Comparison of Some Herbicides on Causing the Dwarfness on Rice

In order to investigate the effect of most common paddy selective herbicides on causing growth disorder called dwarfism two experiments were laid out in 2007 in Rice Research Institute of Iran, Rasht. The dose response study was carried out by herbicides thiobencarb "TB", butachlor and oxadiargil and Hashemi rice cultivar in Petri dishes containing agar and Youshida growth medium. Visual evaluation of herbicides toxicity on seedlings showed that TB caused specific growth irregularity including dark greenish leaves, dwarf stature of seedlings, short thick leaves and brown color roots that are known as symptoms of dwarfism. Regression analysis of rice seedlings height, root length and fresh weight by fitting sigmoidal logistic dose response equations showed that the inhibitory effect of thiobencarb on rice growth was significantly more than two other herbicides (P-1) in a split split plot design with factorial arrangement of treatments in 3 replications. Also two other herbicides were included as control. Toxicity of herbicides were evaluated visually based on 0-100 where 0 indicates no effect, and 100 was complete plant death. Results of this experiment also confirmed the results of previous one and symptoms of dwarfism were observed only in pots treated with TB. Data analysis showed that toxicity of TB was affected by dosage, time of application and water level significantly (P2>95%) between dosages and toxicity of TB. Spraying TB after transplanting compared with before transplanting decreased dwarfness intensity significantly. Based on the results of this study for management of TB toxicity in paddy rice it is needed the herbicide to be applied in flooded soils, and after transplanting in minimum recommended doses