Effects of Planting Density and Nitrogen Management on Light and Nitrogen Resource Utilization Efficiency and Yield of Summer Maize in the Sichuan Hilly Region

Abstract

The low efficiency of light and nitrogen resources, poor yield and profit, and environmental pollution of maize production are main problems in many areas of China. We hypothesized that optimizing nitrogen fertilizer density management strategies could alleviate the above issues. To address this, a 3-year on-site experiment with three planting densities and four nitrogen rates was conducted in the Sichuan Hilly Region. The results indicated that increasing the planting density could increase the extinction coefficient and solar radiation interception of maize populations as well as enhance the utilization efficiency of light and nitrogen resources and yield. For every 100 kg ha⁻¹ increase in nitrogen fertilizer, RUE increased by 0.16%, NUE decreased by 25.0%, and soil apparent nitrogen loss quantity increased by 67.8 kg ha⁻¹. There was a certain interaction between planting density and nitrogen rate. The appropriate planting density and nitrogen rate combination was 67,500 plants ha⁻¹ with 180 kg N ha⁻¹ under the experimental condition. Excessive close planting in weak-light areas and excessive nitrogen reduction after densification are not advisable. This study indicated that nitrogen–density strategies should be matched with the local natural resources such as sunlight. The results provide a theoretical for high-yield and high-quality maize production in these areas.

1. Introduction

Maize (Zea mays L.) is the most cultivated grain crop both worldwide and in China. Increasing maize production is extremely important for food security [1]. Due to the limited arable land area, increases in maize yield have to be achieved by increasing the yield per unit area [2], which has become the main way to increase maize yield [3,4]. Optimizing management strategies such as fertilizer rate and planting density is an important technical measure to improve maize yields [5,6].

Solar radiation provides energy for crop photosynthesis. Enhancing the radiation use efficiency (RUE) of maize is crucial for increasing its yield [7,8,9]. A proper maize population structure (especially canopy structure) allows for better use of light and heat resources and increases RUE [1,10]. The adjustment of planting density and row spacing configuration is often used as an important measure to directly optimize the canopy structure of maize populations, as well as to improve light distribution and utilization [11,12,13]. Increasing planting density augments the population leaf area, enabling maize to better intercept and utilize solar radiation, thereby enhancing RUE to obtain higher yields [11,14]. The average planting density among the nine winning groups of the 2021 US maize high-yield competition stood at 98,800 plant ha⁻¹, with the highest exceeding 120,000 plant ha⁻¹ [15], while the density of the Chinese highest record maize-yielding field, with 24,948.75 kg ha⁻¹, created at Qitai farm in Xinjiang, is as high as 135,000 plant ha⁻¹ [16]. However, the average planting density in large-scale production in China is approximately 59,000 plant ha⁻¹, which is equivalent to the level of the United States in the 1980s [17]. This suggests that there is ample room for increasing maize planting densities in China, but the appropriate density for different ecological areas and environmental condition needs to be further explored [18,19].

Nitrogen fertilization is an effective technical measure to increase crop yield [20]. However, billions of small farmers in China often overuse nitrogen fertilizer to increase maize yields [21,22]. This not only increases production costs but also reduces the nitrogen fertilizer utilization efficiency (NUE), giving rise to environmental pollution problems [20,21,23]. Therefore, it is necessary to reduce N rates in these areas to improve these issues. However, the nitrogen fertilizer management strategies vary not only by cultivars [24] and soil conditions [25] but also by planting density [23]. Many studies have suggested that densification should reduce nitrogen rates to improve NUE and alleviate environmental pollution [26,27,28]. However, some studies believed that densification may lead to increased nitrogen demand due to the greater number of plants, so the nitrogen rate should be increased [29]. Therefore, how to optimize the nitrogen–densification management strategy to balance yield and NUE needs to be further studied, especially in the southwestern region of China.

