According to the United Nations, the global population is projected to reach 9.7 billion by 2050 (United Nations, 2022). To meet the rising demand for food, global crop production will need to increase by 70-100% (Nelson et al., 2010; Huang et al., 2019). However, the expansion of arable land to boost crop yield is limited (Folberth et al., 2020; Tian et al., 2021), and climate change poses significant risks to monoculture systems (Hoegh-Guldberg et al., 2019; Thornton et al., 2014). In this context, rational intercropping emerges as a viable strategy to enhance yields through improved resource utilization without expanding the cultivated area (Raza et al., 2025). It also mitigates the risks associated with climate change by diversifying crop production (Li et al., 2020, 2023; Raza et al., 2021). Intercropping systems hold substantial potential to address challenges such as diminishing cultivable land (Chai et al., 2021), declining soil fertility (Feng et al., 2021), and depleting water resources (Yi et al., 2022; Zhang et al., 2022). They consistently outperform monoculture systems in terms of land and water productivity, as demonstrated by intercropping combinations like maize (Zea mays L.)/soybean (Glycine max (L.) Merr.) (Raza et al., 2021), maize/potato(Solanum tuberosum L.) (Wu et al., 2012), maize/peanut (Arachis hypogaea L.) (Feng et al., 2021), and maize/wheat (Triticum aestivum L.) (Li et al., 2020). The success of intercropping, however, largely depends on the compatibility of the intercropped species in terms of spatial-temporal trade-offs (Zhao et al., 2023), above- and below-ground interactions (Wang et al., 2024), and effective weed control prior to canopy development (Liebman and Dyck, 1993).

Intercropping is a vital agricultural practice in developing countries, particularly in East and South Asia (e.g., China, India, Pakistan), where the challenge of feeding large populations is compounded by limited land resources (Raza et al., 2025). Cereal-legume intercropping leverages interspecific complementarities by integrating diverse crop combinations to achieve higher yields and increased net profits (Xu et al., 2020). For example, the global average land equivalent ratio (LER) of maize-soybean strip intercropping (1.32 ± 0.02) indicates its efficient utilization of light (Liu et al., 2017; Yang et al., 2014), water (Raza et al., 2021), and fertilizer resources (Li et al., 2020; Raza et al., 2020; Zhou et al., 2021) per unit of land. Despite the synergistic benefits of maize-soybean strip intercropping, the differing genetic and biological characteristics of maize and soybean present challenges in the use of postemergence herbicides, limiting effective weed control. Additionally, the antagonistic interactions between herbicides targeting grasses and broadleaf weeds pose significant obstacles to simultaneously managing weeds in both crops. Currently, farmers rely on labor-intensive and cost-effective methods such as hoeing and dual-system chemical weed control with spacers. However, labor shortages and the potential for handling errors make these practices difficult to implement on large-scale farms. To fully realize the benefits of maize-soybean strip intercropping, an effective and convenient weed control method, rooted in genetic herbicide tolerance, is essential. This approach would enable more efficient and large-scale adoption of this intercropping system, maximizing its potential for sustainable agriculture.

Previous studies have demonstrated that differences in crop tolerance to herbicides primarily stem from the herbicide’s mechanism of action or the crop’s ability to metabolize and detoxify these substances (Sterling and Balke, 1989; Usui, 2001). Leveraging this understanding, herbicide-tolerant crops can be engineered to enable the application of herbicides across multiple crops within strip intercropping systems. Furthermore, developing crops with tolerance to multiple herbicides, combined with rotating herbicides with different mechanisms of action, could significantly delay the evolution of herbicide resistance and effectively address the challenges of weed control in intercropping systems. Research in this area has predominantly focused on monoculture and rotational cropping, with limited studies exploring its application in intercropping systems. For instance, imidazolinone-tolerant maize has been developed over the past four decades (Anderson and Hibberd, 1988; Shaner et al., 1996). Using agronomic practices and plant growth regulators has been shown to control weeds and maintain crop quality by reducing senescence and improving various quality attributes during storage (Feng et al., 2020; Ban et al., 2024). However, there is a notable gap in the literature regarding the use of imidazolinone-tolerant maize in strip intercropping systems, including its application methods, herbicidal efficacy, and economic viability.

