The North China Plain (NCP) faces severe groundwater depletion and a widening protein-forage deficit. Sustainable alternatives to the dominant wheat–maize double-cropping system are therefore urgently needed. Based on a three-year field experiment in the NCP, this study evaluated the sustainability of three cropping systems: conventional wheat–maize (WM), perennial alfalfa (ALF), and oat–silage maize (OS). The assessment used an integrated framework that combined protein water productivity, economic analysis, energy evaluation, and life cycle assessment (LCA). Results showed that the ALF system achieved the highest crude protein yield (5.2 t ha −1 ), followed by the OS (2.6 t ha −1 ) and the WM (2.5 t ha −1 ) systems. Compared with the WM system, the ALF system increased PWP mainly through high crude protein yield during the mature production years, while OS reduced total ET and water use. The ratio of economic profit to total cost increased by 237.6% and 101.2% under the ALF and OS systems, respectively. Both forage-based cropping systems represent promising alternatives to the WM system for alleviating forage shortages and improving economic efficiency. Crude protein-based energy productivity was highest in the ALF system (80.8 kg GJ −1 ), followed by the OS system (43.6 kg GJ −1 ) and the WM system (31.4 kg GJ −1 ). On a land basis, forage-based cropping systems reduced most LCA environmental impacts, primarily due to lower synthetic nitrogen inputs. When impacts were normalized to a protein-based functional unit, the advantages of the ALF system became more pronounced across all impact categories. Relative to the WM system, the ALF system reduced global warming potential by 47.8%, whereas the OS system showed no significant difference. Overall, forage‑based cropping systems improved economic and environmental performance compared with the conventional wheat–maize system. Perennial alfalfa achieved the greatest protein production, whereas oat-silage maize reduced water use, highlighting the importance of matching cropping systems to regional resource constraints and production objectives.
Cheng et al. (Tue,) studied this question.