Research shows that horses eating fresh pasture produce significantly higher levels of beneficial bacterial chemicals called short-chain fatty acids compared to horses eating hay or grain-supplemented diets. A 2026 study of 24 horses found that pasture-fed horses had the most active bacterial fermentation in their digestive systems, while oat supplements primarily changed bacterial activity in the stomach area. These bacterial byproducts are important for intestinal health and energy production, suggesting that pasture grazing may support better gut bacterial function than other feeding methods.
According to Gram Research analysis, a new study shows that what horses eat dramatically changes the helpful chemicals their gut bacteria produce. Researchers compared horses eating grass all day, hay, or hay mixed with oats, then examined their digestive systems to measure bacterial byproducts. They found that pasture-fed horses had the most beneficial bacterial activity, while oat supplements changed how bacteria worked in the stomach and upper digestive tract. These findings matter because the chemicals bacteria produce affect horse health and can influence digestive problems. Understanding these differences helps horse owners choose better feeding strategies.
Key Statistics
A 2026 study of 24 horses published in PLoS One found that pasture-fed horses produced significantly higher total short-chain fatty acids in their digestive systems compared to hay-fed or oat-supplemented horses, with the largest differences appearing in the back sections of the large intestine.
Research reviewed by Gram showed that horses receiving oat supplements at 1 to 2 grams of starch per kilogram of body weight per meal demonstrated increased protein breakdown activity in the stomach area, suggesting grain supplementation alters where bacterial fermentation occurs in the digestive tract.
The 2026 equine study found clear separation in bacterial metabolite production between the front digestive system (stomach and small intestine) and the back section (large intestine), with the large intestine producing substantially more beneficial bacterial chemicals regardless of diet type.
The Quick Take
- What they studied: How different horse diets (grass, hay, or hay with oat grain) change the types and amounts of chemicals that gut bacteria produce in different parts of the digestive system.
- Who participated: 24 horses total: 4 eating pasture grass 24 hours daily, 8 eating hay only, 6 eating hay plus a smaller oat supplement, and 6 eating hay plus a larger oat supplement.
- Key finding: Horses eating fresh pasture had significantly higher levels of beneficial short-chain fatty acids (SCFA), which are the main chemicals bacteria produce. Oat supplements changed bacterial activity mostly in the stomach area, suggesting different types of bacterial breakdown happening there.
- What it means for you: If you own horses, pasture grazing appears to support the most beneficial gut bacterial activity. However, this was a small study on horses that were euthanized for examination, so results may not apply to all horses or all situations. Talk to an equine veterinarian before making major diet changes.
The Research Details
Researchers divided 24 horses into four groups with different diets: one group grazed on pasture 24 hours daily, another ate hay without limits, and two groups ate hay plus oats at different amounts (1 gram or 2 grams of starch per kilogram of body weight per meal). After several weeks on these diets, the horses were euthanized and their digestive tracts were carefully examined. Scientists collected samples from multiple locations along the entire digestive system, from the stomach through the small intestine to different sections of the large intestine (colon). Each sample was tested for moisture content, acidity level, and the specific chemicals produced by bacteria, including short-chain fatty acids, ammonia, and lactate.
This research approach is important because it shows exactly where in the digestive system different bacterial chemicals are produced and how diet changes these patterns. By examining the actual digestive contents at multiple locations rather than just looking at feces, researchers could see the complete picture of bacterial activity. This detailed information helps explain why different diets affect horse health differently.
This study has both strengths and limitations. The strength is that researchers examined the actual digestive tract contents directly rather than making guesses from outside measurements. The limitations include the small number of horses (24 total), the fact that horses had to be euthanized to collect samples (meaning results are from a single time point), and that results may not apply to all horse breeds or ages. The study was published in PLoS One, a peer-reviewed scientific journal, which means other experts reviewed the work before publication.
What the Results Show
Horses eating fresh pasture showed the highest total levels of short-chain fatty acids, the main beneficial chemicals produced by gut bacteria. These fatty acids are important because they provide energy for the horse’s intestinal cells and have anti-inflammatory effects. The pasture group also had higher levels of specific types of fatty acids (n-valerate and iso-SCFA) in the back sections of the large intestine, suggesting their bacteria were breaking down proteins differently than in other groups. The study found clear differences between the front part of the digestive system (stomach and small intestine) and the back part (large intestine), with the large intestine producing much more bacterial metabolites overall. Horses receiving oat supplements showed changes mainly in the stomach area, with evidence of increased protein breakdown by bacteria in that location.
The study found that ammonia levels, a byproduct of protein breakdown, were significantly higher in the back sections of the large intestine for pasture-fed horses. This suggests these horses’ bacteria were processing proteins more actively in that area. Moisture content of digestive material also varied significantly by diet type and location in the digestive tract. The hay-only group showed intermediate levels of bacterial metabolites, falling between the pasture group and the oat-supplemented groups in most measurements.
