Eco-Friendly Pre-Emergence Weed Control from Cosmos sulphureus Leaf Extract: An Innovation for Sustainable Rice-Based Systems

Objective This study evaluated the pre-emergence bioherbicidal potential of Cosmos sulphureus leaf extract against major weed species while assessing its selectivity toward common crop plants. Methods A methanolic extract of C. sulphureus leaves was tested at graded concentrations on three weed species ( Cynodon dactylon, Ludwigia octovalvis, Eragrostis amabilis ) and four crop species ( Vigna radiata, Zea mays, Ipomoea aquatica, Brassica juncea ). Germination and early growth responses were quantified, and EC 50 values were calculated for both shoot and root inhibition. Qualitative phytochemical screening was performed to identify bioactive compound groups. Results Inhibitory effects were strongly concentration-dependent and varied among species. C. dactylon showed complete inhibition at ≥0.24 g/mL (EC 50 : 0.0009 g/mL for shoots; 0.0002 g/mL for roots), whereas E. amabilis reached total inhibition at 0.12 g/mL (EC 50 : 0.0600 g/mL). L. octovalvis exhibited higher sensitivity in the roots, with 88.19% inhibition at 0.12 g/mL and EC 50 values ranging from 0.01 to 0.09 g/mL. Crop species varied in tolerance: V. radiata and Z. mays were least affected (maximum root inhibition 64.04% and 37.87%, respectively), while B. juncea showed complete inhibition at 0.48 g/mL. I. aquatica displayed moderate sensitivity with 76.36% root inhibition at 0.48 g/mL. Phytochemical screening confirmed the presence of alkaloids, flavonoids, tannins, and phenolics are compound groups known for allelopathic activity. Conclusion Cosmos sulphureus leaf extract demonstrates strong bioherbicidal potential, with marked selectivity across weed and crop species. Optimizing application concentrations will be crucial for maximizing weed suppression while minimizing adverse effects on crop plants. These results support the development of C. sulphureus –based eco-friendly bioherbicide systems for sustainable weed management.