A smile does wonders. Soft words turn away anger. I realize that might be a challenge for you, but consider this a battle strategy, not a concession of defeat.
Just putter out into the field, approaching their work zone. Drive up to where their trucks are parked and the staging area.
Let them come to you. If they don't come trotting over, then take some pics of the labels on their herbicide tanks. They will be along shortly, if they see you.
Chances are excellent that they will approach you somewhat hostile. Just be nice, and explain that you are concerned about what they are putting down. If you get the "none of your business, lady" approach, then just ask them if they would prefer to deal with your complaints through the Dept. of Agriculture.
Trust me! NOBODY wants to even have their name mentioned to the Dept. of Ag. Even when they are making faultless applications.
If they threaten to call the cops for trespassing, just smile at them and offer your phone with 911 already punched in. Let 'em know that they can choose to be friendly, and the interaction will be much more pleasant.
Your interactions will be a lot more agreeable if you quit treating them like invaders. From their perspective, they have done you no harm, and you are just the crazy lady next door. Assure them that you want them to keep farming the adjacent property, but that you have concerns that they can alleviate if they would just play nice. Then smile a bit more, and let 'em call the cops if they wish.