Turning horses out on pasture is one of the best ways to encourage natural grazing behaviour. Fresh forage can be a valuable part of a balanced equine diet, but grass does not have the same nutritional value throughout the year.

In the spring, growing grasses can accumulate high amounts of hydrolyzable carbohydrates (HC) that might be unsafe for certain horses to consume. If your horse has limited pasture access during the winter, a sudden change in diet when spring arrives can increase the risk of digestive health problems and laminitis.

Proper pasture management, including spring grazing restrictions, can help limit these risks. Some particularly sensitive horses may need to be housed in dry lots and only fed low-HC hay.

This article will discuss the risks associated with horses grazing lush grass in the spring and the precautions to take when transitioning horses to spring pastures.

Spring Grass Safety for Horses

It can be tempting to turn horses out to pasture at the first signs of spring after a long winter.

Especially if your horse primarily eats hay during the winter, warmer temperatures and green fields usually mean easier management and a lower feeding budget.

However, it is important to introduce spring grazing slowly, both for the health of the horse and the health of the pasture.

Grass growth occurs rapidly in the spring when pastures become lush and green. Horses find these young grasses extremely palatable due to low fibre content and high higher non-structural carbohydrate (NSC) content. Horses let loose on fresh pasture can quickly consume large amounts.

But these same characteristics can impact gut function in horses. Shifts in microbial populations have been observed within 4 days of transitioning from ensiled forage to fresh pasture, and within 14 days when transitioning from hay to pasture. [6][18]

Changes in microbial populations are also implicated in hindgut acidosis, colic and laminitis. [22] However, it would be highly unusual for pasture to induce sufficient hindgut acidosis to cause laminitis. A more common scenario is soft green manure.

Pasture grasses also need enough time to grow and establish their root structure before allowing horses to graze, or they will die off and be replaced by unwanted weeds. [23]

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Carbohydrates in Grass

Plants use photosynthesis to convert light energy into chemical energy in the form of carbohydrates. This process is essential for plant growth, development and survival.

In grasses, this process occurs within the chloroplasts, which are specialized organelles found within cells of the leaf.

Sugars, fructans, and starches are all types of non-structural carbohydrates (NSC) produced and stored by growing plants. [2]

Hydrolyzable carbohydrates are the components of NSC that are digestible in the small intestine, namely starch and simple sugars. These contribute to insulin spikes in the animal.

Grasses are classified as either cool-season or warm-season grasses based on their growing cycle and how they metabolize these carbohydrates. [1]

Cool-season grasses, which make up most early spring pastures, store carbohydrates as fructans. Warm-season grasses rely on starch as the primary form of carbohydrate storage. [2]

Fructans can be stored outside chloroplasts, but starch storage is limited to within the chloroplast. As a result, pastures comprised of cool-season grasses can store significantly more reserve carbohydrates. [2]

High NSC Levels

Spring grasses generate large amounts of sugar from both photosynthesis and breakdown of storage carbohydrates to support growth. [1] This results in higher NSC levels in fresh spring forages compared to more mature forages.

Fructans are stored in the lower portions of the plant while sugar and starch are abundant in the portions the horse eats.

When horses consume the hydrolyzable portion of non-structural carbohydrates, digestive enzymes convert them into simple sugars such as glucose. These types of sugars are easily digested by horses, so they cause rapid increases in blood sugar unlike the complex carbohydrates such as fibre and fructans which are not digestible.

High blood glucose levels cause the body to release the hormone insulin. This hormone facilitates the uptake of glucose from the bloodstream into other tissues. [3]

Over time, diets high in hydrolyzable non-structural carbohydrates can contribute to insulin resistance in which cells become less responsive to the effects of this hormone. This results in more insulin being released to keep blood glucose normal. [4]

Abrupt Dietary Changes

Compared to grass hays that horses consume over the winter, spring pasture has a very different nutritional profile, with higher moisture content, higher protein, lower fibre content and higher NSCs. [5]

Transitioning from hay to pasture too quickly can impact your horse’s gut health and increase the risk of colic. [6]

The horse is a hindgut fermenter that relies on microbes in the gastrointestinal tract to break down fibre-rich forages and process them into usable energy.