The Sichuan Hilly Region is an important part of the southwest China maize region and has traditionally been characterized by the predominant cultivation of spring maize. As in North China, sole cropping summer maize has been widely promoted to meet the development needs of mechanized production and large-scale operation in recent years. Nevertheless, studies on its high-yielding management strategy are rather limited. Low planting density and excessive nitrogen rate are major reasons for the low yield and low economics of maize. The average planting density in this region is only 48,000 plant ha⁻¹, which is significantly lower than other maize cultivation areas [17,18], and more than 60% of farmers apply excess nitrogen fertilizer [30]. Therefore, how to optimize fertilization and densification management is crucial for achieving high yields, efficiency, and sustainable development of summer maize in this region. To address this, a three-year field experiment was conducted to (1) explore the response of maize RUE and NUE to planting density, nitrogen rate, and their interaction and (2) establish the quantitative relationship between RUE and NUE with dry matter accumulation and grain yield. The main objective is to find out the optimized maize density and nitrogen fertilizer management strategies to enhance grain yield while minimizing nitrogen fertilizer pollution in the region.

2. Materials and Methods

2.1. Experimental Site

Field experiments were conducted at Zhongjiang Experimental Station of Sichuan Agricultural University, Sichuan, China (30°95′ N, 104°63′ E), in the 2017–2019 maize growing seasons. The test material was Zhenghong 6 (ZH6), a maize hybrid widely grown in the region and provided by Sichuan Zhenghong Biotechnology Co., Ltd (Chengdu, China). This site generally experiences a humid subtropical monsoon climate with a frost-free period of 286 days. The temperature, precipitation, and solar radiation during the maize growing period are shown in Figure 1 and

Table 1. Solar radiation, average daily temperature, and total precipitation during maize growth period.

Year	Solar Radiation (MJ/m2)	Average Daily Temperature (°C)	Total Precipitation (mm)
2017	2015.5	26.6	410.7
2018	1869.4	26.5	1344.2
2019	1929.6	25.1	564.8

. The experimental soil was purple clay and the characteristics of the 0–30 cm layer of soil, which were measured by the Walkley–Black and Olsen methods [31], were as follows: 21.03 g kg⁻¹ organic matter, 1.39 g kg⁻¹ total N, 21.96 mg kg⁻¹ alkali-hydrolyzable N, 5.83 mg kg⁻¹ available P, 112.68 mg kg⁻¹ available K, and pH 7.60.

2.2. Experimental Design and Field Management

A two-factor split-plot experimental design was used. The main factor was planting density with three levels: 52,500 plant ha⁻¹ (the local customary density; D₁) [18], 67,500 plant ha⁻¹ (D₂), and 82,500 plant ha⁻¹ (D₃). The secondary factor was N fertilizer rate with four levels: 240 kg ha⁻¹ (N₂₄₀, the local customary rate which was higher than the required rate under customary density in our previous study and the recommendation by Chen et al.) [32], 180 kg ha⁻¹ (N₁₈₀), 120 kg ha⁻¹ (N₁₂₀), and 0 kg ha⁻¹ (N₀). Twelve treatments and three replications were performed. The area of the plot was 3.2 m × 6 m in 2017 and 5.5 m × 4 m in 2018 and 2019. Maize seeds were sown on 16 May 2017, 13 May 2018, and 10 May 2019, and the harvest dates were 4 September 2017, 1 September 2018, and 7 September 2019, respectively. The row spacing was 80 cm in all treatments, while the plants’ spacing varied according to density.

The N fertilizer was urea (46% N), which was supplied according to different treatments, 50% as basal fertilizer and 50% as spike fertilizer applied at the V13 stage. In addition, 600 kg ha⁻¹ calcium superphosphate (12% P₂O₅) and 150 kg ha⁻¹ potassium chloride (60% K₂O) were applied as basal fertilizer. All basal fertilizers were applied in a furrow parallel to the plant row, and spike fertilizer was placed in a hole dug near the maize plant. Other planting and management measures were conducted according to local high-yield requirements.

2.3. Sampling and Measurements

2.3.1. Population Light Distribution

On a clear day during the silking stage, the photosynthetic active radiation of the maize population above ground (0 cm, 100 cm, 150 cm, 200 cm, and 250 cm (I)) and outside the canopy (I₀) were measured using an LI-1500 photometric meter (LI-COR, Lincoln, NE, USA), and the light transmittance [33], extinction coefficient (K) [34], and canopy solar radiation interception [13] were calculated.

$$Light transmittance \left(LT, \%\right)=\left(\frac{I}{I_0}\right)\times 100\%$$

$$K=-ln\left(\frac{I}{I_0}\right)/LAI$$

$$Solar radiation interception rate \left(SRI, \%\right)=1-Light transmittance$$

2.3.2. Population Dry Matter Accumulation and Radiation Use Efficiency

Six representative plants were cut at maturity stage, dried at 105 °C for 30 min, dried at 80 °C to a constant weight, and weighed, and the population dry matter accumulation (DMA) and radiation use efficiency (RUE) were calculated [35].

$$RUE=\frac{q\times DMA}{\sum R}\times 100\%$$

q is the coefficient of conversion of biomass to heat (0.0175 MJ g⁻¹ for maize) [26], and ∑R is the total radiation during the growing season.

