Optimizing Stem Strength and Yield Stability by Combining Controlled-Release Nitrogen Fertilizer and Urea Application Across Different Sowing Dates

Abstract

The delayed sowing date and basal internode lodging caused by climate change are major constraints on wheat productivity. To investigate the effects of varying sowing dates and fertilization application regimes on wheat yield and lodging resistance, a two-year field experiment was conducted with two sowing dates and five fertilization application regimes. Results revealed that the T2 sowing period caused grain yield reductions of 43.82% and 29.82% over two consecutive years, accompanied by shortened second basal internode length and decreased plant height, although lignin content increased significantly. Among fertilization treatments, S4 effectively enhanced the mechanical strength of the second basal internode, achieving both higher yield and superior lodging resistance. We propose combining controlled-release nitrogen fertilizer (CRNF) with urea across different sowing dates to optimize productivity and stem stability. These strategies tackle climate-driven sowing delays and lodging while maximizing yield potential.

1. Introduction

Wheat (Triticum aestivum L.) is one of the most globally important crops and provides more than 20% of the dietary protein for humans, yet current production levels remain insufficient to meet worldwide food security needs [1]. In 2024, the global population reached 8.2 billion and continues to rise, with projections estimating a peak of approximately 10.3 billion by 2084 [2]. Despite this growth, around 733 million people still suffer from hunger [3]. Under climate change, the stability of the entire food system faces significant risks [4], which may impair wheat growth and productivity, ultimately reducing yields [5]. To mitigate the compounding pressures of climate change, population expansion, and environmental degradation, enhancing wheat yield per unit area remains a pivotal agricultural priority [6,7]. In recent years, the optimal sowing time for wheat has gradually shifted to later periods in various parts of China [8]. While the delayed harvesting of previous crops contributes partially to this trend, the dominant factor lies in altered summer climatic conditions preceding winter wheat planting [9]. Furthermore, global warming has prolonged the pre-winter vegetative phase of Chinese winter wheat, necessitating strategic delays in sowing [8]. However, excessively late sowing exposes seedlings to low temperatures, resulting in stunted growth, weakened plant vigor, and compressed developmental stages [6,10]. Additionally, delayed sowing increases the likelihood of heat stress during grain filling, curtailing this critical phase and resulting in yield losses [11].

Lodging is a critical constraint in unlocking yield potential and remains a major limitation to cereal productivity [12]. As a complex stress phenomenon, lodging is influenced by multiple factors, including plant population density, excessive nitrogen fertilization, adverse weather conditions, soil type, and disease pressures [4]. Cereal crops exhibit two primary lodging types: stem lodging (the most prevalent and yield-damaging) and root lodging, with stem lodging predominantly driven by structural failure at the second basal internode—the segment farthest from the center of gravity and thus subjected to maximal mechanical stress [13,14,15]. In the 20th century, dwarf wheat cultivars bred for reduced center of gravity height significantly mitigated lodging risks and stabilized yields [16]. However, modern yield-maximizing strategies increasingly conflict with dwarfing traits, as studies demonstrate a positive correlation between grain yield and dry matter production [17]. Higher straw yields can lead to higher grain yields [18], which means that more nitrogen input is required.

Nitrogen fertilization plays a pivotal role in boosting wheat yields, serving as a key strategy to address food supply challenges amid global population growth [19]. Urea is the most widely used nitrogen fertilizer globally and has made tremendous contributions to world food production over the past several decades [1]. Moreover, applying urea in the main production area of wheat has been proven to effectively increase grain yield [20]. Surface-applied urea loses nitrogen as ammonia, polluting the air. Its breakdown products (ammonium and nitrates) leach into water, causing eutrophication, while rapid soil nitrogen accumulation accelerates acidification [21]. Additionally, repeated split applications incur substantial labor and time costs. Controlled-release nitrogen fertilizers (CRNFs) are coated urea granules that provide a sustainable alternative aimed at minimizing direct contact with soil urease [22]. By gradually releasing nutrients over extended periods, CRNFs improve the synchrony between nitrogen availability and crop demand, thereby enhancing nitrogen use efficiency (NUE), crop productivity, and environmental sustainability [23,24]. One-time CRNF applications have emerged as a labor-efficient practice with demonstrated efficacy across multiple studies [22,25]. Nevertheless, CRNF performance varies regionally due to climatic and edaphic factors [26,27]. A meta-analysis revealed that one-time CRNF significantly increases yields in potato, maize, and rice compared to split urea applications, yet shows limited efficacy in winter wheat, which is a crop with growth cycles typically exceeding 200 days [28]. This discrepancy persists despite the “S”-shaped nitrogen demand curve of wheat, which is consistent with CRNF release patterns. Crucially, the peak nitrogen demand of wheat often occurs during the jointing stage [29], but few CRNF formulations sustain release beyond 180 days [28], leading to potential nitrogen deficits during critical growth stages. To bridge this gap, researchers are exploring hybrid approaches: blending CRNFs with conventional urea or combining CRNFs with differing release durations [30,31,32,33]. Notably, Ma et al. demonstrated that a split-CRNF strategy, applying a second dose at the wheat greening stage, significantly improved yield outcomes [29], underscoring the potential of CRNF application timing. These technological innovations have effectively bridged the efficacy gap of CRNFs in wheat compared to other crops.

