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How Can Pasture Cause Laminitis?


By Leigh Ballard

There are several risk factors that are known to cause laminitis in horses. One of the common causes of laminitis is carbohydrate overload, too much sugar and starch or nonstructural carbohydrates (NSC) in the diet.
There are several environmental factors which can have an effect on the carbohydrates concentrated in pasture grasses. These factors are stresses on the grass, and include cool or cold temperatures, drought, lack of nutrients and overgrazing or improper mowing.

Sugars are the basic building blocks for plant growth. Plants make sugars during the day by the process of photosynthesis. Plants use sugars at night during the process of respiration, turning the sugars into cell walls and other structures for plant growth. The plant is only able to achieve its growth if environmental conditions allow. If temperatures, water or nutrients are not optimal for the process, the plant stores the sugars for later use. In other words, if photosynthesis is higher than respiration, sugars don’t get used and they accumulate.

Any stressed plants accumulate sugars this way. It is a survival adaptation so the plant can be ready for rapid growth when the stress factor goes away. The plant holds the sugars in the base of the stems or the crown, so even what seems to be sparse grass can be dangerously full of sugar for a horse that’s prone to laminitis.

Typically grasses are lower in sugars in the morning because they have used their accumulated sugars overnight. However, cool or cold night temperatures, about 40F, prevent this, and sugars remain high. This is one reason we see so many spring grass founders. Another reason we see so many spring pasture founders is the type of grass. Cool season grass species such as fescue, orchard grass and perennial rye are adapted for winter hardiness and therefore can sustain growth (produce and use sugars) in cooler temperatures. Green grass that continues to exist during cold temperatures will be very high in sugars. This is good for the grass but bad for certain horses! Warm season grasses such as Bermuda and Crabgrass have different structures and adaptive processes and do not hold and use their sugars the same way, nor do they contain some of the same sugars as cool season grasses.

Assuming there is no temperature, water or nutrient stress on the grass, the safest time to graze is early morning when a grass is in a vegetative state with mostly leaves and not producing seed heads.

The most dangerous time for horses to graze on a carbohydrate overload is late afternoon on a sunny day when the full accumulation of sugars is in the plant. If the grass is heading or flowering it is full of sugar.  Newly forming seed heads are “horse candy” with the highest sugars, so grazing during this stage can be dangerous. If the night before didn’t allow the plant to use its sugar, or if the grass is stressed from lack of water or nutrients, it is high in sugar. If the grass is stubble left from mowing or overgrazing it is high in sugar because the crown of the plant is its growth point and uses or stores the most sugar.

How does one keep a pasture as safe as possible? Environmental factors such as water and temperature can’t be controlled but other factors can be managed. The species of grasses in the pasture can be chosen wisely. The pasture can be properly maintained. It should kept fertilized, and not overgrazed, and it should be cut properly. Know your weeds. Certain weeds, dandelion for example, are extremely high in sugar, because after all, they are adapted to survive in poor conditions.

Of course, not all horses develop laminitis from pasture grass. Some horses look their best, bulky and shiny, when the grasses start to grow. But for horses that are genetically pre-disposed to founder or who have other underlying conditions or previous bouts of laminitis, pasture grass intake should be closely monitored and controlled to keep them safe.

  I highly recommend this site for owners of laminitic horses!
Excellent articles by Kathryn Watts including:
“Forage and Pasture Management for Laminitic Horses”
“Factors Affecting NSC Levels in Grass”
“Risk Factors for Laminitis”
“Founder Fodder: High Risk Weeds”

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