Maize is one of the world’s and China’s major crops in both plant area and yield and plays a pivotal role in national food security. With the development of the Chinese economy and population growth, the demand for grain has shown a steady increase, and maize output will need to rise to fulfill the needs (Ghose, 2014; Tigchelaar et al., 2018). Increasing planting density and the number of effective harvested ears per unit area is one of the key cultivation measures to increase maize yield (Ma et al., 2014). In China, the average planting density of maize rose from 15,000 plants ha⁻¹ in the 1950s to 60,000 plants ha⁻¹ in 2010, and the average grain yield elevated from 1,000 kg hm⁻¹ in the 1940s to 6,000 kg hm⁻¹ in 2010 (Li et al., 2016). In recent years, the planting density of maize in China has grown from 99,500 plants ha⁻¹ to 105,000 plants ha⁻¹ (Kamran et al., 2018; Liu et al., 2021; Zhang et al., 2022).

Increasing planting density is an effective approach to increase maize yield with minimum input. However, as density increases, increased competitive pressure between plants changes the structure and function of maize plants and populations, thus affecting maize stalk morphology, carbohydrate accumulation and distribution, chemical components, root morphology and structure, and mechanical strength of stalks, and increasing the risk of lodging (Xue et al., 2020a; Sher et al., 2018; Zheng et al., 2021; Qi et al., 2023). Stalk lodging causes annual yield loss of up to 75% (Li et al., 2015) and reduces the quality of grain production (Da et al., 2016), becoming an important concern in modern maize production. The direct mechanized harvest and grain drying into storage can simplify the production link, improve production efficiency, aid in achieving large-scale production, and help realize progressive advancement in agricultural practices (Xue et al.,2018a). However, the high lodging rate hinders the large-scale popularization and application of whole-mechanized processes. Grain loss due to straw fall is higher than root fall because straw fall disrupts the transport of photosynthetic products, nutrients, and water (Shah et al., 2021). Lodging at silking and milk-ripening stages (R1 and R3) influence grain filling and yield, while stalk lodging (R6) at the dewatering stage after physiological maturity adversely impacts mechanical yield (Xie et al., 2022). Morphological characteristics of the plant, including plant height and ear height, and mechanical strength of the stalk, including crushing strength (CS) and bending strength (BS), are often strong indicators of maize stalk resistance, and these indicators are often closely related to tillage practices (Shi et al., 2016; Li et al., 2023).

Tillage affects soil structure, moisture, fertility, gases, heat, and other factors that influence crop root growth and yield formation. However, there is little information on how maize stalk traits respond to tillage practices and their effects on maize stalk resistance to lodging. Deep tillage (e.g., subsoiling, deep ploughing, or ripping, to a depth > 20 cm) has been shown to enhance the structure and health of compacted soils (Hamza and Anderson, 2005; Hall et al., 2010). Tillage affects various soil physical, biological, and chemical properties and thus also impacts crop yield. Previous studies have shown that deep tillage improves the soil properties in the tilled layer by reducing soil bulk density and penetration resistance (Varsa et al., 1997), increasing soil porosity, hydraulic conductivity, and infiltration rate (Laddha and Totawat, 1997), and creating a more favorable soil environment for root growth and crop production than shallow tillage. Conventional tillage with soil compaction resulted in barren summer maize, leading to low root stress resistance (Bian et al., 2016). As compared to shallow tillage and no tillage, subsoil (SS) tillage can break up the compacted hardpan layer without inverting it, decrease the soil bulk density and soil compaction, and increase the soil water content. This improves soil structure and, thus, promotes root growth and boosts maize yield (Sun et al., 2018; Xu et al., 2019; Wang et al., 2020b). Root fixation properties and stalk development complement each other, promoting the accumulation of material and mechanical strength formation of aboveground stalks, effectively reducing the rate of lodging. Liu et al. reported that subsoil significantly improved the vascular transport efficiency of the stalk and enhanced stalk strength. On the other hand, it also improves root vigor, facilitates water and nutrient uptake, promotes dry matter accumulation, and increases the mechanical strength of the stalks, ultimately improving resistance to lodging (Liu et al., 2013). Ma et al. demonstrated that subsoil can reduce the ear height of maize under dense planting conditions, slow down the densification pressure, and increase maize stalk puncture strength, CS, and BS to varying degrees, so as to optimize stalk morphology and mechanical traits, which is conducive to improving the resistance to lodging and efficiently accommodating larger populations (Liu et al., 2013; Ma et al., 2014; Zhang et al., 2015). Yu et al. showed that subsoil improves the soil environment, which promotes the growth of the root system and helps to absorb water and nutrients from deeper layers of the soil, and that the use of subsoil technology can further increase the planting density, giving full play to the yield potential of varieties (Yu et al., 2013). It can be seen that SS tillage can be one of the effective ways to exploit the potential of maize for densification and resistance to lodging. Therefore, an in-depth investigation of the effect of SS tillage on maize stalk resistance after densification will be of great significance for the application of SS tillage measures to improve maize resistance in production and for the realization of the full mechanization of maize.

