The limited expansion of arable land poses a significant impediment to the augmentation of total grain production in China, and enhancing the productivity per unit of land remains an ongoing challenge (Han et al., 2022). Intercropping can establish a stratified and distinct crop canopy structure above the ground, intercept more light energy, improve ventilation and light transmission conditions and take advantage of the side rows (Gong et al., 2020). Cereal-legumes can improve the soil nutrient microenvironment through nitrogen fixation of belowground legume rhizomes, and significantly improve the productivity of the land per unit area, which has become an important strategy for organic and smallholder agriculture (Maingi et al., 2001; Li et al., 2003; Yu et al., 2015). Intercropping common vetches with oats has been widely used for forage production in alpine regions such as the Tibetan Plateau and along the foothills of the Yinshan Mountains (Lithourgidis et al., 2006). This production model can be better adapted to the arid and cold environments in northern China, guaranteeing the production of high-quality green forage in alpine pasture areas (Wang et al., 2020). Recent studies have suggested that intercropping systems effectively address challenges in common vetch, such as weak stems and poor erect growth, while providing structural support for common vetch clover growth and facilitating the mechanical harvesting of legume seeds (Lithourgidis et al., 2006; Nguyen et al., 2020). These studies have provided new ideas to solve the problems of low grain yield and high harvest loss of common vetch; however, less attention has been currently paid to the seed production capacity of the oat intercropping common vetch model.

Increasing the sowing density is an effective measure to increase crop yield, but it is often accompanied by higher water and nitrogen demands within the crop population, resulting in increased competition for both aboveground (light) and below-ground (water and nutrients) resources (Zhai et al., 2018). Owing to interspecific spatial and temporal complementarities, intercropping has a higher resource use efficiency than monocropping, which helps optimize the sowing density of combined crops to maximize the yield benefits of intercropping systems (Willey and Osiru, 1972). Previous studies have shown that increasing the density of maize can improve the photosynthetic capacity of intercropping systems, improve the microstructure of leaves (Yang et al., 2022), increase the nitrogen use efficiency of intercropping systems (Xu et al., 2021), and thus increase the total yield of intercropping systems (Fan et al., 2019). As sowing density increase, the dry matter partitioning into the leaves and stems of crops increases, as increased competition for light, water, and nutrients may also partition too much dry matter into the roots (Hauggaaed-Nielsen et al., 2006; Freschet et al., 2018; Zhang et al., 2020). For Poaceae crops, an excessively high sowing density could lead to the development of ineffective tillers, wasteful nutrient and water utilization, and premature senescence owing to shading, all of which are detrimental to high yields (Jiang et al., 2023). However, few studies on density tolerance in intercropping Poaceae crops have been reported. Therefore, further research is necessary to determine whether increasing the density of intercropped oats in oat and common vetch intercropping systems can enhance the overall productivity.

The main reason for intercropping to increase yield is to provide a basis for ecological niche separation for crops with different demand characteristics, which contributes to competition and complementarity between crops and improves resource utilization; that is, crop interspecific relationships are important factors affecting the yield of intercropping. Intercropping systems in a certain range of interspecific competitiveness tend to be enhanced with an increase in the density of a particular crop, which is an important breakthrough point to increase competitive advantage. Further optimization of interspecific relationships is the ecological basis for improving the yield advantages of intercropping (Hauggaaed-Nielsen et al., 2006; Abdul Rahman et al., 2021). According to the study of Mead and Willey (1980), any attempt to explain intercropping advantages using a single index may be problematic, and different aspects of intercropping data should be explained by different indices. The current indicators for evaluating intercropping systems mainly include biological, economic efficiency and interspecific relationships, which have been applied to evaluate potatoes intercropped with legumes (Gitari et al., 2020) and peppermint intercropped with soybean (Machiani et al., 2018) and provide a more comprehensive evaluation of intercropping interspecific relationships. The application of these indicators to evaluate the effect of the sowing density of oats on the grain yield of the common vetch intercropping system has positive significance in improving the intercropping productivity of this model.

We hypothesized that oats intercropped with common vetch would exhibit better tolerance to planting density compared to sole cropping, and that different oat sowing densities would alter the interspecific relationship. This study aimed to (1) assess the grain production performance of oats intercropped with common vetches under various oat sowing densities and (2) elucidate the changes in interspecific competition intensity between the two crops under different oat sowing densities. The findings of this study can serve as evidence in the context of inter-specific competition, thereby contributing to the enrichment of forage planting models and the judicious densification of intercropping systems involving Poaceae crops.

