Legume/Maize Intercropping and N Application for Improved Yield, Quality, Water and N Utilization for Forage Production

Abstract

Legume/maize intercropping has been practiced in many countries as a sustainable cropping system, but the effects of intercropping legumes with maize together with N application rates on biomass yield, quality, water-use efficiency (WUE), and nitrogen use efficiency (NUE) are limited under arid conditions in Northwest China. Field experiments were carried out in arid areas of Northwest China from 2019 to 2020 with three planting patterns (LM: Dolichos lablab (Lablab purpureus L.)/silage maize (Zea mays L.) intercropping; FM: Fodder soybean (Glycine max L.)/silage maize intercropping; M: silage maize monoculture) and four N application levels (N1: 0 kg N ha⁻¹; N2: 120 kg N ha⁻¹; N3: 240 N kg ha⁻¹; N4: 360 N kg ha⁻¹). The results showed that nitrogen fertilizer had a significant (p < 0.01) effect on total yield, WUE, and various nutrient parameters and the interaction between planting mode and nitrogen fertilizer had no significant effect on the above indicators, but had a significant (p < 0.01) effect on NUE. Compared with N1, the N3 and N4 treatments significantly increased fresh and hay yield, crude protein yield, crude protein concentration, and crude fat concentration of maize, legumes, and the whole silage system, and decreased the concentration of neutral detergent fiber (NDF) and acid detergent fiber (ADF). In comparison with N1, the 2-year average total biomass yield of N3 and N4 increased by 60.38% and 56.45%, respectively, and the total crude protein yield increased by 106.71% and 100.00%, respectively. High N input treatments (N3 and N4) significantly increased WUE_B (the WUE of legume and maize biomass), N concentration, N uptake, and NUE than N1, and the 2-year average NUE of N3 was 59.52% greater than that of N4. The results also show that LM and FM increased crude protein concentration and decreased NDF and ADF concentration compared with M, and the forage quality of LM was greater than that of FM. In contrast with M, LM and FM increased biomass yield by 3.70% and 1.72%, crude protein yield by 32.05% and 22.82%, and WUE_B by 10.49% and 6.02%, respectively. Application of 240 kg N ha⁻¹ in the Dolichos lablab–maize intercropping systems produced better dry biomass yield with increased forage qualities than other treatments, but the economic analysis is needed before making a recommendation.

1. Introduction

The global population is expected to continue to grow, which will result in a significant increase in food, feed, and fuel demand by the middle of the 21st century [1,2]. With the rapid development of animal husbandry in China, feed shortage is increasing [3]. Therefore, it is urgent to accelerate the development of the feed production industry in China [4]. Maize (Zea mays L.) is a major crop in China, has a wide range of uses, including as food, feed, industrial raw materials, and bioenergy [1]. With the development of green agriculture, maize will play an increasingly important role in crop production in China [5]. In addition, silage maize is also essential as animal feed for animal-derived products. Therefore, it is important to develop efficient management strategies for silage maize to improve yield and quality and to efficiently utilize water and fertilizers.

As a cultivation method commonly used by Chinese farmers, intercropping can be divided into two types: strip intercropping, in which crops are planted on different strips at the same time, and relaying intercropping, in which late season crops are planted in rows later in the early crop growth season [6]. Compared with monoculture, intercropping can make better use of nutrients, water, and light and can improve land productivity [7,8] and resource-use efficiency [9]. At the same time, intercropping increases farmland biodiversity [10]. Silage maize, as a kind of roughage, has been widely planted globally, but silage maize generally has the problem of low protein concentration [11]. However, intercropping legumes with maize allows us to take advantage of species complementarity to increase the crop productivity as well as the crude protein concentration of forage, improving the nutritional quality of forage crops [12,13]. Studies have shown that soybean (Glycine max L.)/maize intercropping can lead to a greater land equivalent ratio (1.25–1.46), which indicates that intercropping can increase crop yield [14]. Intercropping is an effective way to stabilize crop yield and reduce N input [15,16]. In addition, the use of legumes in the intercropping system can improve N-use efficiency (NUE) and total biomass under reduced chemical fertilizer input [12]. This is because legumes can not only fix N in the atmosphere through biological N fixation [17], but can also make effective use of chemical fertilizers through interspecific N competition [18]. In addition, intercropping can help to control diseases, pests, and weeds [19].

