a horses diet should consist mainly of forage, hay. If your horse is old, i would keep her on the sr feed beet pulp and pellets. A few horses have had problems with alfalfa, the develop stones, enteroliths, I have had horses on alfalfa for many many years and only had one horse get enteroliths but only after he was around 25 yrs old.
Hay will lose some of its TDN after drying and baling, but it should not lose a lot . It depends on the hay, and how and when it was put up. Some grass hays can have higher proteins etc than alfalfa, again it depends on the hay, where it was grown, how it was grown, when it cut baled etc etc ..poor soil, no fertilization, no water, poor hay