The average plough layer is 18.5 cm in China (Li and Wang, 2010) and lower than 35 cm in the United States (Li and Wang, 2010). At a high density, constructing a reasonable soil topsoil layer can promote material accumulation during the formation of stem mechanical strength, and thus reduce the lodging risk. Therefore, it was hypothesized that SS tillage could improve the lodging resistance of maize stalks under high-density conditions. The present study was carried out to (1) elucidate the responses of plant morphology, dry matter accumulation, and mechanical characteristics to subsoiling tillage; and (2) evaluate the response of different maize varieties to SS tillage at different densities. The study’s findings provide ideas and technical assistance for the mechanized harvesting of maize as well as the densification and yield increase.

Maize crop lodging is detrimental to both yield and quality and is influenced by maize varieties and cultivation practices (Ma et al., 2014). Increased planting density will increase the risk of lodging. Implementing subsoil tillage measures has a substantial impact on reducing lodging. In this study, as the planting density grew, so did maize lodging rates. However, after SS tillage, the lodging rate of ZD2 decreased by 6.04%, indicating alleviation of the lodging pressure induced by increased plant density.

Research on maize stalk lodging has mainly focused on plant morphology, stalk mechanical properties, chemical composition, and anatomical structure, among other aspects (Zhang et al., 2021a). Plant height and ear height (EH) are considered the most crucial morphological features related to stalk lodging. Previous studies have also reported that lowering plant height can enhance a crop’s resistance to lodging (Shekoofa et al., 2008; Peng et al., 2014). Additionally, maize with high resistance to stalk lodging should also have a lower ear position to decrease its center of gravity (Shah et al., 2021). Xie et al. (2022) concluded that reducing the ear ratio significantly decreased the length of internodes below the ear, thereby improving the lodging resistance of maize stalks in China over the past 70 years. Furthermore, maize lodging is closely associated with SDMA (Xue et al., 2020b), stalk diameter, and mechanical properties of the stalk (Xue et al., 2020a). Currently, enhancing stalk mechanical strength, stalk diameter, RPS, and the unit length basal internode dry weight (DWUL) are considered important approaches to improve maize lodging resistance (Robertson et al., 2014; Xue et al., 2018b; Wang et al., 2020a). The significant positive correlation between plant lodging resistance to lignin content and its related enzyme activities and mechanical properties has been generally confirmed (Zhang et al., 2014; Sekhon et al., 2020). In this study, the correlation analysis indicated a significant negative correlation between plant height, ear height, and stalk lodging rate. Stalk thickness, SDMA, RPS, BS, and compressive strength exhibited a significant positive correlation with lodging rate. There was a significant positive correlation between lignin, hemicellulose, and cellulose within the stalks and the mechanical strength of the stalks. These findings are in basic agreement with previous studies.

