1. Introduction
Ratoon rice refers to a rice planting mode where, after harvesting the first-season rice, specific cultivation and management measures are taken to stimulate dormant axillary buds on the stubs to germinate into tillers [
1], which ultimately produce ears, flowers, and set seed, enabling a second harvest from the same planting. This method offers advantages such as double harvest from one planting, labor and cost savings, increased yield and efficiency, and superior rice quality in the second season [
2,
3], enhancing the net economic benefit of the ecosystem [
4]. Currently, the cultivated area of ratoon rice in southern China has exceeded 1.3 million hm
2 [
5], with a potential for further expansion of 3–6 million hm
2. However, during machine harvesting, the straight-line crushing rate (the ratio of the crawler crushing area to the harvesting area) is as high as 40–50% [
6,
7], and the turning area is 70–80%, as shown in
Figure 1, which results in delayed development of regenerative axillary buds and severe yield reduction in the second season (with the ratoon yield in the crushed area accounting for about 30% of the uncrushed area) [
8,
9,
10]. Therefore, increasing the yield of the second season of ratoon rice is of great significance to the development of ratoon rice [
11,
12].
In order to address the technical challenge of significant yield reduction in the second-season rice caused by the high rolling compaction rate during mechanical harvesting of the first-season ratoon rice, Yang et al. attempted to enhance the yield and quality of the ratoon season through optimized variety selection and cultivation techniques [
13]. However, this approach failed to effectively reduce the compaction rate and had limited success in increasing yield. On the other hand, scholars such as Fu and Zeng tackled the issue from a different perspective by improving the design of the harvester. They developed a specialized harvester for ratoon rice through adjustments to the cutting table width and optimization of the chassis form [
14,
15,
16,
17,
18]. This approach achieved notable success in reducing the straight-line compaction rate and significantly contributed to increasing the yield of the second-season rice. However, it is worth noting that they did not consider the potential impact of compacted stubbles on the yield of the second-season rice.
To enhance the yield of second-season rice from compacted stubbles, Chen et al. conducted an experiment comparing various indicators such as total dry weight, number of panicles, and axillary buds between manually straightened and unstraightened compacted stubbles during the second-season harvest [
19]. The results demonstrated that straightening the compacted stubbles can contribute to increasing the yield of the second-season rice. Additionally, the team designed an innovative double-layer chain-driven claw-type straightening device for ratoon rice aimed at lifting the compacted stubbles, achieving a straightening rate of approximately 55% [
20,
21]. However, due to the need for the machine to re-enter the field during operation, which can potentially cause secondary damage to the stubbles, and the lack of direct yield tests in the second season to verify the device’s effectiveness in increasing production, further research and validation are required to assess its practical application.
With the development of computer technology, the Discrete Element Method (DEM) has become a significant tool for studying the interaction mechanisms between crops and mechanical devices, and its application in agricultural machinery research is increasingly widespread [
22,
23,
24]. Sun-integrated geometric and mechanical models to construct a DEM-based wheat plant model with flexible characteristics. By comparing experimental data with simulation results, the feasibility and effectiveness of the DEM in building flexible plant models were fully verified [
25]. During the design process of the lawn mower, Shen utilized ADAMS 2022 and EDEM 2020 software to establish a coupled simulation platform for dynamic analysis and optimization of the device. Field measurements confirmed that the operational performance of the device meets national standards and the practical demands of agricultural production [
26]. Wang employed the MBD-DEM coupling method to conduct simulation experiments on a trenching device. The accuracy of the simulation tests was validated through comparisons with on-site measurements, providing strong support for exploring the complex interactions between mechanical movements and soil [
27]. In summary, the coupled application of DEM and dynamics software has emerged as an effective simulation testing tool for exploring the impact of mechanical device movements on crops, offering technical support for the design and optimization of agricultural machinery.
Addressing the challenge of yield reduction in the second season of regenerated rice caused by stub crushing during mechanized harvesting in the first season, this study proposes a stub straightening device for regenerated rice, considering the requirements of agricultural machinery and agronomy. The device has been designed and its feasibility has been verified through DEM-MBD coupled simulation methods. Key factors influencing the straightening performance have been identified. A prototype has been made and field tests have been conducted, using the straightening rate and yield of the second-season rice in the crushed area as operational indicators. The aim is to provide a basis and reference for the research and development, as well as the improvement of the stub straightening device for mechanized harvesting of regenerated rice, ultimately aiming to enhance the mechanization level of regenerated rice harvesting operations.
4. Discussion
The planting mode of ratoon rice is an effective means to ensure stable grain production and promote grain yield increase. However, the high rolling rate of the first-season rice harvested by machines has always been a difficult problem in ratoon rice planting, leading to a decrease in the yield of the second-season rice. Through experiments, Zhang and others have shown that manually lifting the rolled rice stubs can promote the germination of axillary buds and increase the yield of the second season. They designed a chain-row tooth-claw type correcting device for ratoon rice and conducted bench tests. However, the device has a complex structure and no relevant field experiments have been conducted.
To promote the germination of the second season of the rolled and reserved stubs and ensure the yield of the second season of ratoon rice, this paper designs an eccentric parallel four-bar linkage type correcting device for the rolled and reserved stubs of ratoon rice. Simulation analysis and field experiments are conducted to obtain the optimal operating parameters of the device. Parallel experiments are also conducted, which show that the designed correcting device can effectively increase the yield of the second season in the rolled areas of ratoon rice.
However, field experiments have also revealed that the device needs to be manually lifted during turns in the field, lacking adaptive adjustment capabilities. In the future, issues such as height monitoring, speed measurement, and displacement sensors will be added to the device to enable it to operate automatically. At the same time, to further improve the correction rate and promote the increase in the yield of the second season in the rolled areas, the relationship between the ground clearance of the device, the length of the correcting teeth, and the height of the reserved stubs will be further explored.
5. Conclusions
This study addresses the challenge of reduced yield in the second season of ratoon rice due to stubble damage during the mechanized harvesting of the first season. Based on the observation that straightening the crushed stubs can promote germination and increase the yield of the second-season rice, relevant research has been conducted.
By observing and analyzing the biological morphology of the crushed stubs, an eccentric parallelogram-type ratoon rice stub straightening device was designed. The key components of the straightening device were designed, and a preliminary analysis was conducted. Through the construction of a discrete element model of ratoon rice stubs using multi-scale spherical particle aggregation, combined with the Recurdyn multibody dynamics method, a single-factor simulation analysis was performed. This preliminary work determined the parameter range for field experiments and verified the feasibility of the proposed solution.
Using the straightening rate of crushed stubs and the yield of the second-season rice as evaluation metrics, field response surface experiments were conducted to determine the optimal experimental conditions. After refinement, the forward speed was set at 1.4 m/s, the device rotation speed at 75 r/min, and the stub entry angle at 39°. Three parallel experiments were conducted under these conditions, resulting in a straightening rate of 90.35% in the mechanized harvesting crush zone and a second-season yield of 2202.64 ± 10.25 kg/hm2. The difference between these results and the predicted outcomes was within 5%, indicating high reliability of the parameter optimization results. Additionally, yield measurements of crushed but unstraightened stubs and uncrushed stubs revealed that the yield of straightened crushed stubs was approximately 70% of the uncrushed stub yield, representing an increase of about 80% compared to the yield of crushed and unstraightened stubs.
These results indicate that the designed ratoon rice stub straightening device can meet the requirements of field operations and effectively improve the yield of the second season of ratoon rice, which has a positive significance for promoting the cultivation of ratoon rice.