Feeding Horses Straw Instead of Hay? What the Widely Cited Study Really Shows

This article was translated using AI.

Why a sensible weight-loss study is currently being reinterpreted as a supposed hay-saving model – and what really belongs in everyday stable management

Key Takeaways

• The frequently cited study (Dosi et al. 2020) shows that a hay-straw mix can help overweight, hardy pasture ponies lose weight – not that you can simply replace half of the hay with straw for every horse.
• Crucial and often overlooked: The ponies remained on pasture throughout the winter and had access to supplementary forage via grass – a common management practice in the UK, but rather rare in Germany.
• It was an observational field study, not a controlled feeding trial: The two groups were only marginally comparable, and individual feed intake per horse was not measured at all.
• The straw group was offered less supplementary forage by volume to begin with and also left straw behind – the weight loss was therefore also the result of a lower total intake.
• Only body weight was recorded, no Body Condition Score – whether the horses lost fat, fluid, or muscle mass cannot be determined from the study. Important metabolic parameters such as insulin or liver values were also not evaluated.
• The straw used was barely lower in energy than the hay; the claim that “straw has much less energy” simply did not apply here.
• For overweight, easy-keeping horses with healthy teeth, hygienically sound straw can be a sensible component of the ration; however, the study does not justify using straw as a general hay substitute during shortages.
  
  

What is Currently Being Debated So Heatedly

Hay is scarce and expensive this year, and promptly a study is making the rounds that seemingly offers the solution: simply replace half of the hay with straw, and the horses will even slim down. In feeding forums and comment sections, this quickly turns into the rule of thumb: “50 percent straw instead of hay is completely safe.”

The study does exist, and it is by no means poor science. It simply answers an entirely different question than the one currently being attributed to it. It is therefore worth taking a close look at what was actually investigated – and what was not.

What the Study Actually Did

The study examined 40 predominantly hardy native ponies and cob crosses, as well as one Thoroughbred, one Warmblood cross, and one mule, who spent four winter months outdoors on mixed pastures of around 1.6 hectares, unworked and unblanketed. And here lies the first crucial point when transferring the findings to conditions elsewhere: These horses lived on pasture for the entire duration of the trial. In the UK, year-round pasture turnout is standard, whereas in Germany it tends to be the exception. This means that alongside hay and straw, the animals were continuously able to graze – and no one measured how much grass each individual horse consumed. For this reason alone, the situation cannot simply be transferred to a horse kept in a winter paddock that receives exclusively conserved forage.

One group received a mix of barley straw and hay in addition to pasture, while the other received only hay. The actual amounts offered are crucial: The straw-hay group (25 animals) was fed around 7 kilograms of straw and 10 kilograms of hay per group of five, meaning approximately 1.4 kilograms of straw and 2 kilograms of hay per pony. The hay-only group (15 animals) received around 20 kilograms of hay per group of five, or about 4 kilograms per pony – each on top of pasture grazing. Four kilograms of supplementary hay per pony plus free pasture access is already generous for medium-sized hardy horses; accordingly, this group even gained a slight amount of weight on average, whereas the straw-hay group lost an average of around 27 kilograms, or just over six percent. No cases of colic or laminitis occurred in either group during the four months (Dosi et al. 2020).

So far, that sounds like a clear result. However, the critical limitations lie in the study design. Forage was fed from round feeders shared by groups of five horses – who ate how much hay, straw, and grass remains completely unknown. What carries even more weight is that the two groups were not truly comparable. The animals were not randomly assigned; instead, they remained in their existing herds – and they differed massively in body size: the straw group weighed an average of around 453 kilograms, while the hay group averaged only about 238 kilograms. Thus, the straw group consisted of significantly larger, heavier animals, yet received less supplementary forage per head relative to body size (3.4 kg of hay-straw mix for 453 kg body weight), and therefore substantially less than the small ponies in the hay group (4 kg of hay for 238 kg body weight). This disparity alone explains why the large animals lost weight and the small ones gained weight, entirely independent of any special property of the straw. Horse type, baseline weight, herd hierarchy, and pasture quality were all confounded with the feeding group. For an observational field study, this may be acceptable – but as the basis for a universal feeding guideline, it is on very shaky ground.

Why the Weight Loss is Not What It Appears to Be

The obvious explanation – “straw has less energy, so the horses lose weight” – barely holds up in this study. The analyzed straw provided approximately 7.4 megajoules of digestible energy per kilogram, while the hay provided 7.6 – a difference of just 0.2 megajoules. Surprisingly, the straw used here was almost as energy-dense as the hay.

Where did the weight loss come from, then? Primarily from factors that have little to do with “straw instead of hay.” First, the straw group was offered far less supplementary forage on winter pasture to begin with – approx. 0.75% of body weight compared to 1.68%, before a single stem was even eaten. Second, the horses left a portion of the straw uneaten. This was not a sign of reduced appetite: the study itself explains that the bulky straw, which is eaten much more slowly, created a faster feeling of gut fill, and straw is also less palatable. However, this bulkiness and poor digestibility is precisely why too much straw can cause impaction colic – the amount of straw a horse will voluntarily consume is naturally limited, and forcing it higher would be risky. Bottom line: the animals simply consumed less total feed. Whether they lost weight because it was straw or simply because there was less overall feed cannot be clearly separated.

