Energy Intensity of Alternative Wheat-Based Crop Rotations.

Non-renewable energy intensity and net energy gain have been proposed as indicators of sustainability for food and biofuel production. Non-renewable energy use associated with the manufacture and use of farm inputs was calculated for alternative crop production systems representing plot scale research and farmer field surveys. Farm inputs included were: seed, fertilizer, pesticides, & farm machinery operation. Energy intensity is defined as the amount of input energy (MJ) divided by the crop grain yield (kg) and expressed in units of MJ kg^-1. Net energy gain is defined as energy content of the harvested material minus the input energy and is expressed in MJ ha^-1.

In a 2005-2006 plot study at Bozeman, MT, nitrogen fertilizer was the largest energy input to wheat production systems, accounting for up to 84% of total input energy. Reduction of N fertilizer below recommended levels generally resulted in reduced energy intensity but also reduced yields and net energy gain. This effect highlights the challenges of measuring changes in soil organic N, which should be included in such energy budgets. When wheat was grown following pea, at full recommended N fertilizer rates, energy intensity was reduced 31%.

Energy intensity and net energy gain was calculated for paired wheat-wheat and pea-wheat crop sequences from fields within the same farm for 12 field pairs on farms in NE Montana. Wheat grown following pea had 34% lower energy intensity and 67% higher net energy gain than wheat grown following wheat. This result is likely due to a combination of nitrogen availability, water use efficiency, and disease suppression in the pea-wheat rotation.

Diversification of continuous wheat cropping systems to include pulse crops in rotation has significant potential to improve sustainability through reduced energy intensity without sacrificing productivity.