What your horse actually needs to eat (and what you can stop buying)

You walk into a feed store and the wall behind the till is half supplements. Joint scoops. Calmer powders. Hindgut blends. Coat oils. A senior balancer next to a metabolic balancer. Maybe a thyroid powder somebody at the barn told you was a game changer. Your tack room at home looks the same, the feed bill is creeping up, and your horse is still the horse it was last year.

So which of those tubs is doing real work, and which one is paying for marketing?

The peer-reviewed equine nutrition research has reasonably clear answers. Some of those answers are uncomfortable for the supplement industry, because trials have been done and the products have failed at the doses on the label. Some are uncomfortable for owners who have been told for two decades that a particular product is essential. And some are uncomfortable for everyone, because the most boring interventions (cut the calories, fix the forage, change the meal size) are the ones with the strongest evidence and the smallest profit margins.

This guide sorts the field into four buckets: what is worth doing, where the studies are genuinely mixed, where the research is absent or shows no effect, and what you should actually do tomorrow morning. Sources are linked inline. The bottom line sits at the end.

What actually works

The strongest signals in equine nutrition cluster around four things: managing insulin trouble through carbohydrate restriction, getting weight off overweight horses, treating squamous gastric ulcers through forage and feeding-pattern changes, and respecting how starch digestion works. None of it is glamorous. All of it is high-leverage.

Does cutting sugar and starch really matter for an EMS horse?

Worth it. Cutting non-structural carbohydrates (NSC, the sugars and starches in feed and hay) in horses with insulin trouble lowers their post-meal insulin spike and reduces the risk of metabolic laminitis. The mechanism is well established and the lever is yours to pull.

When a horse's body is fighting high insulin all the time (called hyperinsulinaemia), the tissue layers inside the hoof start to fail. That is the most reliably identified driver of endocrine-related laminitis, which accounts for the majority of pasture and management-related laminitis cases in modern research.

The diagnostic side has shifted too. Vets have moved away from fasting glucose toward the oral sugar test, which catches the dynamic insulin response that fasting tests miss. Newer work has added gut hormones (GLP-1, GIP) and pharmaceutical helpers like SGLT2 inhibitors, but the dietary lever is still the foundation. Keep total dietary NSC low, avoid high-sugar feeds and pasture flushes, and feed hay tested for sugar and starch where you can.

Where the studies are genuinely mixed: whether high-protein ration balancers are neutral, helpful, or slightly insulin-spiking in dysregulated horses. Recent work suggests protein-driven gut hormone secretion can amplify insulin spikes in horses on corticosteroids, which complicates the older guidance to feed protein freely while restricting sugar.

What the studies do not support: any "metabolic herbal blend" that promises to substitute for cutting NSC. The active ingredient is the diet, not the supplement.

Is your horse really overweight?

Worth it. Feeding roughly 1.25 to 1.5 percent of your horse's ideal bodyweight in dry forage produces reliable weight loss in fat horses. Combining body condition score (BCS, the 0 to 5 or 1 to 9 fat-cover scale) with a tape measure and ultrasound where available improves accuracy.

Body condition scoring is the most studied tool in equine nutritional assessment, and it is also imprecise. The research keeps showing that BCS alone underestimates fat in specific places (cresty neck, tailhead, sheath fat). Combining BCS with a morphometric tape and, where you can get it, ultrasound rump fat measurement substantially improves accuracy. A horse with a neck score of 4 out of 5 is at metabolic risk even at a moderate overall BCS. Regional fat carries metabolic risk that whole-horse fat does not.

The intervention with the strongest evidence is also the simplest. Restricting dry forage intake to roughly 1.25 to 1.5 percent of ideal (not current) bodyweight produces reliable weight loss across multiple controlled trials. Diet alone and diet-plus-exercise produce similar weight loss, but exercise adds insulin-sensitivity gains beyond the weight loss itself.

What the studies do not support: thyroxine for weight loss in adult horses with normal thyroid function. Recent reviews are explicit that adult equine hypothyroidism is rare, that the diagnosis is over-applied, and that thyroxine for weight loss is not evidence-based. This is one of the cleanest "evidence of no effect" verdicts in the field.

Should you change how you feed an ulcer horse?

