Effects of Two Straw Return Methods Coupled with Raising Ducks in Paddy Fields on Stem Lodging Characteristics

Abstract

Lodging has a negative effect on rice production and leads to a great loss in yield and quality. It is necessary to clarify the effects of straw return measures coupled with rice-duck co-culture on lodging and to explore a measure that can improve lodging resistance. A randomized block experiment with six treatments (rice monoculture (RNN), rice-duck co-culture (RND), direct straw return and rice monoculture (RSN), direct straw return coupled with rice-duck co-culture (RSD), straw carbon and rice monoculture (RBN), and straw carbon coupled with rice-duck co-culture (RBD)) was conducted to investigate the mechanism of the change in lodging resistance. RNN’s rice yield was 6258.02 kg ha⁻¹. The yield of RND, RSN, RSD, and RBN increased by 15.51, 3.06, 10.23, and 1.59%, respectively, while RBD decreased by 5.01% relative to RNN. Direct straw return and straw biochar return coupled with rice-duck co-culture has both negative and positive effects on lodging resistance because of its properties. The stem’s mechanical properties were mainly decided by weight, length, plumpness, and culm anatomy. The increased bending moment at breaking, lodging strength, and bending strength with the RND, RSN, RSD, RBN, and RBD treatments increased, indicated an increase in lodging resistance. Our results clearly demonstrate that direct straw return and straw biochar return coupled with rice-duck co-culture could increase the lodging resistance. In total, rice-duck co-culture could increase the lodging resistance with a higher yield. Compared to straw biochar application, straw return can stabilize the yield and improve the lodging resistance of rice. Thus, direct straw return coupled with rice-duck co-culture should be explored for improving lodging resistance under the condition of ensuring yield.

1. Introduction

Lodging has a negative effect on rice production and may cause a great loss in yield and quality [1]. Lodging can be classified as stem lodging and root lodging. Root lodging is common in upland rice because roots are shallower in the soil and the substratum root is poor; it rarely occurs in rice cultivation [2]. Stem lodging is the main type of lodging in cultivated rice, which can be caused by a loss of balance for the plant and the biotic community (insects and diseases), together with abiotic factors (soil conditions, heavy rain, and wind); it usually occurs in the late season of the heading or grain development stages at the stem’s base. In rice production, lodging of the stem occurs frequently because it is related to environmental factors, for instance, soil conditions, CO₂ [3], water [4], wind, disease management, and input management. Although there have been no direct relationships between lodging and rice cultivation techniques, cultivation techniques can provide a suitable environment for rice to reach a suitable balance for plant growth by changing the soil conditions and by controlling insect and weed density. Thus, it is crucial to improve rice yield and reduce lodging by reasonable rice cultivation techniques.

China has a huge amount of various crop straw [5]. The straw is mainly used as fuel, fertilizer, feed, industrial raw material, and base material [6]. About 62% of the total straw has been returned to fields during 1995–2005 [7]. Previous studies show that straw return to soil can increase nutrients and SOC [8], bacterial community, and diversity of soil [9]. If there is not enough time for straw biodegradation, this is unfavorable for the root penetration and may aggravate the plant infestation [6]. The majority of farm land is a double-cropping or triple-cropping system in China, implying that the crop straw need to be handled quickly after harvest and not leave too much time for it to degrade before late rice transplanting. It also means that the rice penetration is not ideal and may increase the density of insects, which is unfavorable for rice, also increasing the possibility of lodging after strong winds.

Besides direct straw return models, straw can be collected and converted into organic fertilization or biochar before return to soil. Previous studies found that biochar amendments in paddy soil increases soil pH and organic carbon, total nitrogen [10], total phosphorus [11] contents, and also significantly changes enzyme activity and microbial community composition [12]. Biochar can improve silica content [13] and the resistance of diseases caused by a variety of fungi and bacteria [14]; reduce the survival rate, hatching rate, and fecundity of larvae; and reduce insect pests [15]. Hence, biochar application can promote crop growth, uptake of N and P fertilizers, and crop production. Ref. [16] found that that the appropriate application of biochar can increase plant height, the length of the second internode, the diameter of internodes, and wall thickness. Therefore, we speculated that biological carbon could improve the lodging resistance but related studies were not systematic.

