Why does eating grass make my horse irritable?

About the Author

As a career toxicologist, I'm Dr. Rosemary Waring. My work spans research, teaching, and service on international committees, including positions with the European Food Safety Authority (EFSA) and the UK's Pesticide Residues in Food (PRiF) committee.

Toxicology focuses on understanding cellular death mechanisms. My research examines how environmental compounds interact with human tissues, with special attention to digestive processes and gut microbiome chemistry. When the delicate balance of bacteria, viruses, and fungi in our digestive system is disrupted, it can produce harmful compounds that affect health and performance.

Research Background

Our initial development of EquiNectar® stemmed from studying digestive metabolites in athletic horses. A key discovery emerged: horses naturally produce limited amounts of amylase, the enzyme responsible for starch breakdown. When horses consume excessive starch, most passes to the hindgut where harmful bacteria ferment it. We've successfully identified several specific bacteria and their toxic byproducts involved in this process.

Our research expanded to examine horses on primarily pasture-based, low-starch diets. We found similar digestive disruptions occur when grass grows rapidly or contains high levels of fructans (fructose-based compounds). This can overwhelm the digestive system and significantly alter the hindgut's bacterial composition.

A collaboration with Redwings Horse Sanctuary yielded important findings: While horses on spring grass showed major changes in their gut microbiome, those receiving EquiNectar® maintained more stable digestive populations. The complete study, "Modulation of the equine microbiome by pasture and feed supplements: A metabolomics approach," details these results.

Recently, many horse owners have asked why fresh grass causes behavioral changes in their horses. Here's what our scientific review revealed.

Scientific Findings

Understanding Fresh Grass Composition

Fresh grass contains simple sugars (glucose, fructose, and sucrose) that create sweetness, along with complex fructans - long chains of fructose molecules (Kagan 2022). Optimal grass growth occurs in warm, moist conditions typical of spring and fall. These seasons' cool nights trigger fructan production as both cold protection and energy storage. Research shows fructan levels peak in afternoon hours, though frost can cause elevated morning levels (Weinert-Nelson et al 2022, Kagan et al 2020).

Digestive Impact

While fructans serve as natural plant protection, horses can't digest them directly. These compounds concentrate in the hindgut, where bacterial enzymes break them into fructose units. This process allows harmful bacteria to multiply, creating an acidic environment that damages gut walls and allows increased absorption of toxins and fructose into the bloodstream.

Metabolic Effects

The body typically converts fructose to glucose, but high concentrations can overwhelm this system, leading to elevated blood fructose. While equine studies are limited, research in other species shows fructose affects metabolism (Spagnuolo et al 2020) and increases appetite (Payant and Chee 2021). This may explain why horses overconsume fresh grass, which they already prefer for its sweetness and tender texture (Cameron et al 2022).

Behavioral Effects

Research indicates fructose influences brain function, potentially increasing anxiety and behavioral issues through mechanisms still being studied (Tang et al 2022, Payant and Chee 2021). Fortunately, short-term exposure doesn't cause permanent damage, based on rodent studies (D'Ambrosio et al 2023). Studies have linked fresh grass consumption to behavioral and performance issues in horses, alongside factors like obesity and competition stress (Buckley et al 2013).

Behavioral Changes

Horse owners frequently report:

Sensitivity during cinching/girthing
Heightened anxiety levels
Increased irritability
Aggressive field behavior toward humans and other horses
Unpredictable riding behavior (spooking, bucking, rearing, bolting)
Reduced forward movement
Signs of discomfort under saddle

These pasture-related changes likely result from:

Modified gut microbiome affecting mood (Mach et al 2020)
Physical discomfort
Temporary brain function changes from high fructose levels
Excess calorie intake from abundant fresh grass combined with regular feed

Management Solutions

Limited Grazing

Restrict access to fresh pasture to early morning hours and maintain shorter grass height (Kagan 2022). However, weather variations affect grass sugar content throughout the day, and horses often compensate for limited grazing time by eating more quickly.

Intake Control

Grazing muzzles can help limit consumption, though horses often adapt their grazing patterns to maintain intake levels.

Managed Grazing Systems

Creating designated grazing tracks with controlled access areas proves highly effective when facility layout permits.

Dietary Support

EquiNectar® contains enzymes that help manage fructose release earlier in digestion, preventing hindgut fructan metabolism. Users typically report improved temperament and riding behavior.