11 min read Last updated: January 2026 If your horse feels stiff after work or bloods show a CK spike, selenium could be the missing piece. This guide shows how to test, calculate safe doses, and choose UK-ready supplements to protect muscle and avoid toxicityaiming for 0.10.3 mg/kg across the total diet. Quick Summary Short on time? Here are the key takeaways. Area: Test Se + CK What To Do: Ask your vet for whole-blood or serum selenium with CK/AST and vitamin E. Re-test after ration changes and at end-winter and mid-summer. Why It Matters: Confirms status and muscle impact so you supplement precisely, not by guesswork. Common Mistake: Supplementing before testing or relying on a single off-season result. Area: Pair with vitamin E What To Do: Test vitamin E alongside selenium and, if low, correct both using measured products. Why It Matters: Selenium and vitamin E work together in antioxidant defence and deficiencies often co-exist. Common Mistake: Raising selenium alone while leaving vitamin E low. Area: Set diet target What To Do: Aim for 0.1 mg/kg selenium in the total diet at rest, rising toward 0.3 mg/kg for harder work or broodmares. Why It Matters: Matching intake to workload supports muscle protection and performance. Common Mistake: Using a fixed daily dose regardless of work level or forage result. Area: Avoid toxicity What To Do: Keep dietary selenium well below ~2 mg/kg of any feed, and audit all sources before adding more. Why It Matters: Over-supplementation risks hoof and coat problems and more serious toxicity. Common Mistake: Stacking a balancer, lick and performance supplement that all contain selenium. Area: Choose supplements What To Do: Pick targeted selenium (yeast or sodium selenite), selenium+E, or a balanced multi-mineral based on tests and workload; dose only to fill the gap. Why It Matters: The right format simplifies accurate dosing and avoids duplication. Common Mistake: Buying on price or brand without checking form, strength and overlap with existing feeds. Area: Wintersummer plan What To Do: In winter, analyse hay and run bloods; in summer competition, consider the upper range (0.3 mg/kg) and re-test if stiffness or CK stay high. Why It Matters: UK forage is selenium-poor in winter and oxidative demand rises with summer work. Common Mistake: Keeping the same dose year-round despite big changes in forage and workload. Area: Spot deficiency What To Do: Watch for stiffness, crampy feel, poor recovery, or raised CK; in foals, act fast on weakness/WMD signs and call your vet. Why It Matters: Early action prevents performance dips and reduces risk of nutritional myopathy. Common Mistake: Dismissing recurrent high CK as fitness without checking selenium and vitamin E. Area: Calculate daily dose What To Do: Weigh feeds/forage, convert label selenium (mg/kg) to daily mg by multiplying by actual kg fed, and compare with 23 g/kg BW/day (11.5 mg/day at 500 kg). Why It Matters: Simple maths turns labels into accurate, safe dosing. Common Mistake: Assuming mg/kg equals per-day intake or guessing from unweighed nets and scoops. In This Guide Why selenium matters for UK horses How much selenium your horse needs (and the safe upper limit) Spotting deficiency and when to test Supplement the smart way: forms, doses and label checks Seasonal UK plan: winter yards and summer competition Worked examples for a 500 kg horse When low selenium becomes disease: white muscle disease What to buy (and how to choose) from Just Horse Riders Is your horse stiff after work, or have bloods shown a spike in CK? In the UK, one quiet culprit is often selenium a small but vital trace mineral that our soils simply dont supply in abundance.Key takeaway: UK forages usually contain only around 0.1 mg/kg selenium, so test first, then supplement to achieve 0.10.3 mg/kg across the total diet and never exceed roughly 2 mg/kg of feed to avoid toxicity.Why selenium matters for UK horsesUK soils and forages are typically low in selenium (around 0.1 mg/kg), so many horses need selenium added to the diet to meet requirements. This shortfall is well documented across the UK and Ireland, especially in coastal and northern areas.Selenium is an essential part of the antioxidant enzyme glutathione peroxidase, which helps protect muscle cells from