Rice-duck co-culture has many economic, environmental, and ecological benefits [17], in which is ducks live directly in the field by creating a mutualistic relationship between rice and ducks that yields benefits to both entities [18]. Ducks eat weeds and insects in the field to control insect and weed density, increase soil nutrients through metabolism, and increase ventilation to promote root growth. Through an anatomical study, ref. [19] found that duck activity can increase the areas of vascular bundles and the thickness and density of the culm wall and the layer of the culm wall. We found that duck activities can promote the integration of straw and soil and accelerate the degradation of straw. Therefore, straw return coupled with raising ducks in the field can reduce the negative effects of inadequate straw degradation to improve lodging resistance. However, the role of straw return and biochar return coupled with raising ducks in the field is unknown in lodging, and there are gaps regarding which measure of direct or indirect (carbonization is unknown) straw return has a greater effect on rice lodging. Therefore, it is necessary to conduct this study.

Numerous studies have reported variations in lodging resistance among culm morphology and anatomy, such as culm diameter, culm wall thickness, and the number and sheath of large vascular bundles, were the major factors found to influence stem-breaking strength [20,21]. Duan et al. (2004) found that the middle and rigid stem have the highest lodging resistance. In natural conditions, lodging does not occur, so it is different to identify the effect of field management measures on rice lodging. Improvements in lodging resistance in rice can be realized by indirect selection for lodging-associated traits. Thus, it is a prerequisite to understand the relationship between culm morphology, anatomy, and lodging resistance, and we tried to account for the effect of straw return coupled with raising ducks on lodging resistance by culm morphology and anatomy.

Most study on lodging involves genes, lodging traits, lodging model construction, etc. Little is known about the association between culture techniques, such as direct straw return and indirect straw return. In considering implementing clean and green agricultural production and inheriting a traditional farming culture, the present study was carried out to understand the associations between rice-duck co-culture and indirect and direct straw return to the field on lodging, and the benefits of the application of straw return and rice-duck co-culture.

2. Materials and Methods

2.1. Experiment Site

In the south of the middle reaches of the Yangtze River, the site for the field experiment was located in Mingyue village (28°40′N and 113°29′E), Changsha Municipality, Hunan Province, China, which has cultivated rice continuous crops for 10 years. The climate is a subtropical monsoon humid climate. The mean annual precipitation is around 1500 mm. The soil is red-yellow mud with a pH of 5.1, organic matter is 17.49 g kg⁻¹, total nitrogen is 0.98 g kg⁻¹, total phosphorus is 0.56 g kg⁻¹, total potassium is 36.53 mg L⁻¹, alkali-hydrolyzable nitrogen is 77.58 mg kg⁻¹, available phosphorus is 26.30 mg kg⁻¹ and available potassium is 8.67 mg L⁻¹.

2.2. Experiment Design and Treatment

The field experiment was carried out for two years (2020–2021 later rice seasons, July–November in each year) under rice cultivation. The experimental design employed a randomized block design with 3 replicates and plot sizes of 5 m × 9 m. The field experiment has two components, the first is raising ducks and not raising ducks; the second is no straw return to the field, direct straw return, or straw carbon (biochar) return. The treatments included:

• RNN—no straw return and no ducks in the paddy field;
• RND—no straw return and ducks in the paddy field;
• RSN—direct straw return and no ducks in paddy field;
• RSD—direct straw return and ducks in paddy field;
• RBN—straw carbon and no ducks in paddy field;
• RBD—straw carbon and ducks in paddy field.

The individual plots were separated by production rows that were 0.8 m in width, each with an irrigation and drainage outlet.

2.3. Straw Processing

Straw with small pieces of 4–5 cm used for the field experiment was produced on the same plot after the early maturity rice harvest. Biochar used for the field was produced with rice straw at the Baiwei Technology Company, Hunan, China. The production process of biochar is as follows: straw was collected by cutting tractor, followed by air drying and pyrolysis carbonization at 400 °C. Laboratory experiments showed that 30% of the biomass was converted into biochar. Biochar is a powdered carbon powder with small particles, a developed pore structure, and adequate adsorption capacity.

