Flooding and application time effects on thiobencarb herbicide efficacy in paddy rice

Growth irregularity and yield reduction in some transplanted rice growing area treated with thiobencarb (TB) has been reported particularly if TB is applied in non-flooded conditions, without any more information. This experiment was carried out in order to study the effect of time of herbicide application and time of flooding on response of rice and weeds to TB doses. The experiment was laid out as a factorial split-plot in a randomized complete block design with 3 replications. Treatments included factorial arrangement of time of TB application (before and after transplanting) and time of flooding (before and after TB application) as main plot and TB doses (0, 1, 1.5, 3, 4.5 and 6 kg.ai.ha-1) as subplot. Results showed that TB efficacy in barnyardgrass and sedges control in 4 week after transplanting was similar and in 6-week after transplanting TB efficacy on barnyardgrass control was better. Irrespective of time of herbicide application, the dosage required for 50 or 90% barnyardgrass and sedges biomass reduction was less in non-flooded condition compared with flooded condition, but herbicide selectivity on rice decreased in non-flooded situation. In waterlogged soil that the experiment was carried out, TB caused mortality of transplanted rice seedlings, the maximum 12% and 4% when TB was applied before and after flooding and also 9% and 5% if TB was applied before and after transplanting respectively, which occurred in maximum investigated dose. By increasing TB dose 50% more than recommended dose (3 kg.ai.ha-1), and better control of weeds rice leaf area increased and if TB dose was increased more, despite better weed control, rice leaf area decreased indicating phytotoxicity of TB. Overall rice produced 25 and 15 % more LAI and tiller respectively, by applying TB at flooded plots compared with non-flooded plots. Rice height response to TB dose and time of application was similar to leaf area and decreased more if herbicide was applied before flooding. Also productive (panicle-bearing) and unproductive (nonpanicle-bearing) tillers were affected by time of flooding and TB dose, if applied after and before flooding the most productive and unproductive panicles were produced respectively. Grain yield, biological yield and harvest index trends were presented by three parameters Gaussian model (R2≥84%). Based on these models, by increasing the TB rate these traits increased and then decreased, but reduction in lower doses of TB occurred if TB was applied before flooding. Overall, based on the results of this research, TB cannot be recommended in paddy waterlogging soils if they are facing flooding limitation in early season when TB is applied. Albeit better weed control in non-flooded treatments TB causes delayed growth irregularity and yield reduction when fields are treated in non-flooded condition. Continuous application of TB in successive years exacerbated TB phytotoxicity.