A selenium trace mineral salt block is probably ideal in the PNW for your horse.
With Winter approaching, here are some tips for you:
Horses need additional warmth in the Winter months, and even if she's stalled and blanketed at night where you board, please consider giving her all the grass hay free choice at night to keep her warm, especially when it gets below, say, 25 degrees. It will also keep her preoccupied as well and content.
Blanketing horses: it's best to not blanket them unless it's extra cold or they're turned out in weather which is particularly cold, wet, windy. If you blanket, please be sure to buy a good quality waterproof turnout.
A little extra feed in the form of grain also will help maintain her weight in the Winter.
Something to learn which is pretty important is how much to feed by weight, not by the "flake," or "scoop." This way you are giving her what she needs according to her ideal weight for her age, breed, and any other considerations, and not providing too little or too much. If there's a scale at the barn, or you can borrow one, you can get a very good idea of how much you're giving, and after awhile you won't need a scale :)
But in the the Winter, typically it's a very, very good idea to provide extra hay to keep your girl nice and toasty.