2.4. Experiment Details

2.4.1. Crop Management

Rice (Oryza sativa L.) was cultivated for the field experiment with Nongxiang 42 in each year. The cultivars were bred and locally and conventionally cultivated in the area by local farmer. Rice seeds were sowed in a nursery bed for seeding on 15 May, transplanting to the plot on 26 and 15 July in 2020 and 2021, respectively (to adjust for the local climatic conditions), with the spacing of one wide hill and one narrow row hill (wide row: 40 cm × 24 cm; narrow row: 20 cm × 24 cm). Rice was harvested on 1 November 2020 and 15 October 2021, respectively. No pesticides were applied to the whole field in each year.

2.4.2. Straw and Fertilization Management

The straw and biochar were applied at 7000 kg ha⁻¹ (total return of straw to field) and 2100 kg ha⁻¹ (30% of aboveground biomass converted to biochar), respectively, before transplanting for each cycle. The N fertilizer was applied at 120 kg ha⁻¹, of which 50% was applied as a base fertilizer before transplanting, 30% at the tilling stage, and the other 20% at the panicle stage. Amounts of 75 kg ha⁻¹ phosphorus pentoxide and 75 kg ha⁻¹ potassium chloride were applied as a base fertilizer before transplanting during the rice growing cycles of 2020 and 2021, respectively. After the base fertilizer was applied and before transplanting, the field was ploughed and flooded for 2–3 days.

2.4.3. Duck Management

We took Cairina moschata as the experimental duck variety. Ducklings were released into treatment plots of RND, RSD, and RBD at a density of 225 duck individuals per hectare, from fourteen days after the rice seedlings were transplanted to the full heading stage. Each plot was surrounded by a 150 cm nylon mesh fence to prevent ducks from escaping. The ducks were fed one times in once per day. The ducks were taken out of the paddy field on 1 October and 15 September in 2020 and 2021, respectively. The water layer was kept at 5 cm while the ducks were in the field, and before 15 days of harvest, the irrigation was stopped.

2.5. Parameters Measured

2.5.1. Yield and the Compounds of Yield

In the maturity stage, three hills with an average number of panicles and tillers were randomly selected from each plot, and the number of filled grains, the total number of grains, and 1000 rice grains were individually measured. The percent of filled grains was calculated by dividing the number of filled grains by the total number of grains.

All plants in an area of 2 m × 2 m for each plot were harvested, threshed, and air-separated for filled grains to determine the yield per unit area. Grain yield was adjusted to 14% seed moisture content.

2.5.2. Culm Morphology

For samples taken at maturity, 3 hills were harvested, according to the average tillers from each replicate, to measure lodging-related traits. From 3 hills, 15 representative main stems were selected to measure characteristics related to lodging in each plot. Plant length, the height of central gravity, and the length of the first and second internodes (Figure 1) were measured. The weight of each fresh internode, main stem, and spike was measured by micrometer scale. After measuring, the main stems were measured and transported to the laboratory, placed in an oven to dry to a constant weight at 80℃, then measured by micrometer scale. Finally, according the following formula, the fullness of each internode was calculated:

The fullness of each internode (FI, g cm⁻¹) = The dry weight of each internode (g)/The length of each internode (cm).

2.5.3. Culm Anatomy

A determination of the culm anatomy measurements were performed on the first and second internodes of three additional randomly selected plants within each plot. The diameter of the culm was measured by Vernier calipers. The middle portions of the first and second internodes were cut by hand-held slice method to produce transverse sections. Under a light microscope, the widths of the culm wall (from the epidermis to the cavity), the large vascular bundle area, and the culm wall were measured. The number of large vascular bundles per unit area within culm tissues were counted.

2.5.4. Lodging Resistance

The bending load (N) at breaking of the basal first and second elongated internode was measured at a distance of 5 cm between two supporting points with a stalk strength tester (YYD-1A, Top Instrument Co., Ltd, Zhejiang, China). The physical parameter calculations were performed according to the measurements made in previous studies [22,23,24], as follows:

• Bending moment (WP, g cm) of each internode = Length from the basal of the internode to panicle (cm) × Fresh plant weight of plant part from the basal of the internode to panicle (g).
• Bending moment at breaking (M, N cm) of each internode = Break load (N) at each internode × Distance between fulcra (cm).
• Lodging index (LI, %) = Bending Load of each internode/The bending load of each internode.
• Crossing section model(Z, mm³) = π × D3/32 × (1 − α⁴) where D is the outer diameter in an oval cross-section and α is the ratio of the inner diameter and outer diameter in an oval cross-section.
• Bending strength (BS, N mm⁻²) of each internode = Bending moment at breaking/Crossing section model.

