Horse Feed Glossary

A metabolic disorder in which a horse's cells become resistant to the hormone insulin — the body still produces insulin in response to sugar and starch, but the cells don't respond normally, so blood insulin levels remain chronically elevated. High insulin is directly toxic to the sensitive laminar tissue inside the hoof wall, making EMS horses prone to laminitis. EMS horses are almost always easy keepers with a tendency to gain weight. The most important dietary intervention is a very low-NSC diet — total starch and sugar below 10%, ideally below 7%. Feeds like Triple Crown Lite (9.3% NSC), Triple Crown Low Starch (13.5%), Nutrena SafeChoice Special Care, and Buckeye Safe N Easy (12.5% NSC) are formulated for horses with EMS. EMS is different from PPID (Cushing's disease), though the two conditions can occur together in the same horse.

The underlying mechanism of EMS — the horse's cells fail to respond normally to insulin. When a horse with IR eats sugar or starch, the pancreas produces insulin as normal, but the cells ignore it, leaving blood insulin levels elevated long after a meal. Over time, this high insulin damages the laminae. IR horses need total diet NSC below 10%. IR is often used interchangeably with EMS, though technically EMS is the broader syndrome and IR is one component of it. See also: EMS, PPID, Laminitis, NSC.

A hormonal disorder caused by dysfunction of the pars intermedia region of the pituitary gland. Affected horses overproduce ACTH (adrenocorticotropic hormone) and related peptides, leading to elevated cortisol and impaired regulation of insulin. The result is similar to EMS in terms of dietary management — horses with PPID are prone to laminitis and need very low-NSC diets below 10%. PPID is most common in older horses (typically 15+ years) and causes a characteristic long, curly coat that fails to shed normally (hypertrichosis), along with muscle wasting, excessive water intake and urination, and a pot-bellied appearance. Unlike EMS, which is primarily a dietary/lifestyle condition, PPID has an underlying neurological component and is often treated medically alongside dietary management. For feeding: same low-NSC approach as EMS — Triple Crown Lite, Nutrena SafeChoice Special Care, or similar low-NSC complete feeds.

Inflammation of the laminae — the interlocking tissue layers that bond the hoof wall to the coffin bone (pedal bone). The laminae are the structural support system of the hoof. When they become inflamed and damaged, the bond weakens and the coffin bone can rotate or sink within the hoof capsule — this is called "founder." Laminitis can be caused by high insulin (from EMS or PPID), hindgut acidosis from grain overload, systemic infection, or concussive trauma. The dietary component is most relevant for horses with EMS or PPID. For a horse with any laminitis history, the most important dietary rule is keeping total diet NSC below 10%, avoiding molasses and grain-forward feeds, and prioritizing fiber-based energy sources. "Laminitis" refers to the acute inflammatory episode; "founder" refers to the structural damage (rotation/sinking) that results from severe or repeated laminitis.

A genetic muscle disorder in which horses abnormally accumulate glycogen (stored sugar) in their skeletal muscle cells. There are two types: PSSM1 (caused by a specific mutation in the GYS1 gene that makes the glycogen synthase enzyme overactive) and PSSM2 (a group of related muscle disorders with different genetic causes). In both, excess muscle glycogen causes muscle cell damage during exercise. Clinical signs include reluctance to work, muscle stiffness, tying-up episodes, and in severe cases, muscle trembling and sweating at rest. The most important dietary management is a very low-starch, low-sugar diet — total NSC ideally below 10% — combined with adequate fat for energy. High-fat, low-NSC feeds like Triple Crown Low Starch, Buckeye Safe N Easy, or Nutrena SafeChoice Special Care paired with added fat (stabilized rice bran, vegetable oil) are the standard dietary approach. Regular, consistent exercise is equally important. PSSM is also sometimes written as EPSM (Equine Polysaccharide Storage Myopathy).

A muscle condition, distinct from PSSM, in which horses experience episodes of muscle damage (rhabdomyolysis) during or after exercise. Unlike PSSM, RER is not caused by abnormal glycogen storage — the mechanism involves disordered regulation of calcium within muscle cells during contraction. RER is most common in Thoroughbreds, Standardbreds, and Arabian horses, and episodes are often triggered by stress, excitement, inconsistent exercise, or high-grain feeding. Dietary management mirrors PSSM: low-starch, high-fat feeds reduce the risk of episodes. RER horses should not be fed high-grain, high-NSC diets. Historically called "azoturia" or "Monday morning disease" because episodes occurred when horses were rested over the weekend and then worked on Monday after eating full grain rations through the rest period. Also called "tying-up," though that term applies to both RER and PSSM.

A progressive neurodegenerative disease directly linked to prolonged Vitamin E deficiency, typically in horses without access to green pasture. The motor neurons (nerve cells that control muscle movement) degenerate, causing symmetrical muscle wasting, low head carriage, a characteristic stance where the horse shifts weight between hind feet, and abnormal sweating. EMND is most common in horses stabled without pasture for more than a year. Treatment and prevention center on Vitamin E supplementation — natural d-alpha-tocopherol at high doses (often 5,000–10,000 IU/day) under veterinary guidance. Supplementation can halt progression and improve signs if started early. For prevention: ensure any horse without daily green pasture access receives adequate Vitamin E from feed or supplements — at minimum 1,000–2,000 IU/day for an average-sized horse.

