Two things happened that make me feel I have some advice:
1. I had a heavy/cresty 4 year old who had a laminitis issue last spring, so she needs to be off grass.
and 2. A vet's advice at a clinic last summer.
Rosie, my laminitis one, has lost a lot of weight--she has no crest now; her hoof has one more month to grow out for her to be 100% okay.
The difference is she gets no grass. My farrier says grass to horses is not salad, it's ice cream. I have found this to be true. We killed the grass in a good sized pasture, and will do more dry lotting this summer. Since all my horses have a tendancy to keep weight on, the grass will be limited to everyone. They will get more hay. They do not need grain, so mine get Triple Crown 30--just a tbsp twice a day. That's it. Well, despite extra hay, Rosie's ribs at one point were not covered like I wanted them to be
, so I added a 1/4 of black oil sunflower seeds to her dinner. Thhis helped pretty quickly, so now she gets them every other night. I check rib covering on mine a lot.
At the clinic I went to, when I asked the vet about keeping weight off minis, he sighed in a resigned way, and said the best advice he has for minis is to keep them off grass. The vets also talked about owners who insist on feeding their overweight horses grain when they should not be. One vet said the owners feed the grain because it makes them feel better and she hears "but, he likes it!" a lot.
Weather and time permitting, mine also get exercised. I walk them down the lanes and through the fields and down the road, and we work to a jog so we both get exercise!
I lunge the two mares in between walking. It's a lot of work to keep five minis contented, but it sure helps me to stay in shape!
Hope this helps... at the end of the day, you have to follow your gut/heart and do what you think is healthiest for your horses
.