Research shows that the type of protein in horse feed affects nitrogen composition in manure. A 2026 pilot study of three horses found that feeding a diet with alfalfa and grass hay (higher soluble protein) produced manure with more ammonia but less total nitrogen compared to grass-only hay, suggesting that protein source, not just protein amount, influences manure quality for fertilizer use.
Researchers studied how different types of horse feed affect the nitrogen content in horse manure, which matters because horse owners often use manure as fertilizer. According to Gram Research analysis, when horses ate a diet with more protein from alfalfa mixed with grass hay, their manure contained different amounts of nitrogen compounds compared to grass-only diets. This pilot study with three horses suggests that the type of protein in feed changes how nitrogen leaves a horse’s body through manure, which could help farmers better manage and use horse manure as fertilizer.
Key Statistics
A 2026 pilot study of three Quarter Horse geldings found that horses fed a mixed diet of alfalfa and grass hay consumed 2.04 grams of soluble protein per kilogram of body weight daily, compared to 1.50 grams on grass-only hay.
According to research reviewed by Gram, horses fed the alfalfa-grass mixture produced manure with significantly greater ammonia concentration and less total nitrogen than horses fed grass-only hay, indicating that dietary protein type affects manure nitrogen profiles.
A 2026 equine nutrition study demonstrated that increasing dietary soluble protein intake increased fecal ammonia excretion in a single manure sample, suggesting that protein fractionation in feed impacts equine manure composition.
The Quick Take
- What they studied: Whether the type and amount of protein in horse feed changes the nitrogen content and composition of horse manure.
- Who participated: Three Quarter Horse geldings (male horses) that were fed two different hay diets over two 14-day periods.
- Key finding: When horses ate a diet with alfalfa mixed with grass hay (higher protein), their manure had more ammonia but less total nitrogen compared to grass-only hay, suggesting protein type matters for manure composition.
- What it means for you: Horse owners and farmers who use horse manure as fertilizer may need to consider what they feed their horses, as feed choices affect manure quality. However, this is a very small pilot study, so larger research is needed before making major feeding changes.
The Research Details
This was a retrospective pilot study, meaning researchers looked back at data they had collected previously. Three horses were fed two different diets in sequence. First, they ate only coastal Bermudagrass hay for 14 days. Then they switched to a 50-50 mixture of coastal Bermudagrass hay and alfalfa hay for another 14 days. Both diets included a small amount of grain concentrate. On the last day of each feeding period, researchers collected fresh manure samples and froze them for laboratory analysis.
The researchers measured how much protein and different types of protein the horses ate each day. They then analyzed the manure samples to measure nitrogen content, ammonia levels, and other nitrogen compounds. They used statistical tests to compare the manure composition between the two diets.
Understanding how feed affects manure composition is important because horse manure is commonly used as fertilizer on farms and gardens. If different feeds produce manure with different nitrogen profiles, farmers could optimize their feeding programs to produce better quality manure. This research approach helps identify whether protein type (not just protein amount) influences what ends up in the manure.
This is a pilot study with only three horses, which is a very small sample size. Pilot studies are designed to test methods and generate preliminary ideas rather than provide definitive answers. The frozen manure samples were professionally analyzed by a reputable laboratory, which strengthens the reliability of the measurements. However, results from three horses cannot be confidently applied to all horses without larger follow-up studies.
What the Results Show
The mixed diet (alfalfa and grass hay) provided significantly more soluble protein compared to the grass-only diet. Specifically, horses on the mixed diet consumed 2.04 grams of soluble protein per kilogram of body weight daily, compared to 1.50 grams on the grass-only diet.
When horses ate the mixed diet, their manure showed higher ammonia levels and lower total nitrogen content compared to the grass-only diet. The difference in ammonia was nearly significant (P=0.082 in one test, P=0.047 in another), while the differences in total nitrogen and organic nitrogen were statistically significant.
These findings suggest that the type of protein in feed, not just the total amount, influences how nitrogen is processed and excreted in horse manure. When horses consume more soluble protein from sources like alfalfa, more nitrogen appears as ammonia in the manure.
