In spring it is, and has high fructran levesl. It has high fructan levels in frost too, but I wouldn't have thought that it was deficient in magnesium unless your land is. You could just feed magnesium oxide as a precaution if you are worried. Alternatively, if you are asking because your horse is playing up a bit, it could be the fructans, like giving a child a sugared drink!
i am assuming that you are in the uk. the majority of our soil in this country is deficient in magnesium, at all times of the year. but yes the grass levels are even lower at this time of year due to the fact that it is in a dormant/non growing stage. if you require any furthur help.. let me know.