Apparently this common symptom of loose stool (often formed manure with copious watery content) has multiple causes. Clearly if antibiotics assist, then somehow pathogenic bacteria have become a problem (could the source be the chicken manure fertilizer?) and appears that food allergies can cause similar issues.
To add another potential solution here, consider mineral balance and bio-availability. I’ve boarded a Quarter horse gelding at our place for over 10 years and he’s now almost 32 and doing great. But in his late 20’s he developed this same symptom and the vet just passed it off as very common in older horses and nothing to worry about. But I sure can relate to the constant mess, which first began seasonally and then became year-around!
Just to support a few older horses in my care, I ordered a product called Hiland’s Big Sky Minerals (http://bigskymineral.com/hilands-big-sky-mineral/) and after only a few weeks noticed this guy’s loose stool was completely gone! In the months and years following it has never returned, even after having had to gradually switch him to timothy/alfalfa hay pellets as his hay chewing ability rapidly declined (he’s on pasture 24/7). And this is feeding him just half the manufacturer’s recommended amount! Gave Big Sky an enthusiastic “Five Star” rating on their website and continue to consider it an essential part of my horses’ daily ration. Also saw improvements in all horses both with hoof condition/growth and enhanced coat color (no more fading even in Texas sun!).