The experimental site (41°28’24” N, 115°1’3” E, altitude 1450 m) for this study was situated at the Comprehensive Experimental Station of the National Forage System in Zhangjiakou, as depicted in Figure 1A . This area was typically a continental monsoon climate with a frost-free period lasting 100 days. The tested soil was classified as chestnut soil. The chemical properties of tested soil in 0-20 cm layer were recorded as follows: Organic matter, 24.6 g kg^-1, alkali-hydrolyzable N, 50.1 mg kg^-1, available P, 13.2 mg kg^-1, available K, 130.2 mg kg^-1, and pH, 8.0. The soil bulk density in 0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, and 80-100 cm layer was 1.4, 1.5, 1.6, 1.5, and 1.5 g cm^-3, respectively. The total precipitation amounts were 4675.3 mm and 3921.9 mm, and the average temperature was 17.0°C and 17.2°C during the growing seasons in 2018 and 2019 (shown in Figures 1B , C ).

The agronomic traits of silage maize were measured at the milk maturity stage. For this purpose, 10 consecutive maize plants were selected from each plot to determine their plant height, stem diameter, and maximum leaf area. The maximum leaf area was calculated according to leaf length × leaf width × 0.75 (Zhao et al., 2020; Sezer et al., 2021).

For the determination of yield and quality of silage maize, two square meters from each plot were randomly chosen at the milk maturity stage, and the silage maize was cut at a height of 3 cm above the ground. The harvested maize was weighed on site, and recorded. Approximately 1 kg of the mixed sample of chopped was randomly taken for precise weight recording. All plant samples were first oven-dried at 105°C for 30 min and subsequently at 70°C until reaching constant weight. The dried samples were ground. Measurements were then conducted for crude protein content (using the kjeldahl method), acid detergent fiber content (ADF, using acid detergent method), neutral detergent fiber content (NDF, using neutral detergent method), crude starch content (using rotation method), and total nitrogen content (using the kjeldahl method), as detailed by Zeng et al. (2022).

Soil samples from 0-20 cm soil layer were collected and then air-dried before sowing to determine the basic physico-chemical properties. Soil organic matter content and alkali-hydrolyzable nitrogen were determined using the potassium dichromate-external heating method and alkali-dispersion method, respectively. Available phosphorus and potassium were determined using vanadium molybdenum blue colorimetry method and flame photometry method, respectively. Soil pH was measured using the glass electrode method with a water-to-soil ratio of 2.5:1. At the milk maturity stage of silage maize, post-harvest soil samples were collected randomly from three sites in each plot using an auger (inner diameter 2.0 cm) to a depth of 100 cm at 20cm intervals. The three samples from each plot at the same depth were thoroughly mixed to form one composite sample per depth and then transported in coolers on ice to laboratory. Fresh soil samples were immediately analyzed for nitrate nitrogen using ultraviolet spectrophotometry, and soil water content was determined after drying at 105°C for 24 h. Soil bulk density of each layer was measured using the ring knife method (Bao, 2000).

The significance among treatments was analyzed by analysis of variance (ANOVA) using the SPSS 13.0 software (SPSS Inc., Chicago, IL, USA). For multiple comparisons, we used Least Significant Differences (LSD) at the 5% level. Data analysis in this study was conducted using Microsoft Excel for computation and OriginPro 2022 for graphical representation.

The formulas used for various calculations are as follows, in line with the methodology described by Zhang et al. (2008):

Note: Chemical N refers to nitrogen brought by fertilizer input. The irrigation-N and atmospheric deposition-N are based on the research findings of Ju et al. (2006) and Liu et al. (2013), respectively. Mineralized-N refers to the amount of nitrogen absorbed by plants under the condition of no fertilization.

Note: The expenses of fertilizers, the price of fresh grass, and other costs including irrigation, labor, pesticides, and machinery were calculated in accordance with the prevailing market prices of 2019.

To comprehensively evaluate the effects of different management strategies, the data pertaining to yield, crude protein, nitrogen uptake, nitrogen use efficiency, nitrogen surplus, and economic benefit were all standardized (Wang et al., 2020) according to the formula x - x ¯ Std . Among this formula, x ¯ stands for average and Std stands for standard deviation.

The yield, plant height, stem diameter, and leaf area of silage maize were significantly influenced by both variety and density. However, the interaction between these two factors only has a significant effect on leaf area, as depicted in Figures 2A–D . Among the four examined varieties, Xunqing 858 exhibited a significantly higher yield by 11.5%-51.6% than the other varieties. Moreover, the yield of all varieties initially increased with planting density, but subsequently decreased, and reaching its peak value at a planting density of A2 (79,500 plants ha^-1) ( Figure 2A ).

Concerning agronomic traits, it was observed that the plant height, stem diameter, and leaf area of silage maize progressively diminished as the planting density increased, as illustrated in Figures 2B–D . The stem diameter showed no significant differences among the four varieties. Xunqing 858 was distinguished by having the tallest plants and the largest leaf area. In comparison to the other varieties, the plant height of Xunqing 858 was significantly greater by 8.7%-22.6%, and its leaf area was substantially larger by 30.9% than that of Xuntian 3171.

