The reliable identification of productive and nutritionally valuable Bromus inermis Leyss. germplasm requires multi–year evaluation because forage performance is strongly influenced by genotype, stand age, and annual environmental variation. We evaluated four experimental genotypes and the cultivar WUSU as a control over three production years at a fixed alpine site on the Qinghai–Tibet Plateau. Agronomic traits, forage yield, dry matter accumulation, and nutritional quality were measured annually. A multi–criteria TOPSIS model was used to integrate yield and quality traits for genotype ranking, while random forest analysis and piecewise structural equation modeling were applied to identify key traits and potential pathways influencing forage performance. Genotype, year, and their interaction significantly affected most agronomic, yield, and nutritional traits. Most traits reached their highest values in the third production year, indicating that this stage was critical for evaluating full productive potential. Among the tested materials, genotype 4–4 showed consistently high biomass production and favorable nutritional performance, whereas WUSU and genotype 1–10 generally ranked lower. Plant height and grass height were positively associated with fresh and hay yield, while fresh forage yield, crude protein content, and stem diameter contributed strongly to model prediction. The SEM results suggested that genotype–year interaction influenced hay yield mainly through changes in stem diameter and acid detergent fiber content. These findings indicate that combining multi–year field evaluation with multi–criteria ranking and pathway analysis can improve the identification of promising B. inermis germplasm. Genotype 4–4 represents a useful candidate for further multi–site validation and breeding for high–yield, high–quality forage production in alpine regions. These findings provide a theoretical basis and candidate germplasm for the genetic improvement of Bromus inermis Leyss. adapted to the Qinghai–Tibet Plateau.

Early senescence of alpine pasture grass, manifested as rapid yellowing at the onset of autumn on the Qinghai–Tibet Plateau, constrains the sustainable development of grassland animal husbandry. Breeding stay-green forage germplasm is key to mitigating this problem. To identify superior stay-green germplasm and preliminarily elucidate the main drivers of senescence, we evaluated six stay-green lines of Elymus sibiricus with non-stay-green materials as controls. Fixed-site field observations were conducted for three consecutive years in Haiyan County, Qinghai Province. We quantified dynamic changes in phenotypic, photosynthetic, and physiological traits during senescence, applied mixed-effects models to identify factors associated with stay-green, and used the TOPSIS model for comprehensive evaluation. The results showed that plant height, green leaf area, chlorophyll content, net photosynthetic rate, and root activity of stay-green E. sibiricus were significantly higher than those of non-stay-green materials at all planting years, and the senescence rate was significantly slower. All traits performed optimally at the third year. Relative to HB-2, HB-4, HB-8, HB-10, HB-11, and HB-15 (stay-green E. sibiricus), plant height, green leaf area, chlorophyll content, net photosynthetic rate, and root activity of CK (non-stay-green E. sibiricus) were 0.83, 0.95, 0.79, 0.82, 0.80, and 0.78; 0.37, 0.37, 0.34, 0.35, 0.31, and 0.35; 0.82, 0.84, 0.80, 0.86, 0.82, and 0.75; 0.86, 0.86, 0.74, 0.89, 0.77, and 0.70; and 0.72, 0.74, 0.66, 0.78, 0.70, and 0.61, respectively. Mixed-effects modeling identified chlorophyll, root vitality, soluble sugars, and photosynthesis as the primary determinants of stay-green in E. sibiricus. The TOPSIS model indicated that HB-15 maintained the highest fitting degree values in years 2–4. These values were 0.69, 0.62, and 0.71, respectively. Therefore, HB-15 was the most ideal stay-green germplasm. These findings provide a theoretical basis and elite parental materials for breeding new stay-green varieties of E. sibiricus.