Later Incorporation of Astragalus sinicus with Flooding Reduces Rice-Associated Weed Infestation and Increases Rice Yield in the Green Manure–Rice Rotation System

Chinese milk vetch (CMV; Astragalus sinicus L.), serving as winter green manure in rice cropping systems, is widely adopted in the southern China. Field experiments including different incorporation regimes (CMV incorporation, urea substitution incorporation and fertilizer-free incorporation), times (45 days, 30 days and 15 days before rice transplanting) and methods (no flooding, intermittent flooding and continuous flooding) were conducted from 2022 to 2024 to determine the optimal time and method for CMV incorporation that could improve soil nutrients, reduce rice-associated weed infestation, and increase rice yield. Delaying CMV incorporation was beneficial to the accumulation of dry matter and organic matter content in CMV shoots and the increase in the total nitrogen content of the soil before rice transplanting. Broadleaf weed infestation was significantly influenced by flooding method, CMV incorporation and incorporation time. Delaying CMV incorporation combined with flooding significantly reduced the density of broadleaf weeds. Grassy weed infestation was only significantly affected by the flooding method, with significantly lower density under flooding conditions compared to non-flooding conditions when other treatments were consistent. Sedge weed infestation was not affected by any of the experimental treatments. Compared with conventional CMV incorporation (incorporated 30 days before rice transplanting without flooding), incorporating CMV 15 days before rice transplanting with flooding (continuous or intermittent flooding) resulted in a 59.20–66.86% reduction in rice-associated weed infestation. Rice yield was also increased with a delay in CMV incorporation, which mainly manifested in increases in panicle number and seed setting rate. Incorporating CMV 15 days before rice transplanting increased rice yield by 5.34–13.24% compared to conventional CMV incorporation. Therefore, considering the comprehensive effects on soil nutrients, weed infestation and rice yield, incorporating CMV 15 days before rice transplanting combined with intermittent flooding is a recommended green manure management practice in green manure–rice rotation systems.