During Cornell Equine Hospital and Cornell Cooperative Extension’s monthly seminar series, Nathalie Trottier, PhD, professor of animal science in Cornell’s College of Agriculture and Life Sciences, in Ithaca, New York, took an evolutionary and physiological approach to explaining the importance of forage for modern equids.

The Evolution of the Horse

The horse family Equidae adapted extremely well to grassland as they evolved from browsers to grazers over millions of years. Trottier explained this is evident in their rich fossil record, which reveals equids’ teeth, feet, and more adapted to give them an evolutionary advantage over other herbivores.

As we’ve domesticated the horse throughout the past 5,500 years, she said, we’ve not only bred them selectively for performance, appearance, and temperament, but we’ve also added high-starch cereal grains and concentrate feeds to their diets in place of full-time forage. The result? An increased prevalence of gastric ulcers, dental issues, metabolic disease, and digestive upset.

Forage and Equine GI Tract Physiology

Trottier reviewed the horse’s digestive anatomy—from the teeth to the large intestine—and how each component is uniquely designed to process forage.

Dentition

Horses’ teeth erupt slowly and continuously to compensate for constant grinding (16 to 20 hours per day for some, said Trottier). “Grinding is important because it wears away about 2-3 millimeters per year to counter erupting,” she said. “Attrition from an herbivorous diet tends to keep pace with eruption rates.”

Allowing horses to consume free-choice forage all day reduces their risk of uneven tooth wear and dental overgrowth, she said. Feeding concentrates, on the other hand, introduces more sugars and starches that are detrimental to dental health.

Salivary glands

Saliva comprises not only water but also antimicrobial agents, digestive enzymes, and electrolytes critical for maintaining pH balance. The most important stimulant of saliva secretion is chewing, said Trottier.

“In dogs, the sight and smell of food is a stimulant, but in horses it’s not,” she said. “Chewing is the only thing that stimulates parotid gland saliva secretion.”

When we feed horses grains, however, they secrete less saliva because they’re chewing less. Trottier said the average 1,100-pound horse chews 18-30 minutes per pound of hay versus 5 minutes per pound of grain.

Saliva is also critical to dental health because it lowers horses’ risk of developing cavities. “We know low pH is a leading cause of cavities,” said Trottier. “High grains equal more starch and less saliva. More bacteria then grow and ferment.”

Jaws

The horse’s lower jaw evolved to be narrower than the top jaw. This allows the horse to move his jaws left and right and consume fibrous feedstuffs more effectively. “A larger upper jaw allows for movement to chew and consume grasses,” said Trottier. “Feeding of concentrate does not allow horses’ jaws to move as intended, resulting in uneven wear.”

Stomach

Compared to all other domestic animals, the horse’s stomach is the smallest segment of its GI tract. “It’s incredible that this animal has this physiology where the stomach is so small yet is able to consume 20 pounds of hay a day easily,” said Trottier. “The reason is because the horse is a continuous grazer, trickling a small amount of feed at once as forage.”

The stomach is divided into glandular and nonglandular regions, with the latter lacking mucous secretion and being particularly susceptible to gastric ulcers. The reduced surface area of secretory mucosa of the glandular region is an evolutionary adaptation to consuming low-protein grasses, she explained.

“The horse’s small stomach size and capacity favor continued movement and gastric passage,” Trottier said. “Pasture grasses are high in moisture content, which increases gastric rate of passage. A high passage rate accommodates the large quantities of forage the horse needs daily. Continual passage of food is very important to maintain balanced pH in the stomach.”

Feeding forage before grain protects the horse’s stomach from the potentially harmful effects of high-starch concentrates because it:

• Increases salivation and bicarbonate ions that buffer pH.
• Reduces grain starch fermentation in the hindgut (cecum and large intestine).
• Increases water flow through the stomach and, thus, the passage rate of the grain bolus. Otherwise, the starch in grains binds water and reduces the rate of passage, promoting bacterial fermentation.

Large intestine

The horse’s large intestine capacity also aligns with the forage nutrient composition of their intended diet. “The slow passage rate here favors greater fermentation efficiency of cellulose and hemicellulose (insoluble fibers in plant cell walls),” said Trottier. “(The large intestine) is designed to ferment slowly fermentable fibers. We can help it flourish by providing sufficient forages and fibers.”

Take-Home Message

When creating a feeding strategy for your horse, consider how he evolved, and focus on a forage-first diet tailored to his gastrointestinal physiology. In doing so, you can help prevent many dental and digestive issues common among domesticated horses.