Herbicides for monochoria (Monochoria vaginalis) control in transplanted rice

Abstract

In Iran, monochoria is a noxious weed in fields of transplanted rice. Two field experiments were conducted to assess the efficacy of soil-applied and foliar-applied herbicides to control monochoria in transplanted rice. Prepackaged herbicides (triafamone plus ethoxysulfuron applied at 40 g ai ha⁻¹, pyrazosulfuron-ethyl plus pretilachlor applied at 382.5 g ai ha⁻¹, and pendimethalin plus clomazone applied at 1,200 g ai ha⁻¹) reduced monochoria biomass by 100%, 100%, and 14%, respectively; and a single application of flucetosulfuron at 30 g ai ha⁻¹, pendimethalin at 990 g ai ha⁻¹, thiobencarb at 2,750 g ai ha⁻¹, and pretilachlor at 1,000 g ai ha⁻¹ reduced monochoria biomass by 100%, 99%, 75%, and 56%, respectively, compared with a nontreated control. Tank-mixed bensulfuron-methyl at 45 g ai ha⁻¹ applied with pretilachlor, thiobencarb, or pendimethalin provided 100% control of monochoria. Rice height, and straw and grain yield were greater after herbicide treatments than those of the nontreated and hand-weeded controls, indicating the advantages of chemical control of monochoria over manual weeding. Full-season monochoria interference reduced rice grain yield by 32%. In the second study, the herbicides triafamone plus ethoxysulfuron, flucetosulfuron, 2,4-D at 1,080 g ai ha⁻¹, dicamba plus 2,4-D at 928 g ai ha⁻¹, bispyribac-sodium at 31.25 g ai ha⁻¹, bentazon plus MCPA at 1,150 g ai ha⁻¹, pyribenzoxim at 30 g ai ha⁻¹, and propanil at 5,400 g ai ha⁻¹ applied to foliage at 4- to 5-leaf seedlings of monochoria provided ≥97% control and prevented 100% of its regrowth, with the exception of propanil. This study shows that monochoria control can be achieved by using a variety of residual and foliar-applied herbicides with different mechanisms of action.

Introduction

The area under rice cultivation in Iran varies from 450,000 to 650,000 ha, depending on annual precipitation. Two common methods of rice cultivation in Iran are direct seeding and transplanting. About 75% of rice cultivation in Iran occurs in the north, which has a moderate and humid climate (FAO 2020). Transplanted monoculture rice has been historically practiced in this area (Yaghoubi et al. 2022). The average precipitation in this region reaches 1,200 mm, and about 80% of that occurs after the cropping season in autumn and winter. Flooding occurs in the first half of the year during the cropping season, and then continuous rain in the second half makes the rice fields suitable for subaquatic and aquatic weeds to invade the transplanted rice paddies (Yaghoubi et al. 2022). Currently, some aquatic or semiaquatic weeds such as bulrush [Bolboschoenus planiculmis (F.Schmidt) T.V.Egorova], pondweed (Potamogeton nodosus Poir.), azolla (Azolla filiculoides Lam.), flowering rush (Butomus umbellatus L.), watergrass [Echinochloa oryzoides (Ard.) Fritsch], monochoria, barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], knotgrass (Paspalum distichum L.), arrowhead (Sagittaria trifolia L.), waterplantain (Alisma plantago-aquatica L.), and yellow nutsedge (Cyperus esculentus L.) are the most prevalent weeds in transplanted rice in Iran.