The increasing frequency of climate change and extreme weather events has compelled farmers to adopt adaptive measures to mitigate crop damage [34]. Among climatic factors, temperature, precipitation, and CO₂ concentration constitute critical determinants of wheat yield [35]. Although sowing date adjustment represents a straightforward management strategy, it exerts a significant influence on wheat cultivation by serving as a key determinant of productivity [36]. This approach primarily aims to optimize climatic conditions during critical growth stages of wheat under global warming through the strategic alignment of sowing dates. In certain European regions, early sowing enables wheat to utilize autumn rainfall more effectively, thereby enhancing yield potential [37]. In contrast, delayed sowing has been shown to improve wheat growth and productivity in northern China [38]. These findings highlight significant regional variations in optimal sowing dates. However, limited research has been reported regarding the synergy between CRNF application and sowing date optimization for enhancing wheat yield across different planting dates. Based on these findings, our study further explores the feasibility of applying CRNFs in twice-split applications under different sowing dates. The objectives of this study are to evaluate the effects of different CRNF application regimes on yield and lodging characteristics under both conventional and delayed planting conditions, provide recommendations for ensuring stable wheat yields and efficient fertilizer use across different sowing times, and ultimately safeguard sustainable food security stability.

2. Materials and Methods

2.1. Planting Location and Experimental Materials

Field experiments were conducted during the 2022–2023 and 2023–2024 winter wheat growing seasons at Xiangfeng Farm in Yangzhou, Jiangsu Province, China (32°45′ N, 119°68′ E). The previous crop in the experimental field was rice, with full straw incorporation (approximately 8000 kg·hm⁻² annually) after harvest. The soil type was classified as sandy loam. This study was conducted in the middle–lower reaches of the Yangtze River, one of the Chinese primary wheat production regions. This area features a humid subtropical climate, with temperature and precipitation patterns particularly suitable for winter wheat cultivation. The monthly average temperature and precipitation during the two experimental seasons are shown in Figure 1. The experimental wheat cultivar, Nongmai 88, is a locally promoted and widely cultivated variety in this region. This cultivar exhibits a relatively high protein content and demonstrates pronounced responsiveness to nitrogen application, making it ideally suited for the objectives of this study. Before fertilization, soil samples from the 0–20 cm cultivated layer were collected using the five-point sampling method for analysis. The baseline soil fertility results are presented in

Table 1. Soil nutrient levels before planting in the two-year experimental season.

Items	Soil Properties	Year
		2022	2023
Soil basic fertility	Organic matter (g kg−1)	23.1	21.7
	Alkali-hydrolyzable nitrogen (mg kg−1)	131.0	116.1
	Available phosphorus (mg kg−1)	18.9	16.8
	Available potassium (mg kg−1)	80.0	89.2
	pH	6.7	7.8

. The content of soil alkali-hydrolyzable nitrogen was determined using the alkaline hydrolysis diffusion method; soil available phosphorus was extracted with sodium bicarbonate and measured using the molybdenum-antimony anti-spectrophotometric method; soil available potassium was extracted with ammonium acetate and quantified via flame photometry; and soil organic matter content was analyzed using the potassium dichromate volumetric method (external heating method). The CRNF tested was sulfur-coated urea (SCU; 37% N, 90–120-day release period) provided by Hanfeng Slow-release Fertilizer Co., Ltd. (Taizhou, China). Conventional fertilizers included urea (46% N), calcium superphosphate (12% P₂O₅), and potassium chloride (60% K₂O), which were sourced locally.

2.2. Experimental Design and Planting Management

This experiment adopted a two-factor split-plot design, with sowing dates as the main plots and fertilization application regimes as subplots. All treatments were replicated three times. The first sowing period (T1) occurred on 5 November 2022 and 3 November 2023, while the second sowing period (T2) was implemented on 12 December 2022 and 4 December 2023. The control treatment (CK) used conventional urea split into four applications. The S1 treatment involved a one-time basal application of the CRNF. Treatments S2–S4 utilized two split applications of the CRNF, with detailed timing and rates provided in

Table 2. Fertilizer application timing and rate by growth stage in the experimental design.

Treatment	Growth Period
	Sowing	Tillering	Regreening	Jointing	Booting
CK	50% U	10% U		20% U	20% U
S1	100% SCU
S2	60% SCU		40% SCU
S3	60% SCU		40% U
S3	60% SCU		40% U
S4	60% (50%SCU + 50%U)		40% (50%SCU + 50%U)

Note: U represents conventional urea with a nitrogen content of 46%; SCU denotes sulfur-coated slow-release urea containing 37% nitrogen, with a release duration of 90–120 days.

. All fertilization application regimes supplied a total nitrogen (N) rate of 225 kg ha⁻¹, with phosphorus (100 kg P₂O₅ ha⁻¹) and potassium (120 kg K₂O ha⁻¹) applied as basal fertilizers before sowing. Individual plots measured 15 m² (5 m × 3 m) and were sown mechanically with a row spacing of 0.27 m. Seedlings were thinned to 225 plants m⁻² at the three-leaf stage (GS20). No supplemental irrigation was applied due to adequate rainfall; other field management followed local high-yield practices.

