Not a terrible idea initially, blend in with farm equipment, excellent off road capability, small and nimble. However there's a reason Geneva conventions require military personnel and equipment to be distinct from civilians.
Do you want all your civilian farmers blown up? Because this is how you do that.
I though the only distinction was for medical vehicles and military vehicles. There is a whole class of vehicles called Technical which is pretty much a pickup with a heavy machine gun mounted to it.
https://en.m.wikipedia.org/wiki/Technical_(vehicle)
Reminds me of this old Eastern Bloc joke (read it in a thick slavic accent):
Pravda reports imperialist forces unjustly and underhandedly attacked peaceful farm tractors of great USSR doing peaceful harvesting work. Our brave farmers returned fire and flew back to USSR territory.
Ancient John Deere, while they were still good. I highly approve.
Bringing a modern day John Deer to the battle field would result in being stalled while waiting for a proprietary software update, which is suboptimal while under artillery fire.
Not sure when it started, plus my personal experience with JD stems from a tractor that was built in the 70's. I think the model was 1033 or thereabouts.