A disruption of the hindgut's normal pH environment, caused when undigested starch reaches the cecum and large colon and is rapidly fermented by bacteria. Normal hindgut fermentation produces volatile fatty acids (VFAs) slowly — this is healthy and provides the horse with energy. But starch ferments much faster than fiber, producing lactic acid that drops the hindgut pH. This acid environment kills off beneficial fiber-fermenting bacteria, potentially damages the gut lining, and releases bacterial toxins into the bloodstream. Consequences can include loose manure, colic, laminitis, and behavioral changes. Prevention: feed grain in multiple small meals, avoid large single grain feedings, prioritize fiber-based energy sources over grain, and introduce any grain gradually over 7–14 days.

Equine Gastric Ulcer Syndrome (EGUS) refers to ulceration of the stomach lining, most commonly in the squamous (upper, non-glandular) region of the stomach. The horse's stomach produces acid continuously, and without regular forage buffering that acid, the squamous mucosa is exposed to acid damage. High-grain diets, irregular feeding schedules, stress, and strenuous training all increase risk. Dietary management includes feeding forage frequently (ideally constant access to hay), adding alfalfa (which has a documented gastric buffering effect due to its calcium and protein content), and reducing meal size. Feeds with Outlast (Purina's gastric buffer), EquiShure, or built-in buffering agents support gastric health. Definitive diagnosis requires gastroscopy.

The sum of starch + water-soluble carbohydrates (WSC/sugar) — the total fast-digesting carbohydrate load of a feed. NSC is the single most important number for horses with EMS, PPID, laminitis history, or PSSM. It tells you how much carbohydrate will drive an insulin response. Target: below 10% for metabolic horses; below 20% is fine for healthy horses in work. NSC is often not listed on commercial feed labels — it must be calculated from starch + sugar values, or requested from the manufacturer. Note: NSC as calculated from label values (starch + WSC) differs slightly from laboratory-measured NSC, which includes ethanol-soluble carbohydrates. The label-calculated version is a useful estimate.

Simple sugars and fructans — the fraction of carbohydrates that dissolve in water and are rapidly absorbed or fermented. On commercial feed labels, WSC is typically labeled as "Sugar (max)." Fructans, found mainly in grass (not feeds), are a type of WSC that ferments in the hindgut rather than being absorbed. In feed context, WSC primarily refers to simple sugars including sucrose, glucose, and fructose. Target for metabolic horses: sugar (WSC) below 7%, ideally below 5%. High WSC is a primary concern in feeds containing molasses, which is typically 45–55% sugar.

A measurement of the less-digestible fiber fractions in a feed — primarily cellulose and lignin. ADF is determined by boiling the feed in an acid detergent solution and measuring what remains. The higher the ADF, the lower the digestible energy content — ADF is inversely related to feed quality and digestibility. For hay, ADF above 45% indicates mature, stemmy forage with low energy content. For commercial horse feeds, ADF values above 20% are typical of high-fiber, forage-replacement feeds. ADF is a required guarantee on AAFCO-compliant horse feed labels.

A measurement of the total structural fiber in a feed — cellulose, hemicellulose, and lignin together. NDF is always higher than ADF (NDF includes everything ADF measures, plus hemicellulose). NDF is used to estimate gut fill and passage rate. Higher NDF means more gut fill, slower passage, and greater time spent chewing — all generally positive for digestive health. High NDF in a commercial feed indicates significant forage character. NDF above 30–35% in a feed suggests it functions more like a hay supplement than a grain concentrate. NDF is a required guarantee on AAFCO-compliant horse feed labels.

The required panel on every commercial horse feed label that lists specific nutrient levels the manufacturer legally guarantees. In the United States, AAFCO requires horse feeds to guarantee at minimum: Crude Protein (minimum), Crude Fat (minimum), Crude Fiber (maximum), ADF (maximum), NDF (maximum), Calcium (minimum and maximum), Phosphorus (minimum), Copper (minimum, if added), Selenium (minimum), Zinc (minimum), and Vitamin A (minimum, if added). Some manufacturers also guarantee NSC, starch, sugar, specific amino acids (lysine, methionine, threonine), and other vitamins and minerals. Minimum guarantees mean the feed contains at least that much. Maximum guarantees mean it contains no more than that. The actual value may be higher (for minimums) or lower (for maximums) than stated.

Commercial horse feeds in the United States must list ingredients in descending order by weight before processing — the first ingredient is present in the largest amount, and each subsequent ingredient is present in a smaller amount than the one before it. This is the single most useful thing to read on a feed label for understanding what the feed actually is. If beet pulp and soybean hulls are #1 and #2, this is a fiber-based feed. If corn and oats are #1 and #2, this is a grain-based feed. Ingredient position tells you more than the product name or marketing claims on the front of the bag. Note: ingredients are listed by weight before processing — some ingredients (like beet pulp) expand significantly when water is added, which can affect their relative proportion in the finished product.

A highly concentrated feed formulated to provide protein (particularly essential amino acids), vitamins, and minerals in a small daily serving — typically 1–2 lbs per day for a 1,000-lb horse. Ration balancers are not calorie feeds — they provide negligible energy. Their purpose is to fill the nutritional gaps in a hay or pasture-based diet, which typically provides adequate calories but may be deficient in lysine, copper, zinc, selenium, and vitamins E and A. The high crude protein percentage (often 30–35%) looks alarming but is misleading — at 1–2 lbs/day, total protein delivery is modest. Examples: Purina Enrich Plus, Nutrena Empower Topline Balance, Triple Crown 30%, Tribute Essential K. Best suited for easy keepers who need nutritional fortification but cannot have extra calories.