Another factor is not mentioned in the publication at all, but would be crucial for the assessment: How was the forage fed? In feeders, yes, but with or without hay nets? Especially with slow-feeder nets mixed with straw, feeding speed can drop so significantly that a horse consumes considerably less than it actually requires. Whether and how nets were used remains unstated – another reason to view the simplistic equation “straw makes horses slim” with caution. 

Losing Weight Does Not Automatically Mean Losing Fat

A point almost entirely overlooked in the discussion: the study exclusively recorded body weight using an equine scale. There was no Body Condition Score, no measurement of fat coverage, no differentiation regarding fluid or lymph retention, and no assessment of musculature. Yet the scale only indicates that the horse has become lighter – not what made up the weight loss. 

This is far from trivial. A straw-heavy ration is low in protein, and when feed intake decreases at the same time, protein supply drops even further. If the body lacks sufficient usable protein, muscle tissue is broken down. A pony that has “lost 27 kilograms” may well have lost part of that in muscle mass and topline – which is precisely not the goal of a weight-loss program. Without evaluating fat and muscle, it remains unclear whether the study documented healthy fat loss or partly just muscle wasting.

More on this topic: Assessing the correct body condition in horses

What the Study Explicitly Does Not Show

This brings us to the core of the misinterpretation. The actual research question was: Can replacing part of the hay ration with straw help overweight, hardy pasture ponies lose weight during the winter? On the internet, this gets turned into: Hay is scarce, so you can replace half of it with straw for every horse. These are two fundamentally different issues.

In a hay shortage, we suddenly also talk about normal-weight horses, senior horses and dental patients, hard keepers, sport horses, and young horses – and above all, horses without the pasture turnout featured in the study, but rather horses kept in stalls or paddocks receiving exclusively conserved forage around the clock. For none of these situations does the study provide evidence that this feeding practice is healthy or species-appropriate. Nor was a single metabolic marker evaluated – no insulin, no blood glucose, no documented EMS status, no liver enzymes or triglycerides that would help contextualize the weight change. The popular conclusion that “50 percent straw protects insulin-dysregulated horses” is not supported by any data; the hay used in the study was already exceptionally low in sugar, with only around five percent water-soluble carbohydrates.

Straw is Not Simply “Low-Calorie Hay”

Nutritionally, feeding a high proportion of straw is not an insignificant substitution. Straw contains significantly more lignin and poorly digestible structural carbohydrates, and typically provides less protein and essential amino acids. An excessively low protein content in the total ration can even inhibit fiber-fermenting bacteria in the hindgut – precisely the microbes the horse relies on to utilize crude fiber.

Anyone permanently replacing a large portion of the ration with straw must closely monitor protein intake, mineral supply, water consumption, dentition and chewing efficiency, as well as fecal consistency and total feed intake. And most crucially: hygienic quality. Moldy or dusty straw does not become good horse feed simply because hay has become expensive – on the contrary, it poses a real hazard to the respiratory tract, the gut, and ultimately the horse's overall health.

What This Means for Everyday Stable Practice

The balanced message is neither “straw is dangerous” nor “straw replaces hay.” For overweight, easy-keeping horses with sound teeth, the controlled addition of clean, hygienic wheat or oat straw can certainly be useful: straw mixed into hay prolongs feeding time (if only through sorting out the straw), maintains chew time and satiety, and simultaneously lowers the energy density of the ration. That specific application is supported by the study – nothing more.

In practice, a cautious upper limit has proven advisable. Various equine nutritionists recommend capping straw at about a quarter of the total forage amount, and even generous sources draw the line at no more than half. At the same time, a minimum of 1.5% of body weight must be provided as hay, which is already the “diet version” – to maintain a horse's weight, we are looking at 2–3% of body weight. The blanket takeaway drawn from the study – “in a hay shortage, simply replace half with straw” – is completely off the mark here, especially for horses that already receive marginal protein, have poor teeth, and above all, have no pasture access.

Anyone considering adding straw this winter due to hay prices is best advised to follow a simple rule of thumb: straw is a useful tool for overweight horses with healthy teeth and as a supplementary source of chewing activity and structural fiber – but not a blank check to swap out half of every horse's forage ration from hay to straw. The amount, the straw quality, and the individual horse are what decide, not the price of hay.

Source

Dosi, M. C., Kirton, R., Hallsworth, S., Keen, J. A., & Morgan, R. A. (2020). Inducing weight loss in native ponies: is straw a viable alternative to hay? Veterinary Record, 187(8), e60. https://bvajournals.onlinelibrary.wiley.com/doi/10.1136/vr.105793