Agricultural tractor is one of the most important energy and power sources in agricultural mechanization1. It requires high initial capital investment. The introduction of modern technology during the last century resulted in rapid growth of farm production. Tractors and farm machinery are important samples of this modern technology2. Tractor costs have great influence on farm business profit. Knowledge of tractor costs for farm operations has a prime importance in making management plans and decisions especially in comparing different tractor types and models thereby assisting in the selection of a more appropriate farm tractor. Costs of owning and operating farm machinery represent 35% to 50% of the costs of agricultural production when the land is excluded3. The repair and maintenance (R&M) cost is an important item in the costs of ownership and operation. R&M cost is a function of machine age and use4. In general, the costs other than those for R&M usually decrease with increasing usage, but the reverse is true with respect to R&M costs. The cost of R&M is usually about 10% of the total cost; as the machine age increases the cost increases until it becomes the largest cost item of owning and operating the farm machines5. Agricultural engineers have carried many studies regarding R&M of farm machines. Several studies were conducted in both developed and developing countries either to develop models to determine the cost during a certain period or to get absolute numbers to represent owning and operating certain equipment 6789. Using of American and European mathematical relations to estimate R and M costs in under developed and developing countries produced unrealistic and misleading results and therefore, these countries developed their own mathematical models101112. Poor and irregular maintenance reduce tractor reliability, increases fuel consumption, deceases engine power and life and increases exhaust emission13.

In Sudan, agricultural tractors introduced in early nineteen-nineties and there are many tractor makes, models and sizes now distributed between irrigated and rain-fed agricultural farms. These tractors are owned by people from both the private and public sectors and even some are owned by individual farmers and they often work for more than 1200 hours per year14. Tractors have been used in Sudan, as a power source in agriculture for many years. The total number of tractors officially imported into the country between 1984 and 1994 increased from 23,590 units to 32,096 units (FAO, 1995)14. In Sudan, machinery repair, maintenance, and fuel and lubricants consumption is not given enough attention. About 40% of farm machinery was out of work very quickly due to lack of proper maintenance and un availability of genuine spare parts or using of spurious and non-genuine spare parts of low prices. There are some prediction models for tractors repair and maintenance costs in the Sudan were developed 151617. They decided that the correlation between repair and maintenance costs as a percent of tractor initial purchase price and the tractor accumulated hours of use would be best described by a power function equation. There were variations between these models in the predictions for the different tractors. They were varied in structural components due to differences in tractors specifications and conditions and locations of work, therefore, the present study was carried to develop computer models for repair and maintenance costs estimation in relation to hours of use and age for two sizes of diesel engine agricultural tractors in Dongla area.

The data of the study was collected from Dongola area-northern Sudan. The soil is sandy and clay particles, characterized by very low perm ability, deep cracking when dry, poor in nitrogen and organic matter. The climate of the area is classified as dry desert zone.

There are many makes of tractors working in the area, but the Massey Ferguson, tractors are the most common makes available in the area, and are chosen to carry-out this study. Their sizes are mostly in the range of 75–150hp. The total number selected for this study was (44), from which twenty two were (75hp), and the other twenty-two were (150hp). The tractor's technical specifications are shown in Table 1 below.

Many sources were used to collect data concerning tractors repair and maintenance costs they included Elshamalia Company of Agricultural input, Mechanics, Engineers, Agricultural Engineers, individuals and operators, and the workshops. The resources were interviewed,

A questionnaire was prepared to collect the required data then a survey was carried out in the site of the study to interview the target sources of data. The questionnaire included information about: Tractor age, model and make, initial purchase price ($), annual hours of use (hr), annual area covered (fed), annual repair costs ($), annual maintenance costs ($), annual number of repairs, labor cost/year ($).

Using the SPSS computer program, the relationship between the accumulated annual repair and maintenance costs as a percentage of initial purchase price and accumulated annual hours of use was computed.

The following conclusion may be drawn from the present study

1. The relationship between accumulated repair and maintenance costs as percentage of the initial purchase price of the tractor and accumulated hours of use and age in years for the two tractors in Dongla area could better be described by the power function equation.

2. The accumulated repair and maintenance costs increase with tractor age and hours of use.

3. The predicted models for repair and maintenance costs of tractors in Sudan were lower than those of other countries; therefore, each area or country develops its own models of R&M costs to its operational and field conditions.