I'm in Australia so this probably doesn't help at all...
But my mare has free access to an Oaten/meadow round bale.
For a hard feed, she gets 1kg lucerne/wheaten chaff, 2kg of Breeda (formulated stud feed, has 11% fat and 17% protein from memory), and 600g of KER Equi-Jewel (balanced Rice Bran) a day.
I put this diet into FeedXL Horse Nutrition
and it came back balanced :)
I am currently putting weight on her, but she is in minimal work.