Breathe Easy This Winter – Even If Your Horse Is Stuck Inside

Now, onto the big winter dilemma.

If your horse is on a livery yard – or even at home – you’re probably no stranger to turnout debates at this time of year. I see it from both sides. We all know horses are happiest and healthiest with plenty of time outdoors, but yard owners are understandably trying to protect paddocks from becoming complete mud baths. (Looking at what was once my own field, I can’t really blame them… it’s been a very wet winter so far).

Still, keeping horses in for long periods brings extra work, extra cost, and some very real health and welfare considerations. One in particular that’s been on my mind recently is respiratory health.

So many horses start coughing once they’re spending more time indoors, and it turns out this is far more common than most people realise. I rolled up my sleeves and dug into why these winter stable coughs happen, and what we can realistically do about them. Here’s what I found...

What You Can Do – Backed by Research

Mild airway inflammation affects over half of stabled horses, and some studies suggest that as many as 60–80% of horses living indoors show signs of respiratory stress. So no, it’s not just your horse or your yard – this is a widespread issue in winter stables everywhere.

Why does it happen, and what can we do to help our horses breathe more easily? Let’s break it down.

 

🐴 That “Healthy” Hay Is Often the Biggest Source of Air Pollution

It’s a bit ironic, but the biggest contributor to dust in your horse’s environment is often the very thing they spend hours chewing on: dry hay.

Even the cleanest-looking hay releases clouds of microscopic particles every time your horse pulls at the net or takes a mouthful. You’ve probably seen it yourself – a shaft of winter sunlight catches the air and suddenly you can see dust floating everywhere. With every breath, your horse can be inhaling large amounts of tiny particles that irritate and inflame the delicate tissues of the lungs.

The good news is that this is one area where relatively small changes can make a big difference. Steaming hay using a high-temperature hay steamer can remove up to 99% of respirable particles. Don’t have a steamer? Even soaking hay for around 30 minutes can reduce airborne dust by roughly 70%. And if neither of those options is practical, haylage is a viable alternative, as it’s naturally higher in moisture and typically much lower in airborne dust.

The aim is simple: with every mouthful, your horse shouldn’t be engulfed in an invisible cloud of irritants. Their lungs will thank you for it.

 

🐴 Your Cosy Stable May Be Trapping a Perfect Storm of Irritants

We all want our horses warm and comfortable in winter, but sealing a stable up too tightly can backfire.

When doors and windows are closed to keep the heat in (or the rain out), dust from hay and bedding, mould spores, and ammonia from urine all get trapped inside. Experts recommend 4–8 air changes per hour in a stable, yet a closed-up winter barn rarely achieves anything close to this. The result is a build-up of irritants right at nostril height.

The fix is ventilation – even when it feels counterintuitive. A rugged horse in a well-ventilated stable is far better off than an unrugged one breathing stale, dusty air in a sealed shed. Even cracking a top door or window can significantly improve airflow. In shared barns this can take a bit of cooperation, but even modest improvements make a noticeable difference. Your horse – and likely their stablemates too – will breathe easier for it.

 

🐴 Use Low-Dust Bedding and Skip Out Thoroughly

Straw and some older-style bedding types can produce a surprising amount of dust and hold onto ammonia if not managed carefully. Here, it’s not just about how often you muck out, but how well you do it.

The less urine and faeces left in the bed, the better for your horse’s lungs. Skipping out thoroughly every day helps reduce ammonia levels, and switching to dust-extracted options such as hemp, large-chip shavings, or rape straw-based bedding can further improve air quality. Always follow the manufacturer’s instructions.

Personally, I’ve moved my own horse, Ben, onto Aubiose with wood pellets underneath to soak up the wet. It’s been a real game changer. Like many hemp beddings, it can get a bit dusty when very dry, so each week when I add fresh bedding I lightly damp it down with the hose on a gentle shower setting. This improves absorbency and keeps the dust down. Since making these changes, Ben hasn’t coughed at all this winter. Small changes, big improvement.

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Bottom Line: A Breath of Fresh Air Is the Best Medicine

That persistent winter cough is often your horse’s way of saying, “Something isn’t right with my environment.” While medications and supplements can help in the short term, they’re often just sticking plasters if the underlying stable conditions don’t change.

Improving forage management, ventilation, and bedding creates a healthier environment that works around the clock to protect your horse’s lungs. If turnout is limited – as it is on many yards – focusing on the stable environment is one of the most impactful things you can do, and often the quickest to improve your horse’s comfort. Even with good turnout, paying attention to stable air quality during indoor hours is well worth it.

 

✴️ A Note on Short-Term Vitamin C Support

Horses normally make their own vitamin C, but stress, hard work, stabling, and sudden dietary changes can sometimes leave them short. Moving from summer pasture (naturally high in vitamin C) to hay-only diets can temporarily compromise levels while the liver adjusts.

Some performance, race, and stud feeds already contain added vitamin C, so it’s always worth checking what you’re already feeding. For horses that are coughing, older, dealing with PPID, or under particular stress, short-term supplementation may help – but always talk to your vet and/or nutritionist first to establish the right dose. And if you do supplement, taper off gradually so the horse’s own production can safely upregulate again.

I promised to keep this email concise, so I’ll wrap things up here. If you’d like to explore the detail, I’ve pulled together a full article with the research, explanations, and practical guidance, as well as a separate piece on vitamin C and when it may be useful.

The detailed articles are below:-

Respiratory Health in Stabled Horses During UK Winters

Horses kept in enclosed stables during the damp, cold UK winter months are at heightened risk of respiratory issues. Common conditions include mild inflammatory airway disease (IAD) and more severe equine asthma (formerly known as RAO or heaves), often manifesting as chronic cough or nasal discharge. Non-infectious lower airway inflammation is very common in stabled horses – studies estimate severe equine asthma affects roughly 10–17% of horses in the UKhaygain.us, while milder forms of IAD/asthma occur in well over half of stabled horsesfrontiersin.org. In fact, some surveys have found signs of airway inflammation (excess mucus, cough, etc.) in as many as 60–80% of horses when housed in barns, especially during winterfrontiersin.orgmdpi.com. The risk increases with age and with more hours spent confined indoorshaygain.us. Seasonal effects are notable – one study reported that the prevalence of mild asthma signs jumped from ~60% in summer to 87% in winter in a group of horses, even when kept at pasture with hay feedingfrontiersin.org. In short, respiratory disease is a major health concern for stabled horses in winter, and it can significantly impact equine welfare and performance.

