Response of lowland weeds and direct-seeded lowland rice (Oryza sativa L.) to varying herbicide and surfactant dose mixtures

Herbicide efficacy could be enhanced by herbicide combinations and addition of surfactants. These substances improve absorption of herbicides by lowering the surface tension of spray droplets resulting in increased area of contact of the herbicide solution with the leaf surface. This experiment determined the tolerance of rice and efficacy of three post-emergence herbicides in varying combinations with surfactants against Echinochloa crus-galli (L.) Beauv., Cyperus iria L., and Ludwigia octovalvis (Jacq) Raven. The study was conducted at the Cambodian Agricultural Research and Development Institute (CARDI), in Cambodia from January to February 2019. Rice at 10 days after herbicide application (DAHA) showed some degree of susceptibility to Pyribenzoxim- and Quinclorac 50%+ Pyrazosulfuron-ethyl 7% + Fenoxaprop-P-ethyl 13% pre-mix-surfactant mixture but was fully recovered at 20 DAHA. Generally, rice was more tolerant at the 4-6 than at the 2-3 leaf stage. Phytotoxicity of the herbicides to Echinochloa crus-galli depended on the herbicide and surfactant concentration and the stage of growth of the weed at application time. Cyperus iria and Ludwigia octovalvis were relatively more sensitive to the pre-mix alone and where, in most instances, did not merit addition of surfactant.