About 22 years ago we put our garden in what once was a horse coral. Our only issue was a ton of rock. It's not like your situation because it had been a long time from when animals were in the area and when we planted our garden.
Now a days you have to be very careful with
straw and hay that is fed to the animals. A lot of it has weed killer in it. It doesn't go away when the animals eat it, it gets passed in the manure. So you may want to ask what was fed to the horses, where they got it, and what kind of herbicides were used. I have heard stories of people who lost hole crops using straw, or manure that had bad chemicals in it. I hope someone with direct
experience chimes in to help you out. Good luck to you.