2.3.3. Nitrogen Uptake and Utilization Characteristics

The plant samples for the determination of DMA were crushed and sieved through an 80-mesh sieve, and their nitrogen content was determined using a fully automated Kjeldahl nitrogen meter (UDK169VELP Inc., Milan, Italy) to calculate the following indices [36,37,38]:

Plant nitrogen accumulation (kg ha⁻¹) = DMA (kg ha⁻¹) × Nitrogen content (g kg⁻¹) × 10⁻³

Nitrogen dry matter production efficiency (NDMPE, kg kg⁻¹) = DMA/Plant nitrogen accumulation

Nitrogen fertilizer apparent use efficiency (NUE, %) = (Nitrogen accumulation in nitrogen application plot − Nitrogen accumulation in blank plot)/Total nitrogen application × 100

Partial fertilizer productivity of nitrogen (PFP_N) = Grain yield/Total N application;

Nitrogen harvest index (NHI) = N accumulation in grains/Plant nitrogen accumulation.

2.3.4. Analysis of Apparent Soil Nitrogen Losses

Before sowing and after harvesting maize, three points were selected diagonally in each plot, and fresh soil samples were taken in 0–30 cm layers by the soil auger method to determine the nitrate and ammonium nitrogen rate by the 2 mol KCL leaching method and the soil bulk weight by ring knife method at the same time [39]. The soil nitrogen supply and balance were analyzed [40]:

Soil nitrate nitrogen or ammonium nitrogen content (kg ha⁻¹) = Soil nitrate or ammonium nitrogen rate (mg N kg⁻¹) × Soil layer thickness × Soil bulk weight

Soil available nitrogen content (kg ha⁻¹) = Soil nitrate nitrogen content + Soil ammonium nitrogen content

Apparent nitrogen mineralization (kg ha⁻¹) = (Plant nitrogen accumulation at harvest of the no-N treatment + Soil available nitrogen content at harvest of the no-N treatment) − Soil available nitrogen content before sowing of the no-N treatment

Total soil nitrogen input (kg ha⁻¹) = Soil available nitrogen content before sowing + Fertilizer nitrogen input + Apparent nitrogen mineralization

Total soil nitrogen output (kg ha⁻¹) = Plant nitrogen accumulation at harvest + Soil available nitrogen content after harvest

Soil apparent nitrogen loss quantity (ANLQ, kg ha⁻¹) = Total soil nitrogen input − Total soil nitrogen output

Soil apparent nitrogen loss rate (ANLR, %) = ANLQ/Total soil nitrogen input × 100.

2.3.5. Yield and Its Composition

At physiological maturity, all ears in each plot were collected, dried, threshed, and weighed to calculate the grain yield under the standard moisture content of 14%. Before harvest, the number of effective ears was counted. After harvest, 20 representative ears were selected using the average ear weight method, and the number of grains per ear and 100-grain weight were examined.

2.4. Statistical Analysis

SPSS 27 (IBM Inc., Version 27.0, Armonk, NY, USA) software was applied for ANOVA, and means were examined using the least significant difference test at the p < 0.05 level (LSD 0.05). Origin 2023 (OriginLab Corp., Northampton, MA, USA) was applied for plotting and regression analysis.

