Spring turn-out (grazing initiation) should be determined by: stocking rate (how many horses/total pasture acreage), pasture species and condition, ability and availability of mowing/haying equipment for paddocks that may get too mature for effective pasturing, and species height and maturity.
On average, 2 acres of well-managed pasture can provide the forage needs for one horse from spring to fall. "Well managed" means subdivided into multiple paddocks, fertilizing according to soil tests, and controlling weeds. If you have that much or more acreage per horse, you may want to start grazing early to get a jump on the spring pasture growth. If you have less than 2 acres per horse, the pasture cannot be expected to meet all the forage needs for your horses during the grazing season. In this case, plan to provide some hay and designate a sacrifice area/paddock to feed horses as needed to allow adequate rest (on average 2 weeks in spring and 6 weeks in summers) for the pasture.
Grass pastures with good stands of Kentucky bluegrass or smooth bromegrass can handle early spring grazing. "Early" means when bluegrass is 3-4" tall and bromegrass is about 6" inches tall. These grasses are sod-forming and tolerant of horse hoof damage. Pastures dominated by bunchy-growing grasses like orchardgrass and timothy should be taller, about 10" in height. These grasses are more easily damaged by hoof action and grazing. If conditions are really wet, it's best to wait, regardless of plant height. Introducing horses to spring pasture gradually will reduce the chance of laminitis. Slowly begin grazing (15 minute grazing periods at first), working your way up to a full day over a couple of weeks.
By Paul Peterson, PhD, University of Minnesota