Talk to your VET about this pasture, rapid growth and grass founder (in the Spring.) My Vet gives me a head count on the grass founder cases that she treats every Spring when she gives yearly shots. My north pasture--3 acres worth--grows grass, clover, dandelions and other native weeds, and the weeds are safe for them to consume. When I moved there in October, 1999, the previous owners had it planted in corn, so my horses seeded it from the hay I was feeding them. I monitor it's use. When it's pouring in the spring or the snow is melting as it's coming up, I dry lot my herd of 3 to keep them chopping it up too much. This year it's up early, and they are doing a good job clipping it down. I have not had any grass founder.
Grass founder is caused by a rush of carbohydrates. The carb content is higher in the afternoon than in the morning. High, fast growing grass is super-high in carbs and can cause it if the horse grazes on it unmonitored for even 24 hours straight. When my South Pasture (about 1/2 acre) is lush, I will let them on it for about 1-2 hours before they get their evening grain bc they'll leave the pasture for the grain willingly and I can shut the gate behind them in the dry lot behind the barn.
Your Vet can give the BEST advice about this. There is ALWAYS the option to feed your horse hay. =D