One of my mares nearly choked to death just from nibbling up grass clippings that my stepfather had mowed into the paster. (he had the blower facing towards the pasture). By the time the vet got there she was nearly dead, couldn't breath. It was absolutely terrible.
As for the grass... 3 of my horses are out practically 24/7, so we don't have to worry about them. If you're actually around, you can probably leave them out longer than a half hour the first time. A lot of times they're just going to run around like crazy and not eat any grass, and the next day go out and gorge themselves. Make sure they at least eat some before you bring them in.
It sucks that you have to be so careful, wouldn't it be a perfect world where all horses could just be horses?