Chemical weed control in paddy fields under intermittent irrigation

Water resources shortage threatens the sustainability of continuous flooding rice culture. To investigate the effect of initial flooding duration on herbicide efficacy and rice yield loss,a field experiment was conducted at Rice Research Institute of Iran, Rasht, Iran, in 2015. The experiment was arranged in a factorial arrangement based on randomized complete block design with three replications.The experimental factors were two rice cultivars (Hashemi and Gilaneh) and five initial flooding durations (flooding for 2, 4, 6, 8 and 12 weeks after transplanting and then applying intermittent irrigation). Consistent with the paddy fields irrigation method in northern Iran, 12-week flooding duration was considered as continuous flooding irrigation. Results showed that there was no significant difference in herbicide efficacy between Hashemi (87.9%) and Gilaneh (85.6%) cultivars. No significant reduction in herbicide efficacy was observed when initial flooding duration reduced from 12 to 6 weeks, but with further reduction in initial flooding duration to four and two weeks, herbicide efficacy was significantly reduced. The highest paddy yield (3989.2 kg.ha-1) was observed in permanent flooding(flooding for 12 weeks) + herbicide application treatment and the paddy yield was decreasedabout 10% by reducing theinitial flooding duration to six weeks but it was not statistically significant. With further decrease in initial flooding duration to four and two weeks, rice paddy yield reduced significantly by 16% and 31%, respectively. In conclusion, results of this experiment revealed that under water shortage conditions, it is necessary to apply initial flooding duration at least for 6 weeks after transplanting for obtaining favorable herbicide efficacy (more than 90%) without paddy yieldloss compared to permanent flooding treatment.