Common Contributing Factors to Poor Respiratory Health in Stables

Several environmental and management factors contribute to respiratory problems in stabled horses. These factors largely revolve around air quality – specifically the build-up of dust, mould spores, and irritant gases in the stable environment. Key contributors include:

• Poor Stable Ventilation: In UK winters, barn doors and windows are often kept closed to retain heat, but this greatly reduces air exchange. Inadequate ventilation allows dust and noxious gases to accumulate. Guidelines recommend at least 4–8 air changes per hour in horse stables, yet this is rarely achieved in tightly closed barnsmdpi.com. When stables are sealed up on cold days, levels of airborne irritants (dust, endotoxins, ammonia, etc.) rise substantiallymdpi.com. Research confirms that winter months bring higher risk of asthma exacerbation because horses spend longer indoors breathing stagnant, contaminated airmdpi.com. Simply put, if fresh air cannot circulate, respirable dust and gases persist in the horse’s breathing zone for extended periods, increasing inhalation of harmful particleshaygain.us.
• Dusty or Mouldy Bedding: Straw, the traditional bedding, is one of the worst offenders for a horse with respiratory issues. Dried straw can carry a heavy load of dust, mould, and fungal spores. It also tends to become damp with urine, releasing ammonia (an irritant gas). Studies have identified straw bedding as a significant source of respirable dust and allergens in stableshaygain.us. By contrast, low-dust bedding materials (for example, dust-extracted wood shavings, paper/cardboard bedding, or processed straw pellets) produce far fewer airborne particles. They also emit less ammonia because they are more absorbent than straw. In fact, wood-based bedding tends to result in considerably lower ammonia emissions compared to strawmdpi.com. Additionally, specialized materials like peat moss have been shown to bind ammonia effectively, limiting its release into the airmdpi.com. The type of bedding is thus critical: one peer-reviewed study found that horses inhaling fungal particles (from mouldy bedding or feed) were at significantly higher risk of developing IAD, with bedding type being a notable risk factoronlinelibrary.wiley.com. Stable cleaning practices also play a role – mucking out stirs up dust and bioaerosols. Even though removing soiled bedding improves hygiene, it transiently spikes particulate levels in the air. It’s recommended to remove horses from the stall during cleaning, since studies show manure removal and sweeping can briefly increase airborne dust and ammonia concentrations in the stablemdpi.com. Good stable design (e.g. high roofs, good drainage, separate storage for hay/bedding) can mitigate some of these issues by promoting airflow and keeping dust sources away from stalls.
• Forage Type and Quality: Hay is a major source of respirable dust and mould for stabled horses. Traditional dry hay, especially if not of high quality, contains fine particles, mould spores, and even mites that become airborne during feeding. Horses typically bury their noses in hay for hours, inhaling whatever dust is present. It has been documented that stable air can contain millions of microscopic dust particles per breath, largely due to feeding dry hayhaygain.us. Moreover, epidemiological studies have shown that feeding dry, un-treated hay significantly increases a horse’s risk of developing mild to moderate equine asthmahaygain.us. By contrast, haylage (semi-fermented packaged forage) or well-cured late-cut hay tend to have much lower dust and mould content. One large survey of sport horses (over 700 horses) found that those fed dry hay had substantially higher odds of fungal particles in their airways, whereas feeding haylage or steamed hay reduced that riskhaygain.us. Essentially, poor-quality or dusty forage introduces a huge allergen load to the stable environment, contributing directly to airway irritation.
• Airborne Irritants and Allergens: The stable environment is rich in organic dusts and microorganisms that irritate the respiratory tract. Research has identified over 50 species of fungi and bacteria floating in stable air, attached to dust particleshaygain.us. About 70% of the particulate matter in barn air is of organic origin – coming from sources like hay, bedding, dried manure, moulds, pollen, and even bits of skin/hairpmc.ncbi.nlm.nih.gov. These particles often carry endotoxins (toxins from gram-negative bacteria) and β-glucans from fungal cell walls, which can provoke inflammatory reactions in the lungs. Inhalable mould spores are particularly dangerous: their small size (often 1–5 µm) means they can reach deep into the horse’s lungs (down to the alveoli)pmc.ncbi.nlm.nih.gov. Horses breathing air with high levels of fungal spores or dust show elevated inflammatory cells in their airways and are prone to developing chronic coughs. In fact, one study found that horses with evidence of inhaling mould/fungal elements were twice as likely to be diagnosed with inflammatory airway diseaseonlinelibrary.wiley.com. All these allergens (dust, moulds, mites, ammonia, etc.) act in combination to overwhelm the horse’s normal respiratory defences when exposure is constant in an enclosed stable.
• Ammonia and Stable Gases: The accumulation of ammonia gas from urine is another key factor. Ammonia (NH₃) is released as bacteria break down urea in soiled bedding. In poorly ventilated stables, ammonia levels can rise above safe thresholds – one report notes that ammonia should stay below 10 ppm in stables, as higher concentrations damage the respiratory tractmdpi.com. Ammonia has a sharp odour and causes irritation of the horse’s eyes, nose, and airway lining. Constant low-level exposure inflames the respiratory mucosa and can reduce the horse’s local immunity, making infections more likelymdpi.com. This gas tends to accumulate near the stall floor (where it evaporates from wet bedding), right in the zone where horses breathe while resting or eating. Thus, if stalls are not cleaned frequently or if drainage/absorbency is poor, ammonia build-up can contribute to chronic coughing and airway inflammation. Reducing ammonia requires diligent stall hygiene (removing wet spots daily) and sufficient ventilation. As mentioned, bedding choice matters too – more absorbent materials (wood shavings, peat, paper) help keep ammonia lower by soaking up urine and slowing its decompositionmdpi.com. Overall, controlling stable ammonia is essential for protecting the delicate respiratory membranes in the horse’s lungs.