In addition to intercropping, N application is an important factor affecting forage crop yield, quality, and water and N utilization. At present, the amount of N fertilizer applied per unit area in China has far exceeded the world average [20]. The heavy use of N fertilizer not only leads to low NUE, but also produces a range of environmental problems, such as nitrate pollution of groundwater [21], intensification of river eutrophication [22], and acidification of farmland soil [23]. Therefore, optimizing N management to improve NUE is a vital step toward realizing the sustainable development of agriculture in China. Reasonable N application can promote the absorption of N by plants [24], leading to improvements in yield, water-use efficiency (WUE), and NUE of forage crops [25,26]. Increasing the amount of N application can also increase the protein concentration and reduce the starch concentration of maize grain [27,28]. An appropriate N application rate is important for maize production in arid areas, especially for ensuring N supply after the silking stage [29,30]. However, applying N fertilizer beyond that needed to meet the N demand of crops will not increase crop yield and N absorption, but will reduce NUE [31,32]. Xu et al. [12] reported that a higher N application rate can increase maize yield, but may lead to reduced NUE and WUE. Given these previous research results, there remain critical gaps in our understanding regarding the effects of N application on maize yield, WUE, and NUE. Furthermore, information is lacking regarding the arid region of Northwest China. Therefore, the appropriate N application rate for silage maize needs to be elucidated. The objectives of this study are three-fold: (a) Can the maize/legumes intercropping increase the biological yield compared with maize monoculture? (b) Between Dolichos lablab and forage bean, which species will help maize achieve a greater yield in an intercropping system? (c) What is the appropriate nitrogen application rate for the maize–legume intercropping system? We hypothesized that the lablab bean/maize intercropping could increase forage yield and improve quality, WUE, and NUE under appropriate nitrogen application conditions.

2. Materials and Methods

2.1. Site Description

This experiment was carried out in the Linze Grassland Agricultural Experiment Station, State Key Laboratory of Grassland Agroecosystem, Lanzhou University (Figure 1). The area is located in Linze County, Zhangye City, Gansu Province (100°02′ E, 39°15′ N). According to the Köppen classification [33], the region has the three-levels climate classification of BSk (Dry–Dry Summer–Cold arid) and lies at an altitude of 1390 m above sea level. The annual average temperature was 8.94 °C over 30 years (1981–2010), the average annual potential evaporation is 2337.6 mm, and the average annual rainfall is 114 mm over 30 years. The rainfall is mainly concentrated in the summer and autumn, accounting for more than 60% of the total annual precipitation. In 2019, the annual precipitation of the experimental station was higher than usual at 167.7 mm (Figure 2a). The annual precipitation in 2020 was normal at 122.1 mm (Figure 2b). In 2019, we analyzed the soil in the 0–20 cm soil layer before sowing. Soil pH was 7.85, soil bulk density was 1.22 g cm⁻³, soil organic matter was 10.62 g kg⁻¹, soil total N was 0.79 g kg⁻¹, soil total P was 0.86 g kg⁻¹, soil total K was 0.53 g kg⁻¹, available N was 0.06 g kg⁻¹, available P was 0.05 g kg⁻¹, and available K was 0.17 g kg⁻¹.

2.2. Field Management and Research Design

A randomized block experimental design was used to set up three planting patterns: lablab bean/silage maize intercropping (LM), fodder soybean/silage maize intercropping (FM), and silage maize monoculture (M). Four N application levels were set for each planting pattern: no N application (N1: 0 kg N ha⁻¹), low N application (N2: 120 kg N ha⁻¹), medium N application (N3: 240 kg N ha⁻¹), and high N application (N4: 360 kg N ha⁻¹). There were 12 treatments in the experiment, and each treatment had three replications. There were 36 plots in the experiment, and the plot area was 77.0 m² (length × width: 7.7 m × 10 m). A 1.2 m-wide isolation belt was set to prevent water leakage between plots. Silage maize was sown on 26 April 2019 and 2 May 2020 seed rate of about 86,500 plants ha⁻¹ (the spacing was 21 cm). The maize was sown in wide and narrow rows with a spacing of 70 cm and 40 cm. Two fodder soybean or lablab bean seeds were planted in one planting spot between two maize plants. In the N1 treatment, 138 kg ha⁻¹ of phosphate fertilizer was applied before sowing; in the N2 treatment, 138 kg ha⁻¹ of phosphate fertilizer and 120 kg ha⁻¹ of N fertilizer was applied before sowing; in the N3 and N4 treatments, the basal fertilizer was the same as that in N2, and 120 kg ha⁻¹ of N fertilizer was applied at the jointing stage in the N3 treatment, and 120 kg ha⁻¹ of N fertilizer was applied at the six-leaf and 12-leaf stages in the N4 treatment. The irrigation amount of each treatment was 400 mm, and 50% irrigation was conducted at the jointing (25 June) and silking (30 July) stages in 2 years.

In this experiment, after the harvest of crops in the previous season, we plowed 20~30 cm in autumn and leveled the land. The fertilizer is urea (Nitrogen content is 46.4%), which is provided by the China Fertilizer Company Limited and applied manually between two rows of maize. In the selection of crop varieties, we selected silage maize (Quchen No. 9, Yunnan Quchen Seed Industry Co., Ltd., Qujing, China); fodder soybean (Songnen fodder soybean, Heilongjiang Animal Husbandry Research Institute), and lablab bean (Highworth, Beijing best grass Industry Co., Ltd., Beijing, China). The seed rate of maize and bean is about 86,500 and 173,000 plants ha⁻¹, respectively. The seeds are plump, and the germination rate is more than 97%. In terms of weed control, we use artificial weeding.

