As your horse pastures spring back to life, it may be wise to consider limiting your horse's pasture grazing time. Theoretically at least, every horse is at risk of developing laminitis if he gorges himself on enough young pasture grass in a short enough period of time to result in severe gut upset. However, a more common scenario is that some horses/ponies within any given group will become laminitic on spring grass, while others never do.
A recently published study performed by the Department of Animal and Poultry Sciences of Virginia Polytechnic confirms that the animals most likely to develop grass-related laminitis are those with a body condition score of 6 or higher and with elevated levels of serum insulin relative to blood glucose, indicating insulin resistance. Ponies, donkeys, miniatures and any horse that gains weight very easily should be considered at highest risk. Any animal that has developed laminitis on grass in the past is extremely high risk.
High-risk groups should be kept off pastures until the period of rapid growth has stopped. If they must be on the pasture then, allow them on it only with muzzles that prevent or severely restrict grass intake. Don't underestimate a horse's ability to get a surprisingly large amount of grass even while wearing a muzzle.