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 but 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. Also the already established lawn grasses would have good population size to regrow out of the damage.