You can only definitively answer this question if your roughage has been tested. It doesn’t need to be an extensive analysis. It happens from time to time that the amount of calcium is a lot higher than the amount of phosphorus, something that is always the case with alfalfa. In general up to 1 kg of alfalfa can always be given safely to a mature horse.
In case you feed your horse beet pulp, we advise you to compensate the calcium with some bran.