3. Results

3.1. Effects of Planting Density and Nitrogen Rate on Summer Maize Light Distribution

Planting density and nitrogen rate significantly affected the light distribution, absorption, and utilization of maize populations (Figure S1). As the planting density and nitrogen rate increased, the population extinction coefficient (K) and solar radiation interception (SRI) increased and the light transmission (LT) decreased. Averaged over 2018 and 2019 (Figure 2), compared to the local customary density (D₁), the increased densities D₂ and D₃ increased K by 8.0% and 18.5%, decreased the average LT at all measured heights by 10.3% and 15.2%, and increased the canopy (averaged over 100 and 150 cm) SRI by 8.1% and 13.3%, respectively. For each 100 kg ha⁻¹ increase in the nitrogen rate, the K and canopy SRI increased by about 0.0012 (R² = 0.9318 **) and 8% (R² = 1.0000 **), respectively, and the average LT decreased by about 6% (R² = 0.9982 **). There was also a certain interaction between planting density and nitrogen rate. Compared with N₂₄₀, the average K of N₁₈₀ under D₁, D₂, and D₃ reduced by 4.5%, 8.8%, and 18.1%, while those of N₁₂₀ reduced by 10.9%, 15.4%, and 27.4%. This suggests that the reduction in nitrogen application under high density led to a greater decrement of K.

3.2. Effects of Planting Density and Nitrogen Rate on Summer Maize Grain Yield and Its Components

The planting density and nitrogen rate had significant effects on maize yield and its components (Figure 3). Increasing the planting density improved grain yields by increasing effective ears. On average, over the three years, compared to D₁, D₂ and D₃ increased yield by 8.9% and 10.6% (except for D₃ in 2018 which reduced by 3.3% due to lodging). Reducing the nitrogen rate decreased yields by declining the number of grains per ear and the 100-grain weight, and this effect increased with the planting density increase. Compared to N₂₄₀, the average yield of N₁₈₀, N₁₂₀, and N₀ decreased by 0.5%, 3.2%, and 8.0% for D1, 1.3%, 6.7%, and 9.9% for D2, and 7.5%, 13.5%, and 19.4% for D3, respectively. For the same nitrogen reduction, yield loss increased with increasing planting density.

3.3. Effects of Planting Density and Nitrogen Rate on Summer Maize Dry Matter Accumulation and Radiation Use Efficiency

Trends in the effects of planting density and nitrogen rate on dry matter accumulation (DMA) and radiation use efficiency (RUE) at the mature stage were similar to those for yield; that is, they increased with planting density and nitrogen rate increase (except for D₃ in 2018) (Figure 4). Compared to D₁, the average DMA and RUE of D₂ in the two years increased by 11.0%, while that of D₃ increased by 20.0% in 2019 and decreased by 1.6% in 2018. For every 100 kg ha⁻¹ increase in nitrogen rate, the DMA under D₁, D₂, and D₃ increased by 1.06 (R² = 0.8854), 1.76 (R² = 0.9623 *), and 2.53 t ha⁻¹ (R² = 0.9968 **), respectively. The improving effect of nitrogen rate on DMA and RUE increased with increasing density. There was a highly significant interaction effect between planting density and nitrogen rate.

Correlation analysis revealed that maize DMA and actual grain yield were negatively correlated with LT and positively correlated with the canopy SRI and extinction coefficient (K). For an increase of 0.001 in K, DMA, yield, and RUE increased by 1.45 t ha⁻¹, 0.39 t ha⁻¹, and 0.13%, respectively. For each 1% increase in canopy SRI, they increased by 0.28 t ha⁻¹, 0.076 t ha⁻¹, and 0.025%, respectively, whereas for each 1% increase in LT (at 0 cm), they decreased by 0.69 t ha⁻¹, 0.19 t ha⁻¹, and 0.062%, respectively (Figure 5).

3.4. Effects of Planting Density and Nitrogen Rate on Summer Maize Nitrogen Absorption and Utilization and Soil Nitrogen Balance at Maturity Stage

3.4.1. Nitrogen Accumulation

The effects of planting density and nitrogen rate on the nitrogen content (percentage) of plants were relatively small (especially for planting density), so the nitrogen accumulation of the population depended mainly on its DMA. They showed a highly significantly positive correlation (R² = 0.6969 **). For each increase of 1 t ha⁻¹ in DMA, the nitrogen accumulation increased by 10.2 kg ha⁻¹. As can be viewed in Figure 6, with the increase in density, the nitrogen accumulation of single plants decreased, but that of the population increased significantly. Averaged over two years, compared to D₁, D₂ and D₃ increased by 4.9% and 16.8%, respectively. With the decrease in nitrogen rate, both single-plant and population nitrogen accumulation decreased. For each 100 kg ha⁻¹ decrease in nitrogen rate, the population nitrogen accumulation under D₁, D₂, and D₃ decreased by 19.7, 25.5, and 31.8 kg ha⁻¹, respectively.