A feed formulated to provide all required nutrients — including adequate fiber — without the need for hay or pasture. Complete feeds are typically high in fiber (ADF above 20%, NDF above 35%) and can be soaked to form a mash, making them ideal for senior horses with dental problems who cannot chew long-stem hay. They are fed in larger quantities (often 10–16 lbs/day) than typical concentrates. Examples: Purina Equine Senior, Nutrena SafeChoice Senior (can replace hay), Triple Crown Senior. Important: a complete feed being used to fully replace hay must be fed at the manufacturer's recommended hay-replacement rate — underfeeding a complete feed while skipping hay results in inadequate fiber intake.

A traditional grain mix of Corn, Oats, and Barley — sometimes with molasses added (Sweet COB). COB is a high-starch, high-energy supplement historically used to add calories to the diet of working horses. It is not a nutritionally complete feed — it provides energy from grain starch but is not balanced for vitamins, minerals, or amino acids. COB is appropriate for horses in hard work that need extra calories above what a complete feed provides, but is not suitable as a primary feed and is not appropriate for horses with metabolic conditions, EMS, PPID, laminitis history, or PSSM.

An estimate of total protein content based on nitrogen measurement — all nitrogen in a feed is multiplied by 6.25 to estimate protein content. Crude protein does not measure protein quality — a feed can have a high crude protein percentage from poor-quality protein sources that don't supply the essential amino acids horses actually need. The most important amino acid is lysine (the first-limiting amino acid for horses), followed by methionine and threonine. For mature horses at maintenance: 8–10% crude protein is adequate. Senior horses: 12–14%. Growing horses, lactating mares: 14–16%+. Performance horses: 12–16%.

A fat-soluble antioxidant essential for muscle function, neurological health, and immune function. Fresh green pasture is the richest natural source — horses evolved eating it constantly. When horses are on hay-based diets, Vitamin E intake drops dramatically because the vitamin degrades during hay drying and storage. Horses without pasture access frequently become Vitamin E deficient. Two forms appear on labels: d-alpha-tocopherol (natural, more bioavailable) and dl-alpha-tocopherol (synthetic, approximately 2–3× less bioavailable per IU). For a horse without pasture: minimum 1,000–2,000 IU/day. Horses with muscle conditions (EMND, PSSM): often 5,000+ IU/day of natural form. Expressed on labels as IU/lb.

An essential trace mineral and antioxidant with the narrowest safe range of any nutrient in horse feeding. Deficiency causes white muscle disease (nutritional myodegeneration) — pale, degenerated muscle tissue, especially in foals. Toxicity causes selenosis — loss of mane and tail hair, horizontal cracking and separation of the hoof wall, and in severe cases, death. Safe total daily intake: approximately 1–3 mg/day for a 1,000-lb horse. Upper limit: 2 mg/day from all sources combined. Organic selenium (selenium yeast, selenomethionine) is more bioavailable and builds larger tissue reserves than inorganic selenium (sodium selenite). Soil selenium varies dramatically by region — the Pacific Northwest and Northeast US are deficient; parts of Wyoming, South Dakota, and Nebraska can be toxic. Regional hay selenium levels are as important as feed selenium. Expressed on labels as ppm (= mg/kg).

An essential trace mineral required for connective tissue synthesis, pigment production (coat color), immune function, and antioxidant enzyme activity. Copper deficiency causes faded, washed-out coat color, poor hoof quality, weak connective tissue, and in growing horses, developmental orthopedic disease. The most common reason horses are copper-deficient despite adequate dietary copper is iron antagonism — excess iron (common in hay and well water in many regions) competitively blocks copper absorption in the gut. Organic copper forms (copper proteinate, copper amino acid chelate) resist iron interference better than copper sulfate. The copper-to-zinc ratio in the diet matters — recommended ratio is approximately 1:3–4 (copper:zinc). Expressed on labels as ppm or mg/kg.

An essential trace mineral involved in over 300 enzyme reactions, hoof horn synthesis, skin barrier function, immune cell development, and wound healing. Zinc deficiency causes poor hoof quality, skin lesions, reduced immune function, and impaired growth. Zinc and copper share absorption pathways and compete with each other — the ideal dietary ratio is approximately 3–4 parts zinc to 1 part copper (Zn:Cu ratio). Excess zinc suppresses copper absorption; excess copper suppresses zinc. Organic zinc forms (zinc proteinate, zinc amino acid chelate) have better bioavailability than zinc sulfate or zinc oxide. Expressed on labels as ppm or mg/kg.

A water-soluble B vitamin required for keratin synthesis — the structural protein that makes up hoof horn, hair, and skin. Research supports biotin supplementation at 15–20 mg/day for improving hoof horn quality in horses with poor feet. Most commercial feeds contain 0.1–0.5 mg/lb of biotin — far below the therapeutic threshold. At 4 lbs of feed/day, a horse receives approximately 0.4–2 mg of biotin from feed alone. For horses with active hoof quality concerns, a dedicated biotin supplement providing 15–20 mg/day is needed. Biotin affects only new hoof growth from the coronary band — results take 6–12 months to be visible at the ground surface. Biotin is safe at high doses — it is water-soluble and excreted.

A fat-soluble vitamin essential for vision, immune function, reproduction, and skin health. Horses can convert beta-carotene from fresh green grass into Vitamin A — horses at pasture generally meet their needs this way. When horses rely on hay and concentrate feeds, beta-carotene content in hay drops significantly during storage. Commercial feeds supplement Vitamin A directly. Unlike Vitamin E, Vitamin A can accumulate to toxic levels — excessive intake (typically from multiple fortified feeds or supplements combined) causes bone fragility, skin problems, and reproductive issues. Maximum tolerable level is approximately 16,000 IU/kg dry matter according to NRC (2007). Expressed on labels as IU/lb.

