Efficacy of some Herbicide for Bulrush (Bolboschoenus maritimus (L.) Palla Control in Paddy Fields of Northern Iran

Weeds are the major constraint on rice production. It has been reported that weeds cause rice yield losses by 30-40% in Bangladesh, 36 to 56% in the Philippines, 40 to 100% in South Korea and up to 90% in Iran. Extensive application of graminicides, flooding, and continuous hand weeding have increased the population of more problematic weed species like bulrush (Bolboschoenus maritimus (L.) Palla in rice fields of northern Iran. Bulrush is a perennial weed belongs to cyperaceae family and resistant to most paddy herbicides, flooding and also hand weeding. Yield reduction of rice was estimated up to 100% due to bulrush competition. Despite environmental concerns, theeffect of herbicides on non-target organisms, and the increase of herbicide-resistance weeds, chemical control is still the most commonly used and reliable method for weeds control in paddy rice production in Iran.
Field experiments were conducted at Rice Research Institute of Iran during 2013 and 2014, Rasht, (36° 54' N, 40° 50' E, -21 masl) to assess the efficacy of paddy herbicides for bulrush control. In the first experiment (2013), commonly used paddy herbicides in Iran including bensulfuron-methyl, butachlor, oxadiargyl, pretilachlor, and thiobencarb were evaluated for bulrush control. Two control treatments (hand weeded and weedy check) were also included for comparison. Based on the result of the first experiment, second experiment (2014) was designed to evaluate the efficacy of some new herbicides including flucetosulfuron, penoxulam, metsulforun-methyl, pyrazolate and also bensulfuron-methyl for bulrush control. Both experiments were conducted as randomized complete block design with three replications. Individual plots were 3m wide by 3m long and rice seedlings (cv. Hashemi) manually were transplanted on 23 May 2013 and 2014, with planting distance of 20cm × 25cm. Visual injury evaluation for rice (chlorosis, and growth stunting) and visual weed control (chlorosis, necrosis, and plant stand reduction) was conducted at 2, 4 and 6-WAT (a week after transplanting) in each plot. Visual ratings of rice toxicity and weed control were based on a scale of 0 to 100%, where 0 means no injury or no weed control and 100 means complete death or complete weed control. Also, bulrush biomass was determined at 2, 4 and 6- WAT in the first experiment and 4 and at 12-WAT in the second experiment from a randomly selected 1m2 quadrate in each plot. Weed samples were oven dried at 70 °C for 72 h and weighted. At the maturity stage, rice grain yield (based on 14% humidity), and rice biomass was determined from 5 and 1 m2 per plot, respectively. Rice biomass from each plot was dried at 75°C for 96 h and weighted. Harvest index was calculated by dividing the dry weight of the grain by the dry weight of aboveground (biological yield). Data were subjected to analysis of variance (ANOVA), and means were compared using Fisher's protected LSD test at the 0.05 probability level. All statistical analyses were conducted using SAS (ver. 9.1) software program.
Results of the first experiment showed excellent control of bulrush by bensulfuron-methyl (≥98) followed by pretilachlor (≥85%) at 6-WAT, at the end of critical weed control period in transplanting rice. The efficacy of other herbicides was ≤60%. The lowest biomass of bulrush was recorded for plots treated with bensulfuron-methyl (1.5 g m-2) and pretilachlor (10.6 g m-2). Results also showed that the highest grain yields were observed in plots treated with bensulfuron-methyl (4000 kg ha-1) or pretilachlor (4231 kg ha-1). This was mainly due to optimum weed control in these plots. Grain yield in weedy plots reduced by 76% compared to hand weeded plots, indicating the severe infestation of the plots to bulrush and the ability of this weed to cause rice yield loss. The reduction in grain yield in weedy plots could be mainly due to severe competition for nutrient, especially nitrogen, and light. Because of adequate moisture in paddy fields, reduction in rice growth due to weed-crop competition for water is negligible. The highest harvest indices were observed in plots treated with pretilachlor (52.4%) and butachlor (50%) and weed-free plots (48.8%), while the lowest one was recorded for weedy plots. This result indicates that assimilate partitioning to grain or photosynthesis is reduced under weedy condition. The result of the second experiment also confirmed the result of the first one and indicated that bensulfuron-methyl had good efficacy (≥85%) on bulrush control. The result also indicated that bulrush was controlled very well by flucetosulfuron (88%) and pyrazolate (85%). In contrast, penoxulam and metsulforun-methyl had fair efficacy (≤ 49%) on bulrush control. Moreover, flucetosulfuron, and pyrazolate showed good compatibility with rice, and grain yield in these treatments did not statistically differ with gain yield in plots treated with thiobencarb bensulfuron-methyl. Grain yield was significantly lower in weed-free plot compared to plots treated with bensulfuron-methyl, flucetosulfuron, and pyrazolate. This may be due to that manual weeding can be performed only when weeds have reached a sufficient size to be pulled out easily by hand. By that time, yield losses may have already occurred. At the same time, it seems that weed control by 2- or 3-times hand weeding induced severe stress on rice plant, resulted in a decrease in grain yield. Grain yield in a weedy plot was reduced by 51 and 62% compared with weed-free plots and with plots treated with thiobencarb bensulfuron-methyl, respectively. These results indicate the importance of bulrush control in paddy fields.
Of the current paddy registered herbicides in Iran, bensulfuron-methyl was the most effective herbicide for bulrush control followed by pretilachlor with partial efficacy on bulrush. Newly investigated herbicides, flucetosulfuron, and pyrazolate, could be registered as good options for bulrush control in paddy rice. In respect to the different mechanism of action of new herbicides and their compatibility with rice, the alternative application of new and current herbicide is recommended for controlling bulrush and delaying weed resistance to the herbicide.