Lush Grass: Good or Bad?By Kentucky Equine Research Staff · March 6, 2012
Horse owners and farm managers frequently use the word “lush” to describe the state of pasture forage as it begins to grow rapidly in the spring. Just exactly what does “lush” mean? Is this new grass good for horses, or dangerous for them to graze?
In defining “lush,” the dictionary uses words like “growing vigorously; lavishly productive; thriving; plentiful; delicious; savory.” Lush pasture, then, is a grazing area with plenty of abundant green forage that tempts horses to graze enthusiastically for hours on end.
Lush new spring grass, mature summer grass, and dried autumn grass contain the same basic ingredients--water, vitamins, minerals, protein, starch, and structural fiber among other things—but the proportions of these ingredients are far different depending on season. Spring grass grows very rapidly, containing a large proportion (up to 80% or more) of water. This grass is generally soft and easy to chew because the amount of indigestible fiber is less than in mature grass.
Because there is so much liquid in new spring grass, all the other components are found in lower proportions compared to mature grass, so the horse gets less starch per mouthful of grass than when grazing in the summer. However, because this soft grass is so palatable, horses tend to ingest a larger overall volume of forage, so their intake of all nutrients may actually be fairly similar in spring, summer, and early fall.
Fructans are specially adapted sugars that are found in cool-season forages. Fructans are produced by photosynthesis that occurs in the leaves during daylight hours. During the dark (overnight) phase of photosynthesis, plants use the sugars to grow more leaves and stems. Extra sugars that are not used for growth are stored within the plant tissues. Many cool-season grasses store fructans in the lower two inches of the stem just above the soil line.
Temperatures at night are critical in determining sugar content of the grass blades. If the temperature is not above 40° F (4° C) at night, the plant will not grow, and sugars remain in the leaves in high concentrations. Research has shown that under certain climate conditions and at some growth stages, fructans may reach very high concentrations (as much as 50% of dry matter). Pastured horses relish the sweet taste and will search out and preferentially graze plants with higher sugar content.
The unique chemical structure of fructans prevents breakdown in the stomach and small intestine. For this reason, these easily fermented sugars pass into the hindgut, a situation that leads to rapid production of lactic acid and an accumulation in the hindgut. This accumulation of lactic acid is a direct cause of colic and laminitis in pastured horses.
Virtually all horses are subject to some digestive upsets associated with lush spring pasture. The content of highly fermentable carbohydrates in lush pasture can be overwhelming to the digestive system. Horses and ponies that are overweight with insulin resistance and associated high levels of circulating pro-inflammatory agents produced by fat (equine metabolic syndrome) are particularly susceptible to pastures with high fructan content. However, many horses are able to handle some amount of pasture turnout if their digestive tracts are allowed time to adapt gradually to the dietary change and if a hindgut buffer is used to help neutralize lactic acid.
How can horse owners minimize the health challenges associated with lush pasture?
- Continue to offer hay even though the grass is growing well. New grass contains a lot of water and little fiber, and horses may crave the fiber found in hay.
- Monitor horses as grass begins to grow in the spring. To allow the digestive system to adapt to lush grass, begin with short periods of grazing and gradually increase time on pasture.
- Check frequently (several times a day) for signs such as warm hooves or horses walking as though their feet may be painful. Horses that have been grazing through the winter and early spring are at somewhat less risk than horses that have been stalled and are suddenly turned out into lush fields.
- Use a grazing muzzle to restrict intake, and consider the use of a hindgut buffer to neutralize lactic acid.
- Overweight horses, horses with known metabolic problems such as Cushing’s disease, and pony breeds may be at increased risk, but any horse may develop problems after grazing lush pasture.
- Spring grass is a known danger, but stressed grasses may store large quantities of fructans during other seasons due to drought, overgrazing, temperature fluctuations, and other conditions. For susceptible horses, there is no safe time to allow unlimited pasture access.
- If grazing horses show signs of problems (colic, warm hooves, reluctance to move because of hoof pain), remove them from the pasture and call a veterinarian.