2.3. Sampling and Measurements

2.3.1. Dry Biomass Yield

In the maize harvest period, a 6-m² quadrat was randomly selected in each plot biomass above the ground was harvested manual, and the fresh weight of legume plants and maize was determined to calculate the fresh grass yield of maize and legume crops. After that, the legume and maize were oven dried at 65 °C for more than 48 h to a constant weight. Then, the dry weight was measured to calculate the dry biomass of legumes and maize. The formula is shown below.

2.3.2. Forage Quality and Crude Protein Yield

The N concentration of legumes and maize was determined by the Micro Kjeldahl method, and the crude protein concentration was calculated by multiplying percent N by 6.25, then the CP yield was calculated by multiplying dry biomass yield by crude protein percent [34]. The content of crude fat was determined by the Soxhlet ether extraction method using an ether extract analyzer (XT15, Ankom, America) [35]. Additionally, the content of neutral detergent fiber and acid detergent fiber in forage crops was measured by Van Soest’s detergent fiber analysis method proposed in 1991, it is mainly through digestion and filtration for reference analysis [36].

2.3.3. Soil Water Concentration and Soil Water Storage

During sowing and harvest time, three soil sample points were randomly drilled from each plot between two maize plants in the same row. We sampled the soil every 20 cm in the 0–200 cm soil layer using the 1 m and 2 m soil drill coordinately and stored it in an aluminum box. Samples were oven dried at 105 °C for more than 48 h to a constant weight and then weighed to calculate the soil water concentration (SWC) and the soil water storage (SWS). The formulae are as follows:

$$\mathrm{SWC}\left(\%\right)=\left(\mathrm{W}-\mathrm{D}\right)/\mathrm{D}\times 100\%$$

(1)

$$\mathrm{SWS}\left(\mathrm{mm}\right)={{\displaystyle \sum }}_1^n{h}_i\times {\rho }_i\times b_i$$

(2)

where W is the weight of wet soil (g), D is the weight of dry soil (g); h_i (cm) is the depth of soil layer i, ρ_i (g cm⁻³) is the bulk density of soil layer i, b_i (%) is the soil mass moisture concentration of soil layer i, and n is the number of soil layers.

2.3.4. Evapotranspiration and Water-Use Efficiency

Evapotranspiration refers to the total process of transporting farmland surface water to the atmosphere, which is an important part of farmland water balance. Additionally, water use efficiency refers to the mass of dry matter produced by evapotranspiration of crops consuming a unit mass of water in the field. The formulae are as follows:

ET (mm) = P + I + SWS_S – SWS_H

(3)

WUE (kg ha⁻¹ mm⁻¹) = Y⁄ET

(4)

where ET is the evapotranspiration, P is the precipitation (mm), I is the irrigation amount (mm), SWS_S is the SWS before sowing (mm), SWS_H is the SWS during harvest (mm), and Y (kg ha⁻¹) is the total hay yield.

2.3.5. N Concentration, N Absorption, and N-Use Efficiency

After measuring the dry weight, the whole legume and maize plants were crushed and stored in self-sealing bags. The N concentration of crushed samples was measured by an Automatic Kjeldahl N determinate (K-375, Buqi, Switzerland), and the N absorption and NUE were calculated according to the dry biomass yield of legumes and maize [37]. The calculation formulae are as follows:

NA (kg ha⁻¹) = NC × Y

(5)

NUE (kg kg⁻¹) = (NA_N − NA_NN)/NA_A

(6)

where NA is the N absorption and Y is the dry biomass yield. NA_N is the N absorption from N fertilized plot, NA_NN is the N absorption of no N application, NA_A is the amount of N application, and NC is the N concentration.

2.3.6. Statistical Analysis

Excel 2010 was used for data processing and mapping, data were analyzed using a residual test method before statistical analysis, and the dot met the assumption of homogeneity of variances and followed a normal distribution. SPSS 13.0 (SPSS Inc., Chicago, IL, USA) was used for the analysis of variance, and Tukey’s method was used for multiple comparisons between different treatments. The significance level was set as p < 0.05.

3. Results

3.1. Fresh Biomass Yield, Dry Biomass Yield, and Crude Protein Yield

As shown in Figure 3a–d and Figure 4a–d, planting patterns (P), nitrogen application (N), and interaction (P × N) has a significant effect on fresh and dry biomass yields of legume (p < 0.01), but P × N has no significant effect on fresh and dry biomass yields of maize and in total (p > 0.05). As shown in Figure 3a,b, Figure 4a,b and Figure 5a,b, under the same planting mode during the two experimental years, the fresh and dry biomass yield and crude protein (CP) yield of legumes treated with N3 and N4 were significantly greater than those treated with N1. The average biomass yield of legumes treated with N2, N3, and N4 was significantly greater than that of those treated with N1, and there was no significant difference between N3 and N4. At N3 and N4 levels, the fresh and dry yield of legumes under LM treatment were significantly greater than those under FM, and the CP yield of legumes under LM treatment was significantly greater than that under FM at N3 level. The average value showed that the fresh, dry, and CP yields of legumes under LM treatment were significantly greater than those under FM treatment.