The microbial population in the hindgut adapts to digest certain types of feed and hay. When changes are made too quickly, some populations of microbes die off while others proliferate rapidly. [6]

This can result in digestive dysfunction, diarrhea or loose stools, malabsorption of nutrients, or acidosis, in which the hindgut pH decreases.

If the intestinal barrier function is compromised, toxins may also be absorbed into the body. If this happens, the horse develops a systemic inflammatory response with fever and is obviously ill.

Consuming large amounts of NSCs in a single meal can also overwhelm the digestive tract. Undigested sugars and starches spill over from the foregut into the hindgut, disrupting fibre fermentation and the microbiome. [5] The usual consequence is some bloating and the familiar green soft manure of horses on spring pastures.

Palatability

Research shows that horses are selective grazers that use their sense of smell to seek out certain desirable plants. Horses find feedstuffs with high sugar content highly palatable and consume more when grazing on rich spring grasses. [20]

One study evaluating pasture dry matter intake during different seasons found that horses had significantly higher intake rates during the spring when NSC levels were elevated. [20]

Unless carefully managed, horses with unrestricted pasture access may consume excessive amounts of spring grass, leading to weight gain and metabolic issues in susceptible breeds.

Spring Grazing Health Risks

Excessive consumption of spring grass can increase the risk of several diseases in horses. [7]

Colic

Turnout on pasture is normally associated with a decreased risk of colic. But grazing on spring grass can increase the risk of colic due to microbial disturbances caused by a sudden dietary change. [8]

When large amounts of NSCs reach the hindgut, microbes that ferment starch and sugars begin to proliferate. These sugar-fermenting microbes produce large amounts of gas. [5]

This excess gas production increases the risk of gas colic in horses eating spring grass. [8]

Hindgut Acidosis

Hindgut acidosis occurs when the horse’s hindgut becomes too acidic. Excessive dietary starch is the leading cause of this condition. [9]

Starch fermentation in the hindgut increases lactic acid production, which lowers intestinal pH. The acidic environment can cause beneficial bacteria to die and release endotoxins. [9] However, this is an extreme condition only seen with feed room break-in scenarios or experimental overdoses.

Pasture Laminitis

Laminitis is a painful condition affecting the hoof laminae, the soft tissue that attaches the coffin to the hoof wall.

One study linked 46% of reported laminitis cases to excessive consumption of NSCs from lush pasture. [10]

Pasture laminitis involves hyperinsulinemia and the risk of laminitis is limited to horses predisposed to exaggerated insulin responses or with known metabolic conditions. [4][25]

The exact manner by which high insulin induces laminitis is unclear but it is distinctly different from other forms of laminitis. [26] We do know that high insulin results in increased levels of the potent vasoconstrictor endothelin-1 which contributes to reduced blood supply to the laminae. [27]

If the coffin bone and hoof wall separate due to laminitis, the coffin bone can rotate and become displaced. [11] At this stage, the condition has advanced to founder.

Obesity

Horses grazing spring pastures can quickly exceed their digestible energy (DE) requirements and gain excess body condition. Pasture grasses typically provide between 1.78 to 2.74 mcal/kg (megacalories per kilogram).

Horses turned out on pasture have a voluntary dry matter intake (VDMI) between 1.5 – 3.1% of their bodyweight per day. For a 500 kg (1100 lb) horse, this is equivalent to 7.5 – 15.5 kg (16.5 – 34 lb) of dry matter. [24]

A typical mature horse not in work needs to consume 16.65 Mcal per day to maintain their body weight. However, a horse could easily consume up to 30 mcal per day or more if turned out on rich spring grasses all day long.

Some horses have a higher risk of gaining excess weight when grazing on lush pasture. Certain easy keepers breeds are genetically predisposed to store more fat and require careful management of pasture turnout. [12]

Metabolic Issues

Obese horses also might have an increased risk of adverse effects on their metabolic health when transitioning to spring grass. The excess fat in obese horses, especially fat in the liver, hypothetically interferes with normal insulin signalling. [13]

Horses with insulin resistance or metabolic syndrome are particularly susceptible to HC-related diseases. The risk of developing insulin resistance increases with age. [12]

Equine metabolic syndrome refers to a collection of clinical signs related to hyperinsulinemia. Cushing’s disease, or pituitary pars intermedia dysfunction (PPID), is a common endocrine disorder which can also cause hyperinsulinemia and signs of metabolic syndrome such as regional fat deposits. [13]

The risk of developing insulin resistance increases with age but may manifest at any time after the horse has stopped growing. 

