I’m so glad she’s eating again!
Hay does actually have quite a lot of nutrients, but it depends not only on type of hay but also on when it is harvested, if the season has been warm and dry or wet and cold, the composition of the soil, (f ex if it has been fertilized, if it’s naturally deficient in essential minerals such as magnesium or selenium) and on how the hay is stored. The best thing is of course to supply your bun with a good variety of high-quality hay grown in different soils. That’s a bit tricky, honestly, so most bunny owners use pellets to ensure that the bun get all it needs – but it’s very easy to over-feed pellets and end up with a chubby bunbun.
Here are nutritional values of many types of hay:
hay chart
Nowadays it has become common to feed non-working horses of the easy keeper-variety (an easy keeper is a horse that easily puts on weight) hay and forage only, possibly with supplemented selenium.
The problem with recommending a bun be fed hay only is problematic imo not because hay has a low nutritional value, but because bunnies don’t do well with sudden radical changes of diet. Hay isn’t yummy like pellets and fresh veg, so the bun might not eat enough, and if a bunny doesn’t have food in their system, the system shuts down. The gut slows, appetite is lost, bun gets weak, stasis ensues unless you start support-feeding the bun.
Fresh spring grass is actually rather a vitamin-injection for a bun, so if you give just a little bit, it’s really probably the best food a bun can get. Again start slow, avoid radical changes.