Previous research suggested that diet affects bacterial communities in horse digestive systems, but this study provides more detailed information about exactly which bacterial chemicals change and where in the digestive tract these changes occur. The findings support earlier observations that pasture feeding supports different bacterial activity than grain supplementation, but this research quantifies those differences more precisely. The study also confirms that the large intestine is the primary site of bacterial fermentation in horses, producing far more metabolites than the stomach or small intestine.
The study examined only 24 horses, which is a relatively small number for drawing broad conclusions. All horses were euthanized to collect samples, meaning researchers could only measure bacterial activity at one point in time rather than tracking changes over weeks or months. The study doesn’t tell us whether these chemical differences actually affect horse health outcomes or disease risk. Results may not apply to horses of different ages, breeds, or health conditions. The study also doesn’t examine how long it takes for bacterial communities to adjust to diet changes or whether the effects are permanent.
The Bottom Line
Based on this research, pasture grazing appears to support the most beneficial bacterial metabolite production in horses (moderate to high confidence). If pasture grazing isn’t possible, hay feeding is a reasonable alternative, though it produces lower levels of beneficial bacterial chemicals. Oat supplementation changes bacterial activity in the stomach area, which may or may not be beneficial depending on the individual horse (low to moderate confidence). Any major diet changes should be made gradually and discussed with an equine veterinarian, especially for horses with digestive problems.
Horse owners and managers should care about these findings, particularly those managing horses with digestive issues or trying to optimize nutrition. Equine veterinarians may find this useful for understanding how diet affects gut health. This research is less relevant for people who don’t own horses. Horses with existing digestive disorders should have diet changes supervised by a veterinarian rather than making changes based on this study alone.
If you change a horse’s diet based on these findings, expect bacterial communities to begin adjusting within days to weeks. However, this study doesn’t specify how long it takes to reach the full bacterial changes shown in the research. Most dietary changes in horses take 2 to 4 weeks to show their full effects on digestive health.
Frequently Asked Questions
Does feeding horses grain supplements change their gut bacteria?
Yes, oat grain supplements alter bacterial activity, primarily in the stomach area. A 2026 study found that horses receiving oat supplements showed increased protein breakdown by bacteria in the stomach, different from pasture-fed horses where bacterial activity was highest in the large intestine.
Is pasture better for horse digestion than hay?
Research suggests pasture grazing produces higher levels of beneficial short-chain fatty acids, the main chemicals bacteria produce for intestinal health. However, this study examined only 24 horses, so individual horses may respond differently. Consult your veterinarian about your specific horse’s needs.
What are short-chain fatty acids and why do horses need them?
Short-chain fatty acids are chemicals produced when gut bacteria break down plant material. They provide energy for intestinal cells, reduce inflammation, and support overall digestive health. Pasture-fed horses in this study produced significantly higher levels than grain-supplemented horses.
How quickly do horse gut bacteria change when you switch their diet?
This study doesn’t specify the timeline, but research generally shows bacterial communities begin adjusting within days and reach major changes within 2 to 4 weeks. Gradual diet transitions over 7 to 10 days help minimize digestive upset during this adjustment period.
Can I use this research to fix my horse’s digestive problems?
This study provides useful information about how diet affects bacterial activity, but it doesn’t prove that changing diet will cure digestive disorders. Work with an equine veterinarian to diagnose the specific problem and develop an appropriate feeding plan for your horse.
Want to Apply This Research?
- Track your horse’s diet type daily (pasture hours, hay amount, grain supplements with starch amounts) and note digestive health observations weekly, including manure consistency, appetite, and energy levels. Compare patterns over 4 to 8 week periods to see if diet changes correlate with digestive changes.
- If using a horse nutrition app, log the specific type of feed and amounts daily. Set reminders to gradually transition between diet types over 7 to 10 days rather than making sudden changes. Track any digestive changes (manure quality, behavior, appetite) to see how your horse responds to different feeding strategies.
- Establish a baseline of your horse’s current digestive health and bacterial activity indicators (manure consistency, coat quality, energy level) before making diet changes. After any diet modification, monitor these same indicators weekly for 8 weeks. Use the app to create a visual timeline showing how your horse responds to different feeding approaches, which you can share with your veterinarian.
This article summarizes research on horse digestive bacterial chemistry and should not replace professional veterinary advice. The study examined only 24 horses and cannot definitively prove that diet changes will improve any individual horse’s health or prevent digestive disease. Before making significant changes to your horse’s diet, especially if your horse has digestive problems, consult with an equine veterinarian who can evaluate your specific horse’s health status, age, breed, and individual needs. This research provides scientific context for understanding how diet affects gut bacteria, but individual horses may respond differently to the same feeding strategies.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.