So your answer is let him spray now, see what dies later?
It doesn't matter who is required to show what to who. There are apparently on-going issues with this farmer and he knows he is causing problems for his neighbors, so you need to be pro-active in your approach. If the farmer is a spraying toxic chemical that is now drifting on to your property, you need to do what you need to do to stop it before something happens. Talk to the farmer, get the police involved etc. Everything you do to stop the farmer is a point in your favor when it comes time for legal action. The farmer is uncooperative, won't talk to you and just keeps spraying herbicides onto your property, document that for use later.
The police don't need to be trained in the pertinent regulations.
Yes, actually they do. In addition to being a legal obligation to not enforce laws they are not empowered to do, they would be exposing the police department to litigation by the farmer. Arbitrarily declaring the farmer a menace to the public would be a can of worms that many police departments don't wish to open.
They should be able to stop the spraying in the interest of public safety until further determinations can be made. In any case there will be a police report that documents the situation and the actions of the farmer which can be used as evidence in court. Letting the farmer do what he wants now, then attempting to pursue legal action later with little evidence is absolutely the wrong way to go about this.
You could call the Dept of Agriculture. Don't count on a rapid response.
You could hire an expert, and prepare for litigation.
You might get a cop to exceed their authority. They do that often enough on a lot of other issues. Sure, there would be a police report, but that doesn't probably amount to anything with respect to the Dept of Agriculture. If you needed a "neutral witness", I'm sure it would be hard to get a better witness for court than a fair minded cop.
Realistically, though, how are you going to expect a local cop to enforce Federal regulations that he is neither authorized to protect nor trained to enforce? You might as well call the local cops when you have a clear case of violating some FAR part 16 regulations
(Federal Acquisition Regulations). NOPE. That would take the FBI. Maybe the GAO. Hell I don't know that stuff either! But I am pretty familiar with pesticide regulations, having been a licensed applicator in 3 categories since 1984.
Now there is always the general consideration that the farmers generally do their scientific best to keep that pesticide application right where they want it. Allowing drift is a big no no for more reasons than causing collateral damage. You end up with an ineffective application and you waste money. That is a really big consideration for most farmers.