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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.
Herbicides were first researched and discovered in both the UK and the US in the context of clandestine wartime efforts to create chemical warfare agents in 1941. The first herbicide, 2,4-D, was co-discovered independently by two teams; one at ICI and one at Rothamsted Experimental Station. The new chemicals ability to kill weeds was entirely incidental to the aim of the research in either country. Because research into chemical warfare was illegal under the Geneva Protocols, the research operated under the plausible cover story of agricultural research. Experimentation had become quite common (in the fashion of British team at Rothamsted Experimental Station, under the leadership of Judah Hirsch Quastel. Some discoveries were able to be eventually used to increase crop yields, although 2-4D was not used until 1946 near the end of the WWII.[2] When it was commercially released in 1946, it became the first successful selective herbicide and allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crops, because it kills dicots (broadleaf plants), but not most monocots (grasses). The low cost of 2,4-D 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 use either an amine salt (often trimethylamine) or one of many esters of the parent compound. These are easier to handle than the acid.
The triazine family of herbicides, which includes atrazine, were introduced in the 1950s; they have the current distinction of being the herbicide family of greatest concern regarding groundwater contamination. Atrazine does not break down readily (within a few weeks) after being applied to soils of above neutral pH. Under alkaline soil conditions, atrazine may be carried into the soil profile as far as the water table by soil water following rainfall causing the aforementioned contamination. Atrazine is thus said to have "carryover", a generally undesirable property for herbicides.