## 4 Discussion  
### Effects of Intercropping on System Productivity and Silage Quality  
Previous studies have shown that alfalfa, as a vigorous perennial legume, is the superior competitor in the alfalfa-silage corn intercropping, and alfalfa could significantly reduce the root growth, phosphorus and nitrogen uptake, and DM yield of the intercropped corn [10; 20; 21; 33]. Different from the system that alfalfa served only as ground cover which is always suppressed by agrichemicals, we should maximize alfalfa production in the spring and corn production in the summer in our intercropping system. Thus, both the alfalfa and silage corn in the AC conducted the same row spacing and production area as the monoculture did in the local area and the interspecific competition between alfalfa and silage corn during the co-existing period was increased. For the silage corn, except the competition from alfalfa, to ensure the alfalfa survival, the lowered corn density (30,000 to 60,000 plants ha\({}^{-1}\)) in intercropping instead of 75,000 plants ha\({}^{-1}\) in monocultured CK also had a significantly negative effect on corn DM yield. As a result, a reduction of 31.5% in the yield of silage corn was found in the AC on average across two years (Table 2). For the reason that the intercropped alfalfa became wilting in late August and regenerated gradually from then, the alfalfa yield during this co-existing period was really low (0.25-1.66 t ha\({}^{-1}\); Figure 2) and the CP content of alfalfa residues was only 175.1 and 170.1 g kg\({}^{-1}\) in 2017 and 2018 (Table 2), which were much lower than that of alfalfa harvested in the bud or early bloom stage [13; 34]. Consequently, the intercropped alfalfa during summer did not contribute much to system yield, while still significantly improving system CP yield (Figure. 2). Besides, the increase in system CP yield also benefited from  
Figure 3: Multicriteria assessment of treatment combinations in alfalfa–silage corn relay intercropping system. A 2-yr system total dry matter (DM) yield, 2-yr system crude protein (CP) yield, and 2-yr average DM content, 2-yr average CP content, 2-yr average pH, 2-yr average lactic acid (LA) concentration and 2-yr average water soluble carbohydrate (WSC) of silage transformed to 0 to 10 scores, 0 being the worst score and 10 the best score. N60, N120 and N180 mean N application rate of 60, 120 and 180 kg N ha\({}^{-1}\). D30, D45 and D60 mean corn seeding rate of 30, 45 and 60 thousand plants ha\({}^{-1}\). -silage corn. The improvement of CP content in corn intercropping with alfalfa had a positive effect on corn CP content which was further improved by N fertilization, especially in 2017 (Table 2), which is consistent with previous studies [35; 36]. Furthermore, we harvested all corn at the same time, but the intercropped corn was less mature than CK (about 10-15 days' delay) for the competition from alfalfa and the CP content decreased with the maturity [37]. The results showed that seven treatment combinations (N60-D60, N120-D30, N120-D40, N120-D60, N180-D30, N180-D40, N180-D60,) achieved the same CP productivity as CK did (Figure. 2), which could compensate for the DM yield losses to a great extent. For silage quality, the addition of alfalfa always improves the CP content of grass silage [12; 13]. Despite the low proportion (\(\leq\)25%) and relatively low CP content (\(<\)19%) of alfalfa raw materials in the AC, the CP of the mixed silage was significantly increased by 16.3% compared with that of CK on average (\(p<0.001\); Table 3). Meanwhile, as mentioned above, the CP contents of corn materials were also significantly increased by intercropping which promoted silage CP concentration (Tables 2 and 3). However, a high level of soluble protein in alfalfa silage can degrade rapidly and yield ammonia which prevents the pH value of silage from reaching the required level [13].