Field experiment was carried out in the Didactic and Experimental Department in Swadzim near Poznań, in years 2004-2005 (52^o26’ N; 16^o45’ E). The experiment was established in a „split-plot” design with two factors and four field replications. The primary factor consisted in three nitrogen doses: 0, 60, and 120 kg N·ha^-1, while the secondary factor included four doses of kieserite (magnesium sulphate): 0 kg fertilizer·ha^-1, 100 kg fertilizer·ha^-1 (25 kg MgO·ha^-1 + 20 kg S·ha^-1), 200 kg fertilizer·ha^-1 (50 kg MgO·ha^-1 + 40 kg S·ha^-1), and 300 kg fertilizer·ha^-1 (75 kg MgO·ha^-1 + 60 kg S·ha^-1). An increase in the nitrogen dose increased grain yield, its moisture, and weight of 1000 grains but it decreased the yield index and the number of production ears. Under the influence of the magnesium sulphate fertilizer (kieserite), maize produced a higher yield of grains while limiting the vegetative biomass. The level of fertilization with nitrogen and kieserite differentiated the nitrogen content in the dry matter of grain and the magnesium content in the dry matter of ears. With an increased nitrogen fertilization, the content of crude fibre in the dry matter of grain increased, while the number of extraction compounds decreased. An increased dose of kieserite decreased the quantity of crude fibre in the dry matter of grain but, at the same time, it increased the amount of N-free extract. The highest yield of net energy, yield of digestible protein, and total protein were obtained for the dose of 120 kg N·ha^-1.

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