The slippage is an essential criterion for evaluating the fuel consumption and the field performance of tractor. The objective of this research was to develop mathematical models using Design Expert software for modelling and predicting slippage of the CASE JX75T tractor (India manufacture) under operational field conditions. In this research, a chisel plough was used as a loading tool for the tractor under four levels of ploughing depths, with three levels of speed and two levels of cone index (CI) in silty clay soil texture. The experiments were carried out in the site of Basrah University. The results obtained from the fieldwork were analysed to evolve mathematical models and equations to predict and evaluate the performance of the tractor when the slippage occurred. According to the obtained results, the single effects of the parameters (CI, tillage depth, and forward speed) on the slippage were highly considerable (P<0.0001). Moreover, the interaction of the parameters were significant (p<0.05). The slippage of tractor increased by 187 and 116 % with increasing ploughing depth up to 25 cm and forward speed up to 1.53 m.s-1, respectively. On the other hand, tractor slippage reduced by 34% when CI increased up to 980 kPa. The data analysis showed that the developed model has passable imitation ability and excellently executed in confront of the actual data. This confirms the accuracy of the model for predicting tractor slippage under different fieldworks.

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The experiments were conducted on farm of the Agriculture College, University of Basrah, in a silty clay soil with moisture content of 16%. The tractor used in this research was (CASE JX75T) tractor for studying and evaluating its agricultural performance. The parameters of study include three level of tillage depths (10, 15 and 20 cm) and four forward speeds (G=1.9, G= 2.44, G3= 3.25 and G= 4.33 km/h) by using randomized complete block design (RCBD) with a split plots. The results showed that the increase in forward speed from (1.9 - 4.33km/hr) led to a significant increased in drawbar pull and wheel slippage by the following percentages (33.33%-15.78%).The obtained results for the range of tests showed that the maximum effective field capacity 2.52 donum/h was obtained at 4.33 km/h travelling speed, while the fuel consumption 7.9 l/hr, required power 29.66 kW, and specific energy 19 kw.hr/don. Respectively the traveling speed and soil moisture , ploughing depth , the most important factors that affecting the effective field capacity and  drawbar pull, fuel consumption, specific energy.

The effect of forward speed, inflation pressure of rear wheels and soil moisture on the tractor field performance has been investigated in this study during ploughing by using moldboard plough in order to evaluate the drawbar pull, tractor wheel slippage tractive efficiency, the required power, specific energy, effective field capacity, field efficiency, and fuel consumption. The experiments were carried out using four different forward speeds (1.8, 2.33, 3.88 and 4.68 km/h) of Massey Ferguson 285S, three inflation pressures of rear wheel (50, 100, 150 kPa), the average soil moisture content Mcdb (14.67%, 24.18 %) dry basis, and the average of ploughing depth (from 10 to 20 cm). The soil texture was found to be silty clay. The results for the range of tests, showed that the maximum attractive efficiency was obtained at 3.67 km/h travelling speed, 14.67%(M cbd ), 100 kPa inflation pressure of tractor rear wheels, while the drawbar pull, wheel slippage, effective field capacity, field efficiency, rate of fuel consumption, required power and specific energy were 10.60kN .5.58%, 1.45 fed /h, 77%, 8L/h, 25.55 kW and 17.02 kW.h/fed respectively.

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