A fat-soluble vitamin essential for calcium absorption and bone mineralization. Horses synthesize Vitamin D from sunlight exposure — horses with regular outdoor access can meet most of their needs this way. Vitamin D is the most toxic of the fat-soluble vitamins when oversupplemented — excess causes soft tissue calcification (calcium deposits in blood vessels, kidneys, and lungs). Horses kept indoors or in limited sunlight need dietary Vitamin D, but stacking multiple fortified feeds and supplements can create excessive intake. The NRC (2007) maximum tolerable level is 44 IU/kg body weight daily. Expressed on labels as IU/lb.

The ratio of calcium to phosphorus in the total diet — one of the most important mineral relationships in horse nutrition. The recommended ratio is 1.5–2:1 (calcium to phosphorus). An inverted ratio (more phosphorus than calcium) is dangerous — excess phosphorus blocks calcium absorption in the gut, leading to calcium deficiency and bone problems (nutritional secondary hyperparathyroidism). Grains are typically high in phosphorus relative to calcium; alfalfa and limestone are high in calcium. Commercial feeds use limestone and dicalcium phosphate to correct the Ca:P ratio of their grain ingredients. The total dietary Ca:P ratio (feed + hay together) is what matters most — a feed with a good ratio can be thrown off by the wrong hay, and vice versa.

An essential trace mineral required for bone and cartilage formation, antioxidant enzyme activity (manganese superoxide dismutase), and carbohydrate metabolism. Manganese deficiency in horses is uncommon but can impair skeletal development in young horses. Typical requirement: approximately 400 ppm in the total diet. Appears on feed labels as manganese sulfate (inorganic, lower bioavailability), manganese proteinate or manganese amino acid complex (organic, higher bioavailability), or manganese oxide (inorganic, lowest bioavailability — rarely used in quality feeds). Expressed on labels as ppm or mg/kg.

An essential mineral for oxygen transport (hemoglobin) and energy metabolism. Unlike most trace minerals, iron deficiency in horses is extremely rare — most hay, water, and soil in North America provide more than adequate iron. The more relevant concern with iron in horse nutrition is excess iron, which is common. High iron intake — from iron-rich hay, iron-heavy water sources, or oversupplemented feeds — competitively blocks the absorption of copper, zinc, and manganese in the gut. Many horses on typical hay-based diets are inadvertently iron-overloaded, which can present as coat color fading, poor hoof quality, and immune dysfunction despite adequate copper and zinc on paper. Well water in many regions is very high in iron — have yours tested if coat and hoof quality are concerns despite a fortified feed. Commercial feeds rarely need to add supplemental iron but may include small amounts for label completeness.

A macro mineral (required in gram quantities daily) essential for over 300 enzymatic reactions, nerve and muscle function, and bone structure. Approximately 60% of the horse's body magnesium is stored in bone. Magnesium is involved in regulating nerve impulse transmission — horses with magnesium deficiency can appear nervous, hypersensitive to sound or touch, or develop muscle tremors. "Calming" supplements frequently include magnesium as the active ingredient for this reason. Most hay-based diets provide adequate magnesium, but deficiency is possible with certain forage types. Appears on feed labels as magnesium oxide (most common form in feeds), magnesium sulfate, or magnesium proteinate. Expressed in the guaranteed analysis as % (minimum).

The primary intracellular electrolyte in mammals — essential for fluid balance, nerve transmission, and muscle contraction. Horses lose significant potassium in sweat during exercise. Most forages are high in potassium, making deficiency uncommon in horses with adequate hay or pasture. Horses in hard work or heavy sweat may need additional electrolyte supplementation. Commercial horse feeds typically contain potassium from natural ingredients (alfalfa, soybean products) plus potassium chloride or potassium sulfate as supplements. Expressed on labels as % (minimum).

An essential trace mineral required for thyroid hormone synthesis, which regulates metabolism, growth, and energy utilization. Both deficiency and toxicity produce similar signs — goiter (enlarged thyroid), reproductive problems in mares, and weak foals. Commercial horse feeds use calcium iodate or potassium iodide as controlled iodine sources. The narrow safe range makes iodine one of the trace minerals where "more is not better" — avoid stacking iodine from multiple sources (feeds, kelp supplements, iodine-based hoof treatments absorbed through the skin). Expressed on labels as ppm.

An essential trace mineral whose primary function in horses is serving as the core atom of Vitamin B12 (cobalamin). Hindgut microbes in the horse synthesize B12 from dietary cobalt — making adequate cobalt intake important for the horse's internal B12 production. The NRC requirement is very small (approximately 0.05 mg/kg dry matter). Appears on feed labels as cobalt carbonate, cobalt sulfate, or cobalt proteinate. Deficiency is rare but can cause reduced appetite and poor growth. Expressed on labels as ppm.

Calcium is the most abundant mineral in the horse's body — approximately 99% stored in bones and teeth, with the remaining 1% essential for muscle contraction, nerve transmission, and blood clotting. Commercial feeds use several calcium sources: Limestone / Calcium Carbonate — inexpensive, pure calcium source with no phosphorus; used to raise calcium without affecting phosphorus balance. Dicalcium Phosphate (DCP) — provides both calcium and phosphorus; used to balance both minerals simultaneously. Monocalcium Phosphate (MCP) — higher phosphorus content than DCP; used when more phosphorus is needed relative to calcium. The key number on labels is the calcium guaranteed analysis (min and max %) and the Ca:P ratio — target 1.5–2:1 calcium to phosphorus across the total diet.