Compared to other treatments, the T7 treatment had the higher yield and crude protein content of silage maize, as shown in Figure 3 . Treatment 1 without nitrogen fertilizer had the lowest yield at 46.2 t ha^-1, which was significantly lower by 27.1%-36.7% than that of other fertilization treatments. T3 and T4 did not show a significant decrease in yield compared to T2, even with a reduced nitrogen application of 80 kg ha^-1. Under the total nitrogen application rate at 240 kg ha^-1, the T7 treatment achieved the highest yield at 73.1 t ha^-1, which significantly higher than the other treatments by 6.1% to 58.1%. Furthermore, T7 also had the highest crude protein content at 11.1% ( Figure 3B ), the highest starch content at 19.1% ( Figure 3C ), the lowest acid detergent fiber at 25.9% ( Figure 3D ) and the lowest neutral detergent fiber at 50.6% ( Figure 3E ).

As the proportion of polymer-coated urea-N was reduced, the plant nitrogen uptake, nitrogen use efficiency (NUE), nitrogen partial factor productivity (PFP_N), and nitrogen agronomic efficiency (NAE) exhibited an initial increase followed by a decrease, as detailed in Table 1 . The T7 treatment displayed the highest values in all these parameters. Specifically, the plant nitrogen uptake reached 200.5 kg ha^-1, increased by 4.9%-51.7% compared to T1-T8, and its NUE was at 28.4%, PFP_N and NAE were recorded at 58.8% and 18.2 kg kg^-1, respectively. These figures underscore T7’s superior performance in terms of nitrogen utilization efficiency.

Significant differences were observed in the accumulation of nitrate nitrogen among different soil layers and treatments. Compared to the unfertilized control (T1), all fertilizer treatments significantly increased the accumulation of nitrate nitrogen in the 0-100 cm soil layer. However, different optimized nitrogen management strategies resulted in a reduction of nitrate nitrogen accumulation in the deeper soil layer of 40-100 cm, as indicated in Figure 4 . Notably, even with the nitrogen application rate of 320 kg ha^-1 in T2, there was no significant difference in nitrate nitrogen accumulation in the 0-40 cm layer compared to T3, T4, and T5, where nitrogen was reduced by 80 kg ha^-1. In the 0-40 cm layer, the accumulation of nitrate nitrogen in T6, T7, and T8 was significantly higher than that in other treatments, but the differences among these three were not significant. In the 40-100 cm soil layer, T2 exhibited the highest accumulation of nitrate nitrogen at 52.4 kg ha^-1. However, there was no significant difference among T3 to T7 treatments. Importantly, T7 showed the lowest accumulation of nitrate nitrogen (33.3 kg ha^-1), effectively reducing the risk of nitrate leaching.

Chemical nitrogen fertilizer is the predominant nitrogen input in agricultural fields, contributing to more than 60% of the total nitrogen input ( Table 2 ). Among all evaluated fertilizer treatments, T7 had the lowest nitrogen surplus at 115.1 kg ha^-1, which was 7.0% to 46.6% lower than that of the other fertilizer treatments. Additionally, T7 also had the highest total nitrogen output, recorded at 283.0 kg ha^-1. This output was 3.2% to 12.5% higher compared to the other fertilization treatments, underscoring its efficiency in nitrogen utilization.

The T7 treatment demonstrated significantly higher economic benefits compared to the other treatments ( Table 3 ). In terms of output value, T7 achieved the highest value at 5102 $ ha^-1, which was significantly greater than that in other treatments (T1-T5) by 8.5% to 58.1%. However, there was no significant difference in output value among T2, T3, T4, and T5. In the aspect of economic benefit, T1 had the lowest net income at 1565 $ ha^-1, whereas T7 had the highest net income at 3094 $ ha^-1, and the net incomes among T2, T3, and T4 treatments did not show significant differences. Among the treatments with different ratios of polymer-coated urea-N to urea-N (T5-T8), the fertilizer cost for T7 and T8 was comparatively lower.

Selecting excellent varieties, optimizing planting density and applying optimized fertilizer management are necessary measures for improving the quality and yield of maize (Cardwell, 1982; Hu et al., 2013; Zhang et al., 2014). Our results demonstrated that the integrated management strategy 7 had the highest silage maize yield (73.1 t ha^-1), with an increase of 6.1%-58.1% compared to other treatments ( Figure 3A ). The primary reason for this yield increase is that favorable plant traits promote the biomass formation (Dong et al., 2023). In this study, Xunqing 858 exhibited a distinct advantage in plant height and leaf area. Its plant height was significantly higher than other varieties by 8.7%-22.6%, and its leaf area was significantly higher than Xuntian 3171 by 30.9% ( Figures 2B, D ). This may be attributed to the strong adaptability and stress resistance of Xunqing 858 under unique climatic conditions characterized by low temperature and a short frost-free period. Additionally, a suitable planting density created a favorable population structure (Borrás et al., 2003; Assefa et al., 2016; Piao et al., 2016), enhanced crop light utilization, and ultimately coordinated individual and population yield (Li et al., 2019). The integrated management strategies for silage maize not only affected yield but also impacted its nutritional value. This study found that the integrated management strategy 7 increased crude protein and starch content by 0.2%-21.5% and 0.6%-7.3% ( Figures 3B, C ) compared to other treatments, respectively. This is mainly due to the optimized treatment increasing nitrate nitrogen accumulation in 0-40 cm soil layer ( Figure 4 ) and effectively increasing plant nitrogen uptake by 4.9%-51.7% ( Table 1 ). These factors promoted the accumulation of enzymatic protein and photosynthesis in plants, leading to quality improvements (Wang et al., 2012; Ma et al., 2013; Zhou et al., 2016).

