Backgrounder on Sulfonyl Urea Herbicides

Sulfonyl ureas (SUs) are a growing class of herbicides, first created in the mid-1970s and on the market since 1982. Produced by a several manufacturers, SUs are touted as "safe" herbicides, because they are said by their manufacturers to be less likely to cause immediate harm to humans and animals than the chemicals they replace. Sulfonyl urea herbicides are extraordinarily potent, requiring application doses one hundred times smaller than the herbicides they were intended to replace.

The first SU— chlorosulfuron— was formulated by DuPont chemist George Levitt in June 1975. Levitt intended the chemical as an insecticide, for killing spider mites. In the course of testing, it was discovered chlorosulfuron has a devastating effect on plants. Altering the developmental plan for the chemical, DuPont patented it as an herbicide in 1978 and promoted it to wheat farmers as "Glean" in 1982. In its commercial incarnation, chlorosulfuron has been refined to kill the broadleaf weeds in wheatfields, but leave the wheat itself unaffected. Since the introduction of Glean, DuPont has introduced other, specifically targeted SU pesticides for corn, barley, soybeans and rice.

Rather than killing plants with chemical burn, as did early herbicides, sulfonyl urea herbicides block synthesis of essential branched chain amino acids (leucine, isoleucine and valine) by inhibiting the enzyme acetolactate synthase (ALS). For this reason, these chemicals are sometimes referred to as SU/ALS  herbicides or merely ALS herbicides. Because the ALS enzyme does not exist in animals, makers of SU herbicides claim their products are safe for humans and animals.