Monochoria is a monocotyledonous, broadleaf, and semiaquatic to fully aquatic weed in the Pontederiaceae family. Monochoria was first recorded in Iran three decades ago (Sharifi 1990). The weed has adapted to the transplanted rice ecosystem in the northern parts of Iran, and it is spreading to new areas each year. Monochoria is native to Asia and has been recorded in Korea, Japan, India, Southeast Asia, and the Pacific Islands (Holm et al. 1977). It is ranked as one of the most crucial weeds of rice in dozens of rice-producing countries worldwide (Holm et al. 1977; Vanangamudi et al. 2013). It was reported as the main weed of rice fields in Bhutan and Taiwan (Parker 1992), and one of the five most prevalent weeds in rice fields in China (Li et al. 2012). Monochoria’s economic damage to rice has been emphasized in many studies (IRRI 1983; Zhou et al. 2021), and it is the main, yield-limiting factor in organic rice production in Japan (Cheng et al. 2015). More than 80% of rice grain yield loss in competition with monochoria has been reported (Ampong and De-Datta 1991). Monochoria is propagated by seeds and stolons (Caton et al. 2010). It is an annual weed in rice fields and a perennial plant in aquatic ecosystems (Vanangamudi et al. 2013). The seeds of monochoria can survive in the soil for more than a decade (Yamada et al. 2007). The weed is an indicator of nitrogen-rich soils and is more invasive in fertile soils (IRRI 1983; Naylor 1996). Monochoria pulls a lot of nitrogen from the soil, thus reducing rice yield (Ampong and De-Datta 1991; Cheng et al. 2010). Gradual and protracted germination, rapid growth, and high plasticity make monochoria a highly competitive weed (Athira et al. 2020). The seeds of monochoria require very little oxygen for germination (Takeuchi et al. 2001). Monochoria seedlings have emerged from 15 cm or more of water in permanently flooded pots (B Yaghoubi unpublished data).

According to a report by Ampong and De-Datta (1991), monochoria is susceptible to many herbicides, including bensulfuron methyl, butachlor, butralin, cinmethylene, clomitoxinil, 2,4-D, glyphosate, MCPA, oxadiazon, oxyfluorfen, paraquat, pendimethalin, piperophos, pretilachlor, and propanil. It is relatively susceptible to thiobencarb and quinclorac and is tolerant to fenoxaprop (Ampong and De-Datta 1991). Also, Kumar and Ladha (2011) reported that bispyribac sodium, penoxsulam, 2,4-D, ethoxysulfuron, and a combination of chlorimuron plus metsulfuron can control monochoria. However, continued use of acetolactate synthase (ALS) inhibitors to control rice weeds has reduced their efficacy, and several weeds, including monochoria, have developed resistance to it (Kuk et al. 2003). Soil-applied herbicides such as mefenacet and pyrazolate, and foliar-applied herbicides such as 2,4-D and bentazon have been reported to be effective in controlling both ALS-resistant and ALS-susceptible biotypes of monochoria (Kuk et al. 2003).

Currently, the transplanted rice weed management program in northern Iran consists of a combination of cultural (transplanting and flooding), mechanical (puddling and harrowing), and chemical (herbicide) interventions, coupled with hand-weeding (Yaghoubi 2017), with herbicides offering the most substantial contribution. The time needed for hand-weeding a rice field is about 450 h per hectare, which can be reduced to less than 40 h by applying herbicides, indicating a more than 90% reduction in time than required for hand-weeding.

After three decades of monochoria existing in Iran, this weed has spread to more than 50,000 ha of transplanted rice fields in the northern part of the country (Golmohammadi 2018). The rapid spread of monochoria and the lack of practical instructions for its management has necessitated this study. Therefore, this research aims to determine the performance of residual and postemergence (POST) herbicides on monochoria control.

Materials and Methods

Study 1: Performance of Residual Herbicides on Monochoria Control in Transplanted Rice

This study was conducted in 2018 and 2019 at the research fields of the Rice Research Institute of Iran, in Rasht (36.54°N, 40.50°E, 24 m above sea level). Transplanted rice had been grown in this field for more than 50 yr. The soil was an Alfisol type containing 44% clay, 52% silt, 4% sand, and 2.2% organic carbon; pH 7.2; with 12.7 mg kg⁻¹ of available phosphorous and 165.63 mg kg⁻¹ of available potassium; and an electrical conductivity of 2.49 dS m⁻¹. In the previous crop season, the site was heavily infested with monochoria. We removed all weeds, except monochoria, by hand, to maintain a uniform and high density of monochoria infestation in the present study.

