Thanks. I wondered today at Tractor Supply about buying oats, but I was not sure if I should bother. I think I will try a slow transition to Alfalfa pellets, rice bran and beet pulp from the sweet feed I buy currently. With the two underweight, should I bother with oats? Looking at what everyone here mentions, I am guessing not. Their pasture is a bit sparser than most, as a pony and the gelding are on one 3 acre lot, so I do give about 2-3 flakes of orchard grass hay in the morning and evening. The very thin mare is on one acre, and the grass is still so high, she didn't eat the hay I was providing in addition, so I stopped offering that to her (she is on a senior feed, no sweet feed).
My Arab mare is stalled and not turned out (I have electric fence, and her prior owner was against turning her out in it because she felt sure she'd go right through it), and she gets about 15 lbs of hay a day (or 1/3 a bale) and about 6 pounds total of sweet feed (split up through the day), but as I mentioned, she looks so great, I hesitate to mess with what I feed her since coats, weight and hooves are fantastic.
It was only recently we got a local Tractor Supply, and until then the only options were small feedstores, and they didn't sell option in feed really. You could buy sweet feed in 10%, 12% and 14% and an equine senior feed, and that was it.