Consult your veterinarian to determine if turnout on spring pasture is safe for your horse before beginning the transition to spring grazing.

How to Safely Transition Horses to Spring Grazing

Slowly introduce spring grazing to help limit pasture-related health risks for your horse.

Once your veterinarian has confirmed it is safe for your horse to be turned out on spring pasture, wait for adequate grass growth and follow a strategic turnout schedule during the transition.

Wait until the grass reaches 6 inches and start with 15 minutes of grazing on the first day. Increase by 15-minute increments until the horse is grazing for 4-5 hours before allowing unrestricted grazing. [17]

Transitioning Gradually

Most horse owners understand that all changes to the equine diet should be gradual. Your horse’s gut microbiota adapts to what your horse eats. Sudden dietary changes can disrupt microbial populations, which increases the risk of colic and gastrointestinal issues. [5]

Horses that spend the winter on pasture naturally adjust to spring grass as it grows. However, continuous turnout throughout the year on the same field can damage pasture health and lead to overgrazing.

Horses that rely on hay as their primary source of forage should transition gradually to fresh grass during the spring to limit the risk of digestive upset. Horses may also require hay supplementation during the spring to provide additional fibre lacking in spring grass. [7]

Increasing Grazing Duration

Limit grazing to less than one hour daily during your horse’s first week on spring grass. Gradually increase the duration by fifteen minutes daily for the next two weeks to reduce the risk of gut problems.

Diarrhea and other signs of digestive upset can indicate that grazing access is increasing too quickly. [5]

Consult your veterinarian to determine if your horse can safely graze longer than 4 hours per day during the spring. Only increase turnout time if your horse has been grazing for at least two weeks without signs of pasture-related health issues.

Sensitive horses may require less turnout time on pasture to restrict NSC intake. Owners can also strategically schedule turnout during the day to avoid grazing when NSC levels are elevated. [1]

Horses known to have metabolic syndrome or PPID, and those with a history of pasture laminitis, should be kept off spring pastures.

Consider Temperature Fluctuations

NSC levels in grass fluctuate throughout the day depending on sunlight and temperature. Photosynthesis only occurs when there is sunlight, so NSC levels increase during daylight and decrease overnight. [1]

When night temperatures exceed 40oF / 4.5oC, NSC levels are at their lowest before sunrise. Therefore, the early morning hours are the safest time for pasture turnout after warm nights.

However, grasses do not use NSCs for growth if the temperature drops below 40 degrees Fahrenheit, leading to increased fructan storage. Sunny days and cool nights during the spring can result in consistently high NSC levels, even during the early morning. [1]

Owners may need additional precautions to manage grass intake after cool spring nights.

Monitor Body Condition

Keep track of your horse’s body condition to ensure he is not gaining weight too quickly on pasture grass. Regular monitoring of body condition will help you determine how your horse’s diet meets his energy needs.

It is typical for horses to lose body condition in the winter and to gain the weight back in the spring and summer. [21] However, rapid fluctuations in weight could indicate metabolic dysfunction or too much access to NSC-rich pasture grasses.

The 9-point Henneke Body Condition Scoring system is an objective way to estimate how much adipose tissue (fat) your horse has.

On the Henneke scale, a score of 1 is considered very underweight, and a score of 9 is considered very obese. Horses should ideally have a body condition score between 4 – 6.

Henneke Body Condition Scoring Scale for Horses | Mad Barn USA

Managing Grass Intake

Gradually transitioning to spring pastures is crucial for all horses. But managing grass intake in the spring is especially critical for horses with metabolic issues.

Feed Hay Before Turnout

Feeding appropriate hay immediately before turning horses out on spring pasture can prevent overeating. The hay provides fibre and gut fill, reducing your horse’s appetite for additional forage. [7]

If your horse is transitioning from hay to pasture as his primary forage, continue feeding hay as you increase turn-out time to help his digestive system adjust.