2.3. Sampling and Determination

2.3.1. Grain Yield and Yield Components

At maturity, a 1.08 m² sampling area (excluding border rows) was harvested per plot. Grain yield was determined by mechanically threshing the samples, adjusting to a standard moisture content of 13%.

2.3.2. Plant Morphological Characteristics

Destructive sampling was conducted at the flowering and maturity stages, with ten uniform plants selected per plot for measurements. Plant height was measured using a ruler as the distance from the base of the stem to the tip of the spike. Internode lengths were subsequently measured, focusing on the first and second basal internodes from the stem base. Gravity center height was determined by measuring the distance from the stem base to the balance point after root removal.

2.3.3. Morphological Traits of the Second Internode

Following these measurements, the stem diameter of the second internode was recorded using vernier calipers. The second basal internode was then bisected longitudinally after manually removing leaf sheaths. A digital vernier caliper was used to measure the outer and inner diameters at the midpoint of the internode. Wall thickness was calculated as:

Wall thickness (mm) = (Outer diameter − Inner diameter)/2.

The second internode was oven-dried at 105 °C for 30 min, followed by drying at 80 °C to constant weight. The dry weight was measured, and the internode filling degree was calculated as:

Filling degree (mg cm⁻¹) = Dry weight/Internode length.

2.3.4. Lodging Resistance Trait

Given that stem lodging in wheat predominantly occurs at the second basal internode, we measured breaking strength (N) using a YYD-1 stalk strength tester (Top Instruments Co., Ltd., Hangzhou, China). At both the flowering and maturity stages, ten representative plants per plot were selected, and force was applied perpendicularly to the midpoint of the second internode until structural failure occurred. The calculation method of the lodging resistance index (LRI) is based on Chen et al. [39]:

LRI = Breaking strength (N)/Gravity center height (m)

2.4. Statistical Analysis

All statistical analyses and correlation analyses were performed using SPSS Statistics 24 (SPSS Inc., Chicago, IL, USA), and figures were generated with Origin 2018 (Origin Lab Corp, Northampton, MA, USA). Correlation analyses were conducted using two-year averaged data. Following F-test verification, means were compared using the Duncan multiple range test at the p < 0.05 significance level.

3. Results

3.1. Grain Yield

Under the two sowing dates (T1 and T2), T2 significantly reduced grain yields compared to T1, with declines of 43.82% (2022–2023) and 29.82% (2023–2024) (Figure 2a,b), demonstrating that delayed sowing (T2) constrained yield formation. Analysis of variance (ANOVA) revealed significant differences in grain yield among fertilization application regimes. Specifically, the yield of the S1 treatment was significantly lower than CK and other fertilizer treatments (S2–S4), indicating inherent yield-limiting factors in the S1 fertilization strategy. Compared to CK, the S2 treatment increased grain yield by 13.34% and 5.98% across the two growing seasons, while S3 showed yield increases of 13.62% and 2.62%, respectively. The S4 treatment increased grain yield by 18.36% and 1.70%, respectively. However, no significant yield differences were observed among the S2, S3, and S4 treatments across both growing seasons.

3.2. Morphological Characteristics

As shown in

Table 3. Plant morphological characteristics under different treatments in the two-year field experiment.

Time	Treatment	2022–2023			2023–2024
		Basal Internode Length (cm)		Plant Height (cm)	Basal Internode Length (cm)		Plant Height (cm)
		I	II		I	II
T1	CK	3.64 ± 0.42 ab	6.21 ± 0.10 de	69.47 ± 2.59 cd	4.81 ± 0.35 a	8.53 ± 0.13 a	74.05 ± 0.95 c
	S1	3.73 ± 0.53 a	6.3 ± 0.51 cde	75.12 ± 2.47 ab	4.92 ± 0.50 a	7.92 ± 0.24 abc	71.6 ± 0.34 d
	S2	3.33 ± 0.28 abc	6.54 ± 0.03 bcd	76.02 ± 2.33 ab	4.72 ± 0.21 a	6.66 ± 0.28 ef	79.87 ± 0.55 a
	S3	3.41 ± 0.52 abc	6.66 ± 0.37 bc	74.54 ± 3.11 b	4.885 ± 0.33 a	7.69 ± 0.23 bcd	77.57 ± 1.01 b
	S4	3.35 ± 0.31 abc	7.33 ± 0.30 a	78.61 ± 2.11 a	4.65 ± 0.17 a	8.18 ± 0.44 ab	73.09 ± 0.59 c
T2	CK	3.01 ± 0.01 abc	5.90 ± 0.45 ef	61.75 ± 1.23 e	3.29 ± 0.16 c	7.23 ± 0.25 de	69.45 ± 1.35 ef
	S1	3.21 ± 0.39 abc	5.68 ± 0.54 f	66.89 ± 3.37 cd	3.24 ± 0.48 c	6.53 ± 0.43 f	68.92 ± 0.46 ef
	S2	2.77 ± 0.22 c	6.04 ± 0.08 ef	69.95 ± 2.58 c	3.86 ± 0.02 bc	7.09 ± 0.11 def	70.29 ± 0.63 e
	S3	2.74 ± 0.38 c	6.75 ± 0.07 b	70.52 ± 0.87 c	4.3 ± 0.58 ab	7.75 ± 0.81 bcd	68.14 ± 0.78 f
	S4	2.95 ± 0.03 bc	6.15 ± 0.15 de	66.11 ± 1.54 d	3.39 ± 0.17 c	7.32 ± 0.25 cd	69.01 ± 0.72 ef
T		ns	**	**	**	**	**
F		ns	**	**	ns	**	**
T×F		ns	**	ns	ns	**	**

