Some history: There is an herbicide named picloram, which was mixed with 2,4-D and was used during the Vietnam war on foliage that Agent Orange didn't kill. The mixture was informally called Agent White. This herbicide is still produced by Dow Chemical, but it usually doesn't kill grasses. Because of this, it's used to spray hay fields to keep the weeds down. But, you say, cows and horses eat hay. Yes - they do. And their manure is composted and sold in bags at every garden outlet or hardware store in town. And yes, it's a persistent chemical, which means traversing the gut of an animal and then being composted do not break down this chemical.
More history: I worked in a school garden a few years ago, and used manure as an amendment one time. The beds produced little to nothing for two years. I won't take the chance again unless I know exactly where the manure came from and what the horse or cow was fed.
That said, there are sources for good manure - any organic farm with horses or cows, or a grass fed beef operation, will have usable manure, so there are work-arounds. The bigger picture here is what really bothers me - what does it say about a food system (and that's what we're talking about) where even the poop we use to fertilize our crops is poisoned?