exercise-induced oxidative damage. Low status is closely linked with muscle problems, particularly when vitamin E is also low. As the laboratory team at Liphook Equine Hospital explains: selenium is an essential component of the antioxidant enzyme glutathione peroxidase manifestation of deficiency may be affected by vitamin E status (Liphook Equine Hospital).UK soils, and the plants that grow in them, are generally low in selenium As a result it is often necessary to add selenium to the diet via hard feed or supplements. Horse & Hound feeding expertsIf your horse is on pasture or hay-only rations especially through wet winters when grass growth is limited its sensible to check selenium status before the competition season ramps up. When you do need to top up, choose targeted options from our curated range of selenium and antioxidant supplements.How much selenium your horse needs (and the safe upper limit)Most mature horses need around 23 micrograms of selenium per kg bodyweight daily (11.5 mg/day for a 500 kg horse), rising towards 0.3 mg/kg in the total diet for horses in hard work; intakes above roughly 2 mg/kg of feed risk toxicity.Two ways of expressing requirement are commonly used:By bodyweight: 23 micrograms/kg BW/day for mature horses (11.5 mg/day at 500 kg), per Liphook Equine Hospital.By dietary concentration: 0.1 mg/kg in the total diet for inactive horses, rising to 0.3 mg/kg for exercising horses and broodmares, per Horse & Hound.These figures align well in typical UK feeding scenarios. Remember the safety boundary too: toxicity becomes a risk when the dietary concentration exceeds about 2 mg/kg of feed (Liphook Equine Hospital). Thats why testing first, then supplementing carefully, is non-negotiable.Because selenium and vitamin E work together in antioxidant defence, its often wise to correct both where needed. Youll find combined options in our supplements collection to simplify dosing while you fine-tune your horses ration for training.Spotting deficiency and when to testDeficiency typically shows as muscle stiffness, weakness, poor performance and elevated CK; UK vets confirm status with whole-blood selenium and CK/AST testing, and hair analysis can indicate longer-term exposure.In adults, you may see a horse that feels crampy under saddle, struggles to recover from work, or repeatedly returns elevated CK on routine bloods. In youngsters, deficiency can progress to nutritional myopathy (white muscle disease, WMD). Studies report that low serum selenium is strongly associated with WMD in foals, with 7 of 8 affected foals under 30 days old showing low serum Se (Equine Vet J Suppl via PubMed).For diagnosis of selenium-related myopathies, the Merck Veterinary Manual notes a pattern of moderately to markedly increased CK and AST alongside low whole-blood selenium (<0.07 mcg/mL). Given how often selenium and vitamin E deficiencies co-exist, ask your vet to check both together this is routine in BEVA-aligned UK practice.How to test wisely:Whole-blood selenium or serum selenium reflect current status; pair with CK/AST to assess muscle impact (Liphook Equine Hospital).Hair analysis provides a picture of exposure over time and can complement bloods (Cawood).Re-test after any ration changes or if stiffness persists into the season, particularly if youre targeting the 0.3 mg/kg diet level for competition (Horse & Hound).Quick tip: If your horse is stabled through OctoberMarch and relies on hay, pencil in selenium and vitamin E bloods at the end of winter and again mid-summer to catch seasonal swings.Supplement the smart way: forms, doses and label checksSupplement only after forage or blood analysis confirms low status, aim for 0.10.3 mg/kg of the total diet, and avoid stacking multiple selenium sources that could push intake upwards.In UK conditions, its common to add selenium via compound feed, a standalone selenium product, or a balanced multi-mineral. Typical options and guide prices include:Targeted selenium supplements (often yeast-based/organic or sodium selenite forms): around 1030 for a 13 month supply.Vitamin E supplements to pair with selenium: around 1540 per tub.All-round multi-vitamin/mineral buckets or licks with selenium: around 2050 convenient for yard horses through winter.At Just Horse Riders, we recommend