What kind of diet did the vet put the horses on?
They are all candidates for metabolic issues and none of them should have any grain. They do need a good quality vit/min supplement; a mineral block is not good enough and actually they should be eating white salt in their conditions.
I heartily support the grazing muzzles and letting them roam your pastures. If you have hills, believe me, they will get more exercise foraging with muzzles on than you can give them hand-walking and lunging or long-lining.
Those things, however, will re-build your bond and be good exercise for you - yuk - you're talking to someone whose barn is 450' from the house and I drive to it - lol
Hopefully you don't live someplace where people that know nothing about horses will stick their nose in your business. When we opened up the last 9 acres, next to a sort-of suburban/city neighbor, I did call them to explain why two of my horses have to wear muzzles.
They are terrific folks but have to young sons that I just knew would be asking questions and I wanted their parents to have the right answers; that way when the boys went to school "tattling", they'd know what they were talking about - lol lol
Are you able to have your hay tested for sugar/starch value? If you buy from the same source yearly or raise your own, you would be able to take samples and send to a lab for analysis.
If they are on a roundbale, they should not be. If possible, they should be fed flakes (or chainsaw the round bale up) and scatter them about so the horses have to "forage" for them. Many folks will turn their drylots into a "Paddock Paradise" where the horses are forced to keep moving to get their hay.
Here's a link that talks about insulin resistance, ems, & cushings. It is very beneficial in terms of keeping these types of horses healthy. Even though you haven't mentioned metabolic issues, they are headed that way if they don't take some weight off. http://www.ecirhorse.com/
This is one of many hits when I Googled "Paddock paradise". It will give some idea of how to make one out of your drylot if you would like to. http://successful-natural-horsecare.com/natural-living/paddock-paradise-grazing/
Hope this helps some:)