The Stomach Doesn't Wait: Why Chewing Matters
Gastric emptying proceeds whether your horse has chewed properly or not. That's exactly why chewing quality matters.
There’s a timing mechanism in the equine stomach that operates largely independent of how well the horse has chewed its food.
The liquid phase of gastric content exits the stomach in approximately 20-45 minutes. The solid phase progresses through at a relatively constant rate. This timing is physiologically determined and doesn’t adjust based on particle size, degree of mastication, or whether the material has been adequately prepared for intestinal digestion.
The stomach empties on schedule. Well chewed or poorly chewed, the material moves on.
This might seem like it makes chewing less important. If the stomach is going to empty anyway, why does it matter how thoroughly the horse breaks down its food?
The answer is that chewing matters precisely because the stomach doesn’t wait.
What Happens When Chewing is Compromised
Horses chew poorly for several reasons. Dental problems are the most obvious: sharp points that lacerate the cheeks, hooks that prevent proper lateral excursion, wave mouth that prevents effective grinding, missing or damaged teeth that create gaps in the occlusal surface.
But dental arcade geometry is only part of the equation. The temporomandibular joint has a specific range of motion optimised for the lateral grinding movement required to process fibrous material. When TMJ function is restricted by pain, inflammation, or mechanical dysfunction, chewing efficiency decreases even if the teeth themselves are fine.
Diet composition affects chewing too. Long-stem hay requires sustained chewing to reduce particle size and mix with saliva. Pelleted feeds can be swallowed with minimal mastication. A horse eating primarily pellets will spend dramatically less time chewing than one eating primarily hay, regardless of dental health.
When chewing is compromised by any of these factors, the material that enters the stomach is larger, less fragmented, and less thoroughly mixed with saliva than it should be.
And then the stomach empties on schedule anyway.
The Downstream Consequences
The small intestine and cecum are adapted to process material of specific particle sizes. Enzymatic digestion in the small intestine requires adequate surface area. Microbial fermentation in the hindgut requires particles small enough for microbes to colonise and break down efficiently.
When larger, poorly processed particles arrive, several things happen.
First, enzymatic digestion in the small intestine is less efficient. Larger particles have less surface area relative to volume. Enzymes work at surfaces. Less surface area means less enzymatic access and reduced nutrient extraction in the small intestine.
Second, material that isn’t adequately digested in the small intestine passes to the hindgut. This isn’t inherently problematic, horses are hindgut fermenters designed to extract energy from fibre through microbial action. But the efficiency of that fermentation depends on particle size.
Microbes colonise feed particles from the outside in. A particle that’s too large takes longer to ferment completely. In some cases, it may pass through partially undigested, particularly if gut transit time is relatively fast or if the microbial population isn’t robust.
Third, fibre that hasn’t been mechanically fragmented during chewing is less accessible to fermentation. The physical structure of plant cell walls requires both mechanical and enzymatic breakdown. Chewing provides the mechanical component. When that’s inadequate, the hindgut microbiome must do more work, and the overall efficiency of fiber digestion decreases.
The Role of Saliva
Chewing stimulates saliva production mechanically. This is different from species like humans and dogs, where saliva production is also triggered by anticipation of food. In horses, it’s the physical act of chewing that drives secretion.
Saliva serves multiple functions. It provides buffering capacity, which matters for gastric pH regulation. It provides initial enzymatic activity through salivary amylase. It lubricates the food bolus, facilitating swallowing and passage through the esophagus.
When chewing time is reduced, less saliva is produced. This has implications beyond just the mechanical aspects of swallowing. Reduced buffering capacity means less protection against gastric acid. Reduced enzymatic activity means less pre-gastric starch digestion.
The composition of saliva also changes based on diet. Horses eating dry forage produce more viscous saliva than those eating wet or processed feeds. This viscosity affects how well material is lubricated and how effectively it’s mixed during chewing.
TMJ Function and Chewing Efficiency
The equine temporomandibular joint is engineered for a specific movement pattern. Unlike carnivores with primarily vertical jaw movement or omnivores with moderate lateral excursion, horses have pronounced lateral grinding capability. The mandible moves side to side with considerable range, allowing molars to shear across each other and pulverize fibrous material.