The second most abundant mineral in the body after calcium — approximately 80% in bone, with the remainder critical for energy metabolism (ATP production), cell membrane structure, and DNA synthesis. Grains are naturally high in phosphorus but low in calcium; forages vary. Deficiency impairs bone development; excess relative to calcium causes calcium to be mobilized from bone (nutritional secondary hyperparathyroidism). Most commercial horse feeds carefully balance phosphorus with calcium through dicalcium phosphate, monocalcium phosphate, or defluorinated phosphate. Defluorinated phosphate is rock phosphate that has been treated to remove toxic fluoride — used in feeds when a high-phosphorus source is needed without adding fluoride.

The primary extracellular electrolyte — essential for fluid balance, blood pressure regulation, nerve impulse transmission, and nutrient absorption in the gut. Horses lose substantial sodium in sweat. Sodium chloride (salt) is the primary sodium source in horse feeds and the primary ingredient in loose salt and salt blocks. Free-choice loose salt access is recommended for all horses — individual sodium needs vary greatly with workload and sweat rate, and horses regulate intake well when given the option. Sodium bicarbonate is an alkalizing agent occasionally added to feeds for horses with hindgut acidosis risk or gastric ulcer concerns — it buffers stomach and hindgut pH. Sodium sesquicarbonate is a related buffering agent. Expressed on labels as % salt (min and max).

Selenium appears in two fundamentally different forms in horse feeds, with different bioavailability and tissue storage characteristics: Sodium selenite (inorganic) — the historically standard selenium source in commercial feeds. Absorbed and used for immediate antioxidant enzyme function but does not build substantial tissue reserves. Less bioavailable than organic forms. Selenium yeast / Selenomethionine (organic) — selenium incorporated into the amino acid methionine by Saccharomyces cerevisiae yeast during fermentation. This form is absorbed and stored in muscle tissue bound to protein, building meaningful selenium reserves. More bioavailable than sodium selenite, and research shows better tissue selenium status with organic forms. Many higher-quality feeds now use a combination of both forms to provide both immediate availability (selenite) and tissue reserve building (selenium yeast). When evaluating a feed's selenium, note which form is listed — a feed with only sodium selenite and a horse without pasture access may not build adequate selenium reserves over time.

Zinc and copper appear in both inorganic and organic forms on feed labels, with significant differences in bioavailability: Inorganic forms — zinc sulfate, zinc oxide, copper sulfate. Less expensive; lower bioavailability; more susceptible to competitive interference from iron and other minerals. Zinc oxide is particularly poorly absorbed. Organic forms (chelates/proteinates) — zinc proteinate, copper proteinate, zinc amino acid complex, copper amino acid complex, zinc methionine complex, copper lysine complex. The mineral is bonded to an amino acid or protein, which improves gut absorption and reduces interference from competing minerals (especially iron). Higher cost but meaningfully better utilization — important for horses with iron-rich diets or known hoof/coat issues. A feed listing zinc proteinate and copper proteinate alongside zinc sulfate and copper sulfate is providing a blend of fast-acting inorganic and better-retained organic forms — common in premium horse feeds.

A water-soluble B vitamin essential for carbohydrate metabolism — specifically the conversion of glucose into usable energy via the citric acid cycle. Thiamine is also critical for nerve function. Horses synthesize some thiamine via hindgut microbial fermentation, but additional dietary thiamine is beneficial for horses in hard work or under stress. Appears on feed labels as thiamine mononitrate or thiamine hydrochloride. Deficiency causes neurological signs — ataxia, muscle weakness, and in severe cases, cardiovascular problems. Bracken fern contains thiaminase, an enzyme that destroys thiamine — horses grazing bracken-infested pastures risk thiamine deficiency. Safe at high doses (water-soluble, excreted). Not typically expressed in the guaranteed analysis.

A water-soluble B vitamin that serves as a precursor to FAD and FMN — coenzymes essential for energy production from carbohydrates, proteins, and fats. Riboflavin is also required for antioxidant enzyme recycling (regenerates glutathione). Hindgut microbes synthesize riboflavin — horses relying on a healthy hindgut microbiome generally meet their needs. Supplemented in commercial feeds as riboflavin. Deficiency signs include poor growth and eye inflammation. Water-soluble and excreted; not toxic at high doses. Appears on labels as riboflavin or riboflavin supplement.

A water-soluble B vitamin that forms NAD and NADP — the central coenzymes in energy metabolism across all macronutrients (carbohydrates, fats, proteins). Every cell in the horse's body requires niacin-derived coenzymes to extract energy from feed. Horses can synthesize niacin from the amino acid tryptophan (as can all mammals), and hindgut microbes also produce niacin. Supplemented in commercial feeds as niacin or niacin supplement. Deficiency is uncommon in horses with adequate protein intake. Appears on labels as niacin or niacin supplement; not typically in the guaranteed analysis.

A water-soluble B vitamin that is a component of Coenzyme A (CoA) — the central molecule for fat metabolism, acetylation reactions, and the citric acid cycle. Present in virtually all food sources. Appears on feed labels as calcium pantothenate (the stable supplemental form). Deficiency is extremely rare in horses on any reasonable diet. Included in commercial feed vitamin packages as part of a complete B-vitamin profile.

A water-soluble B vitamin essential for amino acid metabolism — transamination, deamination, and decarboxylation reactions that interconvert amino acids and produce neurotransmitters. B6 is required for the metabolism of tryptophan to niacin. Appears on feed labels as pyridoxine hydrochloride. Hindgut microbes synthesize B6; dietary supplementation is still included in commercial feeds as part of the complete B-vitamin package. Deficiency is rare in horses.