Intercropping is recognized as an essential planting pattern in China because of its efficient utilization of light, heat, water, and nutrient resources through temporal niche separation and spatial niche complementarity (Li et al., 2014, 2023). The results from the two-year study indicate that intercropping significantly increased oat grain yield by 23.12–30.98% under different planting densities compared to monoculture, and demonstrated a consistent yield advantage, as observed in previous studies (Wang et al., 2020; Li et al., 2021). This also confirms the potential of oat and common vetch intercropping patterns for grain production. The primary reason for the yield advantage of intercropping lies in the combination of the fibrous root system of oats and the taproot system of common vetch in the cereal-legume intercropping system (Wang et al., 2018; Zhang et al., 2021; Peng et al., 2022). Additionally, the rational combination of tall-stem and short-stem crops in the aboveground portion enhances light capture by the leaves, increases canopy interception capacity, and promotes yield formation (Gou et al., 2017; Li et al., 2020; Lu et al., 2023). Increasing the sowing density to enhance the utilization efficiency of light and temperature resources and unlocking collective production potential are crucial for achieving high yield (Yang et al., 2022). In this experiment, as the oat sowing density increased, the grain yield in both the intercropping and monoculture systems exhibited a trend of initial increase, followed by a decrease. In the intercropping mode, oat yield reached its maximum under the ID3 treatment, which was significantly increased by 12.81% and 16.59% over ID1 in the two years, respectively. In the monoculture mode, the yield reached its maximum under SD2 density, with an increase of 8.40% and 11.37% over SD1 in the two years. Across all density treatments, the average yield increases for intercropping compared to monoculture were 24.37%, 27.11%, 31.41%, 28.89%, and 27.11%, indicating that intercropped oats had a greater yield advantage in the ID3 treatment. This suggests that intercropped oats can accommodate higher sowing densities than monocultures, which is consistent with previous research findings (Moreira et al., 2015).

Previous studies have shown that intercropping oats with common vetch significantly increase photosynthetically active radiation interception by oats, while decreasing interception by common vetch, thereby enhancing the overall light use efficiency of the intercropping system (Li et al., 2014). However, overcrowded plant populations exacerbate intraspecies competition, worsen canopy conditions, and reduce individual photosynthetic capacity, resulting in decreased grain yield per plant (Fan et al., 2022). Intercropping oats with common vetch reduced the common vetch grain yield by 1.51–13.77%, which indicating that the yield increase in oats comes at the cost of reduced common vetch yield. Compared to other intercropping systems, such as maize with peanuts (Wang et al., 2021 or corn intercropped with pea (Yang et al., 2021), the reduction in yield in the oat and common vetch intercropping systems is relatively small. This is likely because oats provide excellent support for the growth of common vetches, allowing them to capture more light by increasing plant height and overcoming competition from oats (Li et al., 2020). With an increase in oat sowing density, the common vetch yield reached its maximum with the ID2 treatment in both years, which had a significant increase of 8.16% over ID1 in 2022. This suggests that moderately increasing oat density is beneficial for increasing common vetch yield, possibly by alleviating nitrogen constraints imposed by oats (Neugschwandtner and Kaul, 2015).

Interplanting crops exhibits both competition and complementarity when utilizing space and various resources. When complementarity between species outweighs competition, the intercropping system enhances resource utilization, demonstrates intercropping superiority, and promotes collective productivity (Ren et al., 2016). Effective intercropping coordination enhances crop yield and land-use efficiency, which is primarily reflected in the LER (Raza et al., 2019). In the present study, the average LER of oats intercropped with common vetch was 1.06, indicating a grain-yield advantage in this intercropping model. In comparison, monoculture would require a 6% increase in land area to achieve equivalent production (Yilmaz et al., 2015). The LER in the intercropping system increased with a moderate increase in the oat sowing density. The LER was highest in the ID2 treatment for both years and significantly surpassed the ID1 and ID5 treatments. Cereal crops generally outcompete legumes for soil mineral nitrogen, forcing them to rely more on nitrogen fixation, while also facilitating nitrogen fixation in legumes and the subsequent transfer of nitrogen to non-leguminous crops (Gao et al., 2022). The increase in oat sowing density contributes to enhanced LER, which may be attributed to the intercropping system alleviating nitrogen inhibition by common vetch (Monti et al., 2016). However, as the oat sowing density continued to increase, the LER of the intercropping system decreased. The ID3, ID4, and ID5 treatments showed average LER reductions of 1.28%, 3.36%, and 6.14%, respectively. Compared to the ID2 treatment, the ID5 treatment consistently significantly lower than that of the ID2 treatment over two years. This decrease may be associated with the intensified competition between among plants for water, light, and nutrients, resulting in reduced accessibility to assimilable light (Moreira et al., 2015). LERo was the highest in the ID3 treatment for both years, showing a significant increase of 4.27% to 7.19% compared with the ID1 treatment. LERv peaked in the ID2 treatment and significantly surpassed that in the ID1 treatment. These findings suggest that increasing the oat sowing density can influence common vetch land-use efficiency and highlight the synergistic relationship between the two crops. In all density treatments, the LEC exceeded 0.25 (Jardim et al., 2021), the SPI surpassed that of monoculture oats, and the PYD was consistently positive. All biological indicators reached their maximum values in the ID2 treatment, which emphasized that a moderate increase in the oat sowing density enhanced the biological efficiency of the intercropping system.

