38. While the choice between the handhoe and the animal drawn plow is closely tied to the evolution in farming systems, the choice between animal power and tractor power falls within the choice of techniques framework. Consider a society with a well-established animal drawn plow-based farming system. With the advent of tractors into the region, the farmer has a choice of power sources for performing various operations such as tillage, transport, etc., while holding all other aspects of the farming system constant. In other words, all the farmer wants to do is substitute one power source for another for a particular operation. Whether he chooses animal draft power or tractors depends on: relative costs of labor and capital, interest rate, capacity utilization, farm size, fodder availability, maintenance cost of animals versus tractors, and the difficulty of obtaining spare parts, fuel and repair services. We ought to emphasize that the choice of techniques analysis implies that not all operations are transferred from one power source to another, and that one observes a co-existence and complementarily of power sources for prolonged periods of time.
