A horse needs a healthy calcium/phosphorous ratio-too much phosphorous can lead to nasty bone problems. (In humans, too...stay away from pop if you want to keep your bones full of calcium!).
Calcium should be higher than phosphorous.
Grass has less phosphorous than alfalfa.
Generally, RB's formulated to be fed with grass hay have higher calcium; whereas RB's for Alfalfa hay have less calcium.
Herein lies my issue with equidae (aside from their ridiculous feeding calculator): They label it the way other feed companies do, to lead a person to believe that "Alfalfa" should be fed with Alfalfa hay, and "Coastal" to be fed with grass hay...but when looking at the ca/p, it seems opposite.
How long has your horse been on Equidae? How old is your horse?