The combined effects of soil moisture and plant density on the nutritional quality of alfalfa

A dual-factor pot experiment was conducted to investigate the effects of soil moisture and planting density on the nutritional quality of alfalfa. We analyzed the interactive effects of soil moisture gradients (W₁: 85%～90%, W₂: 75%～80%, W₃: 60%～65%, W₄: 35%～40% of soil field capacity) and plant density (D₁: 10 plants per pot, D₂: 15 plants per pot, D₃: 20 plants per pot) on crude protein, ether extract, crude ash, crude fiber, neutral detergent fibers, acidic detergent fibers, nitrogen-free extract, and relative forage value of alfalfa. We found that the CP, EE, NFE, and RFV of alfalfa were significantly higher under W₂ treatment compared with other treatments. The CF, NDF, ADF, and CA levels were significantly lower in W₂ treatment compared with those in other treatments. The CP and RFV decreased with increasing plant density. EE and CA increased with increasing plant density, with EE being significantly lower in the D₁ and D₂ treatments than in the D₃ treatment. CA was significantly lower in D₁ treatment compared with other treatments. CF, NDF, and ADF showed a trend of initial increase and then decrease with increasing plant density, with CF being significantly lower in D₂ treatment compared with other treatments. The nitrogen-free extract first increased and then decreased with increasing plant density, with no significant difference between the D₁ and D₂ treatments, but significantly higher than the D₃ treatment. There was no significant interaction between soil moisture and plant density for any parameter. Under W₂ soil moisture conditions, alfalfa had higher levels of CP (22.40%), EE (3.40%), NFE (27.04%), and RFV (151.24%) and lower levels of CF (36.26%), NDF (40.23%), ADF (30.53%), and CA (10.91%). Under the D₂ treatment, alfalfa obtained a higher nitrogen-free extract (27.52%) and lower CF (37.79%). Overall, when the soil moisture ranged from 60% to 80% of field capacity and the planting density was 15 plants per pot, the nutritional quality of alfalfa reached its optimum level. Our findings provide a reference for coupled management techniques involving water use and plant density in alfalfa fields.