Soil preparation consisted of flooding the land in early May, followed by one pass of wet plowing using a rotavator (Badeleh®, Iran Ltd) and a small plowing machine in the opposite direction. Plots were then flooded until harrowing to puddle the soil 1 d before transplanting. The plot dimensions were 2 × 5 m with a planting density of 25 hills m⁻² and three seedlings per hill. Rice seedlings (Hashemi, a traditional cultivar of Indica type) at the 3- to 5-leaf stage were transplanted in plots on May 18. After rice had been transplanted, the soil levees between the plots were covered with polyethylene to prevent the infiltration of herbicide into adjacent plots. The experimental plots were flooded to approximately 5 to 7 cm from 1 d after transplanting until 2 wk before harvest. Preemergence (PRE) herbicides were applied 2 d after transplantation when no emerged weeds were observed by pouring the herbicides into flooded plots using a handheld bottle with three holes specifically designed for this purpose. Each experimental plot had an independent water inlet but no outlet, until the end of the season when the plots were drained to harvest the crop.

All herbicides were assessed based on their recommended rate for transplanted rice weed control (Table 1). Nontreated and hand-weeding controls were also included for comparison and to estimate the extent of the yield losses in transplanted rice as a result of monochoria infestation. Weeds in hand-weeded plots were removed manually at 14, 28, and 42 d after transplanting (DAT). In nontreated and herbicide-treated plots, all nontarget weeds were removed by hand whenever their size was big enough for removal. Fertilizers were applied based on soil analyses and included nitrogen (60 kg N ha⁻¹), potassium sulfate (50 kg ha⁻¹ K₂O), and phosphorous (50 kg ha⁻¹ P₂O₅). Potassium and phosphate were used once after puddling and incorporated into the soil by land leveling. The urea fertilizer was applied in three splits: 1 wk after transplanting, at tillering, and panicle initiation in equal proportions. Stem borer (Chilo suppressalis), the most important insect pest in the region, was controlled by applying 15 kg ha⁻¹ of diazinon (granules 10%) at the tillering and panicle initiation stages. The study was arranged in a randomized complete block design with three replications each year. The timing of all agronomic practices over the 2 yr of the experiment were similar, with less than 1 wk difference.

Table 1. Soil-applied herbicides used in field studies to control Monochoria vaginalis in rice.^{a, b}

^a Abbreviations: EC, emulsifiable concentrate; SC, suspension concentrate; TB, tablet; WG, water-dispersible granules.

^b Field studies were carried out at the Rice Research Institute of Iran.

Sampling

As monochoria propagates through the stolon, in addition to seeds, its density was determined before stolon formation at 6 wk after transplanting (WAT). At rice harvest, the efficacy of the treatments in controlling monochoria and their effect on rice height, straw, and panicle were determined by sampling monochoria and rice from a 1-m² area. Both were cut at the surface, air-dried for 48 h, and separated in the laboratory. Rice panicles were counted, and the average height of 10 randomly selected plants per plot was considered as the height of the experimental unit. The height was measured from the beginning of the stem to the tip of the leaf. Samples were oven-dried at 75 C until a constant weight was achieved. Rice yield was measured by harvesting a 5-m² area in the center of each plot by hand when the grain color changed to yellowish. The grain moisture content was about 14%. Grains were separated from straw by a small stationary research thresher.

Study 2: Performance of Foliar-Applied Herbicides in Monochoria Control

The second study was conducted in 2018 and 2019 to investigate the effectiveness of foliar-applied herbicides to control monochoria. Soil preparation and fertilizer applications were similar to those in the first experiment. The experimental site was densely infested by monochoria in the previous crop season, thus in this study, the plots were flooded with about 10 cm of water, which prevented the germination of most weeds and stimulated uniform germination and growth of monochoria. A few unwanted weeds emerged, such as bulrush and pondweed, which were hand-weeded as early as possible. Herbicides (Table 2) were applied when monochoria seedlings had four to five leaves in the first week of June, using a rechargeable Matabi (Antzuola, Spain) sprayer fitted with a TeeJet® (TeeJet Technologies, Wheaton, IL) nozzle calibrated to deliver a spray volume of 170 L ha⁻¹. No adjuvant was used with any of the investigated treatments. A nontreated control treatment was also included.

