America and other industrialized countries receive herbicide treatments.

Herbicides are also widely used in landscaping, mostly to achieve grassy lawns that are relatively free of broad-leaved weeds, which many people find unattractive. Herbicides are commonly used in this way by individual landowners managing the lawns around their home, and by authorities responsible for maintaining lawns around public buildings, along roadways, and in parks. Golf courses rely heavily on intensive use of herbicides. This is particularly true of putting greens, where it is important to have a very consistent lawn. In fact, the intensity of pesticide use on golf-course putting greens is greater than in almost any other usage in agriculture.

Forestry also uses herbicides. Usually, silvicultural herbicide use is intended to achieve a greater productivity of the desired conifer trees, by reducing the abundance of unwanted weeds. However, in most regions forestry usage of herbicides is much smaller than agriculture and lawn uses, typically less than 5% of the total use.

Herbicides were used extensively by the U.S. military during the Vietnam War. Large quantities of these chemicals were sprayed in Vietnam as a military tactic intended to deprive enemy forces and their supporters of agricultural production and forest cover. So-called "Agent Orange," a 1:1 mixture of 2,4,5-T and 2,4-D, was the most commonly used herbicide. Because the intention was to destroy forests and agricultural productivity, herbicides were used at about ten times the rate typically used in forestry for management purposes. This tactical strategy of war was labeled "ecocide" by its opponents, because of the severe damage that was caused to natural and agricultural ecosystems, and possibly to people (there is ongoing debate about whether scientific evidence actually demonstrates the latter damage). For these reasons, and also because Agent Orange was significantly contaminated by a very toxic chemical in the dioxin family, called TCDD, the military use of herbicides in Vietnam was extremely controversial.