2.6. Statistical Analysis

All dates of data were statistically analyzed using analysis of variance (ANOVA), which were compared by mean separation determined using Duncan’s multiple range test and random block design. Factor one and factor two were analyzed using binary logistic regression analysis and multifactor interaction analysis. Pearson’s correlation analysis was used to investigate the relationships between culm morphology, culm anatomy and lodging resistance.

3. Results

3.1. Yield and Yield Components

A large difference in yield and yield components were found in each year among the treatments of direct straw return and biochar return coupled with raising ducks in the rice field (

Table 1. Grain yield and its components in straw return coupled with raising ducks in the field. Data present the mean ± SE.

Year	Treatment	Panicles (m−2)	Spike Number/Panicle	1000-Grain Weight (g)	Filled Spike (%)	Yield (kg ha−1)
2020	RNN	268.52 ± 16.04 abc	200.75 ± 34.30 a	25.32 ± 0.10 bc	64.6 ± 4.28 a	6866.27 ± 1213.9 a
	RND	259.26 ± 21.22 bc	197.86 ± 13.58 a	24.98 ± 0.30 bc	62.87 ± 2.45 ab	7014.24 ± 216.11 a
	RSN	254.63 ± 8.02 c	179.56 ± 20.79 a	24.76 ± 0.51 c	65.06 ± 3.77 a	6802.78 ± 1687.84 a
	RSD	300.93 ± 28.91 a	205.65 ± 49.59 a	25.51 ± 0.19 b	47.73 ± 4.38 c	6910.19 ± 533.61 a
	RBN	259.26 ± 8.02 bc	211.05 ± 31.66 a	25.58 ± 0.44 ab	57.81 ± 16.33 abc	6218.55 ± 705.97 a
	RBD	291.67 ± 13.89 ab	187.22 ± 15.51 a	26.21 ± 0.42 a	49.64 ± 3.48 bc	6015.07 ± 688.32 a
2021	RNN	180.56 ± 0.00 b	133.56 ± 11.83 a	24.83 ± 0.50 ab	80.26 ± 3.07 a	5649.77 ± 268.2 d
	RND	203.7 ± 8.02 ab	165.94 ± 24.11 a	24.35 ± 0.09 b	76.54 ± 7.22 a	7443.73 ± 649.17 a
	RSN	226.85 ± 21.22 a	135.58 ± 6.68 a	25.17 ± 0.54 a	81.71 ± 3.00 a	6096.30 ± 410.83 cd
	RSD	203.70 ± 8.02 ab	164.88 ± 32.68 a	24.59 ± 0.37 ab	75.67 ± 2.12 a	6886.67 ± 225.11 ab
	RBN	222.22 ± 0.00 a	156.1 ± 21.76 a	24.45 ± 0.46 ab	82.04 ± 7.13 a	6496.40 ± 381.81 bc
	RBD	212.96 ± 21.22 a	151.14 ± 17.71 a	24.50 ± 0.07 ab	78.00 ± 3.87 a	5774.23 ± 275.91 cd
F-value
Year(Y)		153.089 **	27.927 **	35.932 **	100.593 **	1.002
Factor1(F1)		3.756	1.163	0.002	10.574 **	1.675
Y × F1		6.422 *	1.215	7.567 *	1.133	1.499
Factor2(F2)		5.689 **	0.112	2.157	1.532	2.511
Y × F2		0.622	0.057	7.838 **	2.632	0.291
F1 × F2		0.089	2.062	3.165	1.545	2.89
Y × F1 × F2		8.622 **	0.285	1.961	0.832	1.614

Different lowercase letters in the same column indicate a significant different at p < 0.05 by DMRT test. RNN, no straw return, no duck; RND, no straw return, duck; RSN, direct straw return, no duck; RSD, direct straw return, duck; RBN, straw carbon return, no duck; RBD, straw carbon return, duck. Factor 1, duck or no duck; Factor 2, direct straw return, straw carbon return, or no straw return. * p < 0.05; ** p < 0.01.