Worth it. Squamous EGUS (equine gastric ulcer syndrome, in the upper part of the stomach) responds reliably to forage continuity, lower starch loads, and feeding hay before exercise. Glandular ulcers further down the stomach do not respond as cleanly to diet, and the two should not be treated as the same problem.

The split between squamous and glandular ulcers is the single most important update in EGUS research in the last decade, because the two diseases respond to different things. Squamous lesions sit in the upper, unprotected part of the stomach and are reliably caused by acid splashing onto bare lining when the stomach empties of forage. Gaps over four to six hours without forage reliably trigger them in research models.

The fix follows naturally. Keep forage in front of your horse. Reduce concentrates and starch. Feed hay before exercise. Consider some alfalfa where appropriate.

Glandular ulcers tell a different story. Alfalfa helps in some glandular trials but not all. Pectin-lecithin and antacid supplements show modest, inconsistent effects. No equine-marketed "gut-soothing" herbal preparation has shown equivalence to omeprazole for active EGUS. The honest read is that diet is foundational for squamous disease and only adjunctive at best for glandular disease.

Why does grain size matter so much?

Worth it. Keep starch loads below roughly 1 g/kg bodyweight per meal, feed forage first, and feed more meals of smaller size rather than fewer large ones. The mechanism is well understood and the outcome data lines up.

Your horse's small intestine can only digest so much starch at once. When a starch load goes above roughly 1 g/kg bodyweight per meal, the excess carries on into the hindgut and ferments fast. That drops the pH in the cecum and colon and shifts the microbe community in ways linked to colic and laminitis risk.

Meal frequency matters more than total daily ration. A horse fed four small grain meals tolerates the same total starch poorly when you cram it into two big meals. Recent work also supports forage-first feeding order. Putting hay in front of your horse before grain slows gastric emptying, blunts the post-meal blood sugar and insulin peak, and reduces the speed at which starch arrives in the hindgut.

What the studies do not support: any supplement that claims to "neutralise" a high-starch diet. The only thing that consistently works is reducing the starch.

How much does the hay actually matter?

Worth it. Forage analysis is informative. A hay-only diet meets maintenance for most adult horses but rarely covers trace minerals without a balancer. Mycotoxin contamination of commercial feeds is more common than most owners assume.

The most overlooked thing on your feed-room shelf is the hay. Forage chemistry (sugars, starches, protein, calcium-to-phosphorus ratio, mineral profile) varies enormously by species, cut, region and storage. A forage analysis from a feed lab is the cheapest single piece of information you can buy.

A hay-only diet meets the energy and protein needs of most adult horses, but rarely meets trace mineral targets without a balancer. Recent European work has documented biotoxin and phytoestrogen exposure in commercial European horse feeds, and feed-induced drooling outbreaks linked to fungal contamination. Worth bearing in mind when the feed itself becomes the unexplained variable in your horse's health.

Where the studies are mixed: the optimal sugar-and-starch threshold for forage fed to metabolic horses (10 percent versus 12 percent versus depends-on-context), and whether soaking or steaming hay produces benefits beyond NSC reduction.

Does your horse need vitamin E and selenium?

Worth it where deficiency is plausible. Hay-only horses are often vitamin E deficient. Selenium status depends heavily on your region and the safe range is narrow. Outside actual deficiency, the case for supplementing weakens fast.

Pasture supplies natural vitamin E. Horses on hay-only diets, especially through winter on stored hay, are commonly deficient, and supplementation closes the gap. This is one of the cleanest deficiency-replacement signals in equine nutrition.

Selenium is the opposite story. Status is regionally variable, both deficiency and toxicity are documented, and supplementation should follow a serum selenium measurement rather than a guess. The therapeutic window is narrow enough that "more is better" is not safe.

Where the studies are mixed: whether selenium supplementation above NRC requirements adds antioxidant or performance benefit. Recent work suggests training adaptation, not supplemental selenium, drives antioxidant gains in young athletes.

What the studies do not support: "mega-dose" vitamin E for general wellness in non-deficient horses. The benefit is real in deficiency and specific neurological conditions. It does not extend to healthy pastured horses.

Where the research is genuinely mixed

These are categories where the body of work is large enough to take seriously, but the answers do not converge. The verdict is not a refusal to call it. It is a faithful description of where the field actually sits.

