Bioassay screening of sawdust obtained from selected tropical tree species for allelopathic properties and their field performance against paddy weeds

Highly diversified tropical tree species could be a potential source of allelopathic compounds, which could lead to development of natural bio-herbicides for sustainable weed management. The present study was therefore designed to investigate into the allelopathic potential of sawdust obtained from eleven tropical tree species available in Bangladesh viz., Azadirachta indica, Swietenia macrophylla, Acacia auriculiformis, Tamarindus indica, Eucalyptus camaldulensis, Syzygium cumini, Mangifera. indica, Albizia. saman, Artocarpus. heterophyllus, Diospyros discolor and Tectona grandis. The aqueous sawdust extract of these plant species were diluted into four different concentrations viz. 1:5, 1:10, 1:15, 1:20 (w/v) and tested for their potentiality in inhibiting seedling growth of allelopathic sensitive plant Raphanus sativus under laboratory condition. A control (distilled water without extract) was also maintained in every cases and the bioassay experiment was replicated thrice. The results of this experiment showed that S. macrophylla, E. camaldulensis, M. indica and A. saman inhibited more than 90% shoot and root growth of R. sativus. The sawdust of these four plant species were selected to evaluate their potentiality in controlling paddy field weeds under filed condition. A total of 16 weed control treatments were considered in the field experiment viz., sawdust of selected four tree plant species at three application rates (1, 2 and 3 t ha-1), manual weeding (three times), chemical control (pre- + post- emergence herbicides), chemical + manual control and season long weedy (control). The results showed that the effect of different sawdust on the weed control varied significantly. Weed growth suppression by the sawdust was increased with the increase in application rate. The results revealed that manual, chemical weed control and application of E. camaldulensis saw dust @ 3 t ha-1 reduced the weed density by 79, 77 and 72%, respectively, and weed biomass by 86, 84 and 79%, respectively. On the other hand, manual weed control offered 100% rice yield increase while, chemical control and E. camaldulensis saw dust @ 3 t ha-1 both resulted in 92% rice yield increase over control. Although, manual and chemical weed control offered higher weed control efficiency and resulted in better rice productivity, but from environmental viewpoint application of E. camaldulensis sawdust @ 3 t ha-1 may be considered for sustainable weed management in rice.