Note: T represents the sowing time, F represents the fertilization application regime, and T×F indicates the interaction effect between the sowing time and fertilization application regime. Different letters represent significant differences at p < 0.05. ** represent significance levels of p < 0.01; ns means not significant.

, we attribute plant height, basal internode length, and center of gravity height to plant morphology. The sowing date has a significant impact on plant height, basal internode length, and center of gravity height. In 2022–2023, neither the sowing date nor fertilization application regimes affected the length of basal internode 1, whereas in 2023–2024, the sowing date significantly influenced internode 1 length. Across both seasons, T2 reduced internode 1 and internode 2 lengths by 15.94% and 24.62% (2022–2023) and 7.63% and 7.85% (2023–2024), respectively, indicating stronger suppression of internode 1 elongation under delayed sowing.

The sowing date, fertilization application regime, and their interaction significantly affected internode 2 lengths in both seasons. Compared to T1, T2 decreased the internode 2 length by 7.63% (2022–2023) and 7.85% (2023–2024). Internode 2 lengths under S3 and S4 treatments were significantly greater than those under S1 and S2 in both seasons.

Compared to T1, T2 reduced plant height (PH) by 8.07% (2022–2023) and 10.31% (2023–2024). In 2022–2023, the S1–S4 treatments produced significantly taller plants than CK; in 2023–2024, only S1–S3 exceeded CK with regards to PH. S2 increased PH by 11.25% (2022–2023) and 4.64% (2023–2024) compared to CK. The T1S2 combination consistently produced the tallest plants across both seasons, although this height advantage may elevate lodging risks.

Figure 3c,d show a progressive increase in gravity center height (GCH) from flowering to maturity, correlating with heightened lodging susceptibility. The height of the center of gravity in the S2 and S4 treatments was significantly higher than in the S1 treatment, and the two-year experimental results showed consistent trends in the flowering and maturity stages. Under T1, S2 and S3 had higher GCH values than CK in the flowering stage; under T2, S3 and S4 led to significantly higher GCH values than CK. In 2022–2023, no significant GCH differences were observed among T1 treatments at maturity, whereas T2S3 showed significantly higher GCH values than T2CK. Compared to the flowering period, the S4 treatment showed the largest increase in GCH during the maturity period of the 2023–2024 experimental season, with a 42.4% increase. The experimental season of 2023–2024 increased the S3 treatment by 38.4%.

3.3. Traits of the Second Internode

The effects of the sowing date and fertilization application regimes on stem diameter (ST) are shown in Figure 2c,d. Compared with T1, the ST under T2 decreased by 21.45% and 5.29% in the two-year experiment, respectively. In the two-year experimental results, the ST of the S1 treatment increased by 11.83% and 2.61%, respectively, compared to CK. In the 2022–2023 experimental season, the ST of CK was smaller, and the S1 and S3 treatments led to significantly higher values than CK. In the 2023–2024 experimental season, the ST of the S4 treatment was significantly higher than CK. During the 2022–2023 experimental season, T1CK led to significantly higher values than T1S2 and T1S3, and T2S1 led to significantly higher values than T2CK, T2S2, T2S3, and T2S4. In the 2023–2024 experimental season, T1CK led to significantly higher values than T1S3, and T2S1 values were significantly higher than T2S2 values.

The two-year results showed that the sowing period had no significant effect on wall thickness, while the fertilization application regimes had a significant effect on wall thickness, and the interaction between the two had a significant impact. The wall thickness under the S1 treatment was significantly higher than that of CK, S2, S3, and S4. Compared with CK, the wall thickness of the S1 treatment increased by 33.64% and 7.07%, respectively, during the two-year experimental season. In the 2022–2023 experimental season, values under T1S1, T1S2, and T1S4 were significantly higher than T1CK; T2S1 led to significantly higher values than T2S2 and T2S3 (Figure 2e,f). During the 2023–2024 experimental season, T1S1 led to significantly higher values than T1S2, T1S3, and T1S4.

Two years of results showed that the sowing date and fertilization application regimes had a significant impact on the filling degree of the second internode (IFD) during the flowering and maturity periods. Compared with T1, T2 showed a decrease of 21.77% and 20.08% with regard to the IFD during the flowering period in the two-year experimental season, and decreases of 17.09% and −13.73% during the maturity period (Figure 3a,b). Compared with CK, the IFD of the S3 treatment was significantly lower during the flowering period, with a decrease of 29.40% and 20.25% in the two years, respectively. Compared with CK in the mature stage, the IFD of the S2 and S3 treatments was significantly lower than CK, decreasing by 10.89% and 14.45%, respectively, within two years (23.17% and 24.01%). Compared with the flowering stage, the IFD in the mature stage showed a decreasing trend, with the highest rate of decrease observed in the T1S2 treatment, with decreases of 52.59% and 52.91%, respectively, over the two years.

