Long-Term Mineral N Fertilization in Permanent Mountain Grassland: Trade-Offs Between Dry Matter Yield and Metabolizable Energy of Grasses, Legumes and Forbs

Permanent grassland management commonly includes moderate mineral N fertilization, which can differentially affect botanical composition, dry matter (DM) yield and forage quality of grasses, legumes and forbs. We hypothesized that increasing N would enhance grass dominance and DM yield but raise fiber and reduce metabolizable energy (ME) content. We evaluated long-term N fertilization, cutting time and their interaction on DM yield and ME of grasses, legumes and forbs in a permanent mountain grassland. The field experiment, established in 1967, used a randomized block design (4 replicates, 3 cuts/year) with five treatments: unfertilized (Control), PK without N (N0), and PK plus 80 (N80), 120 (N120) or 180 (N180) kg N/ha per year; forage was sampled in 2014–2016. Increasing N raised grasses to ~92% of biomass at N180 and reduced legumes and forbs (p < 0.001). Mean DM yield rose from 2.5 (Control) to 9.8 t/ha (N180; p < 0.001). High N (N120, N180) lowered ME of grasses in later cuts (8.9 vs. 10.1 MJ/kg DM in the third cut), whereas legumes and forbs maintained high ME. Moderate N fertilization (80–120 kg N/ha) best balanced DM yield, legume and forbs contribution and ME content.