A water-soluble B vitamin essential for DNA synthesis and cell division — particularly important for rapidly dividing cells (red blood cells, immune cells, growing tissue). Deficiency causes anemia and impaired immune function. Synthesized by hindgut microbes and present in fresh forage. Particularly important for pregnant mares (early embryonic development) and growing foals. Appears on feed labels as folic acid or folic acid supplement.

The only vitamin containing a metal ion (cobalt) at its core. B12 is essential for myelin sheath synthesis (the insulating layer around nerve fibers), DNA synthesis, and red blood cell formation. Unlike most B vitamins, B12 is not found in plant foods — in horses, it is synthesized exclusively by hindgut microbes from dietary cobalt. A horse with a healthy hindgut and adequate dietary cobalt will produce sufficient B12. Disruption of hindgut microbiome (antibiotics, illness, extreme dietary change) can temporarily reduce B12 synthesis. Appears on feed labels as vitamin B12 supplement or cobalamin. Expressed in micrograms (mcg) when guaranteed.

A vitamin-like nutrient (not technically a B vitamin despite being grouped with them) essential for phospholipid synthesis (cell membrane structure), fat transport from the liver, and production of the neurotransmitter acetylcholine. Choline deficiency causes fat accumulation in the liver. Horses synthesize some choline, but commercial feeds supplement additional choline, particularly in high-fat diets where fat transport demands are increased. Appears on feed labels as choline chloride. Often listed near the end of the ingredient list — required in relatively small amounts.

A fat-soluble vitamin essential for blood clotting and bone protein synthesis. There are two naturally occurring forms: K1 (phylloquinone, found in green plants — alfalfa is a rich source) and K2 (menaquinone, synthesized by gut bacteria). Deficiency in healthy horses is very uncommon because hindgut microbes produce substantial K2 and alfalfa provides K1. The main risk is sweet clover silage contaminated with dicoumarol — a naturally occurring anticoagulant that blocks Vitamin K activity, causing internal bleeding. Appears on feed labels as menadione sodium bisulfite complex (synthetic K3, water-soluble supplemental form) — occasionally listed as "a source of Vitamin K activity." Not typically expressed in the guaranteed analysis.

An antioxidant vitamin required for collagen synthesis, immune function, and iron absorption. Unlike humans, horses synthesize their own Vitamin C from glucose in the liver — healthy adult horses at maintenance do not require dietary Vitamin C supplementation. However, synthesis capacity may be reduced in horses under high oxidative stress (intense training, illness, old age), making supplementation potentially beneficial in those cases. Appears on feed labels as ascorbyl-2-polyphosphate (a stable, fat-soluble form of Vitamin C added to some performance feeds) or ascorbic acid. Buckeye and some Nutrena feeds include ascorbyl-2-polyphosphate as an antioxidant. Water-soluble and non-toxic at normal doses.

A trace mineral that enhances insulin sensitivity — chromium potentiates the action of insulin at the cellular level, helping cells respond to insulin more effectively. In horses, chromium supplementation has been studied for reducing the cortisol response to stress and improving glucose metabolism in horses with EMS or insulin dysregulation. Appears in some horse feeds as chromium propionate (the form approved by FDA for use in horse feeds). Required in very small amounts (ppb level). Not yet required to be listed in the guaranteed analysis. An emerging ingredient in metabolic-support feeds — its inclusion signals the manufacturer's intent to support insulin sensitivity.

A mold inhibitor and preservative used in horse feeds and feedstuffs to prevent fungal growth and extend shelf life. Calcium propionate is the calcium salt of propionic acid — a naturally occurring short-chain fatty acid also produced in the hindgut during fiber fermentation. It is safe for horses at the levels used in feed preservation. Commonly seen in textured feeds, beet pulp products, and mixed feeds with higher moisture content. Appears on labels as calcium propionate or propionic acid (preservative). Its presence on a label indicates the manufacturer is actively managing mold risk — a quality control indicator.

A naturally occurring clay mineral used as a mycotoxin binder — it binds to certain mycotoxins (mold toxins) in the gut and prevents their absorption. Common in feeds from regions where grain crops are prone to mycotoxin contamination (aflatoxin, fumonisin). HSCAS has a strong affinity for aflatoxin in particular. It does not bind all mycotoxins equally — it is less effective against trichothecenes and zearalenone. Also used as an anti-caking agent to prevent ingredient clumping. Its presence on a feed label indicates the manufacturer is aware of and managing mycotoxin risk in their ingredients — relevant in feeds using corn, which is susceptible to aflatoxin and fumonisin contamination.

The first-limiting amino acid for horses — the amino acid most commonly deficient in typical hay-based diets relative to the horse's needs. "First-limiting" means that when lysine is in short supply, the horse cannot build protein efficiently regardless of how much total crude protein is available. Lysine is required for muscle protein synthesis, connective tissue, and immune function. Soybean meal is the richest common source of lysine in horse feeds. When a feed guarantees lysine specifically in the analysis panel, it's a sign the manufacturer is focused on protein quality, not just quantity. Growing horses, senior horses, and performance horses benefit most from adequate lysine. Expressed on labels as % (minimum).

The second-limiting amino acid for horses and a sulfur-containing amino acid essential for keratin synthesis (hoof horn and coat), connective tissue, and many metabolic processes. DL-methionine is the supplemental form commonly added to horse feeds. Methionine works alongside biotin and zinc for hoof quality — deficiency of any one limits the effectiveness of the others. Expressed on labels as % (minimum) when guaranteed.