Table 2. Foliar-applied herbicides used in field studies to control Monochoria vaginalis. ^{a, b}

^a Abbreviations: EC, emulsifiable concentrate; OD, oil dispersion; SC, suspension concentrate; SL, water-soluble liquid; WG, water-dispersible granules.

^b Field studies were carried out at the Rice Research Institute of Iran.

Experimental plots were drained before herbicide application, and 24 h after herbicide application they were submerged again at around 5 to 7 cm. Visual estimates of monochoria control were made 3 and 7 wk after herbicide treatment each year on a scale of 0% to 100%, with 0% being no weed control and 100% being complete weed control. Also, the effect of treatments on monochoria biomass was conducted at the same time, using a 0.25 m⁻² quadrat in each plot. Monochoria aerial parts were collected by cutting at the ground surface and oven-dried at 75 C for 48 h or more until dry matter was achieved. Monochoria density before stolon production initiation was counted at 6 WAT. Because monochoria completes its life cycle by mid-November its regrowth was recorded 12 WAT. Late germination of monochoria under a closed rice canopy may prevent optimal herbicide coverage on monochoria foliage when POST herbicides are sprayed; therefore, in order to identify effective herbicides and confirm their effectiveness, this experiment was performed in a monochoria monoculture. The study was laid out in a randomized complete block design with three replications each year. Plot dimensions were 2 × 5 m separated by a protective row (width 0.5 m), and replications were spaced 1 m apart.

Statistical Analysis

Data were analyzed using SAS software (SAS Institute Inc., Cary, NC). Each experiment was conducted for 2 yr. Before analysis, data were subjected to the homogeneity test and after confirming the assumptions of ANOVA, a combined analysis was performed for all the traits. Treatment and year were considered fixed effects. No significant difference was observed between the years and the treatments, and therefore, the data were pooled for analysis. Means of treatments were separated using Fisher’s protected LSD test at a significance level of 5%. Because the results of visual assessment and biomass measurements were similar, visual data are not presented.

Results and Discussion

Study 1: Performance of Residual Herbicides on Monochoria Control

Almost 95% or more of the weed community in the experimental plots consisted of monochoria, bulrush, and barnyardgrass. Those weeds germinated within the first 3 wk after rice transplanting, whereas monochoria seedlings emerged later than 3 wk after transplanting (data not shown). The cumulative temperature (in degrees Celsius) required for monochoria, barnyardgrass, and smallflower umbrella sedge (Cyperus difformis L.) germination was reported as 263, 119, and 103 growing degree days, respectively (Naylor 1996). Also, Obuchi (1991) reported that monochoria emerges later than other common weeds in rice fields.

Monochoria density was 204 plants m⁻² in the nontreated control (Table 3), which is much denser than it was in transplanted rice with 25 hills m⁻². All investigated herbicides reduced monochoria density significantly, ranging from 21% to 100%. The prepackaged herbicides pendimethalin plus clomazone provided the least control of monochoria. A single application of three common grass killers, pretilachlor, thiobencarb, and pendimethalin, reduced monochoria density by 47%, 81%, and 98%, respectively, and when tank-mixed with bensulfuron, monochoria density was reduced by ≥98%. Similarly, flucetosulfuron and two prepackaged herbicides, triafamone plus ethoxysulfuron and pyrazosulfuron-ethyl plus pretilachlor, provided complete control of monochoria (Table 3).

Table 3. Monochoria control at 6 WAT and rice status at 12 WAT. ^{a, e–h}

^a Abbreviation: WAT, weeks after treatment.

^b Monochoria density in the nontreated control was 204 plant m⁻² at 6 WAT.

^c Monochoria dry matter weight in the nontreated control (monochoria interference with rice) and monochoria monoculture was 378 and 3,693 kg ha⁻¹, respectively.

^d Rice height, panicle, straw and grain yield in nontreated control were 126 cm, 247 m⁻², 3,413 kg ha⁻¹, and 2,387 kg ha⁻¹, respectively.

^e Means are averaged over 2 yr and four replicates.

^f All data are expressed as a percentage of the nontreated for the respective treatment.

