Feeding...

Hello
I was just wondering, there's a horse stabled at my aunts tiny farm (she's the only horse) and she's currently getting fed bran every morning and evening (I'm not sure exactly how much, but approx. Two buckets every day), and she grazes the whole day. She also gets a huge bag of fresh grass every evening. Currently I ride her about once a week and her owner rides her about once a month. I'm hoping to get a half lease on her soon so I will work her about 3 times a week...
Is her current diet healthy enough and should it changed when I increase her work load.
It might be a silly question but I've never owned or leased a horse so I'm not very informed about this type of thing...
P.S. She's an American Saddler, about 15hh. Don't know if that makes a difference to the feed?
Any info would be greatly appreciated.

American saddlebred?

One bucket can really vary...depending on the size of your bucket

A good rule of thumb is to not change anything; if the mare begins to drop weight, then add a little bit more...but do not increase feed with anticipation of increased work...does that make sense?

Thanks for the advice, yes it definitely makes sense.
I will bear the "rule of thumb" in mind for the future.