How does Halosulfuron-methyl kill nutsedge, while leaving lawn grass and most weeds undamaged?

Question

I use Halosulfuron-methyl to control yellow and purple nutsedge in lawns. This chemical acts by interfering with the acetolactate synthase enzyme, which quickly slows cell division, and growth at all points.

How does this work without also interfering with enzymes in the lawn grass?

Answer 1

Halosulfuron-methyl is a member of the family of sulfonylurea herbicides. Brown (2006) mentions in his abstract the following, and I quote:

This class of herbicides acts through inhibition of acetolactate synthase [...], thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. [...] Crop selectivity results from rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range from 1–5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives > 20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative O-demethylation and direct conjugation with (homo)glutathione.

Hence, metabolic inactivation activity differs between susceptible and insusceptible species. To get to your question: this means that the enzymes in lawn grass are also inhibited, but to a lesser extent, as the herbicide is metabolized and inactivated more rapidly in grasses.

^{Reference
Brown, Pesticide Science 2006;29:241–378}