Objectives Rice and milk vetch rotation is an important cultivation system in southern China. We investigated the rice yield and economic benefits under the long-term sole application of milk vetch by off-site returning to the field and the combined application with chemical fertilizers, aiming to propose a reasonable green manure input amount for cost effective and high yield rice production.

Methods A long-term experiment was conducted from 2011 to 2023 after the rice harvest. Ten treatments were designed in the experiment, including: neither chemical fertilizer nor milk vetch application control (CK), chemical fertilizers at rate of 375 kg/hm² (CF), milk vetch incorporation rate of 30 t/hm² (GM₃₀), 60 t/hm² (GM₆₀), 90 t/hm² (GM₉₀), 120 t/hm² (GM₁₂₀), and milk vetch incorporation rate of 45 t/hm² combined with 20%, 40%, 60%, 80% of conventional chemical fertilizer input (denoted as GM₄₅+20%CF, GM₄₅+40%CF, GM₄₅+60%CF, and GM₄₅+80%CF). The fresh milk vetch was off-site returned to rice field. Rice yields were recorded annually to calculate rice yield increasing ability and rate, fertilizer substitution rate of milk vetch (FSRMV), the economic benefits of chemical fertilizer and milk vetch, and the direct and comparative benefits of fertilizers due to rice yield increasing of fertilizers.

Results There was no significant difference in rice yield among different fertilization treatments, among which GM45+60%CF and GM45+80%CF treatments had higher rice yield. Rice yield increasing rate of chemical fertilizer and milk vetch were 5.74 kg/kg and 29.19 kg/t, respectively. The FSRMV was 5.76 kg/t. The economic benefit of rice production by milk vetch was 119.17 yuan/t. When milk vetch was the only fertilizer source for rice production, the economic benefits would become negative beyond the off-site incorporation amount of 90 t/hm². Under combined application, the highest comparative benefit of fertilizers was produced at milk vetch incorporation rate of 45 t/hm² and 40%−60% of conventional chemical fertilizer inputs.

Conclusions Consecutive off-site returning of milk vetch at annual input range of 60−120 t/hm² and the combined application of milk vetch and chemical fertilizers do not show significant differences in rice yield effects, and every ton of milk vetch equals to 5.76 kg of chemical nutrient. However, the direct benefit of fertilizers by rice yield increasing varies greatly. Comprehensively considering, the relatively suitable combination is off-site returning fresh milk vetch 45 t/(hm²·a) and combined 150 kg/hm² of chemical NPK fertilizers (reducing 60% of conventional chemical fertilizer input).