Written by: Linaya Pot, MSc, PAS

As the official nutrition partner of Equestrian Canada, Mad Barn extends our appreciation for featuring this story. To read the full version, visit https://madbarn.ca/winter-pasture-grazing-for-metabolic-horses.
Excess sugar intake from grazing green pasture can trigger abnormal insulin responses in metabolic horses, increasing their risk of laminitis. To mitigate this risk, many horse owners keep their metabolic horses off lush pastures during the growing season, typically spring and early summer.
Winter poses unique challenges for managing metabolic horses because cool-season grasses, though not actively growing, can still store sugars.
Read on to learn effective management strategies to ensure your horses are well fed and comfortable all year long.
Risks of Winter Pasture for Metabolic Horses
Grasses convert carbon dioxide and water into sugars through photosynthesis. The sugars are stored as carbohydrates, such as sucrose and fructans, fuel growth and survival for the plant.
In cool-season grasses, sugar levels increase under certain conditions, such as during rapid growth or periods of environmental stress. Similarly, drought, frost, or a sudden temperature drop can stress the plant, causing it to store more sugars as a survival mechanism.
For metabolic horses, consuming grasses high in sugars can lead to harmful insulin levels and increase the risk of laminitis, making some pastures a significant risk.
Winter pastures can also contain high levels of hydrolyzable carbohydrates (HC; sugar + starch). However, predicting HC levels in winter pastures is challenging due to variations in temperature, grass species, and regional conditions.
This variability means insulin responses in metabolic horses grazing on winter pastures can be inconsistent and difficult to manage.

Sugar Levels in Winter Pasture Grass
Plants like pasture grass have a high water content, which leaves them susceptible to freezing during winter. To survive freezing conditions, plants accumulate cryoprotectant compounds, such as sucrose, a soluble sugar. Sucrose and similar sugars reduce the risk of freezing, and different plant species rely on various types of sugars for protection.
However, plants cannot actively produce these protective sugars during freezing temperatures. Therefore, the plant relies on previously stored carbohydrates to safeguard themselves in freezing weather.
Sugar Storage
The amount of sugar in a plant depends on how much it produces, consumes, and stores.
When temperatures drop below 8 – 10°C (46 – 50°F), plants stop growing. However, they can still produce sugars while the temperature remains above freezing. Since growth is no longer occurring, the sugars made during this period are not used but instead stored within the plant.
As the cold season continues, plants accumulate significant reserves of sugar. This process enables them to survive harsh conditions but also results in elevated sugar levels in winter pasture grasses.
Sugar Fluctuations
Sugar levels in cool-season grasses fluctuate significantly with seasonal temperature changes.
In late fall, low temperatures combined with adequate sunlight allow cool-season grasses to continue photosynthesis and sugar production. However, since plant growth halts in cool conditions, these sugars are stored as fructans, which do not trigger an insulin response in a metabolic horse.
As winter sets in and temperatures fall below freezing, sucrose is released from fructan stores to protect the plant from freezing. This results in higher concentrations of HC in the base of the stem, leaves, and crown tissues during freezing temperatures conditions.
During temperature-driven HC spikes, these grasses are considered a high risk for metabolic horses. Some grasses maintain their fructan stores throughout the winter without releasing sucrose. These plants, with high fructan but low HC content, are considered safer grazing for metabolic horses.
However, due to variability in temperatures, seasonal shifts, and grass species, predicting which plants pose a risk can be challenging. For metabolic horses, it is safest to treat all cool-season pasture grasses with caution, especially during periods of fluctuating temperatures.
Once temperatures are consistently below freezing and the grass becomes dormant for winter, metabolic horses may be able to return to pasture turnout safely. However, even when brown and apparently dead, the plant can still store enough sugars to be a risk for sensitive horses.
When in doubt, it’s safest to maintain these horses on a dry lot with no access to pasture, especially in regions with mild winter climates where pastures may not fully enter winter dormancy.
How to Manage Metabolic Horses on Winter Pasture
Winter turnout is highly beneficial for horses as it supports their physical and mental welfare. However, turnout for metabolic horses needs to be managed properly to reduce the risk of laminitis. If your horse is metabolic and has access to winter pasture, consider these methods to minimize risks:
- Provide Hay: Offering hay as an alternative forage may reduce pasture intake.
- Wait for Deep Snow: A thick, hard layer of snow will make it harder for horses to reach the grass.
- Grazing Muzzles: Use a grazing muzzle if you notice your horse pawing through snow to reach the roots or the stem base of plants, as this is where the sugar is stored.
- Monitor Blood Insulin: Regularly testing insulin levels can help determine if your horse has an abnormal insulin response to pasture and should be removed from grass.
For metabolic horses, dry lot turnout with appropriate hay is another excellent alternative to turnout on pasture grass, reducing the risk of consuming sugars while still providing the benefits of outdoor activity.
Feeding hay ensures metabolic horses have a safe and controlled forage source, meeting their nutritional needs without compromising their health.
Types of Grasses
Grasses are categorized into two main groups: warm-season (C4) and cool-season (C3) grasses. These groups differ in how they produce, utilize, and store sugars, as well as their photosynthetic pathways.
Summary of sugar distribution in seasonal grasses

Linaya Pot, MSc, PAS.
Linaya was born on a small dairy farm in Southwestern Ontario. She spent much of her childhood taking care of the farm’s animals, including several horses, pygmy goats and sheep. This fostered a passion for animal husbandry and care, so she attended the University of Guelph where she received her B.Sc in Animal Biology and a M.Sc in Dairy Nutrition. While studying at university, she continued to develop her equestrian abilities by joining the University of Guelph Equestrian Club. After graduating from school, she worked in the agriculture industry as a consulting nutritionist. While most of her nutrition work focuses on cattle and horses, she has also formulated diets for sheep, goats, deer, elk and hamsters. She came to work at Mad Barn where she could unite her passion for both the dairy and the equine industry.