As shown in Figure 3c–f in Figure 4c–f and in Figure 5c–f, under the same planting patterns, the yields of fresh, dry, and CP of maize and in total under N3 and N4 treatments were significantly greater than those under N1, but there was no significant difference between N3 and N4. The average value showed that the fresh and dry yields of maize and in total under N2, N3, and N4 treatments were significantly greater than those under N1, in which the average total fresh biomass yield increased by 27.21%, 60.38%, and 56.45%, the average total dry yield increased by 26.34%, 58.27%, and 52.94%, and the average total CP yield increased by 45.94%, 106.71%, and 100.00%, respectively. Under the same N application level, except for the total CP yield in 2020, there was no significant difference in the fresh, dry, and CP yields of maize. The average value showed that the total fresh biomass, dry, and CP yields of LM were significantly greater than those of M, with an average increase of 11.97%, 8.93%, and 32.05%, respectively. Among the 12 treatments, LM-N3 obtained the greatest 2-year average total fresh grass, dry, and CP yields.

3.2. Crude Protein, Crude fat, NDF, and ADF Concentration

As shown in

Table 1. Crude protein and crude fat concentration of silage maize and leguminous crops under different treatments in 2019 and 2020.

Year	Planting Patterns	Nitrogen Fertilizer Level	Crude Protein Concentration (%)			Crude Fat Concentration (%)
			Maize	Legume	Total	Maize	Legume	Total
2019	LM	N1	6.4 c	17.5 de	7.6 de	2.07 de	3.26 b	2.20 de
		N2	7.4 abc	19.3 bcd	8.7 bcd	2.44 bcd	3.37 ab	2.54 abc
		N3	8.4 ab	22.0 a	9.9 a	2.65 abc	3.64 ab	2.76 ab
		N4	8.7 a	22.0 a	10.1 a	2.74 ab	3.67 ab	2.84 a
	FM	N1	6.5 c	15.7 e	7.4 ef	1.98 e	3.39 ab	2.12 de
		N2	7.2 bc	17.8 cde	8.2 cde	2.27 cde	3.35 ab	2.37 bcd
		N3	8.0 ab	20.0 abc	9.1 abc	2.62 abc	3.61 ab	2.71 ab
		N4	8.4 ab	20.6 ab	9.5 ab	2.77 ab	3.78 a	2.87 a
	M	N1	6.4 c	NA	6.4 f	2.01 e	NA	2.01 e
		N2	7.3 bc	NA	7.3 ef	2.49 abc	NA	2.49 abc
		N3	8.1 ab	NA	8.1 cde	2.81 ab	NA	2.81 a
		N4	8.5 ab	NA	8.5 bcde	2.93 a	NA	2.93 a
	AVOVA	P	ns	**	**	ns	ns	ns
		N	**	**	**	**	**	**
		P×N	ns	**	ns	ns	ns	ns
2020	LM	N1	6.2 e	18.7 de	7.3 e	2.86 cd	3.09 e	2.88 cd
		N2	7.3 d	20.5 bcd	8.6 c	2.92 abcd	3.44 de	2.97 bc
		N3	8.6 a	23.7 a	10.1 a	3.19 a	3.84 cd	3.25 ab
		N4	8.1 abc	22.5 ab	9.7 ab	3.17 ab	4.06 bc	3.27 a
	FM	N1	6.1 e	17.6 e	7.1 e	2.92 abcd	4.18 bc	3.03 abc
		N2	7.5 cd	19.6 cde	8.6 c	2.87 bcd	4.42 ab	3.02 abc
		N3	8.5 a	22.1 abc	9.6 ab	3.00 abc	4.85 a	3.15 abc
		N4	8.3 ab	20.1 bcde	9.3 b	3.16 ab	4.84 a	3.30 a
	M	N1	6.1 e	NA	6.1 f	2.69 d	NA	2.69 d
		N2	7.2 d	NA	7.2 e	2.92 abcd	NA	2.92 cd
		N3	8.1 abc	NA	8.1 cd	3.03 abc	NA	3.03 abc
		N4	7.7 bcd	NA	7.7 de	3.09 abc	NA	3.09 abc
	AVOVA	P	**	**	**	ns	**	**
		N	**	**	**	**	**	**
		P × N	ns	**	ns	ns	*	ns

Note: M, silage maize monoculture; FM, intercropping of fodder soybean and silage maize; LM, intercropping of Dolichos lablab and silage maize; N1, 0 kg ha⁻¹ N; N2, 120 kg ha⁻¹ N; N3, 240 kg ha⁻¹ N; N4, 360 kg ha⁻¹ N. P, N, and P × N represents planting patterns, N application, and interaction between them, respectively; * p < 0.05 level, ** p < 0.01; ns, no significant difference. Different lowercase letters in the same column indicate significant differences among different treatments for the same forage (p < 0. 05). The different uppercase letters in the same column indicate significant differences among different levels of factors for the same forage (p < 0. 05).

