Mature horses should not have a problem with chicken poop. We are very careful about foals and young horses (under 2 years) ever since losing one to Salmonella. We used to get big flocks of Canadian Geese around here in the winter. We always planted winter wheat pasture for our horses and the darn geese would come in and graze it. We had a really nice weanling -- probably 9-10 months old and he grazed out there with the geese. He got Salmonella poisoning, ended up with bloody scours and about $500.00 in Vet bills later he died. Vet said it was the 3rd young horse he treated that winter. All three died, and the Vet told us that ours would. But, we had to try to save him, but it sure did not work.
Birds also carry Coccidiosis, but it is not as deadly. They can recover form that if they are doctored right away. Again, it only bothers young ones. Foals are the most susceptible. All poultry can carry both, but I think ducks and geese are more often infected with the bacteria.