Laminitis is a painful and serious condition of horse's feet and it can lead to founder. Although most folks think of founder developing in horses who are eating large quantities of grain, more horses develop laminitis and founder on pasture grass due to the high concentration of carbohydrates in the grass than those eating grain. Pasture-associated laminitis usually occurs in horses that are susceptible to laminitis and those are horses that are usually overweight, are easy keepers, and have equine metabolic syndrome or Cushing's disease.
In these susceptible horses, it has been recommended to decrease the amount of pasture and so lots of folks try to decrease the amount of time horses are grazing. However, a study has shown that when horses are restricted from pasture for a part of the day, they increase the amount of grass they eat when they are on pasture and so restricting the time on pasture is not effective unless it is severely restricted to just a few minutes a day. Also, there are differences in carbohydrate concentration in the grass depending on the time of day. Because of photosynthesis that occurs with sunlight during the day, grass in the early morning has less carbohydrate than grass in the afternoon. So if you are going to allow a horse to graze for a few minutes per day, it would be safer to allow this grazing in the early morning versus the late afternoon.
Another management practice that should be considered in these susceptible horses is the amount of food the horse receives at a meal. A study out of Ohio State has shown that horses that are fed smaller meals more frequently have lower insulin levels and lower insulin decreases the chance of laminitis developing.