, under the same planting patterns, the CP and crude fat (CF) concentration in maize and in total treated with N3 and N4 were significantly greater than those under N1, and the CP concentration of legumes treated with N3 and N4 was significantly greater than that under N1. The average value showed that the CP concentration in maize, legumes, and in total under N2, N3, and N4 was significantly greater than that under N1, and the average concentration of total CP over 2 years increased by 16.55%, 30.94%, and 31.65%, respectively, compared with that under N1. The CF concentration in maize and in total under N3 and N4 was significantly greater than that under N1, and the average total CF concentration over 2 years increased by 18.47% and 22.49%, compared with that in N1. Under the same N application condition, there was no significant difference in the CP concentration of maize and legume, as well as the total CF concentration of maize and legume, but the total CP concentration of LM treatment was significantly greater than that of M treatment. The average value showed that the total CP concentration of LM was significantly greater than that of M, which increased by 20.81% over 2 years. Among all treatments, LM-N3 obtained the greatest average maize, legume, and total CP concentration during 2-year.

Table 2. Acid detergent fiber (ADF) and neutral detergent fiber (NDF) concentration of silage maize and leguminous crops under different treatments in 2019 and 2020 years.

Year	Planting Patterns	Nitrogen Fertilizer Level	NDF Concentration (%)			ADF Concentration (%)
			Maize	Legume	Total	Maize	Legume	Total
2019	LM	N1	41.43 ab	39.97 ab	41.27 ab	24.89 ab	36.85 bc	26.21 a
		N2	39.97 ab	37.51 bcd	39.71 ab	24.81 ab	36.45 cd	26.05 a
		N3	36.98 ab	35.04 cd	36.77 ab	22.57 ab	32.51 de	23.66 ab
		N4	36.23 b	34.21 d	36.01 b	21.85 b	32.05 e	22.94 ab
	FM	N1	41.82 ab	43.28 a	41.96 ab	25.13 ab	41.20 a	26.66 a
		N2	40.20 ab	41.72 ab	40.34 ab	25.24 ab	40.50 ab	26.62 a
		N3	37.60 ab	40.19 ab	37.83 ab	23.36 ab	35.40 cde	24.46 ab
		N4	37.14 ab	39.31 abc	37.34 ab	22.30 ab	33.80 cde	23.38 ab
	M	N1	43.27 a	NA	43.27 a	26.20 a	NA	26.20 a
		N2	41.75 ab	NA	41.75 ab	25.53 ab	NA	25.53 ab
		N3	39.83 ab	NA	39.83 ab	23.19 ab	NA	23.19 ab
		N4	37.63 ab	NA	37.63 ab	21.92 b	NA	21.92 b
	AVOVA	P	ns	**	ns	ns	**	ns
		N	**	**	**	**	**	**
		P×N	ns	ns	ns	ns	**	ns
2020	LM	N1	44.08 ab	40.39 ab	43.74 ab	27.56 a	33.78 ab	28.12 a
		N2	42.07 abc	40.10 ab	41.89 abc	25.00 ab	31.84 bc	25.63 abc
		N3	38.77 bc	35.54 b	38.45 bc	22.11 bc	26.61 d	22.55 cd
		N4	36.94 c	36.03 b	36.84 c	20.87 c	26.52 d	21.50 d
	FM	N1	43.30 ab	42.74 a	43.25 ab	26.74 a	36.38 a	27.6 ab
		N2	41.15 abc	41.14 a	41.15 abc	24.47 abc	35.44 a	25.49 abc
		N3	39.91 bc	39.34 ab	39.87 bc	22.94 bc	29.77 cd	23.49 cd
		N4	38.72 bc	38.20 ab	38.68 bc	21.30 bc	30.42 bc	22.04 cd
	M	N1	46.09 a	NA	46.09 a	27.08 a	NA	27.08 ab
		N2	44.27 ab	NA	44.27 ab	24.64 abc	NA	24.64 abcd
		N3	42.18 abc	NA	42.18 abc	24.23 abc	NA	24.23 bcd
		N4	39.55 bc	NA	39.55 bc	21.96 bc	NA	21.96 cd
	AVOVA	P	*	**	**	ns	**	ns
		N	**	**	**	**	**	**
		P × N	ns	ns	ns	ns	**	ns

Note: M, silage maize monoculture; FM, intercropping of fodder soybean and silage maize; LM, intercropping of Dolichos lablab and silage maize; N1, 0 kg ha⁻¹ N; N2, 120 kg ha⁻¹ N; N3, 240 kg ha⁻¹ N; N4, 360 kg ha⁻¹ N. P, N, and P × N represent planting patterns, N application, and interaction between them, respectively; * p < 0.05 level, ** p < 0.01; ns, no significant difference. Different lowercase letters in the same column indicate significant differences among different treatments for the same forage (p < 0. 05). The different uppercase letters in the same column indicate significant differences among different levels of factors for the same forage (p < 0. 05).

shows that in 2020, under LM and M planting patterns, the NDF concentration of maize and in total under the N1 treatment were significantly greater than those under N4. Under the same planting patterns for 2 years, the NDF concentration of legumes under the N1 treatment was significantly greater than that under N3 and N4. The average value showed that the NDF and ADF concentration of maize and in total under the N1 treatment were significantly greater than those under N3 and N4, and the NDF concentration of legumes under the N1 treatment was significantly greater than that under N3 and N4 treatment. The 2-year average total NDF concentration under N1 increased by 10.50% and 14.35%, respectively, compared with that under N3 and N4, and the total ADF concentration increased by 14.85% and 21.04%, respectively. Under the same N application level, there was no significant difference in NDF and ADF concentration between maize and in total under different planting methods over 2 years. The average value showed that the NDF and ADF concentrations of legumes under the FM treatment were significantly greater than those under LM. The 2-year average ADF and NDF concentration under the LM-N3 treatment were lower than those under LM-N1 and M-N1, but there was no significant difference.