In summary, a “perfect storm” of winter conditions – closed-up stables (poor ventilation), dust from straw bedding, mouldy or dry hay, and accumulation of irritants – all contribute to the high prevalence of coughing and equine asthma seen in stabled horses. These factors are well documented in both UK-based and international studies of stable management. The good news is that each of these risk factors can be addressed with practical changes. Below, we outline evidence-based solutions to mitigate respiratory hazards in the stable environment.

Evidence-Based Solutions and Mitigations for Healthier Stable Air

Improving a horse’s respiratory health in winter starts with environmental management. Numerous studies and veterinary guidelines emphasize that reducing airborne irritants is the cornerstone of preventing and managing equine asthma. Everyday horse owners and yard managers can adopt the following proven strategies:

• Feed Low-Dust Forage (Soak or Steam the Hay): Since hay is the largest source of respirable dust, treating the forage can yield dramatic benefits. Steaming hay at high temperature (using a commercial hay steamer) is one of the most effective methods – it has been shown to eliminate up to 99% of respirable particles, as well as kill mould and bacteria in the hayhaygain.us. In one controlled study of 482 horses, feeding steamed hay significantly reduced the incidence of mild-to-moderate asthma, whereas horses fed dry hay had a higher risk of developing airway inflammationhaygain.us. If a steamer is not available, hay soaking is a time-tested alternative. Soaking the hay (submerging it in water) for about 10–30 minutes before feeding can remove a large portion of dust and spores. Research from the University of Edinburgh found that simply immersing hay reduced respirable dust levels by around 50–90%, with a 30-minute soak achieving roughly a 71% reduction in airborne dust compared to feeding dry haysciencedirect.com. Owners should be mindful to soak hay for an optimal duration (about 30 minutes is often recommended) – longer soaking can leach out nutrients and create wastewater issues. Another option is to switch to haylage or wrap: high-quality haylage (fermented forage) inherently contains less dust and mould. A study on respirable particles noted that late-cut hay (with ~85% dry matter) and haylage were among the least dusty types of forage, producing minimal airborne fungi and particles when fedmdpi.com. Whatever the choice, the goal is to avoid feeding dry, dusty hay in the stable. Even simply dampening the hay or using hay nets that reduce chaff can help, but proper steaming or soaking yields far greater dust reduction. Because horses spend so many hours eating, improving forage quality is probably the single most impactful step to improve stable air quality.
• Use Low-Dust, Highly Absorbent Bedding: Re-evaluating the bedding is critical for a healthier barn environment. If a horse is suffering from chronic cough or known asthma, avoid using straw bedding, which is notoriously dusty and often laden with mould (especially in a damp climate). Instead, opt for bedding materials that are dust-extracted or inherently low in dust. Dust-free wood shavings or wood fibre pellets, paper/cardboard shreds, peat moss, or specialized chopped straw that has been cleaned are all viable options. Studies have shown that switching from straw to wood-based or paper bedding can markedly decrease the respirable dust in a stablehaygain.us. Moreover, some of these alternatives help with ammonia control. For instance, wood shavings release significantly less ammonia than urine-soaked straw, and peat has an excellent capacity to trap ammonia, keeping ambient levels lowermdpi.com. Whatever bedding is used, ensure stalls are well-managed: remove wet spots and manure daily (preferably when the horse is out, to minimize its exposure to the dust from cleaning). Adding a deodorizer or an acidifying agent to stalls (as some commercial stable products do) can also chemically reduce ammonia productionmdpi.commdpi.com. Another practical tip is to use less bedding if possible – for example, some horse owners install rubber matting or padded stable floors, which allow a thinner layer of bedding. Less bedding means fewer dust sources and less surface for urine to collect (and it’s easier to clean thoroughly). The overarching principle is to keep the horse’s lying area as dust-free and dry as possible. By improving bedding choice and stall hygiene, one can greatly reduce inhalable irritants – one veterinary study noted that these changes are as crucial as medication in managing equine asthma long-termhaygain.ushaygain.us.
• Maximize Turnout and Fresh Air: Full turnout (keeping the horse outdoors) is considered the gold standard in preventing stable-induced respiratory issues. Veterinarians frequently recommend moving asthmatic horses to live outdoors 24/7 (with appropriate shelter) as a first-line management stepfrontiersin.org. Constant access to fresh outdoor air virtually eliminates exposure to concentrated stable dust. Of course, full turnout may not be feasible for every horse or property in winter, but even partial turnout is beneficial. Increasing the number of hours a horse spends outside each day – for example, turning out during daylight hours and only stabling overnight – will meaningfully lower its total inhaled dust burden. One study demonstrated that horses showed significant improvement in lung function and inflammation when they were turned out to pasture after a period of stall confinementsciencedirect.com. Pasture environments (in UK winters) typically have much cleaner air, provided the horse isn’t exposed to other unique allergens (note: a minority of horses suffer from “summer pasture asthma” triggered by pollens, but those cases are specific and usually a summer issuefrontiersin.org). For the typical horse, the more time out of the stall, the better – even an extra few hours a day of turnout can allow the lungs to clear and recover. Yard managers should implement winter turnout rotations if possible, and use all-weather paddocks or arenas to give horses a break from stall air. Additionally, avoid unnecessary confinement: if a horse is on box rest for injury, exhaust every option to improve its stall air (use all other measures here aggressively). In summary, think of stabling as a necessary evil in winter – minimize the time inside when practical, since natural ventilation outdoors is far superior to any indoor environmentmdpi.commdpi.com.
• Improve Barn Ventilation: When horses must be stabled, optimizing ventilation is key. Simple adjustments can make a big difference. Ensure that stable windows, vents, or top doors are open whenever weather allows, promoting cross-breezes. If the stable has air vents or ridge openings, keep them free of blockage. In barn designs with poor natural airflow, consider using mechanical ventilation (extraction fans or ceiling fans) to pull fresh air through. One field study in a closed barn showed that installing a mechanical exhaust fan cut indoor CO₂ (a proxy for poor ventilation) roughly in half, indicating a significant improvement in air exchangemdpi.commdpi.com. The investment in a fan system can thus notably reduce dust and gas accumulation. Even without fancy systems, small steps help: for instance, leaving barn doors ajar at opposite ends can create a wind tunnel effect. Owners should resist the urge to completely seal up the stable on cold days – horses generally tolerate cold air well, especially compared to dusty air. It’s better to provide a blanket for warmth and keep some airflow, rather than shut every door. Monitoring air quality is also useful: ammonia smell or a haze of dust visible in sunlight are signs ventilation is insufficient. In winter, daily mucking out should include fully removing urine-soaked bedding (as mentioned) and perhaps briefly using a leaf blower or broom to clear cobwebs and dust from rafters – but crucially, do this when horses are outside to vent the dust. Afterward, allow time for particles to settle or be flushed out by fresh air before bringing horses back in. The bottom line is a well-ventilated stable will dilute and remove airborne irritants. Aim for that guideline of several full air changes per hour; this might be achieved through a combination of natural drafts and fans. Good ventilation works hand in hand with the other measures (clean bedding, wetting hay) to maintain a cleaner breathing environment for stabled horses.
• Holistic Stable Management: Finally, adopt a holistic approach where all stalls and the entire barn are managed with respiratory health in mind. It’s not very effective to fix one horse’s stall if neighbouring stalls are still dusty – the irritants will drift and undermine your effortssciencedirect.com. If one horse in the barn has asthma, it’s wise to implement low-dust practices barn-wide (for example, switch all horses on that aisle to dust-free bedding and hay, if possible). Store hay and straw in a separate building or at least far from where horses live; dust from a hay loft or bedding pile can permeate the stable air continuouslysciencedirect.com. Schedule barn activities thoughtfully: sweeping aisles, blowing out fans, or handling bedding should be done when horses are out, or at least in a well-ventilated moment. After these activities, allow dust to settle. Also, consider the microclimate – keep humidity in check if you can (excess moisture can promote mould growth), but avoid making the barn too airtight just to stay warm. Many UK yard owners find that simple changes like opening a top door, using rubber mat flooring (to cut bedding needs), or feeding from ground-level feeders (so dust isn’t blown into the horse’s face) all contribute to noticeable reductions in coughing. Veterinary guidance often stresses that environmental management is a long-term solution: unlike medications which are short-acting, a clean stable environment continuously protects the horse’s lungshaygain.ushaygain.us. By combining all these measures – clean, damped-down forage; minimal dust bedding; fresh air and turnout; and rigorous stable hygiene – even horses with chronic “stable cough” can show marked improvement. These steps are practical and proven, and they mirror what peer-reviewed research has found: horses kept in a low-dust, well-ventilated environment have far healthier airways than those in traditional stable conditionsfrontiersin.orghaygain.us.

