For information, I contacted cpa horticulture during my search for manure. They seem to operate at least in part, as a middle man. I asked if I could obtain sample bags for trials and they put me in touch with a farm in my area who bag the manure into a retail product as well as supply it in bulk. I purchased 8 bags for trials. I setup a trial and all of the plants were dead after 30 days which I can only attribute to herbicide having been present in the manure.
What you must bear in mind is that all of these companies supplying bulk manure collect it from many different sources, and they will most likely have no control or knowledge as to whether the grazing fields have been treated with herbicides. They will be collecting it as a waste product as a means of disposal for the farms.