In this research study, disc harrowing operations in a loamy sand soil, on an experimental plot of twenty different soil moisture levels at tillage speeds of 1.94m/s, 2.22m/s and 2.5m/s were conducted, using trace tractor techniques. The independent variables: drawbar pull force, rolling (motion) resistance, wheel slip, moisture content, cone index, wheel numeric, contact pressure, speed, width of harrow, depth of harrow, and dependent variable (Tractive force) were measured. Mathematical models using dimensional analysis, describing the tractor tyre-soil interaction were developed and validated. Regression analysis, was used to depict the relationships between independent variables and dependent variable. Analysis of variance using Randomized Complete Block Design in two way analysis was also used to study the effects and interactions of variables on tractive forces. Validation results of the developed tractive force models conducted, revealed that harrowing operations recorded the highest coefficient of determination, R2 = 0.995 at 2.5m/s tillage speed, while R2 =0.990 and 0. 9 were obtained at tillage speeds of 1.94m/s and 2,22m/s respectively. Analysis of variance between measured and predicted tractive forces showed correlation coefficients, R2 = 0.9308, 0.8999, 0.9958 and standard errors of 0.5844, 0.8628 and 0.78476 for harrowing at tillage speeds of 1.94m/s, 2.22m/s, and 2.5m/s respectively. The residuals analysis ranged from between – 138.95 and 48.7117, -98.6106 and 451.474, -33.3709 and 32.5384,and percentage (%) errors from -0.83458466 and 0.27430385, -0.396874637 and 2.546385 and -0.191731686 and 0.189232 respectively. These indicate that there are no significant difference (P > 0.05) between the measured and predicted tractive forces, which are clear evidences of the test of goodness of fits of the models Tillage speed of 2.5m/s illustrated the highest correlation coefficient of 0.9958 in this tillage operation. The tractive force models developed showed a good agreement between the measured and predicted results. It is therefore, recommended that the models be used for predicting tractive force in disc harrowing operations.
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