Chemotyping of Koelreuteria paniculata Seed Cake with Bioactive and Feed Potential

<i>Koelreuteria paniculata</i> is an amenity landscape tree whose seed extracts and cold-pressed oil are proven biopesticides and biodiesel feedstocks. However, the residual seed cake phytochemical profile has not been systematically assessed or evaluated for multifunctionality across pesticidal, fertilizing, and nutritional domains. Therefore, the aim of this study was to perform a comprehensive chemotyping of <i>K. paniculata</i> seed cake and evaluate its potential for use as a biopesticide, biofertilizer, and feed additive, contributing to sustainable and circular agricultural systems. Detailed analyses of the defatted seed cake included moisture, crude protein, crude ash, crude fat, and crude fiber determination, as well as amino acid and fatty acid composition determination, supplemented with HPLC and antioxidative capacity investigation. Results delivered a comprehensive chemotyping of <i>K. paniculata</i> seed cake, revealing a nutrient-rich profile with moderate protein (20.01%), substantial monounsaturated fatty acids (75.8%, mainly eicosenoic and oleic), and significant phenolic content, including ellagic acid, rutin, catechin, and gallic acid. Antioxidant assays (DPPH and ABTS) confirmed moderate radical scavenging activity, indicating that bioactivity is retained after cold-press extraction. These compositional and functional traits highlight the potential of the seed cake as a raw material for natural biopesticides, biofertilizers, and value-added agro-industrial products. However, due to its unusual fatty acid profile and possible anti-nutritional factors, feed applications should proceed with caution and be preceded by targeted safety evaluations.