3.4.2. Nitrogen Utilization Efficiency

With planting density increase and nitrogen rate decrease, nitrogen dry matter production efficiency (NDMPE), nitrogen fertilizer apparent use efficiency (NUE), and the partial fertilizer productivity of nitrogen (PFP_N) (except for D₃ in 2018) all showed an increasing trend, while the nitrogen harvest index (NHI) showed a decreasing trend, with the largest effect on NUE reaching a highly significant level in both years (Figure 7 and Figure S2). The nitrogen rate had a greater effect on nitrogen utilization efficiency than planting density. For every 100 kg ha⁻¹ reduction in the nitrogen rate, NDMPE, NUE, and PFP_N increased by 5.3 kg kg⁻¹, 25.0%, and 6.5 kg kg⁻¹, respectively.

3.4.3. Soil Nitrogen Balance

As the planting density increased and nitrogen rate decreased, the soil available nitrogen content (SANC) and apparent nitrogen loss quantity (ANLQ) and rate (ANLR) decreased (Figure 8 and Figure S3). On average, over the two years, compared to D₁, the post-harvest SANC, ANLQ, and ANLR in D₂ and D₃ decreased by 12.2% and 18.5%, 10.8% and 19.9%, and 9.5% and 20.8%, respectively. For every 100 kg ha⁻¹ reduction in the nitrogen rate, SANCs under D1, D2, and D3 were reduced by 11.4, 10.8, and 10.1 kg ha⁻¹, ANLQs were reduced by 66.9, 69.9, and 66.7 kg ha⁻¹, ANLRs were reduced by 9%, 11%, and 11%. Moderate increasing planting density and reducing nitrogen application would help the maize to fully utilize the available nitrogen from the fertilizer applied and the soil and reduce nitrogen loss, which would result in more environmental benefits.

4. Discussion

4.1. Optimizing Nitrogen and Density Management to Improve the Light Distribution of Maize Population and Increase the Solar Radiation Use Efficiency and Grain Yield

Solar radiation is a basic climatic resource on which plants rely. Increasing RUE can produce greater yields in maize [27]. In this study, for each 1% increase in RUE, the yield increased by 2762.3 kg ha⁻¹ (R² = 0.931 **).

Solar radiation energy intercepted by maize populations is converted into biomass energy through photosynthesis of the leaves [41], while canopy structure, which is the space formed by the orderly array of leaves, significantly affects maize’s interception of solar radiation, completion of photosynthesis, and dry matter accumulation [42]. Optimizing canopy structure and improving light distribution in maize populations can increase RUE [43,44]. In this study, we further quantified the relationship of the extinction coefficient, field light leakage rate, and canopy light interception rate with RUE and maize yield (Figure 5). This can provide a theoretical basis for high-yield cultivation of maize in the local area.

Increasing the planting density can rapidly establish the canopy in the early growth stage of maize [23]. This reduces the photosynthetic capacity of individual plants but greatly improves the efficiency of the population in intercepting solar radiation and increases the synthesis of photosynthetic assimilates [1,45,46,47]. Consequently, it is considered as an effective strategy to improve maize yield [26]. In this experiment, as the planting density increased, the population light extinction coefficient rose, the LT decreased, the SRI raised (Figure 2), and the yield subsequently increased. Nonetheless, overcrowding can result in leaves shading each other, accelerating senescence in lower and middle leaves and even influencing stem development, which potentially lead to lodging [48,49,50]. These, in turn, are not conducive to raising RUE and affect the productivity and stability of maize yield. Consequently, there should be a certain limit to densification. The suitable density for high and stable yield under the field setting of this experiment was 67,500 plant ha⁻¹ (D₂). Because this area is a poor-sunlight area (11.2 MJ m⁻²), more attention should be paid to maintaining moderate ventilation and light transmission conditions in the field to avoid lodging and, therefore, the suitable density is lower than that in the light-rich northwest (16.6 MJ m⁻²) and northern China (13.1 MJ m⁻²), and even Yunnan province (15.6 MJ m⁻²) located in the southwest [51]. This demonstrates that the appropriate planting density for maize production varies depending on the ecological region and should usually be matched with local natural resources such as solar radiation; that is, the weaker the solar radiation, the lower the density [11].