Poor management on water and fertilizer during the silage maize production not only leads to the wastage of nutrient resources but also contributes to nitrogen losses and decreases the nitrogen use efficiency (Kang et al., 2016; Srivastava et al., 2020; Wang et al., 2021; Zhao et al., 2022). In recent years, improving nitrogen use efficiency by regulating or altering the transformation or release characteristics of nitrogen fertilizer applied to soil has become a prominent research topic domestically and internationally (Sim et al., 2021). However, the nutrient transformation and release characteristics of commonly used slow-released fertilizers are affected by soil temperature, humidity and other factors (Tlustos and Blackmer, 1992; Husby et al., 2003; Ransom et al., 2020; Lin et al., 2021). Previous studies had shown that the integrated management strategies have a positive impact on nitrogen use efficiency and environmental effects in the plant-soil system (Wang et al., 2020; Chen et al., 2021). Similar results were founded in this study. During the harvest of silage maize, the nitrogen surplus in T7 decreased by 7.0-46.6% ( Table 2 ) compared to other management strategies. The main reason was that, under the condition of equal total nitrogen rate of input items, the nitrogen taken up by plants and nitrate nitrogen in 0-40 cm soil were higher in the nitrogen balance output in integrated management strategy 7. Simultaneously, through effectively regulating the soil nitrogen supply and increasing the nitrate accumulation in 0-40 cm soil, the reasonable ratio of polymer-coated urea-N to urea-N in the integrated management strategy 7 (7:3) met the sustained nitrogen demand during the early and later growth stages of silage maize, resulting in higher nitrogen use efficiency. Compared to other treatments, the nitrogen use efficiency, partial factor productivity of N fertilizer and nitrogen agronomic efficiency of T7 increased by 15.4%-94.5%, 3.7%-47.4% and 13.0%-91.6%, respectively ( Table 1 ).

Against the backdrop of increasing pressure on feed supply and environmental resources, achieving further improvement in silage maize yield and quality, as well as environmental protection, poses significant challenges in the development of livestock industry in China (Hu et al., 2019; Randby et al., 2019). Integrated management strategies, through optimizing crop varieties and densities and employing a balanced combination of polymer-coated urea-N to urea-N, provide a practical approach to address the issues of low and unstable yield and quality, as well as nitrogen excess in silage maize production in cold regions, while reducing environmental pollution ( Figure 5 ). However, there are still many obstacles and challenges to applying and promoting these management strategies. Firstly, in terms of personnel allocation, China’s agriculture is primarily operated by millions of small-scale farmers, which poses a significant challenge to the widespread implementation of integrated management technologies (Spielman et al., 2010; Cui et al., 2018; Yin et al., 2019; Zhang et al., 2020). Secondly, in terms of the controllability of technology, rural areas, especially in cold regions, are sparsely populated and unattractive to young generations, making it difficult to adopt new technologies or production transformations (Zhang et al., 2020). Lastly, in terms of economic factors, technology cost is an important driving factor in determining the implementation of this technology (Dreher et al., 2003). Due to the high labor cost of fertilization, farmers are increasingly inclined to apply all fertilizers before sowing (without top dressing). Hence, farmers have a great demand for slow-released compound fertilizers (Li et al., 2022). However, the technology recommended in this study requires a reasonable proportion of polymer-coated urea-N to urea-N, which increases the difficulty of popularization.

To implement this effective technology strategy, we should (1) increase technical services to enable farmers to adapt and adopt more knowledge-intensive agricultural practices (Chen et al., 2014); (2) increase institutional support and infrastructure construction, and transfer integrated management technologies to millions of small-scale farmers (Zhang et al., 2016). (3) Governments should increase policy support and financial investment in the agricultural sector. Importantly, farmers must receive proper remuneration for using this technology. Additionally, the new technology must be evaluated and receive feedback for end-user, including farmers. Also, the technology can then be further improved and better adapted to the specific agricultural conditions relevant to the end-users, reflecting a bidirectional feedback mechanism via iteration (Shen et al., 2024; Zhang et al., 2024).

In summary, concerted efforts from multiple stakeholders are required to conduct sufficient localized studies to achieve widespread application and promotion of these management strategies globally. This is paramount for realizing the green development of livestock industry. The integrated management technology was studied in cold regions in China. However, this idea and the related results can also provide important references for silage maize production in similar regions of other countries. It is only necessary to update management strategies according to local climate and soil properties to promote the sustainable development of the global livestock industry.