Efficient allocation of climatic resources is essential for improving the sustainability and productivity of cropping systems under changing climate conditions. This study aimed to evaluate whether diversified grain–forage rotations could optimize inter-annual growth period configurations and improve annual thermal and radiation resource use in the North China Plain (NCP). A field experiment was conducted from 2023 to 2025 to compare five cropping systems: wheat–maize (W-M), wheat–silage maize (W-SM), wheat–sweet sorghum (W-SS), ryegrass–silage maize (R-SM), and ryegrass–sweet sorghum (R-SS). Growth period characteristics, annual thermal and radiation resource allocation, dry biomass yield, crude protein yield, resource use efficiency, and economic performance were evaluated. The results showed that alternative cropping systems extended annual resource use duration and altered seasonal resource allocation compared with W-M. R-SS exhibited the longest annual growth duration and the most balanced seasonal distribution of thermal and radiation resources, whereas R-SM achieved the highest annual biomass yield. All alternative systems improved thermal and radiation resource use efficiencies compared with W-M, with increases particularly evident during the summer–autumn season. Moreover, diversified systems reduced cultivation costs and improved economic returns. Overall, optimizing inter-annual growth period configurations might provide an effective strategy for enhancing the resource use efficiency, productivity, and economic sustainability of grain–forage cropping systems in the NCP and other regions with similar agro-meteorology in the world.