Some horses paw or are aggressive at feeding time, this is more than likely due to frustration/pain. They are anxious to self-medicate; their stomach does not feel well due to the build-up of acid and/or painful gastric ulcers. Once they start chewing and eating they experience relief.
The equine stomach produces acid 24 hours a day in preparation for constant uptake and can empty in as little as 15-20 minutes.The lower part of the stomach, in addition to producing the acid, receives protection by also producing mucus. The upper, or non-glandular part (squamous epithelium – similar to our skin) has no protection and is even more susceptible to damage/ulcers. Chewing activates saliva production (an alkaline substance), which buffers gastric acid. Under natural conditions with free-choice forage, the horse will produce approximately five gallons of saliva per day (that's allot of buffering!) and eventually “recycle” much of the water content via re-absorption prior to excretion.
Fiber present in the stomach also prevents the “splashing” of acids. Having fiber and saliva in the stomach is especially important during any physical activity/ exercise which causes the splashing of acids.
Additionally, if your horse consumes hay too quickly, the particle size will not be reduced sufficiently or have a high enough saliva-to-forage ratio. Saliva plays a crucial role in digestion. Large amounts of dry matter lacking sufficient saliva can contribute to impaction colic.
Horses may also experience stress (which can cause ulcers) due to isolation; not experiencing physical interaction with other horses! Slow feeding with multiple locations levels the playing field enabling your entire herd to eat and live together full time. Equines are herd animals; they benefit physically and psychologically from direct physical interaction. Dominant members will keep the others moving as they claim various locations. The less dominant individuals will have alternate sources to eat from; this encourages movement and can decrease cortisol levels associated with stress from being physically separated from herd members.
Read more about slow feeding benefits. Here’s to supporting a natural lifestyle for happier, healthier horses!
Why Do Horses Chew On Things? may be of interest.