Hi, wanted to offer this:
I have two IR horses and i do not allow either one turnout on grass, ever. Its too much of a risk. Both are very happy in drylots and are exercised daily.
My gelding is boarded at a barn that keeps horses on drylot paddocks at night and are allowed into grassy paddocks during the day. Having been at this facility for a few years, my gelding knows the routine and since this is the first year he's kept off the grass, he was understandably indignant when his grass gate wasnt opened this spring. To distract him, i scatter about 10lbs of the ODTB cubes all over his drylot so he spends most of day hours "grazing" while his neighbors are on their grass turnout. Only took a day or too for him to forget about the grass (he loves the cubes). There are other enrichment/distraction options, too, for IRs that shouldnt be on grass (round pen & Nose-It balls, etc).
Putting a known IR on grass even muzzled is a huge risk, IMO.
Kerry in NY
Pinky Sept 2014 PPID/IR
Tofurky Nov 2014 IR