3.4. Lodging Resistance Trait

As shown in

Table 4. Lodging resistance characteristics of wheat under different treatments in the two-year field experiment.

Time	Treatment	2022–2023				2023–2024
		Anthesis		Maturity		Anthesis		Maturity
		Breaking Strength (N)	Lodging Resistant Index	Breaking Strength (N)	Lodging Resistant Index	Breaking Strength (N)	Lodging Resistant Index	Breaking Strength (N)	Lodging Resistant Index
T1	T1CK	8.83 ± 0.13 bcd	29.19 ± 4.46 b	8.70 ± 0.21 a	19.68 ± 0.47 ab	11.04 ± 1.29 ab	31.57 ± 4.45 ab	6.56 ± 0.29 cd	14.14 ± 0.38 ef
	T1S1	9.95 ± 0.49 ab	30.32 ± 2.91 ab	7.00 ± 0.28 abc	16.56 ± 0.66 a–d	11.43 ± 1.3 7 ab	33.49 ± 4.40 a	7.64 ± 0.60 b	17.03 ± 1.54 cd
	T1S2	7.31 ± 0.27 d	21.61 ± 3.96 c	5.89 ± 0.37 bc	12.29 ± 0.68 cd	10.32 ± 0.62 bc	28.16 ± 1.84 b	6.82 ± 0.54 bcd	13.72 ± 1.16 f
	T1S3	7.33 ± 1.66 d	22.07 ± 2.23 c	5.24 ± 0.30 c	11.21 ± 0.63 d	7.82 ± 0.91 d	20.90 ± 2.57 c	6.27 ± 0.21 d	13.38 ± 0.25 f
	T1S4	7.54 ± 0.88 cd	21.19 ± 1.23 c	5.86 ± 1.11 bc	12.20 ± 2.25 d	9.11 ± 0.47 cd	26.41 ± 1.29 b	7.19 ± 0.35 bcd	15.04 ± 0.90 def
T2	T2CK	8.62 ± 0.35 bcd	28.77 ± 2.46 b	8.85 ± 1.71 a	21.61 ± 4.17 a	11.88 ± 0.22 ab	35.51 ± 1.16 a	9.02 ± 0.11 a	18.89 ± 0.08 bc
	T2S1	11.23 ± 0.97 a	34.62 ± 3.56 a	7.73 ± 0.17 abc	18.04 ± 1.65 abc	12.16 ± 1.16 a	36.19 ± 3.22 a	9.90 ± 0.81 a	21.55 ± 1.9 a
	T2S2	9.97 ± 0.91 ab	29.49 ± 1.37 ab	5.76 ± 0.84 cd	13.49 ± 1.33 cd	9.16 ± 0.29 cd	27.40 ± 1.03 b	6.94 ± 0.13 bcd	14.95 ± 0.23 def
	T2S3	11.31 ± 0.49 a	32.46 ± 2.73 ab	5.88 ± 1.07 cd	12.35 ± 2.02 cd	8.55 ± 0.37 d	26.58 ± 1.63 b	7.29 ± 0.23 bc	16.19 ± 0.16 de
	T2S4	9.27 ± 1.06 bc	28.28 ± 1.59 b	6.43 ± 0.26 bcd	14.18 ± 0.25 bcd	11.39 ± 1.11 ab	35.85 ± 3.53 a	9.55 ± 1.01 a	20.52 ± 2.13 ab
T		*	**	ns	ns	*	**	**	**
F		*	*	*	**	**	**	**	**
T×F		ns	ns	ns	ns	ns	ns	**	*

Note: T represents the sowing time, F represents the fertilization application regime, and T×F indicates the interaction effect between the sowing time and fertilization application regime. Different letters represent significant differences at p < 0.05. * and ** represent significance levels at p < 0.05 and p < 0.01, respectively; ns means not significant.

, both sowing dates and fertilization application regimes significantly affected the breaking strength of the second internode at the flowering stage across the two-year experiment. Compared with T1, T2 increased breaking strength by 23.06% and 6.9% in the 2022–2023 and 2023–2024 trials, respectively. During the 2022–2023 season, the S1 treatment exhibited a significantly higher breaking strength than S2 and S4 at flowering, along with a notably higher lodging resistance index (LRI) compared to S2, S3, and S4. Similarly, in 2023–2024, the S1 treatment eclipsed S2, S3, and S4 in both breaking strength and the LRI.

The results of the two-year experiment in the mature stage showed that the breaking strength of plants under the S1 treatment was significantly higher than that of S2 and S3. However, in 2022–2023, CK showed the highest LRI at maturity, with S1, S2, S3, and S4 showing decreases of 16.21%, 37.53%, 42.92%, and 36.09% compared to CK, respectively. Conversely, in 2023–2024, S1 and S4 improved by 16.78% and 7.66% with regard to the LRI, while S2 and S3 reduced it by 13.21% and 10.46% compared to CK. Overall, S1 exhibited the best lodging resistance across both years.

