Rice–duck farming is a sustainable food system that provides rice and duck meat and eggs. Excessive application of commercial feedstuff for ducks weakens the sustainability of rice–duck farming. This study aims to investigate green forage (Lolium multiflorum Lam.) as a substitute for commercial grain mix, to attain a maximum economic benefit and pollution-free production in rice–duck farming. The effects of a mixed feedstuff of green forage and commercial grain mix with a stable ratio of 1:4 (MFSR), mixture feedstuff of green forage and commercial grain mix with dynamic ratios (MFDR), and full commercial grain mix (FCGM) were studied. Results showed that Cherry Volley and hybrid ducks were more adaptable to mixed feedstuff than Mud ducks. Growth of ducks was classified into rapid, slow and steady-growth stages. The weight, body length, neck length, and chest circumference of ducks that were fed MFDR were only slightly decreased. The carcass weight, eviscerated weight, and semi-eviscerated weight of ducks fed MFDR were similar to those fed FCGM. MFDR significantly decreased duck fat accumulation. MFDR did not have negative impacts on the agronomic traits of rice yields. Duck profits per 667 m2 were calculated as CNY (Chinese Yuan) 254.91 (MFSR), CNY309.27 (MFDR), and CNY39.36 (FCGM). MFDR effectively improved the rice–duck farming.