References: The information above is drawn from a range of peer-reviewed studies and expert veterinary sources focusing on equine respiratory health. Key sources include research published in Equine Veterinary Journal, Veterinary Record, and Journal of Veterinary Internal Medicine, as well as review articles on equine asthma (Mańkowska et al., 2024) and in-depth stable environment studies (e.g. Clements & Pirie 2007 on dust reduction). UK-based surveys and trials were prioritized, reflecting management practices typical of British yards. For instance, Pirie (2014) estimates UK asthma prevalencehaygain.us, and Holcombe et al. (2001) showed simply stabling young horses induces airway inflammationfrontiersin.org. Environmental solutions like hay steaming are supported by studies at the Royal Agricultural University (Moore-Colyer et al., 2015) demonstrating drastic dust reductionshaygain.us. The consensus is clear that management changes can prevent most stable-related respiratory issues. By following these evidence-backed practices, horse owners can significantly improve the winter air quality for their stabled horses, protecting them from the discomfort and performance loss associated with chronic coughing and equine asthma.

Sources:

• Uchiumi & Sheats. Front. Vet. Sci. (2019) – Seasonal changes in equine asthma prevalencefrontiersin.orgfrontiersin.org.
• Mańkowska et al. Animals (MDPI) (2024) – Review of environmental risk factors in equine asthmamdpi.commdpi.com.
• Clements & Pirie. Res. Vet. Sci. (2007) – Dust in stables and impact of hay soakingsciencedirect.comsciencedirect.com.
• Haygain (2020) – Summary of EVJ and other research on stable dust and asthmahaygain.ushaygain.us.
• Poland Stud Farm Study (Grzyb et al., 2024) – Fungal and dust levels in stablespmc.ncbi.nlm.nih.govpmc.ncbi.nlm.nih.gov.
• Dauvillier et al. (2019, ACVIM Abstract) – Fungal elements doubling IAD riskonlinelibrary.wiley.com.
• Elfman et al. (2008) – Effects of mechanical stable ventilation on gas levelsmdpi.commdpi.com.
• Holcombe et al. Equine Vet. J. (2001) – Stabling vs pasture and airway inflammationfrontiersin.org.
• Moore-Colyer et al. J. Equine Vet. Sci. (2015) – Hay steaming vs soaking effectshaygain.us.
• Ward et al. (2022) – Bedding material comparison for respiratory healthmdpi.comonlinelibrary.wiley.com. (Plus additional references within)

Vitamin C Supplementation in Horses – Evidence and Recommendations

Background: Horses’ Vitamin C Production and Need for Supplementation

Endogenous Synthesis: Unlike humans, horses synthesize their own vitamin C (ascorbic acid) in the liver from glucose. This means true vitamin C deficiency (scurvy) does not occur in normal horsesplatinumperformance.caequestriantradenews.com. There is no established dietary requirement for vitamin C in horses at maintenance, as healthy horses generally produce enough for basic needsplatinumperformance.ca. Fresh forage is also a natural source – green grass is rich in vitamin C, whereas preserved forages like hay contain almost none (vitamin C is unstable and degrades with drying and storage)platinumperformance.cathelaminitissite.org. For example, a grazing horse might ingest an estimated ~25 grams of vitamin C per day from fresh pasture, but a hay-fed horse in winter gets virtually zero dietary vitamin Cthelaminitissite.orgthelaminitissite.org. In most cases, a healthy adult horse on good forage does not require supplementationplatinumperformance.ca.

