Objectives As one of the three main fertilizer sources for rice production, planting milk vetch and returning to the rice field has a long history in south China. We analyzed the large amount of field experimental data in the seven provinces (autonomous regions) of southern China, trying to quantify the effects of milk vetch return on rice yield and economic benefits, and providing a theoretical basis for making policies boosting the development of milk vetch production and utilization.

Methods Using the keywords “milk vetch,” “milk vetch combined with chemical fertilizer application,” and “rice yield,” we conducted a literature search in Web of Science, China National Knowledge Infrastructure (CNKI), and Baidu Scholar databases prior to April 30, 2024. Subsequently, the searched literature was screened based on the following criteria: 1) The experimental sites were located in the southern rice-growing regions of China; 2) The data were limited to field-based, long-term location experiments; 3) The experiments focused on the milk vetch-rice farming system and included treatments such as fallow control without milk vetch, milk vetch combined with chemical fertilizer application, chemical fertilizer application only, and milk vetch application only; 4) The observed results included sample size, mean values, and standard errors. Ultimately, 39 articles were obtained, providing a total of 4,850 rice yield data points. The data normal interval estimation method was used to calculate characteristic values such as rice yield increasing rate of milk vetch, rice yield increasing rate of chemical fertilizers, substitution rate of chemical fertilizer by milk vetch, and output-input ratio of fertilizers for milk vetch and chemical fertilizers.

Results In the rice-growing regions of southern China, the average rice yield increase attributed to milk vetch is 48.3 kg/t. The yield increase is lower for single-cropping rice than for double-cropping rice but higher than for early rice. The average rice yield increasing rate of chemical fertilizers is 6.18 kg/kg. The average substitution rate of chemical fertilizers by milk vetch is 8.52 kg/t. When the fresh yield of milk vetch is 22.5 t/hm², the average output-input ratio for milk vetch is 2.23, and for chemical fertilizers, it is 2.66. There are no significant differences in these indicators among early rice, single-cropping rice, and double-cropping rice. The economic value of milk vetch as a fertilizer averages 187.7 CNY/t. When the fresh yield of milk vetch is below 9.92 t/hm², milk vetch contributes negatively to the economic benefit in the milk vetch-rice system. The rice yield increase potential of the milk vetch-rice system is 729 kg/hm² higher than that of the winter fallow-rice system.

Conclusions In southern China rice production area under long-term milk vetch-rice rotation system, the economic value of milk vetch as fertilizers is 187.7 CNY/t, the average rice yield increasing rate of milk vetch is 48.3 kg/t, equal to 8.52 kg/t of chemical fertilizer in efficiency. Considering the potential of milk vetch on rice yield and profit, planting milk vetch during winter fallow season and adopt production techniques that improve its fresh biomass yield is recommended in south China rice systems.