If she has a problem keeping weight on, I'd probably check her teeth and make sure everything's ok in there...
But if she was just -not fed- , all the pasture you can put her on won't hurt as long as the grass isn't too terribly rich, she might have to adjust. The horse's digestive system is pretty dern fragile so whatever you change or add to her normal diet, just let her adjust to it slowly.
As far as grain, you'll probably have alot of folks tell you to feed any type of weight-gain supplements/grain that is high in fat content. What alot of people don't know is that FAT isn't absorbed into the body even half as much as you might think. Meaning, just because it is Fat in itself, doesn't mean it's going to stick to you. The same goes for horses and believe me, I've had half a dozen weight gain supplements and feed on this place and fat just doesn't get absorbed into the body and "stick" like people might think.
Carbohydrates, such as bread (grains) and sugar really do stick though...(look at America, our main grain is wheat, mexico's is corn, oriental, it's rice etc...) and look at how...overweight america is- *cringing at the looks I'll get from everyone else* -America ranks like country #24 in health!
Oats (not quite like wheat, but easier for a horse to digest) are pretty good in assisting a hrose to pick up weight. And I don't say this because someone told me. I've fed it, experimented, and saw, it works. In a barn of 30 horses... I finally learned that lesson because most of them were being fed high-quality sweet feed with alot of molassas, and other "energy giving" things in it and, 5-6 of them were being fed straight oats.
The horses getting the sweet feed: didn't look terrible, but were very high-strung, FULL of energy and hard to handle because it was alot like a kid that ate all the cookies in the cookie jar and bounces off the walls for several hours.
The horses being fed straight oats: much calmer and not so stuffed with giddiness, and at an average of 500lbs heavier (these were belgian horses)
OTHER than that, the entire barn was fed a grass/alfalfa mix of hay. I thought it was pretty interesting!
If I had good grass pasture I'd put her on it, second would be some good quality hay without dust or mold. Any feed you can find with more oat grains in it will help too (or you can get a bag of each and mix them)
Hope she gets to picking up some weight! I know how it is, been through it several times
Oh, Ps. I've had good luck with "grow colt" you can probably get it at Tractor Supply, being that your horse is 16 months. It has some good vitamin supplements in it that assist in a more healthy growth in young horses. The weanlings that I fed it to, seemed to have a sleeker, healthier looking coat, and grew in height faster than the young horses that I didn't feed it to. Might help!