Exactly !If one was to fret about all of the things that could possibly be in their hay they should probably worry most about the chemicals that are intentionally applied to it. Our hay supplier told us to assume that most/all hay has been treated with herbicides. The amounts vary depending on the cut of the hay, the crop grown and how much money the farmer is willing to spend.
I believe these herbicides stick around long enough to pass through a cow and have the manure it creates contain enough chemical to kill the plants it is applied to.
These days just about every town has a Swap Meet, Flea Market or Auction close by. Usually once or twice a week they will have a large sale with everything you can imagine. This is a GREAT place to unload Agricultural Goods deemed "Unsafe" by FDA inspections, ESPECIALLY HAY. My direct neighbor property has been burning trash, tires for years. A friend of our daughters father is a Captain of the local Fire Department. While speaking with him at a High School game he told me the story of my neighbor. Before I lived here they would burn so often that passers by on the highway would see smoke and call 911 and report a fire. The Fire Departments hands were tied on getting him to stop burning BUT, because of the obvious trash in the piles and surrounding ground the county stripped their "Organic" certificates. This property grows everything from Melons, Vegetables, Hay, Nut & Fruit Trees and sells them all over the California Coastal cities at Farmers Markets for top dollar.