Situations of Increased Demand: Certain conditions can increase a horse’s vitamin C needs or tax its internal supply. Stress, illness, intense exercise, and aging may impair the horse’s ability to maintain adequate vitamin C levelsequestriantradenews.complatinumperformance.ca. In these scenarios, the horse’s own synthesis might not keep up with utilization. For instance, older horses and those with Pituitary Pars Intermedia Dysfunction (PPID) have been observed to have lower plasma vitamin C than young horsesthelaminitissite.org. Likewise, prolonged stress (such as transport or hard work) can deplete plasma ascorbic acid and reduce immune functionresearchgate.net. Because winter often brings both increased stall confinement (with dust exposure) and a lack of fresh grass, horses kept indoors on hay in winter may be at higher risk of suboptimal vitamin C status. This is why supplementation is frequently considered in these cases to support health and immunity.

Vitamin C’s Role in Equine Respiratory Health

Antioxidant Defense in the Lungs: Vitamin C is a potent antioxidant that helps protect lung tissues from oxidative damage. In horses with inflammatory airway disease or equine asthma (which includes mild IAD and severe RAO/heaves), research has shown that the lung’s lining fluid contains significantly less vitamin C compared to healthy horsespubmed.ncbi.nlm.nih.gov. Oxidative stress is believed to play a key role in equine asthma’s pathology, as inflammation in the airways leads to production of reactive oxygen species that damage cellspmc.ncbi.nlm.nih.gov. Supplementing vitamin C can bolster the lung’s antioxidant defenses: one study found that feeding ascorbic acid (~20 mg per kg body weight, ~10 g for a 500 kg horse) for two weeks significantly increased ascorbic acid concentrations in bronchoalveolar lavage fluid of healthy poniespubmed.ncbi.nlm.nih.gov. In other words, oral vitamin C raised the level of this antioxidant in the respiratory lining, which could help counteract oxidative damage in the lungs.

Benefits for Respiratory Conditions: In practice, vitamin C is often used as part of nutritional support for horses with respiratory issues (especially during winter stabling). Antioxidant supplementation has been studied in horses with Recurrent Airway Obstruction (RAO): a controlled trial in heaves-affected horses showed that a mix of antioxidants (including vitamins C and E and selenium) given for 4 weeks improved lung function. Treated horses had better exercise tolerance and lower airway inflammation scores (less endoscopic evidence of inflammation) compared to placebopubmed.ncbi.nlm.nih.govpubmed.ncbi.nlm.nih.gov. While vitamin C was one component of that mix, the results suggest that enhancing antioxidant intake can modulate airway inflammation in equine asthma. Another study of racehorses with mild equine asthma found that a nutraceutical supplement rich in antioxidants led to reduced clinical signs and tracheal mucus after 3 weeks, supporting the idea that antioxidant therapy can benefit airway healthpmc.ncbi.nlm.nih.gov.

Current Expert Consensus: Modern equine asthma management still prioritizes environmental control (reducing dust, improving ventilation, steaming hay, etc.) and veterinary treatments (bronchodilators, corticosteroids) as first-line interventions. However, targeted nutritional support is recognized as a helpful adjunct. UK nutritionists note that supplementing antioxidants like vitamin C (alongside herbs or omega-3 fatty acids) has been an established practice for decades to help “clear the airways”equestriantradenews.comequestriantradenews.com. Vitamin C, in particular, is frequently included in respiratory supplements to support the immune system and neutralize free radicals in the lungs. Owners and vets report that additional vitamin C may help reduce coughing and other asthma symptoms in susceptible horses when used along with proper managementpremierperformance.ie. It’s not a cure for equine asthma, but it supports lung health by mitigating oxidative stress and inflammationjusthorseriders.co.ukjusthorseriders.co.uk. In horses exposed to dusty stables or high levels of airborne irritants, vitamin C supplementation has been noted to reduce respiratory stress and maintain clearer airwaysjusthorseriders.co.uk. In summary, the current thinking is that vitamin C can play a supportive role in respiratory health, especially for horses with inflammatory airway conditions or those in high-risk environments (such as barns in winter)equestriantradenews.com.

Does Supplementation Suppress a Horse’s Own Vitamin C Production?

A key concern is whether giving a horse extra vitamin C will cause its body to produce less on its own (a feedback suppression). Evidence suggests this can be an issue when high doses are given for extended periods. While formal research in horses is limited, one study in weanling horses provides insight: after 50+ hours of transport stress (which lowered their plasma vitamin C), some weanlings were given oral vitamin C (5 g twice daily) for 5 or 10 days. Supplemented horses did show a modest increase in blood ascorbic acid during the dosing period. However, in the weeks after stopping supplementation, their plasma vitamin C dropped significantly below normal – a deficiency rebound that lasted 1–3 weeks post-supplementationresearchgate.netresearchgate.net. The authors suggested this was possibly due to suppressed endogenous synthesis (or increased excretion) caused by the prior high intakeresearchgate.net. In practical terms, this means a horse’s liver might downregulate its own vitamin C production when large amounts are provided in the diet, at least temporarily.

