Feeding Studies in Horses: Why Practical Observations Often Prove Right Regarding Roughage

This article was translated using AI.

This post is the third part of our specialist series on roughage feeding in horses. While Part 1: „Roughage & Fiber Length in Horses: How the Form of Roughage Influences Digestion“ highlighted the physiological mechanisms from chewing kinematics to intestinal peristalsis, and Part 2: „Hay, Grass, Haylage, Chopped Fibre and Cobs: Roughage in Horses in Practice and Science“ evaluated the individual types of roughage, this part is dedicated to the methodological background: Why practical observation and feeding studies often deliver seemingly contradictory results and what science can actually prove.

Classification and Consequences

Why Practice and Studies Can Diverge

We now have a practical observation, some evidence, several plausible explanations, and above all, many gaps for every form of roughage. The question remains why these observations are so poorly reflected in existing studies. In addition to the methodological reasons from the first chapter, there are three others that rarely appear in the discussion.

One Diagnosis, Different Causes

Free fecal water (FFW) is not a disease entity, but a symptom. Behind it can be very different processes: a starch-induced dysbiosis, a stress-related disorder, problematic roughage quality, poorly chewed feed, dental disease, a damaged intestine after antibiotics or deworming, sand accumulation, and much more, as well as any combination of various causes.

A typical case-control study takes ten horses with this symptom and compares them with five to ten horses without it. If there are five different causes, and a feed form only affects one of them, a strong effect in this subgroup disappears in the average. The result is then: no correlation. Correct would be: no average correlation in this mixed population.

Exactly this pattern is shown by the investigations of Lindroth and colleagues. In two case-control studies, the fecal particle distribution between horses with and without fecal water was similar; in one subpopulation, lactic acid and water-holding capacity differed, in the other they did not. The associated feeding study found, among other things, more concentrate, starch, and sugar as well as less NDF in fecal water horses, while no simple signal was found for individual roughage forms. As observational studies with owner-reported data, they can neither identify structured chaff as a cause nor exonerate it.

In practice, one works exactly the other way around. You have an individual horse, change one thing, and see what happens. If the fecal water disappears after switching to long hay and returns when chaff is fed again, this is a significant finding for this specific horse, even if it does not appear in any scientific study or population statistics. Both paths of knowledge answer different questions: the study asks whether a factor is significant on average across many horses; practice asks what helps this particular horse.

For horses showing digestive problems despite a good supply of roughage, an individual assessment by a competent therapist from the Sanoanimal Therapist Network can help clarify causes such as dental problems or dysbiosis.

Further specialist knowledge on feeding correlations and metabolic processes is provided by the Sanoanimal Online Courses.

Feces Is Not the Large Intestine

The second reason concerns the measurement site. Feces are easy to obtain and are therefore used for microbiome and particle analyses. However, direct examination of different intestinal sections shows that the caecum, ventral colon, pelvic flexure, dorsal colon, and rectum are different microbial habitats. Raspa and colleagues explicitly warn, after direct examination of nineteen horses, against drawing conclusions about the entire digestive tract from a fecal sample.

For our question, this means: an inconspicuous fecal microbiome does not prove an inconspicuous large intestine. Changes in an anterior fermentation chamber may appear weakened or altered in the rectum and disappear in the mean values. The same applies to particle sizes because the material is further degraded microbially on the way there.

Study Period vs. Stable Reality

The third reason is duration. Most feeding trials run for two to three weeks, occasionally a few months, which are then already referred to as "long-term studies." In practice, we see horses that have been receiving haylage or chaff for years. A process that builds up slowly, such as a creeping shift in the microbial composition or a gradual irritation of the gastric mucosa, combined with recurring stress phases such as the integration of new horses into the group, cannot be represented in a three-week trial under controlled stall housing conditions. Conversely, practice often shows that recovery also takes months, not weeks.

What We Can Currently Say About the Individual Roughage Forms

The following overview summarizes what we observe, what is proven, what can be plausibly suspected, and what has simply not been investigated.

Grass

• Practical observation: Horses with sufficient grazing and long hay less frequently have stubborn digestive problems; short, restrictive grazing significantly changes feeding behavior.

• Proven: Very long feeding duration; the intake rate increases when pasture access is time-restricted; sward height changes bite size and the number of chewing movements per gram of dry matter.

• Suspected: High water content and plant structure create different conditions in the mouth than dry hay.

