There is a fairly new Dow herbicide called Forefront (aminapyralid) that we hay buyers need to start asking hay supplies about. It kills broad leaf weeds, so it is used on wheat, oats, and hay fields.
It remains active THROUGH the animal, unchanged, and if the straw or manure is used as mulch or fertilizer, EVEN IF COMPOSTED, it will KILL the plants.
I will be asking hay suppliers about this herbicide, and NOT buying any hay from fields where it has been applied. Hay that has been sprayed is not SUPPOSED to be sold outside the US......