Allelopathic potential of yellow cosmos (Cosmos sulphureus) and royal poinciana (Delonix regia) on weedy rice (Oryza sativa f. spontanea WR20)

This study evaluates environmentally friendly methods for controlling weedy rice (Oryza sativa f. spontanea) to reduce reliance on chemical herbicides. Extracts from Cosmos sulphureus (yellow cosmos - YCL) and Delonix regia (royal poinciana - RPL) leaves were tested at concentrations of 0.015, 0.075, 0.15, and 0.3 g fresh weight/mL for 48 hours. Pre-soaked weedy rice (WR20) seeds were treated, and stem and root lengths were measured 7 days post-treatment. At 0.3 g fresh weight/mL, YCL extract completely inhibited WR20 growth, while RPL extract inhibited root and stem by 71% and 75%, respectively. WR20 seeds soaked for 96 hours before YCL treatment showed the highest inhibition. For RPL, 48-hour soaking yielded the highest root inhibition, and 96-hour soaking the highest stem inhibition. Evaluating effects on OM18 cultivated rice revealed YCL extract at 0.3 g fresh weight/mL inhibited roots (45.41%) and stems (1.72%) at 2 days post-treatment, with full recovery by 7 days. However, RPL extraction proportionally impacted OM18's roots (33.35–100%) and stems (33.10–80.10%). These findings suggest that using YCL and RPL extracts for weedy rice management is promising, particularly when combined with strategic timing of rice planting to minimize damage.