In horses’ natural setting, where they eat almost constantly and consume large amounts of forage, a fibrous “mat” develops in their stomachs, floating on top of the stomach acid. This mat helps prevent acid (which is constantly secreted in the stomach’s lower glandular portion, which is protected by a mucus layer) from splashing up and the nonglandular part of the stomach (where the cells of the lining do not produce protective mucus).
When we meal feed horses, this mat is diminished and, therefore, less of a barrier between the stomach acid and the unprotected nonglandular tissue forms. And, not surprisingly, most gastric ulcers are located in the nonglandular portion or along the dividing line (called the margo plicatus) between the two sections.
After your horse has finished eating, it takes only about 6 hours for the majority of that meal to leave the stomach. So, when you ride before meal times and many hours after the horse’s last meal, his stomach lacks the same level of protection that would have existed shortly after he ate.
Additionally, some foods can act as a buffer, helping to raise the pH of the stomach acid. This is especially true of forages, particularly high-calcium legumes such as alfalfa.
Finally, chewing causes saliva to be released, which also helps buffer the stomach acid. So, if your horse eats close to riding, his stomach acid might be less acidic due to this buffering action.
The best thing you can do when your horse hasn’t eaten for several hours before a ride is to feed it something beforehand. This might just mean allowing your horse to munch on hay in a net while you’re grooming or feeding a pound or so of hay pellets. A legume hay or hay pellet such as alfalfa will help more than a grass hay due to the higher calcium levels and, thus, better buffering capacity.
Buffering and coating supplements are also available that have been shown to help either protect the stomach lining or buffer stomach acid. These supplements tend to be somewhat short-lived, meaning their beneficial action only lasts a couple of hours. But this would be fine in a situation where you’re just wanting that buffering protection for the duration of your ride. Some are more effective than others, so do your homework and see whether the manufacturers have any data to prove that their product does effectively buffer stomach acid and for how long.