Equine nutrition experts extrapolate from other species and recommend caution. It is generally advised not to abruptly stop high-level vitamin C supplementation. Instead, if a horse has been on a high dose, taper the dose down gradually over a few weeks to allow its internal synthesis to readjustplatinumperformance.ca. For example, the advice is to wean off over about 3–4 weeks rather than quitting “cold turkey”platinumperformance.ca. This helps avoid any potential “withdrawal” dip in vitamin C status or antioxidant disruption. It’s worth noting that horses can ramp up production again, but the goal is to prevent any period of insufficiency that could affect the immune system or connective tissues. In summary, oral supplementation can potentially downregulate endogenous production, so supplements should be used judiciously and discontinued with a taper when coming off high dosesplatinumperformance.ca. (Short-term or moderate-dose supplementation is less likely to cause a problem; the concern is with long-term high doses leading to a rebound deficiency.)

Recommended Dosages and Duration of Vitamin C Use

There is no official “requirement” or RDA for vitamin C in horses, but guidance exists on effective and safe supplementation levels. Common practice and research studies have used doses in the range of about 3,000 to 10,000 mg per day (3–10 g/day) for an average adult horseplatinumperformance.ca. This roughly equates to 6–20 mg per kilogram body weight. In the UK and EU, many commercial equine vitamin C supplements provide around 5–10 g per serving, intended for daily use during periods of need. Higher doses (e.g. 15–20 g/day) have been tested in studies or recommended in certain cases – for instance, 20 g per day was reported to enhance antibody response in older horses (more on that below)justhorseriders.co.uk. The Nutrient Requirements of Horses (NRC) publication lists an upper safe limit of about 20 g per day for a 500 kg horseplatinumperformance.ca. Doses beyond that may not be necessary and could risk mild side effects. Fortunately, vitamin C is water-soluble, so horses tolerate it well; excess is excreted in urine. The main adverse effect of very high doses would be digestive upset (loose manure or mild diarrhea)platinumperformance.ca, so it’s prudent not to greatly exceed the upper limit.

Duration of supplementation depends on the goal. Many owners target specific high-risk periods rather than supplement year-round. For example:

• In a horse with seasonal winter cough or equine asthma, one might supplement daily throughout the winter months when the horse is stalled on hay, and then taper off in spring when the horse returns to pasture and symptoms subside.
• For a one-off stressor like long-distance transport or a major competition, an owner might start a moderate dose a few days beforehand and continue for a few days after to support the horse’s immune system. (Prolonged continuous supplementation is usually avoided unless warranted, because of the potential downregulation issue noted aboveresearchgate.net.)
• In cases of chronic conditions or senior horses, some veterinarians and nutritionists will include vitamin C in the daily regimen long-term, but often at the lower end of the dose range (e.g. 3–5 g/day) as a maintenance level. If a high dose (10+ grams) has been used during an illness or other challenge, they will gradually reduce to a maintenance dose instead of stopping abruptlyplatinumperformance.ca.

Overall, current guidance is to use vitamin C supplements strategically and for defined periods: use the higher doses when a horse’s needs are elevated, and otherwise stick to a modest supportive dose or rely on the horse’s natural production when the horse is healthy and on green forageequestriantradenews.com. Always ensure the horse’s basic diet (good forage, balanced minerals) is sound, since vitamin C is only one piece of the puzzle for respiratory and immune health.

Special Considerations for Different Horse Groups

• Horses with Equine Asthma (IAD/RAO): These horses suffer chronic airway inflammation and oxidative stress. Research shows they have lower vitamin C in lung fluids during flare-upspubmed.ncbi.nlm.nih.gov, so antioxidant support is logical. Vitamin C supplementation is commonly recommended as part of a comprehensive management plan for equine asthma. UK veterinary nutritionists recognize that during respiratory disease episodes, horses likely have an increased requirement for vitamin C beyond what they synthesizeequestriantradenews.com. Supplementation aims to replenish lung lining fluid levels and assist the immune system in the airways. In practice, asthmatic horses are often kept on a respiratory supplement containing vitamin C (sometimes along with vitamin E, omega-3s, MSM, or herbal extracts) to help reduce coughing and inflammationjusthorseriders.co.ukjusthorseriders.co.uk. Clinical improvements can include fewer episodes of coughing, less mucus, and better exercise tolerance when vitamin C and other antioxidants are added, although individual responses varypubmed.ncbi.nlm.nih.govpmc.ncbi.nlm.nih.gov. It’s important to note that environmental control (dust-free bedding, soaked or steamed hay, turnout as much as possible) is the primary management for equine asthma; vitamin C is a supportive therapy. But given its low risk and the evidence of antioxidant imbalance in these horses, most experts agree it is a worthwhile addition to the care of horses with IAD/RAO, especially during high-risk times (like stalled in winter)equestriantradenews.com. Owners should work with their vet/nutritionist to determine an appropriate dose; often ~5–10 g/day is used for moderate asthma cases, up to around 20 g/day for severe cases under supervision, always coupled with gradual withdrawal when the time comes.
• Older Horses and PPID (Cushing’s Disease): Aging horses tend to have declining antioxidant status. Studies have documented that aged horses have lower blood vitamin C levels than younger horses under the same diet, and this is pronounced in horses with PPID (a common endocrine disorder in older horses)thelaminitissite.orgthelaminitissite.org. The aging immune system may benefit from extra vitamin C. In fact, researchers reported that supplementing ~10 g twice daily (20 g total per day) increased the antibody response to vaccinations in aged horses, with the effect especially strong in PPID horsesthelaminitissite.org. This suggests vitamin C can help boost immune function in seniors. Anecdotally, veterinarians have also found that older horses with chronic infections or wound-healing issues improve with added vitamin Cthelaminitissite.org. In the UK, organizations like Redwings Horse Sanctuary note that vets may advise a vitamin C and B-vitamin supplement for geriatric horses who have little access to fresh grass, as these nutrients support the immune system and overall healththelaminitissite.org. Therefore, it is now common to include vitamin C in the diet of senior horses, particularly those over ~15–20 years old or those diagnosed with PPID. The approach might be a continuous low-to-moderate dose supplement to maintain their levels, since older horses might not synthesize as efficiently. Importantly, no adverse effects are expected at the moderate levels used, and the potential benefits (better immunity, possibly better coat/skin and recovery) make it a sensible precaution for many aged equines.
• Horses Under High Stress or Heavy Work: Stressors – whether physical (intense exercise, long-distance travel, competition) or psychological – can increase oxidative stress and potentially outpace a horse’s vitamin C production. For example, prolonged transport has been shown to drop horses’ plasma vitamin C and weaken immune defensesresearchgate.net. High-performance horses (racehorses, eventers, endurance horses) produce more free radicals during exercise and may have higher antioxidant turnover. Supplementing vitamin C in these cases is aimed at maintaining optimal antioxidant capacity. Some endurance and race trainers report using vitamin C to help horses recover faster and fend off respiratory infections during rigorous training. Research supports that giving additional antioxidants can enhance blood antioxidant status in hard-working horsessciencedirect.com. However, one must balance this with the knowledge that continuous high-dose supplementation might lead to adaptation (as noted earlier). The current thinking is to use vitamin C during peak stress periods – for example, during a competition season or around shipping – rather than nonstop at high doses. In a practical program, a performance horse might get 5–10 g/day of vitamin C during weeks of heavy training or travel, possibly combined with vitamin E and selenium for a synergistic antioxidant effect. This can help offset exercise-induced oxidative muscle damage and support respiratory health if the horse is inhaling dust at competitionsjusthorseriders.co.ukpmc.ncbi.nlm.nih.gov. After the intense period, the dose can be scaled down. Since vitamin C is relatively cheap and safe, many owners of competition horses feel the benefits (fewer coughs, colds, or faster recovery) are worth it. And as one equine health center notes, if supplementation is discontinued, it should be done gradually to allow the horse’s own production to ramp back upplatinumperformance.ca.