From flowering to maturity, both the breaking strength and LRI declined. The sharpest reductions occurred in T2S3 (48.01% and 61.91%) in 2022–2023 and T1CK (40.63% and 55.21%) in the 2023–2024 experimental season.

3.5. Lignin Content

This study conducted a two-year field experiment to analyze the effects of different sowing dates and fertilization treatments on lignin content in the second internode of wheat during maturity (Figure 4). Notably, the S1 fertilization regimen maintained relatively favorable lignin contents under both sowing dates, while S2 and S3 treatments resulted in reduced lignin contents, indicating that fertilizer type plays a crucial role in lignin biosynthesis. The results demonstrated that delayed sowing treatment (T2) significantly increased wheat lignin content, with the T2S1 combination showing the most pronounced effect, which was significantly higher than other treatments. In contrast, all fertilization treatments under the T1 sowing date generally exhibited lower lignin content, particularly the T1S2 and T1S3 treatments, which were significantly lower than other treatments during the 2023–2024 growing season.

3.6. Correlation and Principal Component Analysis (PCA)

As shown in Figure 5, grain yield exhibited significant positive correlations with the center of gravity height, length of the basal second internode at both flowering and maturity stages, and internode filling degree at flowering. Conversely, yield showed significant negative correlations with the breaking strength, lodging resistance index, internode filling degree, and lignin content at maturity. This indicates that higher yields are inherently associated with elevated lodging risks.

Principal component analysis (PCA) of 14 lodging-related traits captured 63.9% of the total variance across the first two principal axes. Figure 6 illustrates that yield clustered within the same quadrant as plant height, center of gravity height, wall thickness, stem diameter, and flowering-stage internode filling degree, with plant height and center of gravity height demonstrating particularly strong positive contributions to yield. In contrast, the internode filling degree, breaking strength, and lignin content predominantly enhanced the lodging resistance index.

Three common factors (F₁, F₂, and F₃) were extracted from the PCA output, collectively explaining 76.38% of the total variance, confirming their representativeness. Factor score equations were derived based on loading coefficients. A comprehensive score was calculated by weighting these factors according to their contribution rates (

Table 5. Comprehensive scores are calculated based on the weighted contribution rates of individual factors.

Treatment	F1	F2	F3	Synthesis Score	Ranking
T2S1	3.75	0.40	−0.64	1.53	1
T2S4	3.01	0.65	−0.15	1.35	2
T2CK	2.56	0.68	−0.17	1.17	3
T1S1	0.41	−0.79	1.93	0.27	4
T1S2	−2.30	4.02	−0.77	−0.21	5
T1S4	−1.33	0.02	2.26	−0.22	6
T1CK	−1.34	−0.45	1.31	−0.46	7
T2S3	−0.21	−2.36	−1.20	−0.75	8
T2S2	−0.74	−1.55	−1.83	−0.88	9
T1S3	−3.81	−0.63	−0.73	−1.80	10

). The results revealed that treatments T2S1, T2S4, and T2CK achieved the highest scores (>1), indicating superior lodging resistance, while T1S3 scored the lowest, representing the greatest lodging susceptibility.

F₁ = −0.322X₁ − 0.311X₂ − 0.106X₃ − 0.139X₄ + 0.309X₅ + 0.014X₆ − 0.006X₇ + 0.357X₈ + 0.011X₉ − 0.122X₁₀ +
0.382X₁₁ + 0.388X₁₂ + 0.277X₁₃ + 0.394X₁₄

(1)

F₂ = 0.313X₁ + 0.238X₂ − 0.453X₃ − 0.292X₄ + 0.182X₅ + 0.141X₆ + 0.332X₇ + 0.099X₈ + 0.376X₉ + 0.463X₁₀ +
0.083X₁₁ + 0.001X₁₂ + 0.121X₁₃ − 0.049X₁₄

(2)

F₃ = 0.057X₁ + 0.038X₂ + 0.349X₃ + 0.478X₄ + 0.223X₅ + 0.545X₆ − 0.113X₇ + 0.181X₈ + 0.476X₉ − 0.048X₁₀ −
0.023X₁₁ − 0.013X₁₂ + 0.112X₁₃ − 0.092X₁₄

(3)

4. Discussion

4.1. Relationship Between Stem Characteristics and Lodging

Wheat lodging typically occurs during the grain-filling to maturity stages, often triggered by storms and heavy rainfall events [12]. The population structure critically influences lodging susceptibility, exhibiting lower plant height and center of gravity and demonstrating enhanced lodging resistance [40]. Such architectural traits reduce the mechanical stress on basal internodes under external forces. Our study revealed a significant negative correlation between the length of the basal second internode and both breaking strength at maturity and lignin content (Figure 5), indicating that optimizing basal second internode length is essential for improving post-anthesis lodging resistance. As the basal internodes serve as primary structural supports for the entire plant, their morphological characteristics and mechanical strength fundamentally determine lodging resistance in wheat [41].