The third-limiting amino acid for horses. Required for protein synthesis, immune function, and gut mucus production. When a feed guarantees lysine, methionine, and threonine in the analysis panel, it indicates a high-quality protein formulation focused on essential amino acid balance rather than just crude protein quantity. Expressed on labels as % (minimum) when guaranteed.

The cecum and large colon — the fermentation chamber that makes up the majority of the horse's digestive system by volume. Unlike simple-stomached animals (humans, pigs) or ruminants (cattle), horses are hindgut fermenters: a dense and diverse community of bacteria, protozoa, and fungi in the cecum and large colon break down plant fiber into volatile fatty acids (VFAs), which the horse absorbs and uses for energy. The hindgut microbial community is sensitive — disruptions from dietary changes, antibiotic treatment, stress, or grain overload can destabilize it and cause colic, diarrhea, or laminitis. Fiber-based feeds support a healthy hindgut; grain-heavy feeds that overflow into the hindgut disrupt it.

The primary energy products of hindgut fermentation — acetate, propionate, and butyrate. When hindgut bacteria ferment fiber (cellulose, hemicellulose, pectin), they produce these short-chain fatty acids, which are absorbed through the gut wall and used by the horse as an energy source. VFAs are produced slowly and steadily during fiber fermentation, providing sustained energy without blood glucose spikes. This is why fiber-based energy (beet pulp, soybean hulls, hay) is described as "slow-burning" compared to grain starch, which digests in the small intestine and enters the bloodstream as glucose rapidly.

Live microorganisms that, when consumed in adequate amounts, provide a health benefit — typically by supporting the hindgut microbial community. Common equine probiotic bacteria include Lactobacillus acidophilus, Enterococcus faecium, Pediococcus acidilactici, and Bifidobacterium species. The key challenge with probiotics in commercial feeds is viability — live bacteria are sensitive to the heat and pressure of pelleting and extrusion, which can kill significant portions of the live organisms before they reach the horse. Enterococcus faecium is more heat-tolerant than Lactobacillus species. Some manufacturers add probiotics post-pelleting (after the pellet cools) to preserve viability — ask the manufacturer if this is important to you. Efficacy is expressed in CFU (colony-forming units) per gram or per lb.

A non-digestible feed ingredient that selectively feeds and supports beneficial bacteria in the gut — as opposed to probiotics, which are the bacteria themselves. Common equine prebiotics include mannan oligosaccharides (MOS), fructooligosaccharides (FOS), and inulin. MOS from yeast cell walls can bind to certain pathogenic bacteria and prevent them from attaching to the gut wall. FOS and inulin selectively feed beneficial Lactobacillus and Bifidobacterium species. Prebiotics survive feed manufacturing heat better than live probiotic bacteria, making them a reliable ingredient in pelleted feeds.

A fermentation product produced by growing Saccharomyces cerevisiae yeast on a substrate, then drying the entire fermentation mass — including the yeast cells, growth medium, and metabolic byproducts. Yeast culture is not live yeast — it is a dried fermentation product that acts as a prebiotic, supporting existing hindgut bacteria rather than introducing new ones. Research in horses has shown yeast culture improves fiber digestibility, stabilizes hindgut pH, and supports microbial diversity. Because it is a fermentation product (not live organisms), it survives pelleting heat better than probiotic bacteria. One of the most consistently effective and well-researched digestive support ingredients in commercial horse feeds.

A mineral that has been chemically bonded to organic molecules (typically amino acids or proteins) to improve its bioavailability — the proportion that is absorbed and utilized by the horse. Common chelated forms include amino acid chelates (e.g., zinc amino acid chelate, copper amino acid chelate) and proteinates (e.g., zinc proteinate, manganese proteinate). Chelated minerals are generally more resistant to competition from other minerals during absorption — for example, chelated copper is less susceptible to the iron antagonism that blocks copper sulfate absorption. The downside is cost — chelated minerals are more expensive than inorganic forms like zinc sulfate or copper sulfate. Feeds listing chelated minerals are typically higher-quality, more thoughtfully formulated products.

The degree to which a feed raises blood glucose levels after consumption. High-glycemic feeds (grains high in starch) cause rapid, significant blood glucose spikes followed by a large insulin response. Low-glycemic feeds (fiber-based feeds, fat sources) produce gradual, modest glucose increases and smaller insulin responses. Glycemic response is the core concern in feeding EMS, PPID, and laminitis-prone horses — the goal is to minimize insulin spikes. Fiber-based energy (beet pulp, soybean hulls) and fat (rice bran, vegetable oil) are low-glycemic; grain starch (corn, oats, barley) is high-glycemic. NSC is the best single proxy for glycemic impact on a feed label.

A byproduct of ethanol production — the non-fermentable residue left after corn (or other grains) is fermented to produce fuel ethanol. During fermentation, the starch is consumed, leaving behind a protein- and fiber-rich material that is dried and used as animal feed. DDGS is a moderate-protein (approximately 27%), moderate-fat (approximately 10%) ingredient with meaningful fiber content. The starch has already been fermented out, so DDGS has a lower glycemic impact than whole corn despite being a corn byproduct. Used in horse feeds as a cost-effective protein and energy ingredient. On labels: distillers dried grains, corn distillers dried grains, distillers dried grains with solubles.