• Not investigated: Three-dimensional comparison of jaw movement between grass and hay with defined plant structure; jaw movement on short versus tall grass.
  
  

Long-stemmed Hay

• Practical observation: The feed form with which the fewest problems occur and which is used as a baseline for chronic digestive problems; individual batches can still cause issues.

• Proven: Largest measured lateral jaw excursion with full contact of the molar rows; highest chewing effort per kilogram; highest amount of saliva per kilogram; botanical composition changes the number of chewing strokes.

• Suspected: The intensive shearing and grinding effect is important for preparing the fiber for microorganisms.

• Not investigated: Particle distribution and fiber fracture in the swallowed bolus, also in comparison of different hay types and with different feeding management.
  
  

Structured Chaff

• Practical observation: Disproportionately common in the rations of horses with chronic fecal water, bloating, and fermenting feces; omission is the turning point of therapy for a portion of these horses.

• Proven: Short, vigorous chewing cycles with a straw-alfalfa chaff mixture; chaff in concentrate slows down its intake; chopped alfalfa is eaten about twice as fast as long hay; in weanling foals, more severe pyloric lesions under coarse alfalfa chaff.

• Suspected: Partial crushing instead of complete grinding; thus a different fracture structure and surface area of the fibers and different conditions for the fiber-degrading microorganisms; possible mechanical irritation of a pre-damaged gastric mucosa.

• Not investigated: Three-dimensional jaw movement with chaff; particle distribution in the swallowed bolus; fermentation and motility in the large intestine depending on the starting material.
  
  

Soaked Hay

• Practical observation: Is often consumed faster than the same dry hay; in some horses, digestion improves when dry hay is fed instead.

• Proven: Treatment changes water content, haptics, and palatability; intake can be faster or slower depending on the trial.

• Suspected: More grass-like chewing movement, faster bolus formation, and less saliva per kilogram of dry matter because the feed reaches the mouth already hydrated.

• Not investigated: Jaw movement, amount of saliva, and swallowed particle size in dry versus soaked hay, also depending on soaking time and hay type, as well as changes in intake and the microbiome over time in the feeder. The only direct chewing study is unusable due to an internal mix-up of results.
  
  

Steamed Hay

• Practical observation: Similar to soaked hay; often consumed quickly.

• Proven: Can significantly increase intake with hygienically good hay, but not with more contaminated hay.

• Suspected: Softer structure changes the chewing process towards that of fresh plant material.

• Not investigated: Same as with soaked hay; additionally, any direct comparison of moistening, soaking, and steaming with recording of chewing kinematics is missing.
  
  

Haylage and Silage

• Practical observation: Very common in the rations of horses with chronic fecal water, bloating, and colic; switching to hay is the turning point for many.

• Proven: Can change chewing stroke number, fecal particle size, passage, and microbial conditions in the large intestine compared to hay; more chewing strokes in grass silage were associated with larger fecal particles and shorter retention time; late-harvested haylage shows higher germ counts after preservation.

• Suspected: Moisture, fiber elasticity, fermentation acids, and introduced microorganisms act together; feed sold as "haylage" without actual fermentation brings an additional hygienic risk.

• Not investigated: Comparison of the same plant batch as hay and as a fully characterized, actually fermented haylage, with chewing kinematics, bolus, microbiome of different intestinal sections and metabolome, as well as long-term effects over months or years of feeding.

Tip from practice: For horses prone to dysbiosis and fecal water due to the lactic acid load in ensiled feeds, the temporary use of OKAPI Prodic has proven effective for physically binding excess acids in the large intestine by means of ion exchange and relieving the microbiome. It is important to maintain a time interval from mineral-containing feeds.

Hay Cobs and Pellets

• Practical observation: Unproblematic for many horses; significant differences between manufacturers; pellets with a diameter of about 6 to 8 millimeters usually work well, thicker ones more often do not; soaked cobs require almost no chewing effort.

• Proven: Significantly smaller lateral jaw excursion than with hay; pellets that differ only in their physical properties change chewing behavior; fine grinding with subsequent pelleting extended retention time and slowed microbial fiber degradation; a pure pellet diet led to more large fecal particles and lower fiber digestibility.

• Suspected: The degree of grinding before pressing could be more important for digestion than the diameter of the finished pellet; the type of size reduction influences the accessibility of the fiber more than its size.

• Not investigated: The same hay batch long, chopped, and pelleted in different degrees of grinding, diameters, and hardnesses, each dry and soaked, with recording of chewing movement, bolus, passage, and fermentation.