^g Means followed by the same letter within a column are not significantly different at P = 0.05 using the LSD test.

^h Numbers greater than 100 indicate an increase in that trait compared with the nontreated control.

Monochoria biomass reduction in response to herbicides was variable. Triafamone plus ethoxysulfuron, thiobencarb plus bensulfuron, pendimethalin plus bensulfuron, pretilachlor plus bensulfuron, pyrazosulfuron-ethyl plus pretilachlor, flucetosulfuron, and pendimethalin all provided ≥98% reductions in monochoria biomass (Table 3). Tu et al. (2011) reported 95% to 100% monochoria biomass reduction by using bensulfuron plus pendimethalin. Also, Menon et al. (2016) reported excellent control of broad-leaved and narrow-leaved weeds by using triafamone plus ethoxysulfuron. Ethoxysulfuron is one of the most important herbicides recommended for monochoria control in India (Choudhury et al. 2016). A mixed application of pretilachlor and pyribenzoxim in direct-seeded rice demonstrated 86% monochoria control (Herath et al. 2007). In a previous study (Bernasor and De-Datta 1986), complete control of monochoria at the 2- to 4-leaf stage was reported when bensulfuron was used. Furthermore, effective control of monochoria with flucetosulfuron (Yu-xiang 2009) and 80% control with pyrazosulfuron (Hazrati et al. 2019) have been reported. Two herbicides commonly used in transplanted rice, pretilachlor and thiobencarb, reduced monochoria biomass by 56% and 75%, respectively, at 6 wk after transplanting. Ampong and De-Datta (1991) and Kuk et al. (2003) reported poor to moderate control of monochoria by using thiobencarb and pretilachlor. Also, Hou (1983) reported that butachlor failed to control monochoria. Butachlor, pretilachlor, and thiobencarb, which are excellent grass killers used in flooded transplanted rice fields in Iran, were ineffective in controlling perennial pondweed, a broadleaf weed in transplanted rice fields (Yaghoubi et al. 2021). However, some reports (e.g., Im et al. 2004) noted effective control of monochoria when butachlor, pretilachlor, and thiobencarb were used. Because butachlor and thiobencarb have been used for nearly four decades in transplanted rice fields in northern Iran, the development of tolerant or resistant biotypes of monochoria to these herbicides may have occurred. The late emergence of monochoria and the reduction of the residual effects of these herbicides are also responsible for reducing their effectiveness. A single application of pendimethalin controlled monochoria by ≥98%, but the prepackaged formulation of pendimethalin plus clomazone produced a reduction in monochoria biomass by only 14%. Clomazone is the most consistent residual herbicide used for barnyardgrass control (Jordan and Kendig 1998), but it is exceptionally weak at controlling nutsedge (Kendig et al. 2003). An application of a broadleaf- and nutsedge-controlling treatment is recommended before clomazone is applied in rice fields (Kendig et al. 2003). Osterholt et al. (2021) reported a synergistic response by barnyardgrass and an antagonistic response by yellow nutsedge (Cyperus esculentus L.) when treated with clomazone plus pendimethalin. Usually, sedges and broadleaved weeds in rice fields have a similar response to herbicides.

Rice height in all of the herbicide-treated plots was higher than that of the nontreated control, ranging from 1% to 26% taller (Table 3). When rice was treated with flucetosulfuron, triafamone plus ethoxysulfuron, pretilachlor plus bensulfuron, and pendimethalin plus bensulfuron (all of which contain a sulfonylurea herbicide) plant height was 20% to 26% higher than that of the nontreated control (Table 3). Compared with the weed-free control, the rice height increase in plots treated with a sulfonylurea herbicide indicates the hormesis effect of the herbicide (Vidotto et al. 2007). When treated with thiobencarb, thiobencarb plus bensulfuron, pendimethalin, and pyrazosulfuron-ethyl plus pretilachlor rice height was 6%, 9%, 12%, and 14% greater, respectively, than that of the nontreated control. Rice plant height in hand-weeded plots was 1% greater than that of the nontreated plot, it was 5% greater with pendimethalin plus clomazone, and 6% greater with pretilachlor.