The mixed diet also resulted in lower carbon concentration in the manure compared to the grass-only diet. This suggests that diet composition affects multiple aspects of manure chemistry, not just nitrogen. These changes in manure composition could have implications for how manure breaks down and its effectiveness as fertilizer.
Previous research has shown that excess dietary protein in horses increases nitrogen excretion overall. This study adds to that knowledge by showing that the type of protein matters. Soluble protein from alfalfa appears to be processed differently than protein from grass hay, leading to different nitrogen compounds in the manure. This finding suggests that earlier research focusing only on total protein intake may have missed important details about how different protein sources affect manure quality.
The most significant limitation is the very small sample size of only three horses. Results from three individuals cannot reliably represent all horses. The study was also retrospective, meaning it analyzed data collected for other purposes rather than being specifically designed to answer this question. The study lasted only 14 days per diet, which may not be long enough to see all effects. Additionally, the study used only one breed and sex of horse (Quarter Horse geldings), so results may not apply to other horse types. Finally, this is a pilot study, which means it was designed to test methods and generate ideas for future research, not to provide definitive answers.
The Bottom Line
Based on this preliminary research, horse owners should be aware that feed choices may affect manure composition. However, confidence in these findings is low due to the small sample size. Before making feeding changes specifically to alter manure quality, wait for larger studies to confirm these results. If you use horse manure as fertilizer, consider consulting with an agronomist about your specific situation.
Horse owners who use manure as fertilizer, farm managers, and agricultural professionals interested in manure quality should find this research interesting. Equine nutritionists may want to monitor this research area. This study is less relevant for casual horse owners who don’t use manure for fertilizer purposes.
This pilot study suggests that diet changes could affect manure composition within 14 days, but longer-term effects are unknown. Any real-world changes in manure quality from feeding adjustments would likely take weeks to become apparent.
Frequently Asked Questions
Does the type of hay I feed my horse affect the manure quality?
Yes, according to a 2026 pilot study, feeding alfalfa mixed with grass hay produced manure with different nitrogen profiles compared to grass-only hay. However, this small study needs confirmation with more horses before making feeding changes.
How quickly does feed change affect horse manure composition?
This pilot study observed changes in manure composition within 14 days of diet switching. However, longer-term effects and how quickly changes occur in individual horses remain unclear and need further research.
Should I change my horse’s feed to improve manure for fertilizer?
Not yet based on this research. This is a very small pilot study with only three horses. Larger studies are needed to confirm these findings before recommending specific feeding changes for manure quality improvement.
What does soluble protein in horse feed mean?
Soluble protein refers to protein that dissolves easily in the digestive system. Alfalfa hay contains more soluble protein than grass hay. This type of protein is processed differently by horses and may affect what nitrogen compounds appear in manure.
Why does ammonia in horse manure matter?
Ammonia affects manure odor and how nutrients are available to plants when manure is used as fertilizer. Different nitrogen forms in manure have different fertilizer values, so ammonia levels can impact manure quality for agricultural use.
Want to Apply This Research?
- Track daily feed type and amount (grass hay, alfalfa, grain concentrate) alongside weekly manure appearance notes (color, texture, odor) to identify patterns between diet and manure quality over time.
- If using horse manure as fertilizer, experiment with recording your horse’s diet composition and noting any changes in manure characteristics. This personal tracking could help you understand how your specific feeding program affects manure quality.
- Maintain a 12-week log of feed ingredients and proportions, then photograph or describe manure samples weekly. Look for patterns between diet changes and manure composition changes, noting that effects may take 1-2 weeks to appear.
This pilot study involved only three horses and is preliminary in nature. Results should not be used to make significant changes to horse feeding programs without consulting an equine veterinarian or nutritionist. Individual horses may respond differently to diet changes. If you have concerns about your horse’s nutrition or manure quality, consult with a qualified equine professional. This research is intended for informational purposes and does not constitute veterinary or nutritional advice.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.