A milling byproduct from flour production — the combination of bran, germ, and flour that is separated from the white flour during wheat processing. Despite containing the word "wheat" (a starch grain), wheat middlings are primarily a fiber ingredient in horse feeds, not a starch source. They are approximately 15% crude protein, 4% fat, and 9% crude fiber, with a relatively modest starch content. Wheat middlings appear near the top of many commercial horse feed ingredient lists because they are a cost-effective, palatable binder and fiber source. They should be classified as a fiber-energy ingredient, not a grain-starch ingredient, despite their name.

A compound found in plant seeds (especially rice bran, wheat bran, and soybean products) that can bind minerals including phosphorus, zinc, iron, and calcium, reducing their bioavailability. Phytate is particularly relevant when feeding large amounts of rice bran — rice bran is high in both phytate and phosphorus, and the phytate can further reduce mineral absorption. Processing (heat treatment, fermentation) can partially deactivate phytate. For horses receiving large amounts of rice bran, the phytate effect should be considered when evaluating total mineral balance.

Heat-treated to deactivate the enzyme lipase, which causes fat oxidation and rancidity. Raw rice bran spoils within hours to days of milling because lipase rapidly degrades the fat. Stabilization — typically through extrusion or dry heat — deactivates lipase and extends shelf life to months. Stabilized rice bran retains its full fat content and nutritional value. Always use stabilized rice bran for horses — unstabilized product goes rancid quickly and has reduced nutritional value and palatability. The same concept applies to stabilized flaxseed/linseed.

Lignin is the structural polymer that gives wood its rigidity — it is completely indigestible by horses (or any mammal) and cannot be fermented by hindgut microbes. Lignin in hay and feeds reduces digestibility because it physically surrounds and protects more digestible fiber fractions (cellulose, hemicellulose) from microbial attack. ADF includes lignin; NDF includes it as well. High lignin = low feed quality and digestibility. Lignosulfonate (a processed form of lignin) is used in horse feed manufacture as a binder to hold pellets together — it is listed on the ingredient label and is safe, but it is not a nutritive ingredient.

Ground ingredients compressed through a die under heat and pressure to form uniform cylindrical pellets. Pelleting produces a consistent product that cannot be sorted — the horse must consume all ingredients as formulated. Pelleting heat (typically 70–90°C) can reduce some vitamin potency and kill live probiotic bacteria. Most commercial horse feeds are pelleted. Advantages: consistency, reduced dust, prevention of ingredient sorting, and good palatability. A horse on pellets consumes them faster than textured feed, producing less chewing time and saliva — frequent small meals are important.

A feed containing whole or processed grains (oats, corn, barley) mixed with molasses and other ingredients, without full compression into pellets. Textured feeds are typically highly palatable. The major disadvantage is that horses can sort ingredients — picking out preferred grains and leaving behind minerals, vitamins, or less-palatable components. If a horse is a chronic sorter on a textured feed, switching to pellets ensures complete nutrition. Textured feeds almost always contain molasses for palatability and binding, making them generally unsuitable for metabolic horses.

Processed through an extruder — a machine that forces the feed mixture through a die under very high heat (110–140°C) and pressure, then rapidly expands as pressure drops. Extrusion gelatinizes starch (breaks down the starch granule structure), significantly improving its digestibility in the small intestine and reducing the amount that reaches the hindgut undigested. Extruded feeds have the highest starch digestibility of any feed form — for horses prone to hindgut acidosis, this is beneficial. However, extrusion temperatures kill probiotic bacteria. Extruded feeds are typically lighter and more voluminous than pellets. Examples: Buckeye Cadence Ultra, some Seminole Wellness products.

Parts per million — the standard unit for trace minerals on horse feed labels. 1 ppm = 1 mg/kg = 1 mg per kilogram of feed. To convert a ppm value to daily mg intake: multiply the ppm by the daily feeding rate in kilograms (lbs ÷ 2.2 = kg). Example: a feed with 0.3 ppm selenium fed at 3 kg/day provides 0.9 mg selenium per day (0.3 × 3 = 0.9 mg). Used for selenium, copper, zinc, manganese, and iron on feed labels.

International Units — a unit of biological activity for vitamins, particularly fat-soluble vitamins (A, D, E). IU values are not directly comparable between different vitamins because each is based on the specific biological activity of that vitamin in a standardized assay. On horse feed labels, Vitamin E is expressed as IU/lb, Vitamin A as IU/lb, and Vitamin D as IU/lb. To calculate total daily IU from a feed: IU/lb × daily feeding rate in lbs = total IU/day. Important: for Vitamin E, natural d-alpha-tocopherol provides more biological activity per labeled IU than synthetic dl-alpha-tocopherol — the same IU number does not mean the same tissue effect.

Colony-forming units — the standard measure of viable (live) bacteria in a probiotic product. One CFU = one living bacterial cell capable of dividing and forming a colony. CFU counts on feed labels represent the minimum guaranteed at time of manufacture — actual counts at time of use may be lower due to die-off during storage. Effective probiotic doses in horses are generally considered to be in the billions (10⁹) of CFU per day, though the research base for specific equine probiotic doses is still developing.

The National Research Council — a U.S. scientific body that publishes consensus reports on nutrient requirements for livestock species. The NRC's Nutrient Requirements of Horses (6th revised edition, 2007) is the primary scientific reference for equine nutrition requirements in North America. When feed labels, research papers, or this tool reference "NRC requirements" or "NRC recommendations," they are referring to this publication. The 2007 edition established dietary requirements and safe upper limits for all major nutrients in horses across life stages and activity levels. Note: the 2007 edition is now nearly 20 years old — some recommendations have been refined by subsequent research, but it remains the industry standard reference.