With spring comes fresh, sweet grass. For your horse, this is a long-awaited food source but one you need to monitor carefully as spring grass can make your horse founder. During spring, worm larvae are emerging and can easily cause colic.
Feed your horse hay before turn out on grass. Consider using a grazing muzzle, for horses with no self-discipline. Ensure you have dewormed your horse prior to turnout with a good dewormer of the Avermectin class (such as Ivermectin or preferably Moxidectin) to get the worm larvae before they emerge from the bowel wall. A dewormer that includes Praziquantel for tapeworms is a good idea too.