The number of rice panicles following all herbicide treatments was 9% to 64% greater than that of the nontreated control (247 m⁻²) (Table 3). The least number of panicles were counted in plots that had been treated with pendimethalin plus clomazone, and the most were counted in the plots that had been treated with thiobencarb plus bensulfuron. This was not statistically different than the number that occurred in plots that were treated with pyrazosulfuron-ethyl plus pretilachlor and pretilachlor plus bensulfuron. Rice panicles in trials that had been hand-weeded and treated with pretilachlor were 13% and 20% greater than panicles in the nontreated control. Rice panicle numbers were 40% greater than the nontreated control when triafamone plus ethoxysulfuron and thiobencarb were applied, 39% greater when pendimethalin was used, 35% greater with flucetosulfuron, and 29% greater when pendimethalin plus bensulfuron was applied (Table 3). All of these herbicides except thiobencarb controlled monochoria by ≥98%.

Rice straw yield was greater in plots that received herbicide treatments than yield from the nontreated control (Table 3). Pendimethalin plus clomazone, hand-weeded control, and pretilachlor produced the lowest straw yields at 1%, 2%, and 12% more than the nontreated control. Rice straw in plots that received a single application of pendimethalin and thiobencarb was 27% and 43% greater, respectively, than rice straw of the nontreated control. These herbicides are generally recommended to control barnyardgrass and other grasses (Driver et al. 2020). Tank mixing pendimethalin, thiobencarb, and pretilachlor with bensulfuron controlled monochoria by 100% and produced more straw than when they were applied individually. These treatments resulted in a similar straw yield to that of flucetosulfuron treatments and premixed herbicides that contain sulfonylurea (i.e., triafamone plus ethoxysulfuron and pyrazosulfuron-ethyl plus pretilachlor).

Grain yield was greater in herbicide-treated plots compared with that of the nontreated control, ranging from 47% to 165% (Table 3). The greatest yield was achieved in plots that were treated with pretilachlor plus bensulfuron, but it was not statistically different from yield in plots treated with thiobencarb plus bensulfuron or flucetosulfuron; those herbicides produced 157% and 142% greater yields, respectively, than yield from the nontreated control. Rice treated with pyrazosulfuron-ethyl plus pretilachlor, triafamone plus ethoxysulfuron, pendimethalin plus bensulfuron, pendimethalin, and thiobencarb yielded 134%, 134%, 136%, 97%, and 104% more, respectively, than the nontreated control. Pendimethalin plus clomazone and pretilachlor alone provided the least grain yield, and both were statistically similar to the yield obtained from the hand-weeded treatment. Greater yields in plots treated with PRE herbicides, compared with yield from the hand-weeded control, indicate the importance of early season weed control before resource consumption, compared with the weeding when seedlings are large.

In this study monochoria density was 204 plants m⁻² (Table 3) and the reduction in grain yield from the nontreated control was 32% less compared with yield from the hand-weeded control. Similarly, Park et al. (2011) reported a 35% rice yield loss with 60 plants of monochoria m⁻², and Zhou et al. (2021) reported a 45% reduction in rice gross return. In another study, rice grain yield loss in competition with monochoria was reported to be much greater than 80% (Ampong and De-Datta 1991). It seems that in warmer climates monochoria emerges in the early crop season and causes more rice yield loss.

Most herbicide treatments demonstrated greater performance than manual weeding. Manual weeding occurs when weeds have grown big enough to be removed by hand. As a result, competition between rice and weeds before weeding can also lead to reduced yields (Chauhan 2012). Physical damage to rice roots during manual weeding (Kebede 2000) and the use of limited and shared resources by weeds before weeding seems to be the cause of a low yield in the weed-free, hand-weeded treatment. On the other hand, the greater yield of rice with the use of herbicide treatments in comparison to the manually weeded control indicates the optimal performance of those treatments in weed control and the high selectivity of herbicides on rice. Greater grain yield in soil-applied herbicides than the hand-weeded control indicated that preventing monochoria emergence is better than its removal after emergence via hand-weeding. Janiya and Moody (1989) reported that hand-weeding could not remove monochoria completely and monochoria was dominant in plots that were weeded by hand.