starting with a vet conversation and a feed audit. If you need to top up, browse our trusted NAF range and wider supplements selection for targeted selenium or selenium-with-vitamin-E options that fit your workload and budget.Pro tip: Check every label on your yard compound feed, balancer, lick, and any performance supplement. Add up the selenium from all sources against your target (0.10.3 mg/kg of the total diet) and the safety ceiling (about 2 mg/kg of feed). When in doubt, bring the tags to your vet or nutritionist for a quick double-check.Seasonal UK plan: winter yards and summer competitionHorses stabled on hay through UK winters face higher deficiency risk, while those competing in summer often need the higher end of the range (0.3 mg/kg of the diet) due to increased oxidative stress.Winter (OctoberMarch): Forage-based rations dominate, and UK hays often sit around 0.1 mg/kg selenium sometimes less. Combine routine bloods with a forage analysis, then decide whether to add a measured dose supplement or a balanced mineral bucket. Keep the horse comfortable in cold, wet spells with appropriately weighted stable rugs and, for turnout days, weatherproof winter turnout rugs so you can maintain a steady exercise plan while you address diet.Spring/Summer (competition season): As you build work on varied terrain, oxidative demand rises. Many eventers and show horses target the upper end of the dietary range (0.3 mg/kg) during peak season (Horse & Hound). If stiffness or CK remains high, speak to your vet about re-testing and, where appropriate, adjusting both selenium and vitamin E intake. When hacking out to condition safely, add hi-vis for rider and horse so you can train consistently whatever the light levels.Throughout the year: Keep a simple log of workload, diet changes, and any muscle signs. Small, timely adjustments and proof from bloods beat guesswork every time.Worked examples for a 500 kg horseAt rest, a 500 kg horse typically needs roughly 11.5 mg selenium per day, which UK forage alone may not reliably supply; in moderate-to-hard work, youll usually aim the total diet toward 0.3 mg/kg.Here are practical scenarios to show how the numbers fit together. Always adapt to your horses actual intake and test results.Rest/light work on hay only: If your horse eats 10 kg of hay (as-fed) at 0.1 mg/kg selenium, thats 1.0 mg/day near the lower end of requirement. If analysis shows your hay at 0.05 mg/kg, thats only 0.5 mg/day, so a small top-up (e.g., 0.51.0 mg/day) brings you into the 11.5 mg/day range for a 500 kg horse (Liphook Equine Hospital).Moderate work on forage plus compound feed: Suppose hay provides 0.8 mg/day and your performance feed adds 0.8 mg/day. Total 1.6 mg/day adequate for many. If CK remains elevated or work increases, you might edge closer to the dietary concentration of 0.3 mg/kg of the total diet (which, at a 10 kg total daily intake, equates to 3 mg/day), under vet guidance (Horse & Hound).Safety check: Whatever the daily total, ensure the concentration in any single feed component doesnt exceed roughly 2 mg/kg of feed the toxicity threshold quoted by Liphook Equine Hospital. This is why we avoid layering multiple selenium-rich products without counting them up.Quick tip: Weigh feeds and forages. Labels often state selenium as mg/kg thats per kilogram of that product, not per day. Multiply by how much you actually feed to get the daily mg figure.When low selenium becomes disease: white muscle diseaseWhite muscle disease (WMD) in foals is linked to low selenium and/or vitamin E, with affected foals showing low serum selenium and high CK; adults can show selenium-associated myopathies but severe WMD is rarer.Retrospective data in the UK and internationally show a clear association between selenium/vitamin E deficiency and nutritional myopathy in foals and young horses (Equine Vet J Suppl). One study reported low serum selenium (<1.26 micromol/L) in 7 of 8 affected foals under 30 days old (PubMed), while the Merck Veterinary Manual highlights the diagnostic pairing of markedly increased CK/AST with low whole-blood selenium (<0.07 mcg/mL).Broodmares and youngstock on UK pastures are therefore priority candidates for proactive testing and balanced supplementation. If your vet suspects nutritional