The shift from the draft animal-powered agronomy of the first half of the twentieth century to the highly mechanized operations practiced today is characterized by a more efficient use of labor and time, but is difficult to evaluate on a GHG emission basis. Nonetheless, in an effort to quantify the difference in fossil fuel input between the 2 systems we have characterized the primary means of work energy within the 2 time periods. Interestingly, energetic inputs associated with fulfilling the requirements of a team of draft animals under moderate work were 12% greater than the equivalent energy cost of the same work supplied by tractor power (Table 3). In an analysis of fossil fuel usage on US farms, Cleveland (1995) demonstrated that the ratio of on-farm productivity to energy use declined from 1910 to 1970 and attributes this to inefficiency promoted by low fuel costs. This trend reversed as intensification, farm sizes, and fuel costs increased in the 1970s, and these factors are likely to further improve energy productivity in future (Cleveland, 1995). Rydberg and Jansen (2002) noted that although man-hours and energy use are considerably reduced when using modern tractors compared with horse traction, the majority (91%) of energy inputs to the tractor-based system originate from nonrenewable fossil fuels, whereas 60% of draft energy inputs are renewable. Thus, not only energy efficiency, but also energy source must be considered when evaluating the environmental impact of agricultural practices.