Do probiotics and yeast help your horse's gut?

Mixed evidence. Live yeast can blunt starch-induced gut disruption in some study designs. Probiotic supplementation produces measurable shifts in gut bacteria of unclear clinical meaning. Whole-horse outcome data is thinner than the marketing implies.

The hindgut microbiome literature has exploded since gene sequencing became routine. The corpus now contains hundreds of papers on Saccharomyces cerevisiae, Lactobacillus and Ligilactobacillus equi isolates, Bacillus species, beta-glucans and butyrate.

The mechanistic finding that holds across studies: high-starch meals destabilise hindgut pH and shift microbial communities. Live yeast can blunt that destabilisation in some study designs, particularly under high-grain challenge.

The harder question is whether routine probiotic supplementation produces clinically meaningful outcomes (performance, colic risk, manure consistency) in horses on otherwise reasonable diets. The studies are heavy on lab models and light on whole-horse outcome trials.

This category also illustrates the difference between "no evidence" and "evidence of no effect." Most equine-marketed broad-spectrum probiotic powders lack independent dose-response data. Buying one means buying a product the manufacturer has not bothered to study at the dose on the label.

Is your senior horse actually getting what they need?

Mixed evidence. Soaked complete feeds and chopped forage substitutes work for seniors with bad teeth. Whether commercial "senior feeds" outperform a balanced standard ration in healthy older horses is unsettled.

Older horses often carry overlapping problems: dental loss reducing how much hay they can chew, rising PPID (pituitary pars intermedia dysfunction, the hormone disorder behind a lot of older-horse weight loss and laminitis) prevalence after age 15, and weight loss driven by underlying disease rather than feed shortcomings.

The studies support soaked complete feeds and chopped forage substitutes for seniors with dental compromise as solid replacements for long-stem hay. What the studies do not yet settle is whether commercial "senior feeds" outperform a balanced standard ration in healthy aged horses without dental problems.

Protein needs in older horses may be modestly higher to maintain muscle, but the dose is under-specified. No supplement has demonstrated reliable extension of healthy lifespan or reversal of age-related muscle loss in horses.

What about feeding broodmares and growing foals?

Mixed evidence. Late-pregnancy nutrition shapes foal immunity. Rapid growth and over-conditioning in weanlings is a documented joint development risk. Feeding probiotics or beta-glucans to the mare to benefit the foal shows limited effect in recent trials.

The breeding-and-young-horse research lands on a few practical points. Late-pregnancy nutrition shapes foal immune transfer and early growth. Mineral and protein balance during the peak growth windows of the weanling and yearling matters more than total calorie provision. Rapid growth and over-conditioning in weanlings raises the risk of developmental orthopaedic disease (DOD, the umbrella term for joint and bone problems in growing horses).

Where the studies stay mixed: supplementing the mare to benefit the foal. Recent trial work on maternal probiotic and beta-glucan supplementation showed limited benefit on foal antibody levels or immune outcomes. No broad-spectrum joint nutraceutical (glucosamine, chondroitin) has shown benefit for DOD prevention in growing foals.

Where the studies have failed the products

These are the categories where studies have either not been done at the dose on the label, or have been done and the product fell flat. The distinction matters.

Do joint supplements actually work?

Evidence of no effect at consumer doses. Glucosamine and chondroitin at the doses commonly sold to owners do not move objective gait outcomes beyond placebo. Bioactive collagen peptides show modest effect in mild to moderate osteoarthritis. The "joint formula" combination category lacks adequate placebo-controlled data.

A 2009 Equine Veterinary Journal review describing low-quality evidence for glucosamine-based nutraceuticals is still influential, because the evidence has not meaningfully strengthened since. The cleanest recent example is a 2026 trial that ran a placebo-controlled, blinded study of glucosamine and chondroitin in 40 geldings with chronic lameness, with objective gait analysis. Six weeks of supplementation at a typical consumer dose did not move stride length or symmetry beyond placebo. Clean null.

There are exceptions worth flagging. Bioactive collagen peptides show modest clinical improvement in mild to moderate osteoarthritis in two-centre trials. Bioavailability of joint nutraceuticals in the horse remains poorly characterised. Most products on the market have never had a head-to-head trial against placebo at adequate power. The honest verdict on the joint aisle is that the studies do not support paying for the category at consumer doses, with a narrow exception for collagen peptides in mild osteoarthritis.