myopathy, expect bloods, a full diet review, and a careful plan to correct selenium and vitamin E while managing exercise.What to buy (and how to choose) from Just Horse RidersChoose a targeted selenium or selenium-with-vitamin-E supplement when tests show a shortfall, or a balanced multi-mineral bucket for yard horses that need a steady trickle of trace minerals over winter; always review labels to avoid doubling up.Popular choices with our UK customers include:Targeted selenium (yeast-based or sodium selenite) for precise top-ups 1030/month depending on dose.Vitamin E powders or liquids to pair with selenium for muscle support typically 1540.Multi-vitamin and mineral buckets/licks for stabled herds through winter 2050 for convenient maintenance.Explore proven options in our supplements collection, including trusted formulations from the NAF range. While you refine training loads during rehab or conditioning blocks, supportive lower-limb protection from our horse boots and bandages can help you keep work consistent as diet changes bed in.At Just Horse Riders, we recommend a simple three-step routine: test, top up, and re-test. Its the fastest, safest route to better muscle comfort and performance.Conclusion: test first, supplement precisely, and ride the resultsBecause UK soils are selenium-poor, many horses benefit from careful supplementation but only once you know their baseline. Work with your vet to check selenium, CK/AST and vitamin E, then use measured products to reach 0.10.3 mg/kg in the total diet without breaching safety limits. Keep training consistent with season-appropriate turnout and stabling, and youll feel the difference in way of going and recovery.FAQsCan selenium deficiency cause tying-up or high CK in UK horses?Yes. Selenium deficiency is associated with elevated CK and nutritional myopathy; while severe white muscle disease is more common in foals, adults can show selenium-associated myopathies with raised CK and poor performance (Equine Vet J Suppl via PubMed; Merck Veterinary Manual).How often should I test selenium in the UK?Twice yearly is practical for many yards: at the end of winter (hay-heavy diets) and mid-summer (peak work). Test sooner if you notice stiffness, poor recovery, or persistently high CK/AST. UK vets routinely run whole-blood/serum selenium and CK/AST, with vitamin E alongside (Liphook Equine Hospital).What are the signs of selenium toxicity, and what intake is unsafe?Over-supplementation can cause brittle or cracked hooves and hair loss, among other issues. Toxicity risk rises when dietary concentration exceeds about 2 mg/kg of feed, so count all sources feed, balancer, lick, and supplements before you add more (Liphook Equine Hospital).Is yeast-based (organic) selenium better than inorganic forms?Both organic (yeast-based) and inorganic (e.g., sodium selenite) forms can be effective when used appropriately. Prioritise accurate dosing and proof from bloods; choose the format your vet or nutritionist recommends, and avoid duplicating across products. Browse both options in our selenium supplements.Should I give vitamin E with selenium?Often, yes. Selenium and vitamin E work together in antioxidant defence, and deficiencies frequently co-occur in horses with muscle issues. Many UK vets test and correct both simultaneously (Liphook Equine Hospital). You can find paired products via our NAF and wider supplements ranges.Will a mineral lick be enough through winter?Sometimes, but not always. Intake from licks varies by horse and herd dynamics, so a measured supplement often provides more reliable dosing for stabled horses on hay. If you use a lick, account for its selenium when calculating your total intake to avoid stacking.My horse is on a performance feed can I still add a selenium supplement?Maybe, but only after you do the maths. Check the feeds selenium (mg/kg), multiply by your actual daily feed amount, add forage contribution, then compare with your target (0.10.3 mg/kg of the total diet) and the safety ceiling. If in doubt, ask your vet; if needed, choose a small, precise top-up from our supplements so you dont overdo it. Shop the Essentials Everything mentioned in this guide, ready to browse. Shop SupplementsShop NAF SupplementsShop Stable RugsShop Turnout RugsShop Boots & Bandages