Restrict Grazing Time

Restricting grazing can help limit NSC concentrations in your horse’s diet. Especially if your horse has had limited turnout over winter, gradually increasing grazing time will give their digestive system an opportunity to adjust. [14]

Consider limiting turnout after a frost or cool night, even if your horse is well-adapted to spring grass. NSC concentrations in the grass are greatest when temperatures dip below 40oF (4.5oC) at night after a sunny spring day. [1]

Some research shows that horses with restricted pasture turnout consume grass faster than horses continuously grazing. One study found horses on 24-hour turnout had lower peak insulin levels than horses turned out for 10 hours overnight. [15]

This suggests metabolically healthy horses may have fewer health problems if turned out on pasture full-time after the adjustment period. But the best turnout schedule will vary for every individual horse.

Dry Lots

For some horses, no amount of access to spring grass is safe. If your horse has metabolic syndrome, your veterinarian may recommend turnout in a dry lot instead.

Dry lots are small paddocks with little to no forage available for grazing. [7] Horses in a dry lot should be provided with appropriately selected hay or straw chaff fed in a slow feeder or hay net. Soaked hay can also be provided which decreases the ESC (sugars) in the forage.

Dry lots are helpful for resting pastures to prevent overgrazing.

If you don’t have access to a dry lot, turnout in early morning hours after warm nights is safest due to lower NSC levels in grasses. [7]

Grazing Muzzles

Horses susceptible to metabolic diseases wear a grazing muzzle when turned out on pasture to limit or prevent grass consumption. [16]

A grazing muzzle restricts or prevents grass intake while allowing horses to express natural foraging behaviours and drink water. Some muzzles attach to a breakaway halter, while others can be worn alone.

Grazing muzzles also support weight management for easy keepers. One study found that pastured ponies wearing grazing muzzles 10 hours daily experienced less weight gain than non-muzzled ponies. [16]

Pasture Management

Good pasture management and grazing practices can also keep pastures healthy for horses. Encouraging pastures to remain in the growth phase minimizes the storage of carbohydrates but increases sugar levels.

Overgrazing can increase the intake of fructans by forcing horses to consume grass stems closer to the ground, where the plant stores most reserve carbohydrates. [17]

If possible, rest pastures to promote regrowth when the grass is grazed down to 3 to 4 inches. Only allow horses to graze the field again once the grass reaches 6 to 8 inches. [17]

The safest grass for horses with metabolic issues is when plants have reached their full height and dropped their seed.

Nutritional Support for Horses on Pasture

Fresh forage is an excellent source of essential nutrients for horses as part of a balanced diet. However, a forage-only diet will not meet all of a horse’s micronutrient needs.

Horses grazing on pasture are commonly deficient in trace minerals, including selenium, copper and zinc. Forage is also not sufficient to meet a horse’s sodium requirement and does not provide the ideal 20 mg per day of biotin to support hoof health.

Ensure that your horse consumes 1 – 2 ounces of plain salt daily and provide free-choice access to loose salt at all times.

Vitamin & Mineral Supplement

Mad Barn’s Omneity is a complete equine vitamin and mineral supplement designed to balance the majority of forage-based diets. Omneity provides full B-vitamin fortification including 20 mg of biotin, high quality organic trace minerals, yeast, and digestive enzymes.

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This supplement provides elevated levels of minerals essential for balancing the diets of horses with equine metabolic syndrome, insulin resistance, or PPID.

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Digestive Support

Mad Barn’s Optimum Digestive Health is a gut supplement that can help restore balance to the gastrointestinal tract while your horse adjusts to spring pasture.

This formula contains prebiotics, probiotics, yeast, enzymes and immune nucleotides that support the beneficial bacteria in your horse’s hindgut. Feed Optimum Digestive Health to maintain a healthy gut microbiome and to reduce the risk of digestive issues. [19]

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Summary

  • Spring grass contains higher NSC levels and the HC portion can contribute to metabolic issues in horses.
  • Horses with metabolic syndrome need to have their access to spring pastures carefully managed.
  • Transitioning horses gradually to spring grazing helps prevent digestive issues caused by abrupt dietary changes.
  • Spring grass isn’t safe for all horses; restricting turnout or using grazing muzzles and dry lots can help to limit NSC intake.

