[SallyintNorth]

Does anyone know how long after application/exposure the effect on compost lasts?

On fresh grass, no I don't.  I do think it survives on hay through the next winter at meat, but whether a metabolising plant would break it down over some weeks, I don't know.

[SallyintNorth]

Aminopyralid, clopyralid, and picloram I think all pass through into manure, and are even present after composting.  Grazon contains clopyralid.

It does say on the label:
"Do not use any plant material treated with GRAZON 90 for composting or
mulching.
Do not use manure from animals fed on crops treated with GRAZON 90 for
composting."

:bookmark:  Thanks for the definitive list of persistent-through-hay-and-digestion weedkilkers, foobar