The effect of different N, P and K supply levels and their combinations on the nutritional values and nutrient yield of an established all-grass sward were examined in 28th year of a long-term fertilization field experiment set up on a calcareous chernozem loamy soil. The fertilizer responses on the development, hay yield and N-uptake were published elsewhere (Kádár, 1994). The soil of the growing site contained around 3% humus, 5% CaCO₃, 20-22% clay in the ploughed layer and was originally moderately well supplied with available K, Mg, Mn and Cu and poorly supplied with P and Zn. The trial included 4Nx4Px4K=64 treatments in 2 replications, giving a total of 128 plots. The fertilizers applied were Ca-ammonium nitrate, superphosphate and potassium chloride. The groundwater table was at a depth of 13-15 m and the area was prone to drought. In 2001, however, the area had a satisfactory amount of 621 mm precipitation with fairly good distribution. The grass was established on 21. September 2000. The main results and conclusions can be summarised as follows:
     1. While the grass herbage yield was determined by the NxP supply levels, the nutritional values were influenced by the NxK interactions. As a function of NxK treatments, the N-free extract decreased from 532 g/kg (control) to 390 g/kg (N₃K₃), Crude-protein increased on the same plots from 64 g/kg to 183 g/kg, so the ratio of N-free extract/Crude-protein tightened from 8.3 to 2.1. Compared to the unfertilized control, the Crude-ash enhanced with 26%, while the N-dependent Metabolic Protein (MFN) with 286% on the N3K3 levels.
     2. The maximum nutrient yields were measured at the first cut on the 100 kg/ha/yr N-treatment with 150 mg/kg ammoniumlactate soluble P-supply soils for Crude-fibre, Neutral and Acid Detergent Fibres (NDF, ADF) N-free extract, Crude-ash and Nettoenergy (NE) parameters. The Crude-fat and the Energy Dependent Protein (MFE) yields gave maximum surpluses using 200 kg/ha/yr N-rate with high level of 333 mg/kg AL-P2O5 supply in plow-layer. Highest crude-protein and the N-dependent Metabolic Protein (MFN) yields were connected, however, to the highest N3P3 levels, where the yield increased 5-times compared to the control.
     3. At the 2nd cut, both the nutritional values and the nutrient yields changed only as a function of N-supply. Maximum nutrient yields were obtain generally at the 300 kg/ha/yr treatment. Assessing the 1st and 2nd cuts together, the 200 kg/ha/yr treatment seemed to be the best fertilization practice. Among the different NPK treatments developed extreme differences. The unfertilized for 28 years plots (N0P0K0 ) gave small nutrient yields. The moderate N-fertilization alone (N1P0K0) enhanced the yields 2-3 times. The moderate balanced fertilization (N1P1K1) gave further dramatic surpluses. The N-dependent Crude-protein and the protein fractions (MFE, MFN) as well as the Crude-fat yielded maxima values at the highest (N3P3K3) treatment.
     4. Summarizing above, we can state that the satisfactory or abundant fertilization on such soil poor in NPK and in a favourable year can increase the Crude-fibre, the fiber fractions (NDF, ADF), N-free extract, Crude-ash, Crude-fat and Nettoenergy (NE) yields 3-5 times, while the N-dependent Crude-protein and protein-fractions (MFE, MFN) even 7-8-times.