3.3. N-Use Efficiency

As shown in Figure 6, under the same planting patterns, the NUE in maize, legumes, and in total under N2 and N3 was significantly greater than that under N4. The average value showed that the NUE in maize, legumes, and in total under N2 and N3 was significantly greater than that under N4, and the average total NUE over 2 years increased by 37.30% and 59.52%, respectively, compared with that under N4. Under the same N application level, there was no significant difference in the NUE of maize under different planting methods in 2019. The average value showed that the total NUE of FM and LM was significantly greater than that of M, with an increase of 33.09% and 27.34%, respectively, and the total NUE of LM was significantly greater than that of FM treatment. Among all treatments, the LM-N3 obtained the greatest average of NUE of maize and legume over 2 years.

3.4. Soil Water Storage, Evapotranspiration, and Water-Use Efficiency

As shown in

Table 3. Soil water storage, evapotranspiration, and water-use efficiency under different treatments in 2019 and 2020 years.

Year	Planting Patterns	Nitrogen Fertilizer Level	Water Storage before Sowing (mm)	Water Storage during Harvest (mm)	Soil Evapotranspiration (mm)	Water Use Efficiency of Legume and Maize Biomass (kg ha−1 mm−1)
2019	LM	N1	367.6 a	352.8 a	553.9 a	38.0 cd
		N2	361.5 a	328.7 a	571.8 a	47.2 ab
		N3	359.9 a	308.9 a	590.1 a	54.5 a
		N4	358.8 a	307.2 a	590.7 a	52.9 a
	FM	N1	368.9 a	354.0 a	554.1 a	35.4 d
		N2	369.8 a	346.4 a	562.5 a	47.1 ab
		N3	367.3 a	312.5 a	594.0 a	53.7 a
		N4	373.6 a	310.7 a	601.9 a	52.5 a
	M	N1	368.9 a	341.2 a	566.8 a	35.8 d
		N2	369.8 a	319.7 a	589.3 a	44.1 bc
		N3	369.0 a	303.2 a	604.9 a	51.5 ab
		N4	372.1 a	296.3 a	614.9 a	50.4 ab
	AVOVA	P	ns	ns	ns	ns
		N	ns	**	ns	**
		P × N	ns	ns	ns	ns
2020	LM	N1	354.1 a	380.5 a	496.1 a	38.2 cd
		N2	339.3 a	362.5 a	494.2 a	46.2 bc
		N3	329.6 a	370.0 a	477.0 a	63.6 a
		N4	334.9 a	386.2 a	466.1 a	61.8 a
	FM	N1	353.3 a	371.2 a	499.5 a	36.5 d
		N2	347.8 a	361.3 a	503.9 a	44.2 bcd
		N3	330.1 a	358.2 a	489.3 a	61.7 a
		N4	348.6 a	378.1 a	487.9 a	58.5 a
	M	N1	349.2 a	381.2 a	485.4 a	38.6 bcd
		N2	341.5 a	375.6 a	483.3 a	47.7 b
		N3	327.5 a	368.6 a	476.3 a	61.2 a
		N4	343.6 a	376.6 a	484.3 a	56.2 a
	AVOVA	P	ns	ns	ns	ns
		N	ns	**	ns	**
		P×N	ns	ns	ns	ns

Note: M, silage maize monoculture; FM, intercropping of fodder soybean and silage maize; LM, intercropping of Dolichos lablab and silage maize; N1, 0 kg ha⁻¹ N; N2, 120 kg ha⁻¹ N; N3, 240 kg ha⁻¹ N; N4, 360 kg ha⁻¹ N. P, N, and P × N represent planting patterns, N application, and interaction between them, respectively; ** p < 0.01; ns, no significant difference. Different lowercase letters in the same column indicate significant differences among different treatments for the same forage (p < 0. 05). The different uppercase letters in the same column indicate significant differences among different levels of factors for the same forage (p < 0. 05).

, under the same planting method, there was no significant difference in SWS and field ET before sowing and during harvest under different N application treatments. The average value showed that there was no significant difference in SWS and field ET between N application treatments. In 2019, the harvest water storage under the N1 and N2 treatments were significantly greater than that of N3 and N4, while in 2020, the harvest water storage under the N4 treatment was significantly greater than that of N2 and N3. Under the same N application level, there was no significant difference in SWS and field ET between sowing and harvest under different planting methods. The 2-year average water storage before sowing and harvest and field ET of the LM-N3 treatment were lower than those of FM-N4, LM-N1, and M-N4, but there was no significant difference.