). On average, the yield with RND, RSN, RSD, and RBN increased by 15.51, 3.06, 10.23, and 1.59%, respectively, and RBD decreased by 5.01% relative to RNN. The yield with ducks in the paddy field (RND, RSD, and RBD) increased by 5.02% relative to rice monoculture (RNN, RSN, and RBN). The yield with direct straw return (RSN and RSD) and biochar return (RBN and RBD) decreased by 1.03 and 9.16% relative to no straw return (RNN and RND). Raising ducks in the paddy field induced an increase in the number of panicles (4.26%) and the spike number of panicles (5.51%), and a decrease in filled spikes (9.51%). The decrease in the filled spikes with biochar return (5.92%) was much larger than direct straw return (4.97%) relative to no straw return.

3.2. Culm Morphology

The plant height, length of central gravity, first internode, second internode, and spike had significant differences between different treatments in 2020 and 2021 (Figure 2(A1,A2)). The highest length of plant and central gravity was observed with RBD; the highest length of the first and second internode was observed with RNN. On average, the application of rice-biochar and direct straw return showed significant decreases in the length of the first internode by 19.25 and 21.15%, respectively, and significant decreases in the length of the second internode by 9.56 and 15.38%, respectively, compared to no straw return. With direct straw return, the plant length significantly decreased by 0.07% and increased by 4.59% with biochar application.

The outer diameter and the inner diameter of the first and second internodes had significant differences between different treatments in 2020 and 2021 (Figure 2(B1,B2)). The highest inner and outer diameters were observed with RND in 2020 and RBD in 2021. On average, the outer diameter of the first internode with RND, RSN, RSD, RBN, and RBD increased by 16.48, 9.13, 9.04, 11.39, and 16.29%, respectively, and the outer diameter of the second internode increased by 27.46, 7.93, 6.93, 9.57, and 14.38%, respectively. The inner diameter of the first internode increased by 14.38, 8.35, 5.29, 10.68, and 14.38%, respectively, and inner diameter of the second internode increased by 23.15, 8.22, 3.56, 9.59, and 14.79%, respectively, compared to that of RNN. The first outer diameter with biochar application and direct straw return significantly increased by 23.09 and 23.95%, respectively, and the second outer diameter with biochar application and direct straw return significantly increased by 22.88 and 19.47%, respectively, compared to no straw return. Accordingly, the first and second inner diameters with ducks in the field increased by 11.15 and 7.46%, respectively, and the first and second outer diameters increased by 6.64 and 4.71%, respectively, relative to no ducks in the field.

The dry weight of the first internode, second internode, spike, and stem had a slight difference (Figure 2(C1,C2)). There was a smaller and no remarkable difference in dry and fresh weight on the stem when comparing treatments, respectively (Figure 2(D1,D2)). The fresh weight of the first internode under treatment ranged from 0.45 ± 0.07 g to 1.16 ± 0.08 g in 2020 and from 1.56 ± 0.15 g to 2.14 ± 0.59 g in 2021. The fresh weight of the second internode under treatment ranged from 2.14 ± 0.16 g to 1.64 ± 0.13 g in 2020 and from 2.06 ± 0.04 g to 3.04 ± 0.88 g in 2021. The spike of the fresh weight and dry weight under treatment ranged from 4.72 ± 0.5 g to 5.31 ± 0.69 g and from 2.85 ± 0.38 g to 3.37 ± 0.2 g in 2020, respectively, and from 4.72 ± 0.5 g to 5.31 ± 0.69 g and 3.87 ± 0.27 g to 5.14 ± 1.06 g in 2021, respectively.