Do tryptophan calmers do anything?

Evidence of no effect. A 2016 dose-response trial across three tryptophan dose rates found no behavioural effect.

Tryptophan has been marketed as a calmer for decades on the basis of a serotonin theory that has not translated cleanly to the horse. The dose-response trial published in a 2016 Veterinary Journal study tested three oral dose rates and found no effect on reaction speed. The trial is small and the design is not the last word on the question, but as a clean null at three doses it is the cleanest evidence currently available, and it points the same direction as the broader behavioural-supplement research.

Does magnesium calm horses down?

Evidence of no effect in the magnesium-replete horse. Magnesium deficiency (rare in horses on balanced forage) does affect nerve and muscle function. Supplementing horses with normal magnesium status has not produced reliable behavioural change.

Magnesium has produced inconsistent behavioural results in horses with normal magnesium status. Some commercial blends combining magnesium with citrus botanicals and yeast show modest cortisol or behavioural attenuation under acute stress in small trials, but the specific contribution of magnesium to those effects is not isolated. The category overall is supported by less independent placebo-controlled evidence than the consumer market for it would suggest.

Should your overweight horse be on thyroxine?

Evidence of no effect for the indication as commonly prescribed. Adult equine hypothyroidism is rare. Thyroxine for weight loss in horses with normal thyroid function is not evidence-based and is over-prescribed.

Thyroxine for weight loss has its own version of the supplement problem. Recent reviews are explicit that the diagnosis of adult equine hypothyroidism is over-applied, and that thyroxine for weight loss in horses with normal thyroid function is not supported by the literature. It comes up in nutrition conversations as an apparent shortcut around dietary restriction. The studies do not support it.

Are omega-3s worth feeding?

Mixed. Fish oil reliably raises blood EPA and DHA. Flax raises ALA but conversion to EPA and DHA in horses is limited. Whether the resulting biochemistry translates into clinical benefit at typical commercial dosing is unsettled.

The omega-3 research in horses is biochemically clear and clinically ambiguous. Fish oil reliably raises blood and milk EPA and DHA. Plant sources raise ALA but conversion to longer-chain omega-3s in horses is limited.

What is less clear is whether the shifts in fatty acid profile translate into the clinical endpoints (joint comfort, skin condition, reproductive performance) that the products are usually marketed for, at the doses typically fed. No demonstrated performance enhancement from omega-3 supplementation has been established outside of an inflammatory or reproductive endpoint.

Here is how to think about it, in order

A defensible decision flow runs roughly like this.

First, body condition. Score your horse with a tape and a careful eye, including the trouble spots (neck, tailhead, sheath, ribs). If your horse is over-conditioned, dropping dry forage intake to roughly 1.25 to 1.5 percent of ideal bodyweight, plus exercise where you can, is the highest-leverage thing you can do before any supplement.

Second, the hay. Get a hay analysis. Most owners do not. The cost is small relative to the rest of the feed bill, and the result tells you whether you need a balancer and which one, whether the sugars are safe for a metabolic horse, and whether you need to correct the calcium-to-phosphorus ratio. Where forage analysis is impractical, work from typical regional values and pick a balancer formulated to that pattern.

Third, the meal mechanics. Forage first. Smaller meals, more often. Starch load below roughly 1 g/kg bodyweight per meal. Hay before exercise for the working horse. These are mechanical changes, not supplemental ones, and they have the strongest evidence in the corpus.

Fourth, targeted supplementation only where deficiency or disease state warrants it. Vitamin E for the hay-only horse. Selenium following a serum measurement, where regional status indicates a need. A balancer to close trace-mineral gaps in a sole-forage diet. Diet-led management for the EGUS or metabolic horse, with pharmacological adjuncts (omeprazole, pergolide, SGLT2 inhibitors) layered in under your vet's direction.

What the studies do not support paying for, at consumer dose, in the otherwise healthy horse: glucosamine and chondroitin combinations at consumer doses, tryptophan calmers, magnesium for behaviour in the magnesium-replete horse, thyroxine for weight loss in the euthyroid adult, and broad-spectrum probiotic powders without independent dose-response data.

