The Rule of bigger is better doesn't always fit when buying a farm tractor.

With the high price of machinery and fuel, you should consider what size tractor will handle your fieldwork most economically. A new software program, called Common Agricultural Tractor Selection System, or CATSS, can help you make that decision.

CATSS determines the least-cost tractor-implement combination. Unlike simple rate calculations, the program factors in all the variables that exist in farming, including the biggest wild card — weather.

“Any farmer will tell you that farming is a gamble,” says ag engineer Reid Pulley, who developed the software as part of his master's program at the University of Minnesota. “Weather can make or break a crop.”

To be safe, many farm managers will oversize machinery to ensure field operations are performed on a timely basis, Pulley says. However, this has several drawbacks: higher overhead and maintenance costs, higher operating costs, and risk of excessive soil compaction.

The program counters the need to oversize. It uses four mathematical models to simulate conditions on your farm:

Climate model

Simulates climate conditions throughout the U.S. using weather data that are statistically similar to historical weather patterns of the past 100 years. The amount of rain, sun, heat, humidity, wind and other factors all help determine the field conditions.

Soil hydraulic model

Soil moisture content is based on the soil type, field conditions (tilled, compacted, etc.) and the recent weather.

Traction model

Tractor performance for all four usual types (2-wd, mechanical four wheel drive, 4-wd, and track) and sizes ranging from 100 to 450 hp are tested simultaneously.

Farm economics model

Farm economics are used to calculate the separate machinery costs: ownership, labor, operating and timeliness.

These mathematical models are applied to determine how big an implement needs to be, and then what size and type of tractor is best to pull it.

Pulley says other farm modeling programs used for machinery selection (like Purdue's Crop Livestock Linear Program and the USDA-ARS Integrated Farm System Model) would have to be rerun hundreds of times to come up with the same pairings. Pulley's program must be run only twice (one time for the implement and the next time for the tractor).

“Farmers can get a reasonable estimate of their machinery needs in about 15 minutes,” he says.

Results are displayed on a simple line graph showing the cost per acre of each combination. Once the most economical size of tractor and size of implement are determined, the program generates a listing of all makes and models that meet the search criteria. The program draws on performance data collected by the Nebraska Tractor Test Lab, University of Nebraska-Lincoln.

In the Midwest, tractors and other farm equipment account for 12% of a farm's total assets, according to the National Agriculture Statistics Service.

“Decreasing machinery costs by even a fraction would free up cash flow for other parts of the farm or decrease the operating loan and the finance expenses for the whole farm,” Pulley says. “Fuel costs would also be minimized by having the tractor properly sized.”

For more information on CATSS, contact Pulley at 612/799-7617,, or