Under the same planting patterns, the WUE of legume and maize (WUE_B) treated with N2, N3, and N4 were significantly greater than those under N1, but there was no significant difference between N3 and N4. The average value showed that the WUE_B of N2, N3, and N4 over 2 years were significantly greater than those of N1. The average WUE_B increased by 24.38%, 55.80%, and 49.80%, respectively, compared with those in N1. Under the same N application condition, WUE_B under different planting methods had no significant difference over 2 years. The average value showed that WUE_B under LM treatment was significantly greater than that under M, with an increase of 10.49% compared with M. Among all treatments, LM-N3 obtained the greatest average WUE_B during 2-year.

4. Discussion

4.1. Effects of Legume/Maize Intercropping and N Application on Forage Crop Yield

Under legume/maize intercropping, legume crops can usually compensate for the yield loss of maize caused by low planting density and can improve forage yield [14]. The study has shown that legume/maize intercropping can improve natural resources use efficiency by coordinating competition and complementarity between the two crops [38]. This may be because legume/maize intercropping can get a greater soil coverage than pure maize and the weeds were reduced by the limited light availability [14]. In addition, compared with maize monoculture, maize yield under cowpea/maize and soybean/maize intercropping increased by 25% and 22%, respectively [39,40]. As a feed crop with high nutrition and good palatability, forage soybean can be mixed with maize to improve crop quality and yield. When the ratio of legume to maize is 1:3 and 1:2, the population yield is 15.5% and 16.4% greater than that of maize monoculture, respectively [41]. Lablab bean is a forage crop with high protein concentration, high-temperature resistance, frost resistance, and late maturity. Studies have reported that intercropping lablab beans with maize can improve crop quality, but reduce maize yield [42], which was confirmed by our study results. In the 2-year experiment, the yield of maize under legume/maize intercropping was lower than that under maize monoculture, but the total yield of the intercropping system increased due to the increase in legume crop yield. The yield of the lablab bean/maize intercropping system was greater than that of the forage soybean/maize intercropping system.

The suitable N application rate for maize differs in different regions of China. The reasonable N application rate for maize in the North China Plain is 150–240 kg ha⁻¹ [20]. Under the soybean/maize intercropping system in Sichuan Province, greater crop yield was obtained by applying 180 kg ha⁻¹ of N [43]. In a pea/maize intercropping system in the arid area of Northwest China, reducing N application from 400 kg ha⁻¹ to 300 kg ha⁻¹ enhanced the interspecific competition between pea and maize, which improved the yield of the intercropping system [38]. The results of our experiment were similar. In the arid area of Northwest China, N fertilizer (240 kg ha⁻¹) significantly increased the fresh hay and CP yield of maize and legumes as compared with no N fertilizer (0 kg ha⁻¹) and low N fertilizer (120 kg ha⁻¹), but there was no significant difference with high N fertilizer (360 kg ha⁻¹). Wang et al. (2014) showed that under maize/soybean intercropping, the application of 150 kg ha⁻¹ of N increased maize yield and promoted dry matter accumulation compared with no N application [44]. In addition, with sufficient water, the application of N fertilizer in the range of 0–210 kg ha⁻¹ increases maize yield with increasing N application [45]. This result is consistent with our results. When N application was 0–240 kg ha⁻¹ in the arid area of Northwest China, the forage crop yield will increase with increasing N application.

4.2. Effects of Legume/Maize Intercropping and N Application on Forage Crop Quality

Legume/maize intercropping is a common intercropping practice. The mixed sowing of legumes and grasses can improve the CP concentration and palatability of forage crops and improve the nutritional quality of forage crops [46]. Zhang et al. (2015) showed that the protein concentration of feed maize was low, N-fixing legumes/maize intercropping can improve the protein concentration of feed crops [47]. In addition, a greater feed quality will show lower ADF and NDF concentration. Legume/maize intercropping significantly reduces ADF and NDF centration compared with maize monoculture, which may be because it reduces the proportion of maize or increases the proportion of legumes in the total feed yield, thus improving forage quality [48]. The results of this study were consistent with those of the above studies. The intercropping of lablab beans, forage beans, and maize increased the CP and crude fat concentration of mixed forage crops, reduced the concentration of ADF and NDF, and improved the nutritional quality. The nutritional quality of lablab bean/maize intercropping was greater than that of forage bean/maize intercropping. Anil et al. (2000) also showed that legume/maize intercropping reduced the concentration of dry matter, starch, NDF, and ADF of forage crops and increased the concentration of CP [49]. This may be due to the high concentration of protein, Ca, and P in leguminous crops in legume/maize intercropping systems, which is conducive to improving the quality of mixed silage [50].

