Within the framework of the research program developed at the Latvian University of Agriculture (LLU), at the Research Institute of Agriculture (ZZI), Agency of LLU field trials were established and research was conducted over the period 1999–2003. In all trials, tetraploid perennial ryegrass cv. ‘Spidola’ was sown to plots at a rate of 12 kg ha^-1. The impact of three factors (NPK) in 17 treatments, replicated four times, were researched according to the so-called „star” scheme added to control treatment N₀P₀K₀ + absolute MAX N₁₂₀P₁₂₀K₁₆₀. The totalplot area was 17.5 m², harvested area 13.5 m². In the trial, five levels of fertilizer werecompared: – 0; 30; 60; 90 and 120 kg ha^-1 of N, 0; 30; 60; 90 and 120 kg ha^-1 of P₂O₅, and 0; 40; 80; 120 and 160 kg ha^-1of K₂O. The goal of current research was to clarify the influence of mineral fertilizer optimization on forage quality formation in perennial ryegrass cv. ‘Spidola’ under agro-climatic conditions of Latvia. It is concluded that N fertilizer mostly influenced herbage yield and quality characteristics of perennial ryegrass where both, crude protein (CP) content of dry matter and total yield of CP per hectare increased. In the treatments N60 and N120, the CP content in grass dry matter increased by 0.14 and 2.66%, and CP yield per hectare by 98 and 226%, respectively, compared with N0 (at P₆₀ K₈₀ background).