3.3. Culm Anatomy

The number of large vascular bundles in the first and second internodes had significant difference between each treatment in 2020 and 2021. Among the treatments, the lowest number of large vascular bundles in the first and second internodes was observed with RBN, except for the second internode in 2020 (Figure 3(A1,A2)). However, on average, the stem’s cross-sectional area with RND, RSN, RSD, RBN, and RBD, in the first internode, increased by 22.73, 18.18, 9.09, 15.91, and 25% and the second internode increased by 3.44, 10.34, 17.24, 12.06, and 20.69% in both cycles (Figure 3(B1,B2)). The area of large vascular bundles had a smaller but significant increase relative to RNN. On average, the large vascular bundle area with RND, RSN, RSD, RBN, and RBD in the first internode increased by 8.11, 3.21, 10.30, 6.93, and 22.30%, respectively, and the second internode increased by 17.55,16.07, 10.99, 24.31, and 10.78%, respectively, in both cycles (Figure 3(C1,C2)).

On average, raising ducks in the paddy field can slightly increase the number of vascular bundles, the area of the cross-section, and the first internode’s area of large vascular bundles, but doing so can also decrease the second internode’s area of large vascular bundles. When compared to no straw return, the first internode’s number of large vascular bundles with biochar application and direct straw return decreased by 2.21 and 0.52%, respectively, while the second internode with biochar application decreased by 0.39% and straw return increased by 0.25%. The first internode of the stem’s cross-sectional area with biochar application and direct straw return increased by 2.04 and 8.16%, respectively, while the second internode increased by 11.86 and 14.41%, respectively. The first internode’s large vascular bundle area with biochar application increased by 4.95%, and with direct straw return, decreased by 6.96%. The second internode’s large vascular bundle area increased by 4.37 and 8.07%, respectively. Overall, there were significant differences between the interactions with Factor 1 and Factor 2 on the first internode’s number of large vascular bundles, the second internode of the stem’s cross-sectional area, and the large vascular bundle area (Figure 3).

Direct straw return and straw biochar return coupled with raising ducks in the paddy field made a great difference on the cross-section of the stem in both cycles (Figure 4). In both cycles, the first internode and the second internode thickness with RND, RSN, RSD, RBN, RBD, were thicker than with RNN; rice-duck co-culture was thicker than with no ducks in 2021 but thinner in 2020. Direct straw return and straw biochar return can increase the cross-section of the first and second internodes.

3.4. Lodging Resistance

The first internode of plumpness with RND, RSN, RSD, RBN, and RBD increased by 25, 25, 37.5, 75, and 12.5%, respectively. The second internode of plumpness increased by 42.86, 57.14, 57.14, 57.14, and 28.57%, respectively, across both years compared to that of RNN. The plumpness internode with biochar application and straw return significantly increased compared to no straw return, on average, and raising ducks in the field resulted in a slight difference in plumpness compared to no ducks in the field (Figure 5(A1,A2)).

There was a significant difference in the crossing section model in 2020 and 2021. The first internode of the crossing section model with RND, RSN, RSD, RBN, and RBD increased by 22.63, 29.44, 33.33, 43.94, and 48.42%, respectively. The second internode of the crossing section model increased by 15.64, 25.37, 19.59, 40.13, and 44.44%, respectively, across both years compared to that of RNN. According to interaction studies, we founded that direct straw return and straw biochar return can significantly (p < 0.01) increase the crossing section model. On average, rice-duck co-culture can increase the crossing section model. The increase in the crossing section model of the first and second internode in straw biochar application was higher than direct straw return when compared to no straw return (Figure 5(B1,B2)).

The first and second internode lodging indices were affected by the treatments. The second internode lodging index was higher than the first internode. There was a significant difference with the direct straw return coupled with rice-duck co-culture in the first internode lodging index. On average, the first internode lodging index of RND, RSN, RSD, and RBN was higher but RBD was lower than RNN. The second internode lodging index of RSN, RSD, and RBN was higher than RNN, but RND and RBD was lower than RNN. Rice-duck co-culture can decrease the first and second internode lodging index compared to no ducks in the field. The first and second internode lodging indices with direct straw return decreased by 12.21 and 12.00% and by 16.22 and 11.57%, respectively, in 2020 and 2021. Straw biochar application decreased by −3.86 and 2.45% and by 6.67 and 9.71%, respectively, in 2020 and 2021, compared to no straw return (Figure 5(C1,C2)).

