Chemistry and fate of triazolopyrimidine sulfonamide herbicides

The triazolopyrimidine sulfonamide (TSA) herbicides were registered in the United States in 1993. Their mode of action is by inhibition of acetolactate synthase (ALS), an enzyme common to plants and microorganisms but not found in animals. ALS inhibitors include other herbicide families such as the sulfonylureas, imidazolinones, and pyrimidinyl thiobenzoates. In the 1970s, sulfonylureas were the first ALS inhibitors to be introduced to the market. Their discovery was greeted as a great achievement because of their ability to suppress weeds at extremely low application rates compared to previously used herbicides and with low toxicity risk to humans and wildlife. By 1991, the market value of ALS inhibitors had reached $1.3 billion (U.S.). However, some of the problems associated with their use include the induction of resistance in weed species to both ALS-inhibiting and alternative herbicides. ALS inhibitors have also been found to damage nontarget plants at residual levels that are below the detection limits of standard analytical methods (Saari et al. 1994; Whitcomb 1999).