I would have to ask you a whole bunch of questions lol to answer yours!
How old is your mare?
Does she have any special needs such as worn molars, requiring a mash diet?
How much exercise, roughly?
Is she otherwise stalled full-time? Is she on pasture or have any turnout?
What is her ideal weight, and is she at that ideal weight and maintaining it on her present diet?
How much is "one flake?" How many pounds of alfalfa per day?
Does she have a salt/trace mineral block?
I've never heard someone feeding a wheat bran mash on a regular basis. Is she getting any type of pelleted feed? I want to pass on this short article and commentary, there's a little info and discussion about alternatives to wheat bran, you can also use well soaked beet pulp (got to be careful as it ferments quickly in warm weather): Warm Mashes for Senior Horses | The Feed Room
Just some ideas for you. If you decide to make any changes, I would check with a vet of course, and make any changes gradually.