Sustainable cropping systems for the Northern Great Plains -- Energetic and economic considerations

Reliance on non-renewable resources is among the fundamental challenges to agricultural sustainability. Quantification of inputs in units of embodied fossil energy offers insight into sustainable use of these resources. Metrics of intensity, efficiency of non-renewable energy inputs to agriculture have been proposed for optimization in search of sustainability in the face of energy scarcity. Such analyses have found controversial results however, and further theoretical understanding is necessary. The research presented here focuses on approaches to sustainability targeting the semiarid northern Great Plains of North America. The 4 million ha of cropland fallowed in this region every year represent both a challenge to sustainability and an opportunity to address that challenge. Long identified as unsustainable when accomplished by tillage and without fertilizer input, the summerfallow -- wheat crop production system is also energy-efficient by definitions that do not account for changes in soil fertility. It is shown here that accounting for lost soil N as an energy input to crop production partially resolves this paradox, but no strategy for energetic valuation of systems that build soil quality is apparent. Alternatives to summerfallow considered here include pulse crops (e.g. pea and lentil ) grown for grain, forage, or as cover crops. In research conducted on farms already growing pulses, the largest effect on cropping system energy productivity was due to increased wheat yield rather than a reduction of inputs. In plot-scale research addressing a wider variety of production practices, neither system-level energy intensity nor productivity provided more insight into energy price exposure than basic economic analyses. Legume green manure (LGM) cover crops increased wheat grain yield and protein, especially when incorporated with tillage, but did not improve cropping system energy intensity. It was found by a novel approach, however, that the energy cost of the short-term N equivalence of LGM cover cropping was less than that of fertilizer N, while the economic cost was greater. Pulse crops were found to have potential for improving the sustainability of cropping systems in the northern Great Plains, both harvested for grain where that is feasible and used as cover crops where water is more limiting.