Studies have shown that nitrogen rates can also affect the canopy structure of maize [42,52]. Adequate nitrogen supply facilitates canopy formation in maize and increases SRI, thereby improving RUE [42,53]. In this experiment, for every 100 kg ha⁻¹ increase in the nitrogen rate, the population extinction coefficient, SRI, and RUE increased by 0.012, 8%, and 0.16%, respectively. Furthermore, there existed an interaction between nitrogen rate and density. This implies that a moderate rise in density can, to some extent, compensate for the negative effect of nitrogen reduction on maize RUE and yield. However, this compensatory effect diminishes as the planting density further increases.

4.2. Optimizing Nitrogen and Density Management to Improve Nitrogen Use Efficiency and Reduce Nitrogen Losses

The pursuit of increased maize yields requires an increase in population DMA [20]. Increasing the nitrogen rate can increase crop nitrogen intake, resulting in more DMA [31,54,55]. The results of this experiment indicated that for each 100 kg ha⁻¹ increase in the N rate, the N accumulation and DMA increased by 25.6 kg ha⁻¹ and 1.7 t ha⁻¹, respectively, which resulted in a higher grain yield. However, many studies have shown that the relationship between maize grain yield and nitrogen rate is not linear, but rather shows a saturation curve or polynomial relationship. Excessive nitrogen fertilization does not contribute to yield increase and even leads to yield decrease [28,56]. Wang et al. [31] pointed out that in the northwestern maize production zone, there was no significant increase in maize yield when N rates exceeded 240 kg ha⁻¹. Moreover, increased nitrogen rates would reduce the NUE, increase nitrogen loss, and cause environmental pollution [21,23]. In this study, we quantified the relationship of NDMPE, NUE, PFP_N, ANLQ and ANLR with nitrogen rate and found that moderate nitrogen reduction (N₁₈₀) based on the local customary nitrogen rate (N₂₄₀) did not have a significant effect on maize yield (especially under the local customary density, D1). However, it could significantly improve NUE, reduce nitrogen loss, and mitigate environmental pollution.

Increasing planting density not only enhances DMA, RUE, and yield, but also improves NUE, reduces nitrogen loss, and mitigates environmental pollution [31,57]. Shao et al. concluded that by increasing planting density, fertilizer nitrogen recovery efficiency, nitrogen remobilization efficiency, and physiological nitrogen utilization efficiency can be improved, thus increasing NUE [58]. In this study, we found that increasing the planting density by 15,000 plant ha⁻¹ (D₂) on basis of the local customary density (D₁) could increase yield by 11.0%, 10.1%, and 7.0% and increased NUE by 29.5%, 27.6%, and 11.4% under N₂₄₀, N₁₈₀, and N₁₂₀, respectively, with a corresponding reduction in apparent nitrogen loss. This indicates that the effect of densification varies depending on the nitrogen rate, and there is a synergistic effect between density and nitrogen rate. In areas and fields with excessive nitrogen rates, moderate densification and nitrogen reduction are effective measures to synergistically improve maize yield and NUE and reduce nitrogen losses. Ju et al. [59] reported that in the North China Plain, the nitrogen rate could be reduced by 30–60% without yield loss through efficient application of nitrogen fertilizer. Our study demonstrated that nitrogen reduction by 25–50% was possible under the local customary density (D₁). However, excessive nitrogen reduction should not be applied after densification (especially under D₃) to avoid yield loss due to N deficiency, and excessive soil N depletion is to the detriment of sustainable development.

In summary, optimizing nitrogen and density management can increase light interception and utilization and grain yield in maize populations, and synergistically enhances nitrogen use efficiency, reduces nitrogen loss, and mitigates environmental pollution. However, the appropriate nitrogen and density combinations should be matched with the local sunlight resources, soil circumstances, etc., and its physiological and molecular mechanisms need to be further systematically explored.

5. Conclusions

A moderate increase in density could increase RUE, NUE, and grain yield, but excessive densification is not advisable in low-light areas. Nitrogen rate is positively correlated with RUE and soil nitrogen loss and negatively correlated with NUE. Moderate nitrogen reduction is advisable in areas with excessive nitrogen fertilizer, but excessive nitrogen reduction should not be carried out after densification. The appropriate nitrogen–density combination under the experimental condition was a planting density of 67,500 plant ha⁻¹ with a nitrogen rate of 180 kg ha⁻¹.