Assuming they have an animal control...sometimes small counties don't have the funding for an animal control, so you have to rely on the sherriffs office...which sometimes can be pretty stinking fruitless.. or the situation has to get soo bad..
First of all TRESPASSING and then snapping photos will get you no where as the photos wont be useable evidence. If you want to get anything done, it has to be done legally. If you can see/ snap photos of the horses while off property that's great! Im assuming that because you have so much knowlege about ages of horses and the conditions of there stalls that you have permission to be on the property?? If so DESCRETLY take photos/video with your phone.
I would continue to call in complaints against these people, and even your states ASPCA, email photos you were legally able to get...just keep harassing thse agencies..