I use MSMA (Monosodium methyl arsenate) to kill crabgrass in lawns. I am not certain of the mode of action.
How does this chemical work? How does it target the annual grassy weeds, without damaging the lawns much?
Please consider this a sort of comment — I tried looking up for articles related to MSMA mode of action almost all the articles are about its toxicity in mammals.
By what I understand trivalent arsenates act as phosphate analogs and therefore interfere in phosphorylation reactions including DNA synthesis [Ref].
The weed is affected perhaps because of its relatively higher growth rate and therefore being affected by the herbicide. Also the already established lawn grasses have good population to regrow out of the damage.