Study 2: Performance of Foliar-Applied Herbicides on Monochoria Control

All herbicides except propanil provided ≥91% reductions in monochoria biomass at 3 WAT and their effectiveness increased to 99% to 100% at 7 WAT compared with the nontreated control (Table 4). Propanil reduced monochoria biomass by 60% at 3 WAT, but its effectiveness was sharply reduced to 24% at 7 WAT.

Table 4. Monochoria control by foliar-applied herbicides. ^{a, b, c}

^a Abbreviation: WAT, weeks after treatment.

^b Monochoria density in the nontreated control (monochoria monoculture) was 902 plant m⁻² at 6 WAT.

^c Monochoria biomass in the nontreated control (monochoria monoculture) at 3 and 7 WAT was 4,373 and 5,695 kg ha⁻¹, respectively.

Flucetosulfuron and triafamone plus ethoxysulfuron provided ≥99% control of monochoria (Table 4). In the first study, these two herbicides were applied when the weed emerged, and both provided 100% control (Table 3). In another study, triafamone plus ethoxysulfuron and flucetosulfuron provided ≥90% control of bulrush, pondweed, and barnyardgrass as soil-applied herbicides in transplanted rice (Yaghoubi et al. 2022). Excellent control of complex weed flora in rice by using flucetosulfuron and triafamone plus ethoxysulfuron applied via various methods, including PRE to soil in flooded fields or foliar-applied POST in nonflooded fields.

Bispyribac-sodium, pyribenzoxim, and penoxsulam plus cyhalofop-butyl provided 100% control of monochoria at 7 WAT (Table 4). Bispyribac-sodium and penoxsulam control barnyardgrass, sedges, and broadleaf weeds in rice, and are effective as soil or foliar treatments (Pourreza et al. 2020). Kobayashi et al. (2007) reported that bispyribac-sodium plus thiobencarb applied at the 3-leaf stage provided complete control of monochoria. Despite being recommended as POST foliar-applied herbicides, residual activities of penoxsulam and bispyribac-sodium have been documented (Pearson et al. 2008). Bispyribac-sodium and penoxsulam were recently registered for use in Iran, and farmers are using them in sequential application with residual herbicides to control escaped weeds. This practice reduces the cost and difficulty of manual weeding.

Other herbicides investigated in this study included 2,4-D, dicamba plus 2,4-D, and bentazon plus MCPA. All provided ≥99% monochoria control. Effective control of monochoria by hormonal herbicides 2,4-D (Lamid 1981), dicamba, and MCPA has been reported (IRRI 1983). Rice cultivars in northern Iran are not tolerant to hormonal herbicides. Piperophos + 2,4-D was registered in Iran to control barnyardgrass and sedges in transplanted rice. Despite its excellent weed control efficacy, this herbicide caused growth disorders, including twisted leaf and stem malformation, whenever the temperature dropped below 12 C. Similarly, Ueji and Inao (2001) reported that 2,4-D demonstrated inadequate efficacy in rice weed control in cold regions in Japan. Ampong and De-Datta (1991) reported that rice is susceptible to 2,4-D at emergence, incipient tillering, booting, and heading. Also, Munir and Abdel Rahman (2012) reported that 2,4-D delayed the heading of barley. According to Radosevich et al. (2007), when plants were treated with hormonal compounds such as 2,4-D, dicamba, triclopyr, and picloram, abnormal tissues and twisted bending plants and epinasty were apparent.

All herbicides except propanil prevented monochoria regrowth (Table 4). Because monochoria propagates through seeds, its regrowth and flowering prevention could reduce its seed bank and result in eradication. Moon et al. (2012) reported that monochoria can produce abundant seeds if left unchecked, and it will build up large populations, leading to a significant reduction in rice yield. In addition, seeds of monochoria can stay viable in the soil for more than a decade (Yamada et al. 2007). Long-term monitoring of monochoria-infested fields is necessary for its successful management.

