Regardless of the stage of mechanization, new power sources are always used first for power-intensive operations. Furthermore, it appears that the price of labor matters less for the mechanization of power-intensive operations than for control-intensive ones-that is, it often pays to move to a higher stage of mechanization in power-intensive operations, even at low wages, when mechanization of control-intensive operations is not profitable.
As long as agricultural wages were relatively low, large farmers found it more efficient to maintain a tractor and truck along with some horses. Horses did virtually all the jobs for which power was not the overriding input. Each power source specialized in the tasks for which it had the greatest comparative advantage.