When to call your vet or a nutritionist

Three bright lines.

A horse with a history of laminitis, a cresty neck, regional fat deposits, an abnormal oral sugar test, or a positive ACTH test for PPID needs a vet-led nutrition plan that combines diet, exercise and where indicated, medication. The owner-only version is harder to get right and the cost of getting it wrong is a laminitic foot.

A horse losing weight that you cannot explain, particularly an older horse, needs a workup before a feed change. Dental disease, PPID, gastric ulcers, malabsorption, parasites and chronic pain all show up as weight loss. Feeding more of the same ration to a horse with an undiagnosed underlying problem rarely fixes it.

A horse on a hay-only diet, particularly without pasture, benefits from a forage analysis and a registered nutritionist's review of the full ration before adding individual supplements. The balancer choice matters more than any single supplement. Most balancers are formulated to address typical regional forage patterns, and a nutritionist can match the two.

Bottom line

Bottom line. The evidence-strong nutritional moves are unglamorous: keep your horse at a healthy weight, feed forage first in smaller more frequent meals with starch loads below 1 g/kg per meal, restrict sugar and starch for the dysregulated horse, treat squamous EGUS through forage continuity, and supplement vitamin E and trace minerals only where the diet leaves a gap. The studies do not support paying for joint supplements at consumer doses, tryptophan calmers, magnesium for behaviour in non-deficient horses, or thyroxine for weight loss in horses with normal thyroid function. Spend on the hay test and the body condition assessment first. The supplement aisle last.

