T go to the Police he may admit what chemical was used that day on the record. Worth a try.
It's a state licensing law that they keep on file exactly what was used and what the reported weather conditions were. The State Ag. agent will have no trouble getting answers, especially if it was a Coop or other contract spraying operation. With rented land and non-owner farmers, I would expect them to use contractual services for the big jobs that take tons of capital investment.