Rice in Mississippi is often in early seedling growth stages when paraquat-based herbicide treatments are commonly applied to corn, cotton, and soybean; therefore, off-target movement of the herbicide onto adjacent rice fields may occur. After an off-target movement event has occurred, weed management in the rice crop is still necessary. Field studies were conducted from 2019 to 2021 in Stoneville, MS, to evaluate rice injury and barnyardgrass control with labeled herbicides after exposure to a sublethal concentration of paraquat. Herbicide treatments were label-recommended rates of imazethapyr, quinclorac, propanil, bispyribac-sodium, cyhalopfop, and florpyrauxifen-benzyl applied following rice exposure to a sublethal concentration of paraquat. Rice injury was detected 7 and 28 d after treatment (DAT) and was ≥35% and ≥14%, respectively, for all herbicides. Florpyrauxifen-benzyl and imazethapyr caused the greatest rice injury at 28 DAT. Following paraquat exposure, barnyardgrass control was similar for all labeled herbicide treatments at 7, 14, and 28 DAT except for florpyrauxifen-benzyl and no herbicide (paraquat alone) at 7 DAT. Across all evaluations, barnyardgrass control was at least 12% greater following paraquat exposure and labeled herbicide treatments than with no paraquat exposure. The current research demonstrates that labeled rates of herbicides applied following exposure to a sublethal concentration of paraquat resulted in <36% injury and provided as great as 95% control of barnyardgrass, depending on the herbicide treatment. Therefore, the labeled herbicides choice following rice exposure to a sublethal concentration of paraquat should be based on weed spectrum.

Florpyrauxifen-benzyl was commercialized in 2018 to target barnyardgrass and aquatic or broadleaf weeds. Field studies were conducted from 2019 to 2021 in Stoneville, MS, to evaluate barnyardgrass control following a simulated failure of florpyrauxifen-benzyl or other common postemergence rice herbicides. In the first field study, florpyrauxifen-benzyl was applied at 0 and 15 g ai ha–1 to rice at the two- to three-leaf stage to simulate a failed application targeting barnyardgrass. Sequential herbicide treatments included no herbicide and full rates of imazethapyr, quinclorac, bispyribac-Na, and cyhalofop applied 7 or 14 d after florpyrauxifen-benzyl treatment. The second field study was designed to evaluate barnyardgrass control with florpyrauxifen-benzyl following simulated failure of postemergence rice herbicides. Initial herbicide treatments included no herbicide and half rates of imazethapyr, quinclorac, bispyribac-Na, and propanil. Sequential applications at 7 or 14 d after the initial herbicide treatments included florpyrauxifen-benzyl at 0 and 30 g ai ha–1. Results from the first study indicated barnyardgrass control 21 d after final treatment (DAFT) was greater with sequential treatments at 7 compared with 14 d after initial treatment (DA-I) with no initial application of florpyrauxifen-benzyl. Therefore, delaying sequential treatments until 14 d after initial florpyrauxifen-benzyl at 15 g ha–1 allowed barnyardgrass to become too large to control with other rice herbicides. Rough rice yield was reduced in plots where quinclorac application was delayed from 7 to 14 DA-I with no initial application of florpyrauxifen-benzyl. The second study suggested that florpyrauxifen-benzyl application should be delayed 14 d after a herbicide failure. Although no differences in barnyardgrass control 21 DAFT were detected whether florpyrauxifen-benzyl was applied 7 or 14 DA-I of any herbicide utilized, >85% control was only achieved when florpyrauxifen-benzyl application was delayed 14 DA-I. These results demonstrate barnyardgrass control options following simulated failed applications of common rice herbicides.

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.

A study was conducted at the Louisiana State University Agricultural Center’s H. Rouse Caffey Rice Research Station in 2017 and 2018 to evaluate the interaction between a prepackage mixture of clomazone plus pendimethalin applied at 0, 760, 1,145, or 1,540 g ai ha−1 mixed with propanil at 0, 1,120, 2,240, or 4,485 g ai ha−1. A synergistic response occurred when barnyardgrass was treated with all rates of clomazone plus pendimethalin mixed with either rate of propanil evaluated at 56 d after treatment (DAT). Unlike barnyardgrass, an antagonistic response occurred in yellow nutsedge used as a control when treated with 760 and 1,540 g ha−1 of clomazone plus pendimethalin mixed with 1,120 or 22,40 g ha−1 of propanil at 28 DAT; however, 1,145 g ha−1 of clomazone plus pendimethalin mixed with 4,485 g ha−1 of propanil resulted in a neutral interaction. At 28 DAT, rice flatsedge treated with all herbicide mixtures resulted in neutral interactions. The synergism of clomazone plus pendimethalin applied at 1,540 g ha−1 mixed with propanil applied at 2,240 or 4,485 g ha−1 to control barnyardgrass resulted in an increased rough rice yield compared with 760 or 1,145 g ha−1 of clomazone plus pendimethalin mixed with propanil applied at 1,120 or 2,240 g ha−1. These results indicate that if barnyardgrass and rice flatsedge are present in a rice field the prepackage mixture of clomazone plus pendimethalin mixed with propanil can be an option for growers. However, if yellow nutsedge infest the area other herbicides may be needed.

