By Dr. Rustebakke
There are few things more beautiful than watching horses enjoying lush green pasture. Unfortunately the lush green pasture may have a dark side. Every Spring we see a few horses that develop a crippling disease called "Founder" or "Laminitis". There are multiple causes of laminitis; however one of the more common causes is excessive exposure to green pasture by horses that have not been properly acclimated to it. The sugars in the green grass pass into the large intestine where they are fermented by bacteria causing an overgrowth of bacteria; the bacteria produce a chemical called "endotoxin" which is toxic to the small capillaries in the feet resulting in swelling of the sensitive laminae inside the hooves. This causes necrosis and weakening of the attachment of the hoof wall to the coffin bone. The hoof can in extreme cases separate from the coffin bone, and actually come off. The x-ray is of a horse we saw last week whose coffin bone actually came through the bottom of his foot and had to be destroyed. Please be careful when turning your horses out on green pasture!
There are a lot of variables on the ideal way to acclimate a horse to green pasture. Like anything in life, there is always going to be some risk, regardless of how careful you are. In the natural state horses come out of winter eating whatever is available, and their diet gradually changes to green grass; this gives the bacteria in the gut plenty of time to adapt. Also there are variables between individual horses's tolerance to sugar in the hay. Most horses can tolerate green pasture very well, however some researchers claim that about 10% of horses are likely to have problems. Horses with a history of previous episodes of laminitis should not be on green pasture at all. The sugar content rises during the day, and decreases during the night; it is generally at its lowest right before sunrise. Putting your horses out early in the morning and locking them away from the green grass by mid morning works for many people.
The following link is a good read for those interested in more information: http://www.thehorse.com/articles/26766/pasture-sugars
Dr. David A. Rustebakke
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