Summer grasses are every bit as bad as spring and fall.
If there's no rain drought grass is even more dangerous.
The only time the muzzles come off my horses is when it finally gets cold enough to frost and freeze. Sadly, they only get about three months of muzzle freedom where I live
I see you are in the southeast somewhere, so you are probably experiencing the same ghastly heat/humidity that I am.
My area hit the mid-90's in May and we have only had a handful of day in the high 80's since. We've hit triple digits a lot (we will today) with humidity around 105.
My two insulin resistant horses go out with grazing muzzles regardless. I absolutely hate it for them, but it's either that or chance the founder.
I really really like these Tough-1 Easy Breathe grazing muzzles. They quit running to the corner of the stall after I got these and now put their noses in them willingly.
I'm not sure these would work for some horses as the grazing hole is also bigger than a Weaver or Best Friend. The breathing holes are huge which is great for my big-headed Tennessee Walkers
I take them off around 7:15 every night to give them some free grazing time after the sun goes down.
Please don't wait to use the grazing muzzle. There is no safe grass for easy keeper horses, as I am learning Saddles Tack Horse Supplies - ChickSaddlery.com Tough-1 Easy Breathe Grazing Muzzle