Imidazolinone herbicides, registered for use on soybean, are effective against a broad spectrum of grasses and broadleaf weeds by inhibiting the enzyme acetolactate synthase (Folberth et al., 2020). These herbicides are notable for their efficacy at low application rates, low mammalian toxicity, and favorable environmental profile (Tan et al., 2005). Grasses that grow within soybean rows are common and challenging weeds in maize-soybean intercropping systems, often difficult to control with a single herbicide. In this context, using a grass herbicide like imidazolinone, which is registered for soybean, while developing maize that is tolerant to this herbicide, could be highly effective. Based on this premise, we hypothesized that weed control strategies employing imidazolinone-tolerant maize would offer a productive, economical, and reliable solution for weed management in maize-soybean strip intercropping systems. We utilized a previously developed non-genetically modified (non-GM) imidazolinone-tolerant maize, intercropped with a shade-tolerant soybean cultivar, to achieve the following objectives: (1) quantify the growth parameters and grain yields of crops in the intercropping system, (2) assess weed populations and weed control efficacy, and (3) analyze the land equivalent ratio and economic outcomes under different weed control strategies.

Cereal-legume intercropping systems, such as maize-soybean strip intercropping, have been introduced to address food security concerns for the growing global population by increasing resource utilization efficiency while reducing inputs (Du et al., 2018). The maize-soybean strip intercropping system, with a land equivalent ratio (LER) greater than 1, demonstrates that vegetative diversity within the field creates an ecological niche that captures resources more effectively, resulting in greater biomass production compared to monocropping (Xu et al., 2020). However, the weeds that survive within this intercropping system compete aggressively with maize and soybean for essential resources, thereby limiting the economic output of the system. Therefore, selecting appropriate herbicides is crucial for effective weed management in modern agriculture (Swanton et al., 2008). The use of effective herbicides is particularly important in intercropping systems, where weed control can be more challenging (Strehlow et al., 2020).

Despite the benefits, the species diversity in maize-soybean strip intercropping and the absence of a common post-emergence herbicide complicate and increase the cost of weed management in a single operation. Farmers have traditionally relied on machine-mounted dual-spraying systems with separators (SW), which are difficult to manage due to the need for careful handling to avoid herbicide mixing, resulting in higher costs and lower efficiency. In response to these challenges, this study presents an inexpensive and effective weed control method based on herbicide-tolerant crops to enhance the yield and land-use efficiency of the maize-soybean strip intercropping system. Our findings demonstrate that the use of imidazolinone-tolerant maize in this intercropping system not only provides effective weed management but also significantly improves yields compared to monoculture systems and the traditional SW method.

Previous studies have shown that weed control systems employing imidazolinone-tolerant cereals and imidazolinone herbicides are effective in managing weeds that are difficult to control with other herbicides, such as red rice in rice (Oryza sativa L.) and goat grass in wheat (Triticum aestivum L.) (Tan et al., 2005). Imidazolinone herbicides target the enzyme acetohydroxyacid synthase (AHAS) or acetolactate synthase, inhibiting the biosynthesis of branched-chain amino acids in plants (Folberth et al., 2020). These herbicides are particularly effective against a broad spectrum of grass, broadleaf, and sedge weeds, including those closely resembling the crop itself (Tan et al., 2005). Importantly, imidazolinone herbicides can be applied to soybean without targeted resistance, as they are rapidly metabolized, preventing toxicity in the soybean plants (Tecle et al., 1993). In our study, we applied imidazolinone as a common herbicide by introducing imidazolinone-tolerant maize into the maize-soybean intercropping system. The maize used in our study is capable of tolerating more than four times the registered dose of imidazolinone. In addition, bentazone, a selective post-emergence (POST) herbicide, has been successfully used in maize-soybean strip intercropping, primarily for controlling broadleaf and sedge weeds, though it is less effective against grasses (Dai et al., 2017; Ali et al., 2020). Therefore, the combination of imidazolinone (imazamox) and bentazone (NSW) in our study expanded the weed control spectrum, enhanced weed control efficacy, and reduced crop-weed competition.

