A long term crop rotation experiment was established at Ottawa, Ontario, Canada in 1992. Rotations included various combinations of corn, soybeans and alfalfa. A subset of plots with continuous corn received annual nitrogen treatments of none, inorganic fertilizer, and stockpiled and rotted manure, each at two rates. The manure treatments were applied annually after harvest and incorporated by moldboard plowing. Draft measurements for moldboard plowing were made each year using Agriculture and Agri-Food Canada's instrumented research tractor.

Previous analysis of plow draft data up to 1999 showed a substantial reduction in tillage energy inputs for plots that received annual manure treatments. The reduction was attributed to a buildup of soil organic matter resulting from the annual manure application. The present paper will track the changes in tillage energy with an additional five years of plowing data since 1999. It will also include an analysis of soil organic matter to test the hypothesis that reduction in moldboard plow draft in manured plots was due to an increase in soil organic matter resulting from annual manure treatments.