Researchers studied how horses’ digestive systems adapt when switching from grass hay to a processed grain-based diet. Twelve horses were gradually introduced to the new food over four weeks while scientists tracked changes in their gut bacteria. The study found that horses’ digestive bacteria change significantly based on what they eat, and keeping some hay in their diet helps maintain a healthy balance of bacteria. When hay was completely removed, the diversity of helpful bacteria decreased, suggesting that horses need some roughage to keep their digestion working properly.

The Quick Take

  • What they studied: How a horse’s gut bacteria change when gradually switching from hay to a processed complete diet, and whether the bacteria stay healthy during this transition.
  • Who participated: Twelve adult male horses (geldings) weighing about 370 kilograms each, taken from pasture and monitored over four weeks.
  • Key finding: Horses’ gut bacteria changed based on diet, but the variety of bacteria stayed stable as long as some hay remained in their food. When hay was completely removed, the bacterial diversity dropped, suggesting roughage is important for gut health.
  • What it means for you: If you feed horses a processed diet, gradually introducing it while keeping some hay may help maintain their digestive health. However, this is a small study on horses, so consult with an equine veterinarian before making major diet changes.

The Research Details

This was a controlled feeding trial where researchers carefully tracked what happened to horses’ digestive bacteria over time. All twelve horses started by eating only hay for one week to establish a baseline. Then half the horses continued eating only hay for four more weeks, while the other half gradually switched to a processed complete diet over those same four weeks. The processed diet was introduced slowly—starting at 30% of their food in week two, increasing to 60% in week three, and reaching 100% by week four. Scientists collected poop samples on specific days (days 7, 14, 21, and 28) to analyze the bacteria living in the horses’ digestive systems. They also tested how well these bacteria could break down food in laboratory conditions. This gradual approach mimics how horse owners would realistically change their horses’ diets.

Understanding how horses’ digestive bacteria adapt to diet changes is important because horses have sensitive digestive systems. Sudden diet changes can cause serious problems like colic (severe stomach pain). By studying how bacteria gradually adjust, researchers can help develop safer feeding strategies. The study also shows that the presence of roughage (hay) might be more important for digestive health than previously thought, even when horses are eating processed complete diets.

This study was well-designed with a clear control group (horses staying on hay only) and an experimental group (horses switching diets). The researchers collected samples at multiple time points, which allows them to see changes over time rather than just comparing before and after. However, the study only included twelve horses, which is a relatively small number, so results may not apply to all horses. The study was published in a peer-reviewed journal, meaning other scientists reviewed it before publication. The researchers used modern laboratory techniques to identify and count bacteria, which is reliable.

What the Results Show

When horses ate more of the processed diet, certain types of bacteria decreased in their poop, including Fibrobacteres, Proteobacteria, and several others. At the same time, other bacteria like Verrucomicrobia and Synergistetes increased. The most important finding was about bacterial diversity—the total number of different bacteria species stayed relatively stable through day 21 (when horses were eating 60% processed diet and 40% hay), but dropped noticeably by day 28 when horses were eating 100% processed diet with no hay. This suggests that having some hay in the diet helps maintain a healthy variety of bacteria. The processed diet itself did affect which bacteria were present and how well they could break down food in laboratory tests, indicating that diet composition directly influences digestive function.

The study found that the processed diet changed not just which bacteria were present, but also how well those bacteria could ferment (break down) food in laboratory conditions. Some bacterial groups that appeared with the processed diet had different abilities to process food compared to bacteria found in hay-only diets. This suggests that diet changes affect not just the types of bacteria present, but also the overall digestive capacity of the system.

Previous research has shown that horses’ digestive bacteria are flexible and change based on diet, but this study provides new evidence about the importance of roughage (hay) in maintaining bacterial diversity. The finding that bacterial diversity declined only when hay was completely removed suggests that even small amounts of roughage may be protective for digestive health. This supports traditional horse feeding practices that recommend always including some hay, even when using complete processed diets.

The study only included twelve horses, all of similar size and type, so results may not apply to all horses, especially different breeds or ages. The study only lasted four weeks, so we don’t know if the bacterial changes would continue or stabilize with longer feeding. The horses were taken from pasture, which may have affected their initial bacterial populations. The study didn’t measure actual digestive health outcomes like weight gain or disease, only bacterial composition and laboratory fermentation tests. Results from laboratory fermentation tests may not perfectly reflect what happens in a living horse’s digestive system.

The Bottom Line

Based on this research, gradually introducing a processed diet over several weeks while maintaining some hay appears to be a safer approach than switching completely to processed diet. The evidence suggests keeping at least some roughage in a horse’s diet helps maintain healthy digestive bacteria. However, this is a small study, so work with an equine veterinarian or nutritionist when making significant diet changes. Confidence level: Moderate—the study is well-designed but small, and results are specific to horses.

Horse owners and caretakers should pay attention to these findings, especially those considering switching horses to complete processed diets. Equine veterinarians and nutritionists can use this information to make better feeding recommendations. This research is less relevant to people who feed horses traditional hay-based diets. The findings apply specifically to horses and may not apply to other animals.

Bacterial changes in the digestive system happen relatively quickly—within one to two weeks of diet changes. However, the full impact on digestive health and overall horse performance may take longer to observe. If making diet changes, monitor your horse for at least 4-6 weeks to assess tolerance and health.

Want to Apply This Research?

  • Track daily hay intake percentage and processed diet percentage for each horse during diet transitions. Record observations about manure quality, consistency, and frequency every 2-3 days to monitor digestive health during the changeover period.
  • If transitioning a horse to a processed diet, use the app to schedule gradual increases: Week 1 (30% processed/70% hay), Week 2 (60% processed/40% hay), Week 3+ (100% processed with hay available). Set reminders to maintain this schedule and prevent sudden diet changes.
  • Create a long-term tracking system that logs diet composition weekly, manure characteristics, horse behavior, and any digestive issues. Compare notes over 4-6 weeks to identify any problems early. Include a reminder to consult with a veterinarian if any digestive issues develop during the transition.

This research describes what happens to horse digestive bacteria during diet changes in a controlled study setting. It should not replace professional veterinary advice. Before making significant changes to your horse’s diet, consult with an equine veterinarian or qualified equine nutritionist, as individual horses may respond differently. This study was conducted on a small number of horses and may not apply to all horses, especially those with existing digestive problems, different ages, or different breeds. Always make diet changes gradually and monitor your horse closely for any signs of digestive upset.