In this study, the combination of imazamox and bentazone (NSW) was used for the first time in the maize-soybean strip intercropping system, and it demonstrated a broader herbicidal spectrum and higher weed control efficacy compared to the existing nicosulfuron + fomesafen (SW) system (Tables 2, 3, Figure 5), particularly in controlling Cyperaceae weeds. For instance, the weed control efficacy and fresh weight control efficacy of 2M4S^NSW for Cyperaceae were over 95%, significantly higher than the 60-70% efficacy observed with 2M4S^SW (Figures 5a-d). Furthermore, our two-year data on LAI, TDM in maize and soybean, and the relative leaf area of weeds (Lw) indicated that 2M4S^NSW effectively reduced competition between intercropped species and weeds compared to 2M4S^SW. Thus, our findings confirm that the introduction of imidazolinone-tolerant maize combined with the NSW system can be adopted as a reliable, effective, and broad-spectrum POST weed control method for maize-soybean strip intercropping.

One of the main reasons farmers adopt the maize-soybean strip intercropping system is its ability to increase land productivity while reducing inputs, as evidenced by its higher land equivalent ratio (LER > 1) compared to sole cropping systems. Our results also demonstrated the yield and land-use advantages of maize-soybean intercropping over monoculture. Additionally, the convenient and effective weed control achieved through the NSW method significantly reduced weed-crop competition, leading to enhanced land efficiency and increased grain yield, particularly for soybean. We found that the 2M4S^NSW method produced 102% higher total grain yield and 99% higher soybean yield compared to the 2M4S^SW method, while the maize yield did not differ significantly between the two weed control systems. The highest total LER values of 1.81 in 2021 and 1.75 in 2022 further confirm that using imidazolinone-tolerant maize for weed management can substantially improve land use efficiency in maize-soybean intercropping without compromising maize yield.

Previous studies have shown that weeds in soybean-based intercropping systems primarily originate from soybean rows and can severely reduce soybean yield (Cheriere et al., 2020). In contrast, the competitive advantage of maize, supported by narrow row spacing and high planting density, significantly suppresses weed biomass (Begna et al., 2001; Marín and Weiner, 2014; Teasdale, 1995). Our results are consistent with these findings, as the average pLERs of 2M4S^NSW was twice as high as that of 2M4S^SW, while there was no significant difference (p-value > 0.05) in pLERm between 2M4S^NSW and 2M4S^SW. These findings indicate that soybean yield in intercropping strongly depends on the effectiveness of weed control. The higher LER observed in 2M4S^NSW is primarily driven by the increased soybean yield, which corresponds to the reduced weed pressure on soybean achieved through the NSW method.

The utilization of herbicide-tolerant maize in maize-soybean intercropping can significantly improve weed control efficacy while reducing associated costs. For example, in the imidazolinone-tolerant maize-soybean intercropping system, imidazolinone herbicides can be applied using drones, unlike the SW method, which requires a hooded sprayer to separately apply post-emergence (POST) herbicides. The SW method has three major drawbacks compared to NSW: a narrower weed control spectrum, increased herbicide usage, and higher mechanical costs, along with the risk of herbicide-induced crop damage. In contrast, the NSW method is five times faster (8 ha h^-¹) than SW (1.6 ha h^-¹), offering potential savings in both time and money. Our economic comparison of the two weed control systems confirmed this, showing that the 2M4S^NSW system saved 37.05 USD ha^-¹ in herbicide and weeding machinery costs compared to 2M4S^SW. Additionally, the net income of the 2M4S^NSW system was estimated at 3414.12 USD ha^-¹ over two years, representing an impressive 57.6% and 101.2% increase compared to 2M4S^SW and MM, respectively (Table 5). These results are consistent with previous studies, which have shown that herbicide-tolerant crops can reduce weed control costs, improve yields, and increase overall benefits (Brookes and Barfoot, 2020; Green, 2012). Our findings, along with the successful use of herbicide-tolerant crops in other systems for weed control and yield optimization (Bonny, 2016; Gianessi, 2008; Liu et al., 2020; Watkinson et al., 2000), suggest that the development of herbicide-tolerant crops is a critical trend for the future of weed management in intercropping systems.

