The aim of our experimental work of developing the fertilisation consultancy system of silo sorghum was to examine the effect of N, P and K supply levels on the yield and quality of silo sorghum at well separable nutrient supply levels in a long-term fertilisation experiment. The long-term fertilisation experiment was established in 1989 on deeply calcareous chernozem meadow soil at 4 different N, P and K supply levels each, applying 64 treatments, i.e., the whole treatment combination. This study presents the results of the effect of N fertilisation (0, 80, 160, 240 kg N ha-1) on the main quality characteristics of silo sorghum based on the examinations performed between 2000 and 2002 which can be summarised as follows:

1. The raw protein content of silo sorghum ranged between 8.05–11.35% depending on the N supply level and the given crop year. Without N fertilisation, the raw protein content amounted to 8.05–9.22% and it constantly increased as a result of better N supply.

2. The raw fat content of silo sorghum was nearly the same in the experimental years of 2000 and 2001. Without N fertilisation, the raw fat content was between 1.20–1.30% which increased to 1.42–1.47% in a statistically significant way until the N supply level of 160 kg ha-1. In a dry crop year, the raw fat content of sorghum was higher, amounting to 2.47% without N fertilisation and it increased to 2.81% as a result of 80 kg ha-1 N.

3. As regards raw fibre, N fertilisation caused a significant change only in one year, when the increasing N supply level resulted in decreasing raw fibre content.

4. At the time of flowering and the beginning of wax ripeness, N fertilisation significantly decreased the sugar content of sorghum. The highest sugar content (15.3% and 15.9%) was reached at the stage of physiological ripeness. In this phase only the 240 kg ha-1 dose of N fertiliser resulted in decreasing sugar content. The sugar yield at harvesting was 5.7 t ha-1 in the case of zero N fertilisation and 6.4 t ha-1 in the case of 80 kg ha-1 fertilisation and it was significantly increased by the 160 kg ha-1 treatment. Only the N over fertilisation resulted in a statistically significant sugar yield decrease in comparison with the highest value of sugar yield.

5. Of the 17 amino-acids examined in the experimental years of 2000 and 2001, only the amount of methione and cysteine did not increase (g per 100 g dry matter) as a result of better N supply. At the same time, the amount of only five amino-acids (isoleucine, leucine, alanine, asparagine acid, glutamic acid) increased significantly at the 240 kg ha-1 N supply level despite the notable increase of raw proteins following the N fertilisation in 2002. The various levels of N supply did not significantly affect the proportion of amino-acids, while crop year did. No consistent N impact was observed in relation to the amino-acid composition of the protein (g per 100 g protein) which could be detected in the change of the proportion of a single amino-acid in all experimental years.

Keywords: N supply, silo sorghum, yield quality, long-term experiment