Personally, i wouldn't feed her in the summer anything other than maybe some pellets which have all the vitamins and minerals in that horses need. You dont say if you are riding her every day.
In my experience with my mare who was diagnosed with tying-up or azorturia as it is more commonly known, recently, we were told by the nutritionist at the Queen's College Veterinary Centre in Cambridge to not feed her anything heating or with fat, and only a quarter of a mug of her no.14 Lo-Cal Balancer pellets.
If we didnt ride, we didnt feed.
Horses are not designed to eat without exercise, and it can have devestating results. As your mare is of the type prone to laminitis, and being a 'good-doer' make sure the paddock she is in isnt huge with lush green grass. Horses of the native breed can live on air if needs must.
So i say, dont feed unless you believe she either needs the weight and energy, or you dont have the grass for her.
You may find it useful to speak to someone at a feed centre. Dodson and Horrel and Baileys both have people who you can speak to and they give brilliant advice.
Hope this has been of help

If you have Pasture you can let her graze in the summer or as long as there grass. You may want to feed her oats to balance her diet.