I FINALLY got to talk to the farmer. Yesterday he was just about to spray his field when we got a hefty downpour with hail. He left his spray tractor(or what ever it's called) in the field and went home. After it stopped raining, I left a note on his tractor with my question and phone number. So today I got a call and he says he's just spraying roundup this year and my horse should be fine. Boy, was I relieved!
He even said I had quite a nice horse!