Selenium co-application is associated with faster quizalofop-p-ethyl dissipation, phyllosphere microbiome restructuring, and partial mitigation of forage-quality changes in alfalfa

Quizalofop-p-ethyl (QPE) is an aryloxyphenoxypropionate herbicide used for selective grass-weed control, but residual QPE and its transformation products may impose sublethal stress on forage crops. Selenium (Se) can regulate plant redox homeostasis and glutathione-dependent detoxification, yet whether Se co-application modifies QPE dissipation and the associated host–phyllosphere response remains unclear. This study investigated alfalfa under a high-dose QPE field exposure scenario by integrating herbicide residues, physiological responses, forage quality, and phyllosphere bacterial and fungal communities. Compared with QPE alone, QPE + Se showed earlier QPE disappearance and lower persistence of its major metabolite, quizalofop acid (QPA). This lower residue burden coincided with reduced malondialdehyde accumulation and consistently higher glutathione S-transferase activity, while the QPE-induced elevation of several antioxidant enzymes was moderated at selected sampling times. Forage-quality responses were trait- and time-dependent, with QPE + Se showing partial mitigation of QPE-associated changes in dry matter and selected fiber-related traits. Phyllosphere alpha diversity was not significantly altered, whereas community composition showed clear treatment-dependent separation for both bacteria and fungi. QPE was characterized by taxa including Pseudomonas and Alternaria, whereas QPE + Se showed a distinct assemblage including Sphingomonas, Methylobacterium, Wallemia, and Aspergillus. QPE showed higher standardized predicted values for pathways related to xenobiotic metabolism, transport, redox regulation, and environmental sensing, whereas QPE + Se showed higher standardized predicted values for carotenoid, sphingolipid, and phenylpropanoid biosynthesis. Exploratory genus–host associations further showed coordinated directional variation with QPA residues, oxidative-stress indicators, GST activity, and forage-quality traits, although none remained significant after false-discovery-rate correction. Under high-dose QPE exposure, QPE + Se was associated with a distinct physiological and phyllosphere state characterized by earlier QPE disappearance, lower QPA persistence, reduced oxidative injury, higher GST activity, partial mitigation of forage-quality changes, and marked microbiome restructuring. Rather than indicating a simple return toward the untreated condition, these coordinated responses suggest that QPE + Se represents a distinct ecological and physiological state under QPE stress. The findings provide a basis for further evaluating Se-assisted approaches to mitigate herbicide stress and residue burden in alfalfa, while the contribution of specific phyllosphere taxa to QPE or QPA transformation requires direct functional validation. QPE + Se was associated with earlier QPE disappearance and reduced QPA persistence in alfalfa. QPE + Se was associated with lower lipid peroxidation and higher GST activity than QPE alone. QPE + Se showed partial, trait- and time-dependent mitigation of forage-quality changes. QPE and QPE + Se were associated with distinct bacterial and fungal phyllosphere community structures. QPE + Se corresponded to a distinct alfalfa–phyllosphere physiological and microbial state.