Sulfonylurea herbicides kill a broad range of weedy grasses and broadleaf weeds. Barley, maize, rice, soybean and wheat are among the cereal crops that metabolize the herbicide safely. Sulfonylurea herbicides are post emergent, meaning that they are applied after weeds are visible in the soil. The rates and method of application vary according to the brand.
In June 1975, entomologist George Levitt of the DuPont Corporation discovered a molecule with potent herbicidal properties that he named R4321. This and the sulfonylurea herbicides that followed were derived from sulfonic acid and urea, an organic compound that helps metabolize compounds containing nitrogen. The next year Levitt created chlorsulfuron, a successor to R4321. DuPont released its first sulfonylurea herbicide, Glean, in 1982. Other early DuPont brand names were Ally, Harmony and Oust.
Sulfonylurea herbicides inhibit acetolactase synthase, an enzyme that is vital to the synthesis of amino acids in plants. Plants use a sequence of amino acids made from carbon, oxygen and nitrogen to make proteins that they need for growth. The acetolactate synthase enzyme is also known as ALS or AHAS, which stands for acetohydroxy acid synthase. ALS is found in both plants and micro-organisms, but not in humans or animals. This makes it safe to use if it is applied according to the manufacturer’s instructions.
Sulfonylurea herbicides may be applied effectively at lower rates than other herbicides. Agro News reports that farmers only use 1 to 5 percent of the amount of herbicides than they did before sulfonylurea herbicides appeared on the market, some 200 million fewer pounds each year.
DuPont’s original patent on sulfonylurea herbicides has now expired. DuPont now markets sulfonylurea herbicides bearing the brand names Express, Glean, Classic, Cropper, Ally, Escort, Answer, Londax, Matrix, Resolve, Tarot and Titus. DuPont researchers assert that their competitors’ brands often contain contaminants and impurities that lessen their effectiveness; however, DuPont based its conclusions on its own research, not that of independent studies.
Any manufacturer can now legally market a sulfonylurea herbicide. Just as in the pharmaceutical industry, herbicide manufacturers mix and match active ingredients in order to obtain a new patent. In 2009, agronomists at the University of Georgia listed 31 brand names of herbicides in the sulfonyurea chemical family including their active ingredients and mode of action. These are: Accent, Basis, Basis Gold, Beacon, Blade, Canopy, Canopy EX, Canopy XL, Celebrity, Celebrity Plus, Celsius, Certainty, Cimarron, Cimarron Plus, Corsair, Equip, Exceed, Finesse, Firstshot, Harmony Extra, Harmony GT, Manor, Maverick, Monument Option, Osprey, Oust, Peak, Permit, Revolver, Sandea and Sedgehammer.
Sulfonylurea herbicides are broken down by water in the soil, a process called hydrolysis; in drought conditions the residues can build up. Sulfonylurea herbicides are also broken down by microbes found near the surface of the soil. Australian studies conclude that sulfonylurea herbicides break down rapidly in acidic soils, those with low soil pH. If the soil is neutral or alkaline they move at nearly the same pace as water, traveling quickly past the microbes needed to break them down, so they may accumulate in the soil. Australian researchers N. Wilhelm and K. Hollaway say a build-up of sulfonylurea herbicides on the roots of cereal crops can "damage" them, without saying how they are sensitive or what degree of damage might be involved. The buildup of sulfonylurea herbicides on roots does not make the cereals unsafe for human consumption.