Previous studies have demonstrated that increased plant height reduces stem flexural resistance while negatively impacting stem diameter and wall thickness [42]. Internode length serves as a primary determinant of plant height, which in turn governs biomass production. Although height reduction may mitigate lodging risks, it concurrently limits dry matter accumulation and yield potential. In this study, the lodging resistance index exhibited a negative correlation with the center of gravity height but showed significant positive correlations with lignin content and internode filling degree (Figure 5). Lignin is not only crucial for plant growth and development, but previous studies have demonstrated a significant positive correlation between lignin content and lodging resistance [43]. These relationships underscore the inherent challenge of balancing high yield with lodging resistance, highlighting the critical need to enhance stem mechanical strength. Our experiment revealed progressive declines in internode filling degree and breaking strength from flowering to maturity. This physiological transition occurs as plants shift their developmental priority from vegetative growth to reproductive growth. During grain filling, nutrient remobilization from stems to developing grains reduces internode filling efficiency and elevates the center of gravity. The observed decline in breaking strength reflects the change in the physical strength of the stem, likely attributable to diminished cell wall synthesis and maintenance capacity during this critical phase, ultimately weakening internode mechanical strength [41]. Furthermore, excessive nutrient translocation during grain filling accelerates stem senescence, while suboptimal population structures exacerbate premature aging and lodging susceptibility [44].

4.2. Effects of Sowing Time and Fertilization Application Regimes on Lodging Resistance

This study systematically reveals the mechanisms through which sowing dates and fertilization application regimes influence crop yield formation and lodging resistance traits. The results indicate that the T2 sowing date significantly reduced wheat plant height and center of gravity height, thereby improving lodging resistance at the expense of a substantial yield reduction (36.81% average decrease over two years). This is consistent with prior studies demonstrating that delayed sowing mitigates lodging risk by reducing internode length, plant height, and center of gravity while enhancing internode diameter, wall thickness, dry weight, and grain filling efficiency [45]. However, more than one month sowing interval in this study exposed plants to distinct climatic conditions across growth stages. Late-sown (T2) wheat failed to develop robust tiller populations before winter, with tillering delayed to the regreening stage. Given identical planting densities between the two sowing dates in this study, the T2 sowing date resulted in stronger individual stems due to delayed tillering initiation, but this came at the cost of reduced total population biomass accumulation. Furthermore, T2 flowering coincided with elevated temperatures compared to T1, which typically shortens the grain filling period and accelerates senescence, contributing to yield losses [46]. Previous studies on the effects of sowing date have shown that minor delays within the locally optimal sowing window can resolve this contradiction, enhancing lodging resistance while maintaining a high grain yield [45]. As shown in Figure 1, the T2 sowing time experienced lower temperatures, resulting in insufficient accumulated heat before winter. This led to a significantly lower number of tillers per plant compared to the high-yield standard. The release of the CRNF was notably influenced by soil temperature, with the 0–20 cm soil layer in T2 being cooler than in T1, potentially reducing its nitrogen release rate. After spring regreening, rising soil temperatures accelerated CRNF release, which may have promoted the development of individual wheat stems, thereby significantly enhancing lodging resistance. As global temperatures continue to rise, necessitating shifts in winter wheat sowing windows [8], producers must strategically balance sowing dates to optimize both yield and lodging resistance.

Nitrogen fertilization is critical for achieving high wheat yields, yet excessive application reduces NUE and exacerbates agricultural pollution [47]. The amount of nitrogen fertilizer application significantly influences the morphological characteristics of wheat, and the effects vary with different types of fertilizers [48]. Increasing nitrogen application can enhance vascular bundle differentiation and stem thickness, but excessive nitrogen input may reduce lignin content [17]. Elevated nitrogen inputs disrupt carbon partitioning, diminishing non-structural carbohydrates and lignin content in stems while increasing lodging susceptibility [49]. Consequently, studies advocate reduced nitrogen inputs combined with higher planting densities to achieve high yields, though excessive density elongates internodes and elevates lodging risk [40,50]. CRNFs address these limitations by synchronizing nutrient release with crop demand. Our research group’s previous work demonstrated that split CRNF applications significantly enhance wheat yields compared to one-time applications [29,51]. Consistent with these findings, the S1 fertilization treatment in our experiment produced significantly lower yields compared to other fertilization application regimes. However, its shorter plant height, thicker stems, and lower center of gravity formed the foundation of superior lodging resistance. The inherent contradiction between higher grain yields and improved lodging tolerance arises primarily from increased stem bending resistance caused by elevated grain weight in aboveground plant parts [40], which physiologically substantiates and confirms the contradiction between high yield and easy lodging. Notably, S2 under T1 significantly boosted yields (9.73% average increase over CK) but increased lodging risk through elevated plant height and reduced internode filling. In the 2023 results, the S3 treatment exhibited elongated basal second internodes with low filling degrees. This phenomenon likely originated from the timing of the second topdressing application during the regreening stage, when nutrient absorption is close to the time of tillering increase. Excessive tillering under these conditions may have promoted internode over-elongation, thereby increasing lodging risk. These findings align with previous studies demonstrating that sufficient nitrogen absorption during stem elongation enhances yield but simultaneously elevates lodging susceptibility through excessive basal internode growth [52]. Additionally, the lower lignin content in the S3 treatment also indicates that excessive elongation of the stem during nitrogen absorption can reduce the synthesis of lignin, damaging the strength of the stem. This is consistent with the report by Li et al. on the weakening of stem mechanical properties under high nitrogen and high-yield cultivation [49]. Reducing planting density can enhance stem lignification, which improves vascular bundle structure and promotes greater lignin deposition in secondary cell walls, thereby strengthening lodging resistance [15,53]. Therefore, the excessively large population size in the S3 treatment may represent a key contributing factor to the observed reduction in lignin content. The S4 treatment significantly outperformed CK and S1 in yield while showing the greatest increase in center of gravity height at maturity (36.6% and 42.4% higher than at flowering in 2022–2023 and 2023–2024, respectively). This “late-stage advantage” may be attributed to the nutrient release profile of the CRNF. The S4 regimen, combining urea with the CRNF, likely prolonged nitrogen remobilization to the spikes and maintained elevated stem metabolic activity during late grain filling. A recent study proves that combined urea and controlled-release fertilizer applications balance yield and lodging resistance [54], with this fertilization strategy simultaneously modifying basal second internode strength while improving wheat population characteristics.