This movement requires full TMJ function. When the joint is restricted by inflammation, pain, previous trauma, or mechanical dysfunction, lateral excursion decreases. The horse may still be able to close its jaw vertically and bite, but the grinding motion that actually reduces particle size is compromised.
TMJ problems often go unnoticed until they’re severe. Unlike dental points that cause obvious clinical signs (quidding, dropping feed, head tossing), TMJ dysfunction can be subtler. The horse may eat more slowly, show preference for one side, or develop asymmetric wear patterns on the teeth. But unless someone is specifically evaluating TMJ function, these signs might not be connected to the underlying joint problem.
Palpating the TMJ during jaw movement can reveal restriction, clicking, or pain responses that indicate dysfunction. Measuring lateral excursion provides quantifiable data about range of motion. These assessments should be part of routine wellness evaluation, not reserved for cases with obvious eating problems.
Hay vs Pellets: More Than Nutrition
The physical form of feed has mechanical implications independent of nutritional composition.
Long-stem hay requires extensive chewing. The horse must grasp, position, grind, mix with saliva, and form a bolus. This process takes time and produces substantial saliva flow.
Pelleted feeds can be swallowed with minimal chewing. The material is already reduced to small particles during manufacturing. The horse may crush the pellets slightly, but the grinding action required for hay is absent.
This difference affects several things beyond just the obvious time spent eating. Dental wear patterns differ. Saliva production differs. The mechanical stimulation that maintains TMJ function differs. The particle size entering the stomach differs.
A horse on a pellet-heavy diet spends less time chewing, produces less saliva, and delivers material to its stomach that, while mechanically processed by manufacturing equipment, hasn’t undergone the biological processing that integrates chewing with salivation and enzymatic activity.
This doesn’t mean pellets are inherently problematic. They serve legitimate purposes in feeding management. But the shift away from long-stem forage has consequences that extend beyond nutritional composition to the mechanical and enzymatic preparation of material for intestinal digestion.
Clinical Assessment
Evaluating chewing effectiveness requires looking at multiple factors, not just dental arcade geometry.
Dental examination should assess functional capacity, not just anatomical structures. Do the molars meet properly across their full surface? Is there adequate lateral excursion to allow effective grinding? Are there asymmetries suggesting unilateral chewing?
TMJ assessment should include palpation during movement, evaluation of range of motion, and attention to clicking or crepitus that might indicate cartilage damage or inflammation.
Observing the horse eating provides information about behavioural patterns. Does it drop feed? Quid? Chew on one side preferentially? These behaviours suggest functional problems even if static examination of teeth and TMJ appears normal.
Monitoring body condition, manure consistency, and signs of digestive upset provides indirect information about whether chewing and subsequent digestion are adequate. Unexplained weight loss, changes in manure (too dry, too loose, containing undigested fibre), or recurring mild colic episodes might trace back to inadequate mastication.
The Practical Takeaway
The equine stomach empties on a schedule that doesn’t wait for proper chewing. This means the burden of inadequate mastication falls on downstream processes: small intestine enzyme activity, hindgut microbial fermentation, and the overall efficiency of nutrient extraction.
Chewing isn’t optional just because gastric emptying is relatively fixed. It’s essential precisely because the stomach will empty whether the food is ready or not.
Everything that happens in intestinal digestion depends on the preparation that happened during chewing: particle size reduction, saliva production, mixing, and formation of a bolus that can be efficiently processed by enzymes and microbes.
When that preparation is compromised by dental problems, TMJ dysfunction, or dietary management that minimises chewing time, the consequences propagate throughout digestion.
The stomach doesn’t wait. That’s exactly why chewing matters.
References:
Lorenzo-Figueras M, Merritt AM. Effects of exercise on gastric volume and pH in the proximal portion of the stomach of horses. Am J Vet Res. 2002.
Lorenzo-Figueras M, Preston T, Ott EA, Merritt AM. Meal-induced gastric relaxation and emptying in horses after ingestion of high-fat versus high-carbohydrate diets. Am J Vet Res. 2005.
Métayer N, Lhôte M, Bahr A, Cohen ND, Kim I, Roussel AJ, Julliand V. Meal size and starch content affect gastric emptying in horses. Equine Vet J. 2004.
Luthersson N, Nielsen KH. Review of equine gastric ulcer syndrome. Equine Vet Educ. 2009.