Crude protein (CP) and fiber are the two most important quality traits of forage crops, and accurately estimating them under water and salt stress is of great significance to the production of high-quality alfalfa. However, there is seldom an applicable quality model. Here, we conducted pot and plot experiments under three irrigation amounts (full irrigation W1, moderate water deficit W2, and severe water deficit W3) and six soil salinity levels (S0, S1, S2, S3, S4, and S5, indicating 0, 1, 2, 3, 4, and 5‰ mass ratio of salt per unit soil mass, respectively), to examine the responses of CP and relative feed value (RFV) and develop CP and RFV models in alfalfa at different growth ages under combined water and salt stress. We found that there was a negative parabolic relationship between relative alfalfa CP (CP_r) and relative soil electrical conductivity (EC_r) with a EC_r threshold (EC_rth) for maximum CP_r. Moderate water stress improved the CP_r, while cutting number related to growth age had the opposite effect. The EC_rth tended to decrease with increasing water stress and growth age, i.e., they increased the sensitivity of CP_r to salt stress. Relative RFV (RFV_r) of alfalfa was improved by water and salt stress, however, at the expense of biomass reduction, and the rate of increase declined with increasing growth age. Based on the above, crude protein and fiber models of alfalfa incorporating growth age under water and salt stress were developed, and performed well under pot (CP model: r² = 0.94; RFV model: r² = 0.98) and plot conditions (CP model: r² = 0.95; RFV model: r² = 0.88). Overall, the developed CP and RFV models would provide an essential framework for the production of high-quality forage in saline soil, conservation of water resources, and suppression of salinization.

粗蛋白（CP）和纤维是牧草作物最重要的两个品质性状，准确估算它们在水盐胁迫下对生产优质苜蓿具有重要意义。然而，很少有适用的质量模型。在这里，我们在三种灌溉量（完全灌溉 W1、中度缺水 W2 和严重缺水 W3）和六个土壤盐度水平（S0、S1、S2、S3、S4 和 S5，表示 0、 1, 2, 3, 4, 和 5‰ 每单位土壤质量的盐质量比），以检查CP和相对饲料值（RFV) 并在水盐胁迫下，在不同生长年龄的苜蓿中建立 CP 和 RFV 模型。我们发现相对苜蓿CP ( CP _r ) 和相对土壤电导率 ( EC _r )之间存在负抛物线关系，其中最大CP _r的EC _r阈值 ( EC _rth ) 。适度的水分胁迫提高了CP _r，而与生长年龄相关的扦插数量则有相反的效果。的EC _RTH倾向于随着水分胁迫和生长的年龄，即下降，它们增加的灵敏度CP_r盐胁迫。水盐胁迫提高了苜蓿的相对RFV（ RFV _r），但以生物量减少为代价，并且随着生长年龄的增加，增加率下降。在此基础上，建立了水盐胁迫下苜蓿粗蛋白和纤维模型，并在盆栽（CP模型： r ² = 0.94；RFV模型： r ² = 0.98）和小区条件（CP）下表现良好。模型： r ² = 0.95；RFV 模型： r ²= 0.88）。总体而言，开发的 CP 和 RFV 模型将为在盐渍土壤中生产优质牧草、保护水资源和抑制盐渍化提供重要框架。