In this study, as the planting density grew, the plant and ear height of maize increased. Compared to RT, SS tillage could reduce both plant and ear height, thereby lowering the center of gravity of maize. A lower center of gravity is more favorable for the lodging resistance of maize stalks (Shah et al., 2021). With increasing density, the stalk internode diameter, SDMA, and mechanical strength of stalk internodes decreased. In comparison to RT, SS practices could increase the stalk internode diameter and SDMA, enhancing internal lignin and cellulose content of the stalks, thus increasing the mechanical strength of the stalks. This effect was more pronounced under high-density conditions, and the response of lodging-resistant varieties to SS tillage was more evident. Moreover, under SS conditions, the negative impact of high-density stress on stalk dry matter and mechanical strength was less than that under shallow rotation. This indicates that SS tillage can attenuate the negative effects of high-density stress on lodging-related indicators of stalk resistance.

SS tillage can improve the lodging problem caused by high-density planting, possibly because subsoil practices effectively break the plough pan, reduce soil volume (Yu et al., 2023), and decrease soil bulk density and compaction (Lou et al., 2021). This enhances soil porosity and cultivation thickness (Wang et al., 2019), altering soil physical properties and establishing a reasonable tillage layer structure. This is more conducive to the downward growth of maize roots, leading to variations in maize root characteristics. Compared to shallow rotation, subsoiling techniques are more favorable for the growth of maize roots, thereby increasing root biomass and anchorage capacity (Sun et al., 2017). Additionally, SS tillage can enhance root vitality, facilitate water and nutrient absorption, and delay root aging (Yin et al., 2021). A strong root system could increase root anchorage and absorptive capacity for water and nutrients, leading to high yield and resistance to root lodging (Manzur et al., 2014). Root development and stalk development complement each other, and well-developed roots promote the accumulation of stalk material and the formation of mechanical strength, ultimately improving maize lodging resistance.

Previous studies have shown that SS tillage is advantageous for regulating soil nutrients and breaking up soil compaction (Zhang et al., 2021b). Bian et al. (2016) reported that under shallow rotation, soil compaction caused poor root development in summer maize, leading to low lodging resistance of summer maize. Moreover, SS tillage can increase maize emergence rate, plant height, and uniformity, and enhance leaf area index (LAI), affecting photosynthetic characteristics. SS tillage enhances the photosynthetic rate, stomatal conductance, and transpiration rate, and promotes the accumulation of aboveground biomass (Wu et al., 2020). Various studies have supported that SS tillage significantly improves the transport efficiency of stalk vascular bundles, influences maize stalk characteristics, strengthens stalk strength, and ultimately enhances lodging resistance (Liu et al., 2013). SS tillage can also increase soil water storage (Brunel-Saldias et al., 2018; Zhou et al., 2019). As maize reaches the high-growth period, the demand for water increases and soil water storage may decrease. SS tillage is more conducive to the downward movement of water after irrigation or rainfall, effectively increasing moisture content in the deep soil layer (Wang et al., 2019), promoting the formation and accumulation of plant biomass. These factors could contribute to improving maize lodging resistance.

Subsoil tillage improved the maize lodging resistance, and its effects were more pronounced under high planting density. The lodging rate of maize was significantly negatively correlated with plant height, ear height, and ear position coefficient. It also significantly negatively correlated with stalk diameter, mechanical strength of the base internode, and biomass accumulation. The CS at the fourth and sixth internodes exhibited relatively high correlation coefficients with the lodging rate. Subsoil increases the mechanical strength of stalks by increasing the internal cellulose, hemicellulose, and lignin content of the stalks. SS tillage under high density reduces plant and ear height while elevating stalk diameter, dry matter accumulation, and the mechanical strength of the stalk, leading to an effective reduction in the maize lodging rate. In combination with stalk lodging-resistant varieties (XY335), SS tillage can effectively alleviate the problem of stalk lodging after intensive planting. The findings of this study provide new ideas for improving maize’s resistance to lodging.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

XF: Data curation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. DM: Conceptualization, Methodology, Writing – review & editing. TL: Investigation, Writing – review & editing. SH: Investigation, Writing – review & editing. XY: Project administration, Writing – review & editing. JG: Project administration, Writing – review & editing.