Tip from practice: If seniors or horses with dental problems lack the necessary grinding function for long-stemmed hay, feeding soaked, molasses-free hay cobs such as OKAPI Heucobs Sugar Light offers a well-tolerated alternative to meet fiber requirements without unnecessary sugar load.

Straw

• Practical observation: Usually unproblematic in limited quantities and good quality; problems mainly when straw replaces hay and the ration is scarce; then increased impaction colics and coarse fibers in the feces.

• Proven: Lowest digestibility among tested roughages with simultaneously larger fecal particles; up to 50 percent hygienically perfect wheat straw did not worsen ulcer scores over three weeks; in a risk factor study, straw as the sole roughage was associated with an increased risk of ulcers.

• Suspected: Strong lignification requires different mechanical and microbial processing; large fecal fibers occur here even without chewing problems.

• Not investigated: Jaw movement and bolus structure with straw; differences between straw types; peristalsis changes under straw feeding (well-known "straw colics"); chewing behavior with straw instead of hay under rationed feeding.
  
  

What the Decisive Study Would Have to Look Like

The necessary trial is surprisingly obvious, yet extremely complex. One would have to harvest a sward as uniform as possible and produce several feed forms from it: long-stemmed hay, the same hay in defined chaff length, the same material pelleted in two grinding degrees, and an actually fermented haylage version from the same growth with documented dry matter, pH value, lactic acid, acetic acid, and other fermentation products. Part of the hay would additionally have to be fed dry, moistened, soaked, and steamed.

The horses would have to receive the same dry matter in a sufficiently large cross-over, with controlled prior feed access, comparable meal sizes, and comparable feeding motivation, because hunger and time pressure change feeding behavior just as much as the feed itself.

And then the decisive steps would have to be measured simultaneously: the three-dimensional jaw movement, chewing number and chewing time, saliva production, but above all, particle size and fiber fracture in the swallowed bolus. In addition, gastric findings, regional transit and motility patterns, particle distribution in different large intestine sections, fermentation metabolites in the horse's blood or urine, and a modern microbiome, ideally over a longer period than the usual three weeks.

Such a trial is complex, but not impossible. Individual building blocks of it already exist, just never together and never on the same animals. The most striking gap is the swallowed bolus. Between the feed in front of the mouth and the start of digestion in the stomach lies exactly the step that it's actually about, and of all things, this is skipped in almost all studies.

That this investigation is missing is not proof for our hypotheses. But it is just as little a disproof.

Our Conclusion: Everything Begins in the Mouth

Perhaps the most consequential error in classical feeding theory is judging feed predominantly by its analyzable ingredients. For the horse, however, feed is more than a nutrient composition. It has length, thickness, elasticity, hardness, moisture, breaking behavior, and surface structure. It tastes different and is colonized differently by microorganisms. And before an intestinal bacterium can even reach a plant fiber, that fiber must pass through the chewing apparatus, through the acid bath of the stomach, and through the enzymatic digestion of the small intestine.

Existing research clearly shows that different feed forms produce different jaw movements, chewing stroke numbers, intake speeds, particle patterns, and passage conditions. It also clearly shows that simple equations do not work. More chewing strokes do not necessarily mean better comminution. Finely processed does not necessarily mean smaller fecal particles or better digestibility. More intestinal contractions do not necessarily mean faster transport. And an equal total retention time does not necessarily mean that no regional sorting occurs in the intestine.

What we have are recurring clinical observations, several physiologically plausible explanations, individual fitting experimental puzzle pieces, and large research gaps. From these, good hypotheses emerge. Science must verify them.

Anyone who dismisses these observations by pointing out that they are not scientifically proven is confusing two things. "There is no study that shows this" is different from "there is a study that disproves this." For structured chaff, soaked, and steamed hay, the very investigation that could answer the most important questions is currently missing. Conversely, the same applies: as long as this investigation is missing, our explanation is a hypothesis and not proof. We consider it the most plausible at present, and we state what it is based on and what it is not.

For practice, a simple consequence follows. For a horse with chronic fecal water, conspicuous gas formation or bloating, recurring stomach problems, or digestion that does not stabilize despite sensible feeding and therapy, it is worth looking not only at the nutrient analysis of the ration, but asking: In what form does this horse get its feed? And what does its chewing apparatus actually have to do with it before it arrives in the digestive tract?

Because potentially, it is not just what a horse eats that decides, but how it can chew that feed.

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