Environmental Engineering Reference
In-Depth Information
10.8 Largest tractors working on the U.S. Great Plains and
the Canadian Prairies rate about 300 kW (
@
400 hp). Photo-
graph courtesy Buehler Corporation, Winnipeg.
combines tillage and planting runs in order to reduce the
number of field passes and the degree of soil disturbance,
has brought major declines in specific fuel needs (Phillips
and Phillips 1984; Baker, Saxton, and Ritchie 1996). Ad-
ditional advantages are reduced soil erosion, improved
water retention, and greater flexibility of land use. Com-
pared to conventional practices, disk-and-plant tillage
needs 66% less fuel and slot-planting 75% less fuel; a fur-
ther source of energy savings is the higher efficiency of
nitrogen fertilization (Wittmuss, Olson, and Lane 1977).
Disadvantages include the need for more herbicides,
increased opportunities for pest damage, and lower soil
temperatures.
Accounting for the energy cost of field machinery is
much more complicated than determining its fuel con-
sumption. The usual approach is to take the average
weights of machines and implements used for a particular
cropping cycle, estimate their typical energy costs, and
then prorate this mass per hectare over a period of
expected service (10-20 years). With the energy cost of
tractors and major implements at 70-120 GJ/t and
most conventionally grown staple crops requiring 10-30
kg/ha, these indirect energy subsidies amount annually
to 0.7-4 GJ/ha. Markups of 5-15% cover the energy
costs of maintenance and repairs. In 2005 there were
about 27 million tractors in use worldwide. With average
power of nearly 40 kW and 500 hours of work per year,
their fuel consumption was about 5 EJ and the annual
energy cost of building and maintaining the machines
and their implements was nearly 1.5 EJ (assuming 100