There were slight differences and significant differences in the bending moment lodging index for 2020 and 2021. The first internode of the bending moment for RND, RSN, RSD, RBN, and RBD increased by 18.60, 19.72, 2.01, 15.65, and 34.18%, respectively. The second internode of the bending moment increased by 21.66, 26.76, 4.71, 24.39, and 39.45%, respectively, across both years compared to that of RNN. The interaction of rice-duck co-culture and direct straw return was significantly different. On average, rice-duck co-culture can increase the bending moment of the first and second internodes. The increase in the bending moment of the first and second internode with straw biochar application was higher than direct straw return compared to no straw return (Figure 5(D1,D2)).

There were significant differences in the bending moment at breaking lodging index for 2020 and 2021. The first internode of the bending moment at breaking for RND, RSN, RSD, RBN, and RBD increased by 40.34, 31.62, 38.47, 20.25, and 29.44%, respectively. The second internode increased by 21.54, 25.37, 34.54, 23.45, and 25.80%, respectively, across both years compared to that of RNN. According to interaction studies, we found that rice-duck co-culture can significantly (p < 0.01) increase the bending moment at breaking of the first internode. On average, rice-duck co-culture can increase the bending moment of the first and second internode. The increase in the bending moment at breaking of the first and second internode with straw biochar application was lower than direct straw return compared to no straw return to the field (Figure 5(E1,E2)).

There were slight differences in the bending strength lodging index for 2020 and 2021. The first and second internodes of bending strength for RND, RSN, RBN, and RBD decreased compared to that of RNN, and RSD increased across both years compared to that of RNN. On average, rice-duck co-culture can increase the crossing section model. Compared to no straw return, direct straw return and straw biochar application can increase the bending strength at the first internode but the second internode with direct straw return decreased compared to no straw return (Figure 5(F1,F2)).

3.5. Partial Correlation Analysis of Factor Affecting Lodging Resistance

Bending strength was negatively correlated with the plant length, height of central gravity, internode length, and crossing section model. The bending moment at breaking was positively correlated with the outer culm diameter; inner culm diameter; dry and fresh weight of the internode, spike, and stem; and the fullness of each internode. However, there was no correlation between the bending moment at breaking and the number of large vascular bundles, area of the cross-section, and the area of the large vascular bundles. The lodging index was negatively correlated with the fullness of each internode but positively correlated with the length of the plant, each internode, and spike length crossing year, and significantly positively correlated with the weight of the stem, internode, and spike in 2021. The bending moment was significantly positively correlated with the plant length, spike length, the inner and outer stem diameter, and the fresh weight of the internode and spike, and positively correlated with the area of the large vascular bundles (Figure 6(A1,A2)).

4. Discussion

Our hypothesis suggested that rice-duck co-culture would increase the yield and have a significant influence on filled spikes and straw return but that straw biochar application would lead to a lower yield (Table 1). Similarly, previous studies [25,26,27] found that the rice-duck farming system can increase the yield by 4.93%, straw return reduced yield by 0.6-7.1% in some trials in Northern China, and short-term biochar application decreased the yield due to the decrease in grain weight and harvest index. Rice monoculture’s rice yield is 6258.02 kg ha⁻¹ The rice-duck co-culture, rice with straw return, rice-duck co-culture with straw return, and rice with biochar application increased by 15.51, 3.06, 10.23, and 1.59%, respectively, and rice-duck co-culture with biochar application decreased by 5.01% compared to rice monoculture (Table 1). However, past research has shown that straw return inhibits the vegetative growth and delays the senescence to promote the formation of rice yield [28]. Thus, the change in yield at straw return might be due to straw degradation; the temperature of the soil increase with straw degradation, which affects the development of roots to inhibit the growth of rice when straw is returned to the field in late rice. The temperature gradually decreased and the straw degradation was slow after July and the straw degradation was again accelerated due to the addition of ducks to make straw have closer contact with soil and oxygen to provide more nutrition for the later growth of rice.

Variation in stem lodging resistance was observed in culture techniques in the present study, as manifested by their different value for culm morphology and anatomy. Although the lodging resistance is determined by the lodging index, understanding the relationship between traits of culm morphology and anatomy is useful for determining the critical characteristics that are associated with stem-based lodging in culture techniques.

