By over fertilising for years with both organic and inorganic fertiliser the soil may contain a surplus of potassium. Dried chicken manure especially contains high levels of potassium.
Too much potassium in the soil, and therefore in the plant, interferes with the ability of grass to absorb calcium and magnesium. As a result, protein production within the plant is disturbed. This leads to the production of excessive amounts of NSC in the grass.
By ingesting too much potassium horses can develop difficulties absorbing magnesium, on top of the already lower available concentrations in the grass. The beneficial effects of a magnesium supplement can therefore be less than expected.
Consult an agricultural laboratory for fertilisation advice that keeps potassium levels as low as possible, without causing potassium deficiency. Both magnesium levels in the grass and the magnesium absorption capacity of the horse will rise once potassium levels are normalised.