Being a trickle-feeder a horse will stress if they run out of forage and have no means to find any more, which for the stabled horse can then lead to developing those well-known stable habits, i.e. weaving, wind-sucking, cribbing, box-walking et al.

Why? Because unlike us, when our stomachs are empty, our acid production stops, whereas a horse’s stomach constantly produces hydrochloric acid - they're a trickle-feeder for a very good reason. Under normal feeding habits, the acid is naturally absorbed into the continually eaten food and buffered by bicarb-containing (acid soothing) saliva. However, if they run out of forage and their stomach empties, the acid keeps on coming.

Without a continual forage supply, the intestinal lining in the stomach degrades. And without a strong protective mucosal gut lining in place, the continual acid splash will burn the fragile intestinal mucosa, creating a raw wound, aka an ulcer.

Meanwhile, the stress involved with both lack of forage and the ulcer pain now triggers the release of the stress hormone, cortisol. This isn’t a situation you want to prolong as cortisol isn’t necessarily a friendly hormone; it’s fine for a perceived short-term threat because it has a vital protective job to do, but for a prolonged state of anxiety it not only permanently floods the body with acid, making the peripheral limbs feel like they’re on fire, but it also switches off several vital metabolic functions ( including digestion), while suppressing immunity and increasing blood glucose levels.

Cue squamous stomach ulcers developing due to food disruption/quality, and glandular/pyloric ulcers which are caused by the stress/anxiety.