Evaluation of Herbicide Combinations for Weed Control and Crop Safety in Astragalus membranaceus: Emphasis on Root Injury

Astragalus membranaceus is an important medicinal crop whose commercial production is frequently constrained by severe weed infestation. However, limited information is available regarding herbicide programs that simultaneously achieve effective weed control and maintain root safety, the primary determinant of total root fresh weight and quality. In this study, three-year field experiments (2022–2024) were conducted to evaluate the weed control efficacy, crop safety, root growth, yield, and major bioactive constituents of different post-emergence herbicide programs. Linear mixed-model analysis showed significant treatment effects on weed control efficacy, underground injury, root growth, and total root fresh weight (p < 0.001), whereas no significant year × treatment interaction was detected for the evaluated response variables under the conditions of this three-year field experiment (p > 0.05). The combination of bentazone 48% AS (2.700–3.000 L·ha−1) and quizalofop-P-ethyl 15% EC (0.375 L·ha−1) achieved >90% total weed control, caused no detectable underground injury, and increased total root fresh weight by 21.04–30.03% relative to the untreated control. In contrast, chloransulam-methyl combined with quizalofop-P-ethyl caused severe underground phytotoxicity, with root injury indices ranging from 63.10 to 80.01, which were associated with substantial reductions in total root fresh weight. Correlation and regression analyses further demonstrated that underground injury was significantly associated with root yield, whereas visible aboveground injury showed little relationship with yield performance. Furthermore, astragaloside IV and calycosin-7-O-β-D-glucoside contents did not differ significantly among CK1, T8, and T9. These findings highlight the importance of incorporating underground phytotoxicity into herbicide safety evaluation and provide a basis for developing effective post-emergence weed management strategies for A. membranaceus. Because all three growing seasons were evaluated at a single experimental site, the applicability of these findings to other environments, soil conditions, and A. membranaceus cultivars requires further validation.