Introduction
Wheat is one of the most important strategic crops in Iran, and rainfed wheat plays an important role in the agricultural production system. Despite the high level of mechanisation in wheat harvesting, mechanical interaction between crop stems and combine harvester components can contribute to crop damage and harvesting losses. The combine harvester reel is one of the first moving components to interact directly with standing crop stems. The intensity of this interaction depends on reel rotational speed, crop mechanical properties, stem geometry, and moisture condition. In rainfed wheat, pre-harvest rainfall may expose mature stems to a wetting–drying cycle, potentially altering their mechanical properties and resistance to reel loading. Therefore, this study investigated the effect of moisture shock, defined as wetting followed by subsequent drying, on the mechanical properties and fracture behaviour of rainfed wheat stems of the Sardari cultivar under simulated combine harvester reel loading.
Materials and Methods
Mature and healthy Sardari wheat stems were collected from rainfed wheat fields in Bijar, Kurdistan Province, Iran. The samples were divided into two groups: conventional stems and moisture-shock-treated stems. Approximately half of the samples were manually wetted with water for 30 min and then subjected to a 72-h drying period under environmental conditions, including exposure to sunlight during the daytime and shade during the night. This procedure was used as a controlled laboratory simulation of a wetting–drying cycle.
Stem moisture content was determined using the oven-drying method at 105 ± 2 °C for 24 h. Physical and mechanical properties, including stem diameter, wall thickness, cross-sectional area, shear force, and shear strength, were evaluated. The moisture content of the tested stems was approximately 11.8%. Stem breaking force was measured using a Lutron FG-5000A digital force gauge. For the final statistical comparison, 30 stems were evaluated in each group. An independent-samples t-test was used to compare breaking force, and Cohen’s d was calculated to determine the effect size.
A laboratory-scale combine reel with a radius of approximately 0.45 m was used to simulate reel-induced loading. The reel rotational speed was varied from 18 to 96 rpm, corresponding to a peripheral speed range of approximately 0.85–4.52 m s^-1. Stems were directly exposed to the moving reel bars, and the rotational speed was gradually increased until stem failure occurred. The critical rotational speed was recorded using a laser tachometer.
To characterise the progressive failure of stems, survival analysis was performed using Kaplan–Meier curves, the Log-rank test, and the Cox proportional hazards model. Reel rotational speed was considered the loading-progression variable, while stem breakage was defined as the failure event. Stems that did not break by 96 rpm were considered censored observations. Statistical significance was evaluated at p < 0.05.
Results and Discussion
Moisture shock substantially reduced the mechanical resistance of wheat stems. The mean breaking force decreased from 1.284 ± 0.12 N in conventional stems to 0.748 ± 0.09 N in moisture-shock-treated stems, representing an approximately 42% reduction. This difference was statistically significant (t(58) = 12.35, p < 0.001), with a large effect size (Cohen’s d = 1.6; 95% CI: 1.1–2.0). A positive relationship between stem diameter and breaking force was observed in both groups, with R² values of approximately 0.68 for conventional stems and 0.61 for moisture-shock-treated stems.
The reel simulation also demonstrated a marked difference in the failure threshold of the two groups. Conventional stems first began to break at 74 rpm, whereas moisture-shock-treated stems began to break at 48 rpm. At the maximum tested speed of 96 rpm, 12 of 30 conventional stems and 28 of 30 moisture-shock-treated stems had broken. These results indicate that moisture shock reduced the ability of stems to withstand the mechanical loading imposed by the reel.
Survival analysis confirmed the increased vulnerability of moisture-shock-treated stems. Kaplan–Meier curves showed a lower probability of remaining unbroken in the moisture-shock group, particularly over the approximate range of 45–75 rpm. The Log-rank test demonstrated a significant difference between the two groups (χ²(1) = 15.7, p < 0.001). The Cox proportional hazards model estimated a hazard ratio of 3.4 (95% CI: 2.1–5.5), indicating that the instantaneous risk of stem breakage was approximately 3.4 times higher for moisture-shock-treated stems than for conventional stems under the tested loading conditions.
Conclusion
The findings demonstrate that moisture shock significantly reduced the mechanical resistance of Sardari wheat stems and increased their susceptibility to breakage under simulated combine harvester reel loading. The approximately 42% reduction in breaking force and the 3.4-fold increase in breakage hazard indicate that a wetting–drying cycle can substantially alter the mechanical condition of mature wheat stems. Under the specific laboratory conditions of this study, conventional stems tolerated reel speeds of approximately 70–75 rpm, whereas moisture-shock-treated stems began to break at lower speeds. Therefore, reducing reel speed to below approximately 45 rpm following a moisture-shock event may help reduce stem breakage under conditions similar to those investigated. This value should be regarded as an experimental threshold specific to the Sardari cultivar and the tested conditions. Further field studies involving different cultivars, moisture regimes, and harvesting systems are required to validate its applicability.
Acknowledgement
This research was supported by Kurdistan University. The authors gratefully acknowledge the cooperation of Bijar farmers and the technical assistance of the Biological Materials Engineering Laboratory staff.