Effective herbicides in monochoria control in this study contained various mechanisms of action, including synthetic auxins (2,4-D), benzothiadiazide (bentazone), aryloxyphenoxy-propionate (cyhalofop-butyl), dinitroanilines (pendimethalin), and ALS inhibitors (bispyribac sodium, pyribenzoxim, flucetosulfuron, and penoxsulam). Also, herbicides with lesser efficacy in monochoria control belonged to several other chemical groups, including dithiocarbamate (thiobencarb) and chloroacetamide (pretilachlor). Kuk et al. (2003) reported that 2,4-D and bentazon effectively controlled a biotype of monochoria that is resistant to ALS herbicides and one that is susceptible to them; therefore, sequential applications of these herbicides with other common herbicides can be helpful in preventing monochoria and other sedges and broadleaf weeds from developing herbicide resistance.

Monochoria flowering and seed production are completed after the rice has been harvested in northern Iran. Bispyribac-sodium, penoxsulam, penoxsulam plus cyhalofop, bentazon plus MCPA, 2,4-D plus MCPA, glyphosate, and 2,4-D when applied after rice harvest were found to control monochoria by 37%, 72%, 90%, 90%, 93%, 94%, and 98%, respectively, and all of them, except bispyribac-sodium, prevented monochoria flowering and seed production (BY, unpublished data). Bispyribac-sodium is more effective at controlling small weed seedlings than weeds that have already developed (Williams 1999; Yaghoubi 2017). Excellent control of monochoria by various foliar-applied herbicides, in addition to residual herbicides, indicates that the chemical management of monochoria in rice fields is very promising.

Despite monochoria emergence about 3 to 4 wk after rice seedling transplantation, the weed caused a 32% grain yield loss in the nontreated control compared with the hand-weeded treatment. Grain yield increases with some herbicide treatments were up 60% compared with that of the nontreated control, indicating the superiority of the herbicides to control monochoria over hand-weeding. Rice yield loss as a result of interference with monochoria in a warmer climate in India and in a cooler one in Korea was reported to be 80% and 35%, respectively (Park et al. 2011), which demonstrates the effect of climate on the interaction between weeds and crops. Therefore, preventing this annual weed from completing its life cycle via early plowing after rice harvest is the most economical and environmentally friendly method for its sustainable management. Also, spraying weed-infested fields after rice harvest using sequential applications of herbicides with different mechanisms of action is recommended. Due to the late emergence and rapid spread of monochoria, many farmers who encounter this weed for the first time in their fields quickly understand that soil-applied herbicides do not effectively control it. Instead, foliar-applied herbicides with soil residuals such as triafamone plus ethoxysulfuron, penoxsulam, or bispyribac are the best options for monochoria control. Because many PRE and POST herbicides effectively controlled monochoria (Tables 3 and 4), the weed’s fast spread in Iranian transplanted rice indicates the imbalanced herbicide consumption in the country. For three decades, more than 80% of Iranian rice fields have been treated continuously with butachlor, pretilachlor, and thiobencarb only, and none of them controlled monochoria effectively, despite being excellent barnyardgrass killers.

Practical Implications

Monochoria demonstrates strong adaptability to transplanted rice ecosystems. It evades residual PRE herbicides by emerging late and thriving under the rice canopy. Additionally, it exhibits tolerance to flooding and resists control by hand-weeding, while its germination process is gradual and protracted. Consequently, implementing an integrated management approach becomes imperative to effectively combat this weed.

If monochoria manages to escape early-season soil-applied residual herbicides due to its late emergence, a sequential application of a different soil-applied herbicide or the use of foliar-applied herbicides becomes unavoidable for successful control. It is crucial that the second herbicide employs a distinct mechanism of action than the initial herbicide.

In regions with a climatic profile akin to northern Iran, characterized by moderate or cold weather, monochoria typically flowers and produces seeds after the rice harvest. In such scenarios, the most effective strategy for sustainable monochoria management entails plowing, mowing, or employing herbicides to control the weed after the rice harvest but before monochoria flowers and produces seed. For long-term management, adopting a crop rotation practice or transitioning from permanently flooded irrigated plots to dry-seeded, nonflooded cultivation methods is highly recommended.