4.3. Comprehensive Evaluation of Yield and Lodging Resistance

The correlation analysis and PCA results further reveal the multifactorial synergistic mechanisms underlying lodging formation, with yield positively correlated with morphological indicators, such as centroid height, and negatively correlated with flexural strength and lignin content, confirming the physiological basis of the contradiction between high yield and easy lodging. With the increasing frequency of extreme weather events caused by climate change, this research holds significant practical importance for guiding wheat breeding strategies. Breeders can integrate genomic selection technologies to develop a new generation of wheat varieties that combine high yield potential with optimal stem strength. Research demonstrates that replacing urea with a CRNF in southwest China can improve the environmental sustainability of rice production systems while increasing nitrogen use efficiency and economic benefits [24]. A separate study revealed that CRNFs enhance the photosynthetic function of leaves in agroecosystems under ozone pollution and boost crop yields compared to conventional urea application [55]. These findings suggest that CRNFs exhibit superior adaptability to climate change compared to traditional urea, offering a dual solution to the challenges of food security and climate resilience. Principal component analysis revealed that three extracted common factors cumulatively explained 76.38% of the total variance, visually demonstrating the dynamic balance between plant architectural development and mechanical reinforcement processes. The T2S1 treatment, achieving the highest comprehensive score, confirmed the lodging resistance advantages of delayed sowing combined with the one-time application of a CRNF. This is consistent with the research findings of Dai et al. in wheat crops [45]. Notably, the S4 fertilization application regime maintained stable yields with superior internode filling degree and lodging resistance indices under both T1 and T2 sowing dates. However, agricultural producers should exercise caution when employing the S2 fertilization strategy, as it elevates the risk of reduced internode filling efficiency. We recommend integrating chemical growth regulators with S2 applications to mitigate this limitation. The combined application of a CRNF and urea offers farmers an optimized solution that balances yield stability and lodging resistance while reducing costs. Although CRNFs are more expensive per unit than urea, a 50% CRNF + 50% urea blend can lower total labor costs compared to split applications of CRNFs. From the perspective of farmers, this fertilization strategy reduces the number of required applications and lowers labor expenses compared to conventional split urea fertilization. In the study of Ma et al, it was found that combining CRNFs with urea not only saves labor costs but also reduces nitrogen loss in farmland [56]. Considering these advantages, the combined use of S4 is highly recommended. However, interannual precipitation patterns may have mediated fertilizer release dynamics. During the 2022–2023 growing season, the T1 sowing period experienced excessive precipitation, while T2 faced limited rainfall. In contrast, the 2023–2024 season exhibited relatively balanced rainfall distribution. Under the rain-fed conditions of this experiment, seasonal precipitation variability likely served as the primary driver for the higher yield reduction rate observed in T2 during 2022–2023 compared to 2023–2024.

Future studies should systematically test shorter sowing windows across multiple locations and planting dates to better evaluate how fertilizer regimes affect yield stability and lodging resistance. Multi-environment trials are needed to verify whether optimal fertilizer combinations maintain their advantages under diverse growing conditions, and research should particularly focus on the balance of yield and lodging resistance. Such work will help develop more robust cropping systems while maintaining economic feasibility for farmers.

5. Conclusions

Our results demonstrate that compared to conventional sowing times (T1), delayed sowing (T2) significantly reduced wheat plant height and shortened the basal second internode length while increasing stem lignin content, leading to improved lodging resistance at the cost of substantial yield reduction. Among fertilization treatments, S1 showed optimal lodging resistance but lower yields, whereas S2 achieved better yield performance at the expense of increased lodging risks due to elevated plant height and center of gravity. The S3 treatment exhibited elongated basal second internodes with poor filling, further exacerbating lodging susceptibility. Notably, the S4 treatment not only enhanced the mechanical strength of basal second internodes but also showed the greatest increase in center of gravity from flowering to maturity while maintaining both high yields and good lodging resistance. Principal component analysis confirmed the negative correlation between yield and stem strength, validating the inherent trade-off between high yield potential and lodging resistance in wheat production. Based on these findings, we recommend adopting a combined application of CRNFs and urea under different sowing dates to achieve an optimal balance between yield and lodging resistance.