References

1. Harbowy R M et al. (2026). Efficacy of an oral chondroprotective joint supplement on stride length and gait symmetry in aged geldings with chronic lameness. Animals. doi.org/10.3390/ani16081230
2. Noble G K et al. (2016). Randomised clinical trial on the effect of a single oral administration of l-tryptophan, at three dose rates, on reaction speed, plasma concentration and haemolysis in horses. The Veterinary Journal. doi.org/10.1016/j.tvjl.2016.05.003
3. Palmer A T et al. (2025). Corticosteroid administration enhances the glycemic, insulinemic, and incretin responses to a high-protein mixed meal in adult horses. Journal of Veterinary Internal Medicine. doi.org/10.1111/jvim.17305
4. Loos C M M and Urschel K L (2025). Current understanding of insulin dysregulation and its relationship with carbohydrate and protein metabolism in horses. Domestic Animal Endocrinology. doi.org/10.1016/j.domaniend.2025.106940
5. Menzies-Gow N J and Knowles E J (2025). Sodium-glucose transport protein 2 inhibitor use in the management of insulin dysregulation in ponies and horses. Journal of Veterinary Pharmacology and Therapeutics. doi.org/10.1111/jvp.13470
6. Elliott J and Bailey S R (2023). A review of cellular and molecular mechanisms in endocrinopathic, sepsis-related and supporting limb equine laminitis. Equine Veterinary Journal. doi.org/10.1111/evj.13933
7. Bamford N J et al. (2019). Influence of dietary restriction and low-intensity exercise on weight loss and insulin sensitivity in obese equids. Journal of Veterinary Internal Medicine. doi.org/10.1111/jvim.15374
8. Garland A et al. (2025). Morphometric changes in overweight horses following 10-week weight loss programs. BMC Veterinary Research. doi.org/10.1186/s12917-025-05032-z
9. Potter S J et al. (2024). Development of a body condition index to estimate adiposity in ponies and horses from morphometric measurements. Equine Veterinary Journal. doi.org/10.1111/evj.13975
10. Delarocque J et al. (2020). Weight loss is linearly associated with a reduction of the insulin response to an oral glucose test in Icelandic horses. BMC Veterinary Research. doi.org/10.1186/s12917-020-02356-w
11. Kritchevsky J et al. (2022). A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses. Equine Veterinary Journal. doi.org/10.1111/evj.13480
12. Kranenburg L C et al. (2023). Changes in management lead to improvement and healing of equine squamous gastric disease. Animals. doi.org/10.3390/ani13091498
13. Sundra T et al. (2024). Comparison of oral esomeprazole and oral omeprazole in the treatment of equine squamous gastric disease. Equine Veterinary Journal. doi.org/10.1111/evj.13997
14. Hewetson M and Tallon R (2021). Equine squamous gastric disease: prevalence, impact and management. Veterinary Medicine: Research and Reports. doi.org/10.2147/VMRR.S235258
15. Sykes B W et al. (2014). Effect of pectin, lecithin, and antacid feed supplements (Egusin) on gastric ulcer scores, gastric fluid pH and blood gas values in horses. BMC Veterinary Research. doi.org/10.1186/1746-6148-10-S1-S4
16. Bustamante C C et al. (2026). Starch overload and cecal alkalinization: impacts on the intestinal microbiota and health of horses. Veterinary and Animal Science. doi.org/10.1016/j.vas.2026.100619
17. Jensen R B et al. (2025). The effect of feeding order of forage and oats on metabolic and digestive responses related to gastric emptying in horses. Journal of Animal Science. doi.org/10.1093/jas/skae368
18. Carter M M et al. (2025). Influence of Saccharomyces cerevisiae CNCM I-1077 on the fecal pH, markers of gut permeability, fecal microbiota, and markers of systemic inflammation in sedentary horses fed a high-starch diet. Journal of Animal Science. doi.org/10.1093/jas/skaf005
19. Kwass L M et al. (2025). Exposure of horses to biotoxins, phytoestrogens, and pesticides from different feed materials and supplementary feeds. Journal of Equine Veterinary Science. doi.org/10.1016/j.jevs.2025.105632
20. Boswald L F et al. (2025). Feed-induced hypersalivation in horses from Austria, Germany and Switzerland. Equine Veterinary Journal. doi.org/10.1111/evj.14433
21. Ensley S and Mostrom M (2024). Equine mycotoxins. Veterinary Clinics of North America: Equine Practice. doi.org/10.1016/j.cveq.2023.10.002
22. Donnelly C G and Finno C J (2023). Vitamin E depletion is associated with subclinical axonal degeneration in juvenile horses. Equine Veterinary Journal. doi.org/10.1111/evj.13907
23. Finno C J and McKenzie E C (2025). Vitamin E and selenium-related manifestations of muscle disease. Veterinary Clinics of North America: Equine Practice. doi.org/10.1016/j.cveq.2024.11.001
24. Owen R N et al. (2022). Elevated dietary selenium rescues mitochondrial capacity impairment induced by decreased vitamin E intake in young exercising horses. Journal of Animal Science. doi.org/10.1093/jas/skac172
25. Sobral G G et al. (2023). Effect of supplementation with Saccharomyces cerevisiae and beta-glucans to mares during late gestation on colostrum quality and passive transfer of immunity in foals. Journal of Equine Veterinary Science. doi.org/10.1016/j.jevs.2022.104168
26. Yamada A L M et al. (2022). Effects of oral treatment with chondroitin sulfate and glucosamine in an experimental model of metacarpophalangeal osteoarthritis in horses. BMC Veterinary Research. doi.org/10.1186/s12917-022-03323-3
27. Dobenecker B et al. (2018). Specific bioactive collagen peptides as supplement for horses with osteoarthritis: a two-centred study. Journal of Animal Physiology and Animal Nutrition. doi.org/10.1111/jpn.12863
28. Leleu C and Couroucé A (2023). Effect of a blend of magnesium oxide on equine squamous gastric disease in young trotter horses under training. Journal of Veterinary Science. doi.org/10.4142/jvs.23118
29. Jarvis N and McKenzie H C (2021). Nutritional considerations when dealing with an underweight adult or senior horse. Veterinary Clinics of North America: Equine Practice. doi.org/10.1016/j.cveq.2020.12.003
30. Herbst A C et al. (2025). Demographics and health of U.S. senior horses used in competitions. Equine Veterinary Journal. doi.org/10.1111/evj.14438
31. Czerner A C et al. (2026). Geriatric horses in Germany: approaches to nutrition, housing and overall care. Animals. doi.org/10.3390/ani16050813
32. Pagan J D et al. (2022). Long-chain polyunsaturated fatty acid supplementation increases levels in red blood cells and reduces the prevalence and severity of squamous gastric ulcers in exercised Thoroughbreds. JAVMA. doi.org/10.2460/javma.22.06.0275