Either way it's on the plant via overspray; if it wasn't, nobody would need the custom seeds.

Looking at it's structure, it's relatively stable. I found many links on it's halflife; ~10-100 days in the environment. If low levels matter, then 10 halflifes might still be a problem.

I think it would depend on the efficiency of the metabolic process and availability of catalysts for that process at site. If it's very efficient then perhaps not very much is present, having been metabolized.

Just some musings from a non-expert.