The plant length, internode length, and height of central gravity correlated to lodging resistance in many cases. In this study, bending strength had a slightly negative correlation with the length of the internode, plant, and central gravity, but the bending moment lodging index had a significant positive correlation with the length of the internode, plant, and central gravity (Figure 6). Similarly, ref. [29] reported a negative correlation between plant length and lodging resistance. We also found that rice-duck co-culture can slightly decrease the length of the plant, central gravity, and internode. Straw biochar application can significantly increase the length of plant and direct straw return had a slight effect on plant length but had a significant decreasing effect on the height of central gravity and internode length (Figure 2A1,A2). As shown in Figure 5, the rice-duck co-culture indicated an increase in the lodging resistance and the increase in the bending moment at breaking in straw biochar application was lower than in direct straw return. This is consistent with the change in plant length, central gravity length, and internode length.

The stem structures are very important contributors to the mechanical properties of the stem [30] but the stem size, wall thickness, and number of vascular bundles indirectly explain the lodging resistance of the stem [31]. So, the bending moment at breaking lodging index and the bending strength of the six-kind stem were analyzed and compared. In the present study, direct straw return and straw biochar application coupled with rice-duck co-culture could increase the outer and inner diameter of the stem (Figure 2B1,B2), number of large vascular bundles, area of large vascular bundles, and cross-section of the stem (Figure 3). As seen in Figure 5, the bending moment at breaking and the bending strength were generally higher than rice monoculture. We detected a negative correlation between the bending strength and stem characteristics and a positive correlation between the bending moment at breaking lodging index and stem characteristics (Figure 6). Rice-duck co-culture could increase those traits of the stem. Direct straw return and straw biochar could increase the diameter and area of the cross-section of the stem but straw biochar return had a lower value in the number of large vascular bundles and area of large vascular bundles compared to direct straw return, so direct return had the higher lodging resistance.

The weight of the stem, spike, and internode are also important contributors to the mechanical properties of the stem [32]. The weight of the spike and the weight of the stem could influence the height of central gravity (Figure 2C1,C2,D1,D2). In this study, a lower spike weight was discovered in rice-duck co-culture coupled with direct straw return. The weight of the spike was not consistent with the yield due to the differences in the panicle, spike number per panicle, and filled spike. Although the weight of the first and second internode slightly decreased, the fourth and third may have had a higher level; otherwise, the length should be taken into consideration. So, we argue that the change in the lodging index was changed by the internode of plumpness. It can be seen in Figure 5(A1,A2) that the internode of plumpness increased compared to that of rice monoculture. We detected a negative correlation between lodging index and plumpness internode and a positive correlation between the bending moment and plumpness internode (Figure 6).

5. Conclusions

In this study, the bending moment at the breaking and bending moments significantly increased and the lodging index decreased. The rice-duck co-culture could increase the bending moment and bending moment at breaking, decreasing the lodging index. Our results are consistent with those of previous studies. Ref. [33] revealed that raising ducks in a paddy field can increase the breaking force by 36.09%, and the lodging index can decrease by 8.16% in the mature stage of rice. Li et al. (2013) reported that straw return to the field can increased the lodging resistance. Biochar application to the field [34] could decrease the lodging index, and increase the lodging resistance. However, we found that direct straw return had a higher lodging resistance than the straw biochar application, possibly caused by the significant increase in plant length and the weight of the spike. The finding of this study highlighted the effect of the lodging resistance of direct straw return and straw biochar return coupled with rice-duck co-culture and provided an insight that facilitates straw return to the field.

Taken collectively, our study demonstrates that direct straw return and straw biochar return coupled with rice-duck co-culture has both negative and positive effects on yield and lodging resistance. All the treatments showed a significant difference in lodging resistance because of their properties. Notably, the morphological and physiologically traits of stems were found to be related to the lodging resistance of the bending moment at breaking lodging index. The stem’s mechanical properties were mainly decided by the weight, length, plumpness, and culm anatomy. Our results clearly demonstrate that direct straw return and straw biochar return coupled with rice-duck co-culture could increase the lodging resistance, rice-duck co-culture could increase the lodging resistance with a higher yield, and the increase in the lodging resistance of direct straw return was higher than that of straw biochar return. Thus, straw return coupled with rice-duck co-culture should be explored for improving lodging resistance under the condition of ensuring yield.