The grip of winter is finally letting up and landscapes are greening. That used to be joyful news for horses and owners but today many are concerned about the safety – or dangers – of grazing spring pastures. What are the facts?
Spring grass is nature’s answer to the malnutrition feral horses suffer over the winter and the perfect food for mares about to foal. It is low fiber, high calories and high protein compared to other growth stages. This works very well for feral horses that are also covering endurance distances of 10 to 20 miles/day but what about domesticated horses on small acreage with more nutrient dense strains of grass?
For most modern day horses, the down side to spring pastures is rapid weight gain. This can be managed by restricted grazing time, use of muzzles and/or adjustment in other elements of their diet.
Potentially more concerning is horses that develop bright green diarrhea on spring pastures. Since spring grass is definitely in the category of a diet change, it is really no surprise that this happens. It may be accompanied by some bloating but frank colic as a result is unusual. This hind gut upset could be a risk factor for laminitis (in the same way as grain overload is) but without fever or colic that’s not likely. Management is the same as for preventing weight gain.
There is a subpopulation of horses (and ponies) that are prone to laminitis when eating spring pastures, even with very limited grazing times. We now know this is caused by insulin resistance (IR) and exaggerated insulin response to the high sugar and/or starch levels in spring pastures. Fat ponies are the poster child for this, but horses are also often affected.
Risk of IR depends on breed. Thoroughbreds, Standardbreds, Quarterhorses, drafts (with some exceptions) and warmbloods are at low risk while very hardy breeds like Shetland, Icelandic, Haflinger, mustang/Spanish, as well as Arabians, Morgans are at higher risk.
Symptoms of IR may include easy weight gain, fatty crest on the neck, fat deposits at the tail base or along shoulders/withers and history of laminitis. Diagnosis is by blood testing for glucose and insulin.
For the insulin resistant horse, spring pasture always poses a risk of causing laminitis. It might not happen to every IR horse/pony or every year, but the risk is always real. Limiting pasture does not work. Avoiding it is the only answer and is a small price to pay for avoiding the agony of laminitis.
Eleanor Kellon, VMD