In addition to the planting patterns, N application is also an important factor affecting the nutritional quality of forage crops. Increased N application has been shown to reduce the grain saturation of crops and increase the protein concentration of grains [51]. In addition, increasing N application increased the protein concentration of maize crops and decreased the starch concentration, which improved the nutritional quality of maize [52]. Yuan et al. (2016) studied the effect of N application on the nutritional quality of maize for many years [53]. Compared with no N application, N application reduced the starch concentration and increased the crude fat concentration of maize. The results showed that high (360 kg ha⁻¹) and medium (240 kg ha⁻¹) N fertilizer treatments increased the concentration of CP and crude fat in maize, legume crops, and in total, reduced the concentration of NDF and ADF, and the nutritional quality of forage crops under high N fertilizer was higher. Zhang et al. (2012) found that N application significantly increased the starch concentration and lysine concentration of maize grains under legume/maize intercropping [54]. In addition, N application increased the forage CP concentration and decreased NDF under legume/maize intercropping, which was conducive to improving the silage quality of forage crops [55].

4.3. Effects of Legume/Maize Intercropping and N Application on Water and N Utilization of Forage Crops

Legume/maize intercropping could improve the WUE and NUE of forage crops compared with maize monoculture. Mao et al. (2012) studied a pea/maize intercropping system in which maize, a C₄ crop, had high physiological WUE, and pea, a drought-tolerant crop, can use water more effectively, so the WUE of the system was improved compared with monocultured crops [56]. This was consistent with the results of our experiment. The WUE in our 2-year intercropping system was significantly greater than that of the monoculture system, and the WUE of lablab bean/maize intercropping was greater than that of forage bean/maize intercropping. In addition, studies have shown that WUE under maize/peanut 1:5 intercropping was 83.2% greater than that of monocultured maize [57]. This is because mixed planting or intercropping can improve the coverage of the ground surface and reduce ineffective evaporation from the soil, thus improving the WUE of the system [58]. In addition, legume/maize intercropping helps to increase the abundance of N-fixing bacteria in the soil, thus improving the ability of crops to absorb N from the soil [59]. A correlativity study shows that symbiotic N₂ fixation increases because legume and maize root interaction significantly increases legume nodulation and maize flavonoids (signaling compounds of rhizobia) exudation [60]. Therefore, compared with monoculture, legume/maize intercropping can improve the soil water conditions of crops, promote the absorption and utilization of N by forage crops, and then improve NUE [61,62]. This is consistent with the results of our study. The NUE of the legume/maize intercropping system was significantly greater than that of maize monoculture. This may be because mixed planting or intercropping changes the spatial structure, thus affecting the competition between crops for resources, and then affecting the utilization of N fertilizers [63].

In addition to the planting patterns, N application also affected the WUE and NUE of forage crops. Studies have shown that in the legume/maize intercropping system, reducing the N application rate at the jointing stage of maize from 135 kg ha⁻¹ to 45 kg ha⁻¹ can prolong the N accumulation time of pea and increase the total N accumulation [64]. Rational application of N fertilizer can improve NUE. If a large amount of N fertilizer is applied, then the yield input ratio of N fertilizer will decrease and the NUE will be reduced [65]. The results of our experiment were similar to those reported by the above studies. Compared with no N application, N application significantly increased the N concentration and absorption of maize, leguminous crops, and the whole system, and the NUE after treatment with 240 kg N ha⁻¹ was significantly greater than that after treatment with 360 kg N ha⁻¹. Studies have reported that increasing N fertilizer can significantly increase the yield of forage crops, reduce field ET, and improve crop WUE [24,66]. The results of our study confirmed this. Compared with no fertilization, high N fertilization (360 kg ha⁻¹) and medium N fertilization (240 kg ha⁻¹) increased the total hay yield, but there was no significant difference in field ET, which significantly improved the WUE of maize. In all treatments, the maximum WUE and NUE were obtained under the application of 240 kg N ha⁻¹ in the lablab bean/silage maize intercropping system.

This model provides a management strategy suitable for silage maize forage production in the arid area of Northwest China. The results of this study can provide guidance for farmers and farm managers and similar regions. However, this study is limited in that it was carried out for only 2 years, and the annual rainfall, temperature, and other climatic conditions may differ over longer periods. Therefore, our results and conclusions have certain limitations, and we will carry out longer-term research in the future.

5. Conclusions

Compared with M, LM and FM treatments significantly increased the total fresh hay yield, CP yield, WUE_B, total N concentration, and absorption, and NUE of forage crops over 2 years. Compared with monoculture, intercropping significantly increased the concentration of CP, reduced the concentration of NDF and ADF, and the forage quality of LM was better than that of FM. Compared with N1, N3 and N4 treatments significantly increased the total fresh and hay yield, CP yield, CP concentration, and crude fat concentration, and decreased the concentration of NDF and ADF. In addition, the N3 and N4 treatments significantly improved WUE_B, N concentration, N absorption, and NUE compared with N1, and the 2-year average NUE under N3 was significantly greater than that under N4. Among all treatments, LM-N3 obtained the greatest 2-year average total fresh and hay yield, CP yield, CP concentration, and NUE, and the concentration of NDF and ADF were low. Therefore, the application of 240 kg N ha⁻¹ (LM-N3) in a lablab bean/silage maize intercropping system is a reasonable model for forage crop production in the arid area of Northwest China, but an economic analysis is needed before making a recommendation.