Because more than one crop is involved in intercropping systems, which typically have different selling prices and production costs, various indices can be employed to assess the economic efficiency of such systems (Machiani et al., 2018; Soratto et al., 2022). In this study, the OEY over two years was lower than that of monoculture oats, and the RVT was consistently less than 1. This outcome primarily stems from the considerably lower grain yield per unit area of common vetch than that compared of oats, despite its price being only 25% higher (Verma et al., 2013). Increasing the oat sowing density enhanced the OEY of the intercropping system, with significant improvements of 11.39% and 11.76% in the ID2 and ID3 treatments, respectively. The MAI is positive in all scenarios, indicating that the oats-common vetch intercropping model is more profitable than the monoculture approach (Soratto et al., 2022). As the oat sowing density increased, the MAI initially increased and then declined, suggesting that a moderate increase in oat density during intercropping could lead to higher profitability. Similar conclusions were drawn for the RVI. The IA represents the economic superiority of the intercropping system (Machiani et al., 2018), with IA_O and IA values exceeding 1 for all treatments, indicating that the intercropping model outperformed the monoculture in terms of profitability. However, IA_V values less than 1 in each treatment in the two years could explain economic disadvantage observed in the OEY, RVT, and RVI values.

The relative crowding coefficient (K) measured the competitive advantage of one crop over another in an intercropping system. In this study, K_O values at all densities were greater than K_V values, indicating that oats were the dominant crop in the intercropping system (Abou El-Enin et al., 2023). The K values for the intercropping system are greater than 1 in both years. This indicated that intercropping systems exhibit less interspecific competition compared to intraspecific competition in monoculture, signifying a productivity advantage of intercropping (Banik, 1996). At different oat sowing densities, the K value of the ID2 treatment was the highest, indicating that the ID2 treatment can effectively coordinate the interspecific relationship between the two crops. K_O and K_V reached their maximum values under the ID3 and ID2 treatments, respectively. This also suggests that optimizing the intraspecific competition of common vetch is an important direction for further enhancing the productivity of the intercropping system. In all treatments, the competition ratio between oat and common vetch was positive, with CR_O being greater than CR_V, indicating that oats were more competitive than common vetch and were the dominant intercropping crop. This is consistent with the results of a previous study of Li et al. (2020). The competition ratio (CR_V) for intercropped common vetch was positive, indicating the feasibility of intercropping at different oat planting densities. Among all density treatments, CR_O was the smallest and CR_V reached the largest in the ID2 treatment, it indicated that ID2 density could coordinate competition between intercropped crops to achieve synergistic yield increases, a trend also reflected in the Aggressivity.

Actual yield loss (AYL) represents the relative yield loss or gain in intercropping systems. In the present study, AYL was greater than 0, indicating intercropping superiority, and AYL in the ID2 treatment is significantly higher than that in the ID1 treatment, suggesting that moderate densification could enhance the efficiency of intercropping systems. In the present study, AYL_O was maximized at the ID3 density, whereas AYL_V reached maximized at the ID2 density. The phenomenon could be attributed to oats increasing their competitiveness with rising crop density to ensure resource acquisition for high-density populations, while excessive competition may impede the yield of common vetch (Neugschwandtner and Kaul, 2015). The positive values of AYL_O for all treatments indicate that oats exhibit a yield advantage in the intercropping system, which is consistent with the result that oats are the competitive dominant crop and common vetch is the competitive inferior crop in the intercropping system (Li et al., 2020).

Our study showed a significant advantage in grain yield production for the oat-common vetch intercropping model compared with sole cropping, demonstrating higher adaptability to increased sowing densities. With a 20% increase in the oat sowing density, both oat and common vetch yields were significant improvement. Moderate increases in sowing density also contributed to the economic benefits of the intercropping system. In the intercropping system, oats exhibited a competitive advantage. The moderate increase in sowing density primarily alleviated the interspecific competition between oats and common vetches, synergistically enhanced the overall yield. This study provides novel insights into the grain production of oats and common vetch in arid and semi-arid regions. In further investigations, we should pay more attention to optimizing crop spatial arrangement and nitrogen usage to determine if it can promote intercropping systems to accommodate larger densities, thereby achieving higher levels of grain yield.

The datasets presented in this article are not readily available because no limite. Requests to access the datasets should be directed to Ying Wang 1269046052@qq.com.

YW: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. XH: Writing – review & editing. XZ: Writing – review & editing. YZ: Data curation, Methodology, Software, Writing – original draft. BQ: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. LL: Writing – review & editing, Conceptualization, Methodology, Supervision, Project Administration, Funding Acquisition.