Effects of adjuvants and soil microbes on the phytotoxic activity of coumarin in combination with p-vanillin on goosegrass (Eleusine indica L.) seedling emergence and growth

Repeated use of herbicides has led to the occurrence of Eleusine indica biotypes evolving resistance to various groups of herbicides. The use of allelochemicals as an alternative method of weed control has been receiving a lot of attention recently. Coumarin and p-vanillin, two phenolic compounds belonging to this large class of allelochemicals, have proven to inhibit germination of E. indica greatly in a previous study. However, the efficacy of these two phenolic compounds is dependent on its absorption and is influenced by adjuvants. Thus, this study was conducted to determine the effectiveness of selected non-ionic surfactants (NIS) and an organosilicon (OS) in enhancing the phytotoxic activity of coumarin in combination with p-vanillin on emergence and growth E. indica seedlings under controlled laboratory conditions. Autoclaved and non-autoclaved soil samples were additionally used in the study to determine soil microbe effect on the activity of the phenolic mixture. Results have shown that the NIS, Tween-20 at 0.2% and Speed-Thru at 0.08% were effective in enhancing the phytotoxicity of the phenolic mixture. Two of the adjuvant concentrations used resulted in a reduced dosage of the phenolic mixture needed to reach 90% reduction in emergence (EM90), fresh weight (FW90), shoot length (SL90) and root length (RL90) by 2 to 3 fold. The NIS, Dupont at low dosage of 0.125% was effective in reducing the EM90 and RL90 values by 2.5 to 3.0 fold whereas the OS, Speedex at a recommended dosage of 0.1%, was found to be able to decrease the EM90, SL90 and RL90 values by 2 to 3 fold. On the other hand, the phytotoxic effects of the phenolic mixture on E. indica grown in non-autoclaved soil were less than those observed in autoclaved soil, demonstrating the role of microbes in reducing the bioavailability of the phenolic mixture in the soil.

► Adjuvants enhance phytotoxicity of the phenolic mixture on E. indica seedlings.
► The phytotoxicity of the phenolic mixture can be enhanced by 1.5–3 fold.
► Soil microbes reduce the phytotoxicity of the phenolic mixture in the soil.