In summary, vitamin C supplementation in horses is a targeted strategy used when circumstances suggest the horse’s natural vitamin C status may be suboptimal – such as in respiratory disease (equine asthma), old age/PPID, or periods of high stress. The current evidence, including peer-reviewed studies and expert veterinary guidance, indicates that while a normal horse on grass doesn’t need extra vitamin C, those in challenging conditions often benefit from additional antioxidant support. Vitamin C can support respiratory health by increasing lung antioxidant levels and possibly reducing inflammationpubmed.ncbi.nlm.nih.govpubmed.ncbi.nlm.nih.gov. It can aid older and immune-compromised horses by improving immune responsesthelaminitissite.org. And it can help stressed horses maintain their antioxidant balanceresearchgate.net. The key points emphasized by veterinarians and nutritionists are: use appropriate doses (typically 5–10 g/day), avoid excessively long high-dose courses, and taper off gradually to avoid any rebound effectsplatinumperformance.caresearchgate.net. This balanced approach ensures that horses get the benefits of vitamin C supplementation when they need it, without unintended drawbacks.

Sources: Recent studies and reviews have informed these recommendations, including research in Free Radical Research (Deaton et al. 2003) on pulmonary vitamin C in horsespubmed.ncbi.nlm.nih.govpubmed.ncbi.nlm.nih.gov, Equine Veterinary Journal (Kirschvink et al. 2002) on antioxidant therapy in RAOpubmed.ncbi.nlm.nih.gov, and Animals (Ralston et al. 2012) on post-transport vitamin C dynamicsresearchgate.netresearchgate.net. Expert guidance from UK-based equine nutritionists (e.g. Kate Hore, NAF) and organizations (Redwings) also reinforces the practice of supplementing vitamin C for horses under respiratory or immune stressequestriantradenews.comthelaminitissite.org. The consensus is that, when used thoughtfully, vitamin C supplementation can be a valuable tool in supporting equine health and is generally safe for horses in need of an antioxidant boost.

 

Effect of Dietary Vitamin C Changes on Equine Vitamin C Status

Pasture vs. Hay – Vitamin C Content

Fresh green pasture is a rich natural source of vitamin C for horses, whereas preserved forages like hay contain very little. Ascorbic acid in plants is unstable and oxidizes during the cutting, drying, and storage of hay, dramatically reducing its vitamin C contentforum.chronofhorse.com. In fact, one estimate suggests a grazing horse might ingest on the order of 25 grams of vitamin C per day from abundant fresh grass (e.g. ~500 mg vitamin C per kg of grass; a 500 kg horse consuming ~50 kg fresh pasture could get ~25 g)thelaminitissite.org. By contrast, hay (especially older, cured hay) is virtually devoid of vitamin Cforum.chronofhorse.com. Thus, horses obtain significantly more ascorbate when on lush pasture than on a hay-only diet.

High Vitamin C Intake and Endogenous Synthesis

Horses, unlike humans, can synthesize their own vitamin C in the liver from glucose. A healthy horse can typically produce enough ascorbic acid to meet its needs under normal conditionsmadbarn.commadbarn.com. However, research indicates that providing high levels of exogenous vitamin C (whether via supplements or nutrient-rich pasture) may down-regulate the horse’s internal production. In other words, the liver’s enzyme (L-gulonolactone oxidase) activity can be suppressed when plenty of vitamin C is available from the dietmadbarn.commerckvetmanual.com. Veterinary sources warn that prolonged high intake of vitamin C could trigger a feedback mechanism reducing the horse’s own synthesis of ascorbatemerckvetmanual.com. This has been observed in studies where horses were given large supplemental doses: their endogenous production seemed to slow in responsemadbarn.comforum.chronofhorse.com. Notably, horses supplemented with ~10 g twice daily showed only modest plasma increases during supplementation, but their liver likely adjusted by making less vitamin C on its ownmdpi.commdpi.com.