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References

  1. White, L. Carbohydrate reserves of grasses: a review. J Range Manag. 1973.
  2. Jensen, K. et al. Seasonal Trends in Nonstructural Carbohydrates in Cool- and Warm-season Grasses. Crop Sci. 2014.
  3. Gordon, M. et al. The Effects of Nonstructural Carbohydrate Content and Feeding Rate on Glucose and Insulin Response to Meal Feeding in Equine. J Equine Vet Sci. 2007.
  4. Kronfeld, D. et al. Insulin resistance in the horse: Definition, detection, and dietetics. J Anim Sci. 2005.
  5. Julliand, V. et al. The Impact of Diet on the Hindgut Microbiome. J Equine Vet Sci. 2017.
  6. Garber, A. et al. Abrupt dietary changes between grass and hay alter faecal microbiota of ponies. PLoS One. 2020. View Summary
  7. Geor, R. Nutrition and Exercise in the Management of Horses and Ponies at High Risk for Laminitis. J Equine Vet Sci. 2010.
  8. Durham, A. The Role of Nutrition in Colic. Vet Clin North Am Equine Pract. 2009. View Summary
  9. Rowe, J. et al. Controlling Acidosis in the Equine Hindgut. Rec Adv Anim Nutr Aus. 1995.
  10. Kane, A.J. et al. A cross-sectional survey of lameness and laminitis in U.S. horses. Proc 9th Intern Symp Vet Epid Econ. 2000.
  11. Geor, R. Pasture-Associated Laminitis. Vet Clin North Am Equine Pract. 2009.View Summary
  12. Norton, E. et al. Genetics of Equine Endocrine and Metabolic Disease. Vet Clin Equine Pract. 2020. View Summary
  13. Frank, N. et al. Equine Metabolic Syndrome. J Vet Intern Med. 2010.
  14. Cameron, A. et al. UK Horse Carers’ Experiences of Restricting Grazing When Aiming to Prevent Health Issues in Their Horses. J Equine Vet Sci. 2021.View Summary
  15. Glunk, E. et al. Effect of Restricted Pasture Access on Pasture Dry Matter Intake Rate, Dietary Energy Intake, and Fecal pH in Horses. J Equine Vet Sci. 2013.
  16. Longland, A. et al. Efficacy of Wearing Grazing Muzzles for 10 Hours per Day on Controlling in Pastured Ponies. J Equine Vet Sci. 2016.
  17. Watts, K. et al. Forage and pasture management for laminitic horses. Clin Techniq Equine Pract. 2004.
  18. Fernandes, K.A. et al. Faecal Microbiota of Forage-Fed Horses in New Zealand and the Population Dynamics of Microbial Communities following Dietary Change. PLoS One. 2014.View Summary
  19. Schoster, A. Probiotic Use in Equine Gastrointestinal Disease. Vet Clin Equine Pract. 2018. View Summary
  20. Dowler, L. et al. Determination of Pasture Dry Matter Intake Rates in Different Seasons and Their Application in Grazing Management. J Equine Vet Sci. 2012.
  21. Giles, S.L. et al. Obesity prevalence and associated risk factors in outdoor living domestic horses and ponies. PeerJ. 2014. View Summary
  22. Fernandes, K.A. et al. Resilience of Faecal Microbiota in Stabled Thoroughbred Horses Following Abrupt Dietary Transition between Freshly Cut Pasture and Three Forage-Based Diets. Animals. 2021.View Summary
  23. Sharpe, P. Horse Pasture Management, 2nd Edition. Elsevier. 2023.
  24. National Research Council Nutrient Requirements of Horses. 2007.
  25. Menzies-Gow, N.J. et al. Prospective cohort study evaluating risk factors for the development of pasture-associated laminitis in the United Kingdom. Equine Vet J. 2017. View Summary
  26. Patterson-Kane, J.C. et al. Paradigm shifts in understanding equine laminitis. Vet J. 2018. View Summary
  27. Gauff, F. et al. Hyperinsulinaemia increases vascular resistance and endothelin-1 expression in the equine digit. Equine Vet J. 2013.View Summary