Herbicides are chemicals, developed since the 1940s, that are used with the intention of killing plants. Herbicides can be used tactically to clear foliage or destroy crops for military purposes; agriculturally, to selectively rid fields of non-desired plants while letting food crops grow; or as landscape maintenance, to clear utility rights-of-way and roadsides of weeds and shrubbery. Chemical herbicides inexpensively replaced large numbers of farm laborers and boosted crop production, but the costs to human health and the environment are just beginning to become apparent.
Weeding, or pulling out the plants you don't want to favor growth of the plants you do want, is likely the oldest form of agriculture. While tactical herbicides have been around for thousands of years--when the Romans sacked Carthage in 146 B.C., they salted the surrounding fields to preclude any survivors from re-establishing agriculture--they were of little use for agricultural purposes because they simply killed all plants. Chemicals that could distinguish between one plant and another did not begin to arise until the 1940s.
The herbicide 2,4-D came on the market in 1945, developed by a British team seeking to increase food production as part of the war effort. It was the first herbicide to distinguish between broad-leaved plants and grasses. It did not damage corn plants, which are grass relatives, but killed the broad-leaved weeds between the corn rows. Historians at Wessel's Living History Farm in York Nebraska report that 2,4-D was called a wonder drug that substantially increased corn yields. Crop Protection Research Institute staff wrote in the Weed Technology journal that herbicide use spread rapidly through the United States in the 1950s and 1960s, replacing millions of agricultural workers formerly employed in weeding. Atrazine followed 2,4-D, registered in 1958 for use on corn crops and later for right-of-way and roadside verge clearance. Roundup, the start of Monsanto's glyphosate herbicide line, was introduced in 1974. Virtually all crop acreage not registered as organic, together with large segments of roadsides, landscapes and home yards, are now treated annually with herbicides.
Selective herbicides like 2,4-D and atrazine kill broadleaf plants, making them effective for treatment of weeds in lawns and cornfields, while other selective herbicides are marketed specifically to kill grass in landscaping. Broad-spectrum herbicides such as Roundup and its progeny kill all living plants, such as for use clearing roadsides or utility-line rights-of-way. Broad spectrum herbicides have also given rise to modern tactical herbicides: The U.S. Army developed chemical herbicides for military applications beginning in World War II and most vividly employed in aerial operations Agent Orange and other 2,4-D and dioxin derivatives in diesel oil base in Vietnam in the early 1970s. Newer tactical herbicides are being used in the war on drugs, with aerial spraying on marijuana, poppy and coca crops.
While Rachel Carson's 1962 book "Silent Spring" addressed the pesticide DDT rather than herbicides, it served as a watershed moment in the perception of agricultural chemical use. The public and scientists quickly recognized the misconception that herbicides could be used to increase crop production without negative impacts to the environment and human health. The U.S. Environmental Protection Agency was founded in good part to regulate herbicide and insecticide use and to protect drinking water resources from pesticide pollution.
The near-universal application of herbicides has given rise to herbicide-resistant weeds. Eradication of weeds that have adjusted to herbicide use means using more, or more lethal, herbicides. Another consideration that has recently developed is that Monsanto and other companies have now made gyphosate herbicides useful for crop application by genetically modifying their proprietary crop seed to resist herbicide applications. In addition to public concerns regarding the safety and ethics of genetically modified organisms, this new practice has lead to substantial increases in the quantity of herbicides released into the environment every year. The history of chemical herbicides is not long, and its effects are just beginning to be understood. Resolution of health concerns and these agricultural practice considerations will determine the next chapter in herbicide history.