Abrupt Dietary Changes and Vitamin C “Lag”

If a horse goes from a high–vitamin C diet (fresh pasture or long-term supplementation) to a low–vitamin C diet (hay-only) suddenly, evidence suggests there can be a temporary shortfall in vitamin C status while the liver ramps back up production. In effect, the horse may experience a lag before endogenous synthesis compensates for the lost dietary supply. A peer-reviewed study demonstrated this phenomenon: weanling horses that received high-dose ascorbic acid for 10 days had normal-to-high plasma ascorbate while supplemented, but when the supplement was abruptly withdrawn, their blood vitamin C plunged below normal and remained low for up to three weeksmadbarn.com. The researchers attributed this to suppressed hepatic enzyme activity that was slow to rebound after sudden cessation of vitamin C intakemadbarn.com. Equine nutritionists therefore advise against an abrupt withdrawal of high vitamin C intake. Instead, any prolonged vitamin C supplementation should be tapered off gradually to allow the liver’s enzyme activity to readjustmadbarn.comforum.chronofhorse.com. By analogy, a horse moving from summer pasture to a hay diet could similarly experience a transient dip in plasma vitamin C until its internal synthesis increases to compensate. While full-blown scurvy does not occur in equines, suboptimal plasma ascorbate during that adjustment period is a concernmadbarn.com. This “rebound deficiency” risk is precisely why careful management of dietary changes is recommended.

Seasonal Variation in Plasma Vitamin C

Although formal studies on seasonal plasma vitamin C levels in horses are limited, clinical observations align with the above principles. Veterinarians note that horses tend to show higher blood ascorbate levels during periods of fresh forage access and that these levels “drop off in winter when [horses are] on hay.”drkhorsesense.wordpress.com In essence, horses often have their peak vitamin C status in spring and summer on green grass, then experience a decline in circulating vitamin C over the winter if fed only cured forages. This seasonal pattern suggests that the transition to a vitamin C-poor ration can leave horses temporarily under-supplied until endogenous production fully adapts. Supporting this, older horses or those under stress (which often coincides with winter confinement or illness) have been found to have lower plasma vitamin C than young, pastured horsesthelaminitissite.orgthelaminitissite.org. Overall, while healthy adult horses usually synthesize enough vitamin C year-round, the seasonal drop in dietary intake may reveal marginal deficiencies – especially in the first weeks of the diet change or in individuals with greater needs.

Managing Seasonal Transitions for Immune and Respiratory Health

Veterinary guidance emphasizes proactive nutritional management when horses transition off pasture, particularly to support immune and respiratory health. Antioxidant vitamins are crucial for immune function and lung tissue integrity, and a sudden decrease in antioxidants like vitamin C and vitamin E (common when pasture access is lost) can leave the horse vulnerableforum.chronofhorse.comforum.chronofhorse.com. To mitigate this, experts often recommend supplementing vitamin C (and other antioxidants) during the autumn/winter switch or any period of stress. For example, Redwings Horse Sanctuary (UK) notes that “your vet may advise a supplement containing B vitamins and Vitamin C if grazing is especially restricted, as these are helpful in supporting the immune system”thelaminitissite.org. This is aimed at bolstering the horse’s antioxidant defenses when natural forage vitamins wane.

Respiratory health in particular can benefit from vitamin C support. Horses with recurrent airway obstruction (RAO, “heaves”) or other chronic lung issues tend to have depleted vitamin C in their lung fluid and plasmaforum.chronofhorse.com. In such cases, veterinarians often recommend high-dose vitamin C supplementation (on the order of 30 mg per kg body weight per day, roughly 15 g for a 500 kg horse) to support respiratory tissue and immune cell functionforum.chronofhorse.com. Research confirms that antioxidant supplementation (including vitamin C alongside selenium and vitamin E) can reduce airway inflammation, improve lung function, and increase exercise tolerance in horses with heavesmadbarn.com. Similarly, studies have shown that giving 10–20 g of vitamin C per day to aged or immune-compromised horses improves their antibody responses to vaccines and may help ward off respiratory infectionsforum.chronofhorse.comforum.chronofhorse.com. These findings underscore the practical advice to provide extra vitamin C during high-risk times (such as winter stabling, after transport, or in older Cushing’s/PPID horses), in order to shore up the horse’s antioxidant status when its own production or stores might be insufficientforum.chronofhorse.comthelaminitissite.org.

When implementing vitamin C support, it is important to do so thoughtfully. Gradual changes are key: if supplementing, introduce vitamin C gradually and likewise taper it off slowly when grass becomes available again or when discontinuing the supplementmadbarn.com. This prevents the abrupt “crash” in plasma vitamin C that can occur with sudden withdrawalmadbarn.com. Many equine nutritionists also recommend using a stabilized form of vitamin C (e.g. ascorbyl palmitate or coated ascorbic acid) for better absorption and stability, and pairing it with a balanced diet that includes adequate vitamin E and other antioxidantsmadbarn.commadbarn.com. By planning ahead – for instance, beginning a vitamin C supplement regime in late fall before pasture is completely gone, or continuing low-level supplementation through the winter – horse owners can help maintain consistent antioxidant levels in their horses. This, in turn, supports the immune system and respiratory tract through seasonal challenges. In summary, a controlled approach to vitamin C during diet transitions can smooth the horse’s adjustment and promote health, but any long-term supplementation should be managed carefully to avoid suppressing the horse’s natural vitamin C synthesismerckvetmanual.commadbarn.com.

References

• Ralston, S.L. & Stives, M. (2012). Supplementation of Ascorbic Acid in Weanling Horses Following Prolonged Transportation. Animals (Basel) 2(2): 184–194. DOI: 10.3390/ani2020184.
• Kellon, E. (2017). Vitamin C for Horses. Dr. K’s Horse Sense (blog)drkhorsesense.wordpress.com.
• Kentucky Equine Research Staff (2013). Vitamin C in Horse Diets. Equinews (KER)forum.chronofhorse.comforum.chronofhorse.com.
• The Laminitis Site (2014). Vit C and PPID (web article)thelaminitissite.orgthelaminitissite.org.
• Merck Veterinary Manual (2023). Nutritional Requirements of Horses – Ascorbic Acidmerckvetmanual.com.
• Mad Barn (2023). Vitamin C for Horses: Benefits, Sources & Supplementatio