Current weed control practices in maize-soybean intercropping primarily rely on pre-emergence herbicides (Prasad and Rafey, 1995; Singh et al., 2005; Gupta and Singh, 2017; Bibi et al., 2019; Singh and Longkumer, 2021; Geng et al., 2023), including acetochlor, metolachlor, S-metolachlor, pendimethalin, flumetsulam, thifensulfuron-methyl, and metribuzin (Gupta and Singh, 2017). However, these soil-applied herbicides face two critical limitations: (1) Narrow application window: Treatments must be strictly timed between sowing and crop emergence (0–48 h), and the soil needs to be at least 50% water, leaving minimal flexibility; (2) Inability to suppress late-emerging weeds: Persistent gaps in weed management occur due to uncontrolled weed growth during later crop stages. Furthermore, post-emergence herbicide options are severely limited in this system. While bentazon, a broadleaf-specific herbicide, is approved for both crops (Dykun et al., 2020; Saad et al., 2004), no graminicide are registered for simultaneous use in maize and soybean. This regulatory gap highlights the urgent need to develop graminicide-tolerant maize or soybean varieties.

There are few reports on simultaneous weed control of herbicide-resistant crops in intercropping, which is related to farming systems in different countries. For example, herbicide-resistant crop of herbicide rotations are more commonly reported in American and European countries (Lamichhane et al., 2017; Marochi et al., 2018), but intercropping with same-season normal crops in Asia (Singh et al., 2005; Singh and Longkumer, 2021; Geng et al., 2023). A German study documented maize and soybean intercropping for weed control, where staggered sowing of maize and soybean enabled sequential pre- and post-emergence mechanical weeding combined with herbicide applications (Andert, 2021). However, in Asia (particularly China), synchronous sowing of maize and soybean makes separate weed management for the two crops highly inconvenient. Although the imidazolinone-resistant (graminicide-tolerant) maize soybean intercropping system described in this study reduces costs and improves operational convenience, it remains imperfect.

It has been reported that the persistence of imidazolinone herbicides in the soil can potentially harm subsequent crops (Gehrke et al., 2021). However, certain soil conditions—such as higher pH, lower clay content, moderate organic matter, higher soil moisture, and elevated temperatures—can promote the dissipation of these herbicides (Gehrke et al., 2021). To mitigate the residual effects on the following season’s crops, it is recommended to use appropriate herbicide concentrations at the correct timing, combined with other broadleaf herbicides like bentazone and fluthiacet-methyl. Furthermore, the development of herbicide-tolerant crops, such as maize tolerant to Acetyl CoA carboxylase (ACCase, EC: 6.4.1.2) inhibitors, soybean tolerant to 4-Hydroxyphenylpyruvate dioxygenase (HPPD, EC:1.13.11.27) inhibitors, or maize and soybean tolerant to non-selective herbicides (e.g., glyphosate and ammonium-glyphosate), could provide new solutions for maize-soybean strip intercropping weed control. These herbicides typically have shorter residual periods and pose less risk to successive crops. Adopting a rotation of these crops and corresponding herbicides could serve as a sustainable practice to address weed management challenges in maize-soybean strip intercropping. Overall, our results suggest that weed control based on herbicide-tolerant maize enhances integrated weed management and is more profitable, yielding higher net income with fewer inputs in the maize-soybean intercropping system. This strategy offers a convenient, economical, and efficient approach to weed control in intercropping and mixed cropping systems.

Imidazolinone herbicides can be safely and effectively applied for weed control in an imidazolinone-tolerant maize-soybean intercropping system, eliminating the need for multiple herbicides and complex spraying equipment like hooded sprayers. The superior weed control efficacy and the ability to mimic monoculture traits for herbicide application suggest that the herbicide-tolerant approach used in our study could greatly simplify weed management in strip intercropping systems. With this approach, growers can uniformly apply a single herbicide to both maize and soybean in a maize-soybean strip intercropping system using any convenient method—whether hand-held sprayers, self-propelled sprayers, or drones—depending on the size of the farmland and available resources. Our results demonstrated that the imidazolinone-tolerant NSW method effectively suppressed weed pressure in soybean rows, leading to improved grain yields (98.6% of MM yields and 79.2% of SM yields in the 2M4S system) and net profits exceeding 3000 USD ha^-¹, which were significantly higher than the profits from individual MM and SM systems. This indicates that effective weed management in maize-soybean strip intercropping can offer substantial returns to farmers, contributing to meeting the food needs of a growing population through sustainable crop production, particularly in resource-limited regions such as China. However, further research is needed to evaluate the suitability of various weed species and control methods across different ecological zones, as well as to assess the impact of herbicides on the growth of subsequent crops.