A paper-based survey was conducted from 2015 to 2017 among stakeholders of the Texas rice industry on current weed management challenges and factors influencing management decisions. A total of 108 survey questionnaires were completed by stakeholders at the rice Cooperative Extension meetings conducted in the rice-growing counties of Texas. In addition, late-season field surveys were conducted prior to harvest in 2015 and 2016 across the rice-growing counties to understand dominant weed escapes occurring in rice fields. Results from the questionnaire survey revealed that rice–fallow–rice was the most common rotation practiced in Texas rice production. Echinochloa spp., Leptochloa spp., and Cyperus spp. were the top three problematic weed issues faced by the respondents. Among the Leptochloa species, Nealley’s sprangletop, a relatively new species in rice fields, was indicated as an emerging concern. Clomazone was the most frequently used PRE herbicide, whereas quinclorac, propanil, imazethapyr, and cyhalofop were the popular POST herbicides. Most respondents (72%) made weed-control decisions on the basis of economic thresholds, whereas 63% made decisions on the basis of weed problems from previous years. Most respondents (88%) expressed moderate to high concern for herbicide-resistant weeds in their operations. Strategies to manage herbicide-resistant weeds and economical weed management practices were among the top suggested research needs. The field survey revealed that jungle rice, Nealley’s sprangletop, and hemp sesbania were the top three late-season weed escapes in rice production in Texas, with frequencies of occurrence of 28%, 19%, and 13%, respectively. Furthermore, average field area infested by a species was the greatest for jungle rice (13%), followed by hemp sesbania (11%) and weedy rice (11%). Findings from the stakeholder and field surveys help direct future research and outreach efforts for sustainable weed management in Texas rice.

A field study was conducted in 2015 and 2016 at the H. Rouse Caffey Rice Research Station near Crowley, Louisiana, to evaluate the interactions of quizalofop and a mixture of propanil plus thiobencarb applied sequentially or mixed to control weedy rice and barnyardgrass. Visual weed control evaluations occurred at 14, 28, and 42 d after treatment (DAT). Quizalofop was applied at 120 g ai ha−1 at 7, 3, and 1 d before and after propanil plus thiobencarb were each applied at 3,360 g ai ha−1. In addition, quizalofop was applied alone and in a mixture with propanil plus thiobencarb at day 0. Control of red rice ‘CL-111’ and ‘CLXL-745’ was greater than 91% when quizalofop was applied alone at day 0, similar to control for quizalofop applied 7, 3, and 1 d prior to propanil plus thiobencarb at all evaluation dates. Control of the same weeds treated with quizalofop plus propanil plus thiobencarb applied in a mixture at day 0 was 70% to 76% at each evaluation date, similar to quizalofop applied 1 or 3 d after propanil plus thiobencarb. A similar trend in control of barnyardgrass by 88% to 97% occurred when quizalofop was applied alone and by 48% to 53% at 14, 28, and 42 DAT when the mixture was used. ‘PVL01’ rough rice yield was 4,060 kg ha−1 when treated with quizalofop alone; however, yield was reduced to 3,180 kg ha−1 when it was treated with quizalofop mixed with propanil plus thiobencarb at day 0, similar to PVL01 rice treated with quizalofop 1 or 3 d following the propanil plus thiobencarb application.

A field study was conducted in 2015 and 2016 near Crowley, LA, to evaluate antagonistic, synergistic, or neutral interactions of quizalofop when mixed with contact herbicides labeled for use in rice production. Quizalofop was applied at 120 g ai ha−1. Mixture herbicides included bentazon at 1,050 g ai ha−1, carfentrazone at 18 g ai ha−1, propanil at 3,360 g ai ha−1, saflufenacil at 25 g ai ha−1, and thiobencarb at 3,360 g ai ha−1. A second application of quizalofop at 120 g ha−1 was made at 28 d after the initial application (DAIT) to evaluate control of weeds escaping the initial treatment. At 14 and 28 DAIT, red rice, ‘CLXL-745’, and ‘CL-111’ treated with quizalofop plus propanil indicated an antagonistic response with an observed control of 69% to 71% compared with an expected control of 92% to 94%. Barnyardgrass treated with the same mixture also indicated an antagonistic response at 14 and 28 DAIT with an observed control of 16% compared with an expected control of 94%. Barnyardgrass treated with quizalofop plus saflufenacil indicated an antagonistic response at 14 DAIT; however, the same mixture produced a neutral response by 28 DAIT. In addition, a second application of quizalofop was not able to overcome the antagonism observed with a quizalofop plus propanil mixture at 14 and 28 DAIT for red rice, CLXL-745, CL-111, or barnyardgrass control. Quizalofop mixed with carfentrazone or thiobencarb produced a neutral response for all weeds evaluated at each evaluation date.

Herbicide resistance to several of the most common weed species in US rice production, such as barnyardgrass and red rice, has made weed control extremely difficult with available herbicide options. No very-long-chain fatty acid–inhibiting herbicides are labeled for use in US rice; however, pethoxamid is one such herbicide under development for soil-applied use to control grasses and small-seeded broadleaves in rice and various row crops. Field trials were conducted in 2015 and 2016 near Stuttgart, AR, for rice tolerance and in 2016 near Colt, AR, and Lonoke, AR, for weed control with the use of pethoxamid-containing rice herbicide programs. Pethoxamid was applied alone and in a program at 420 and 560 g ai ha–1 with other herbicides labeled in rice including clomazone, quinclorac, propanil, imazethapyr, and carfentrazone POST. Injury less than 10% was seen for all treatments 2 wk after application in 2015 and 2016, except for pethoxamid at 420 g ha–1 to clomazone to one-leaf rice. Rice injury dissipated to less than 5% following all treatments by 4 wk after flood establishment. Barnyardgrass was controlled 95% or more near Colt and 93% or more near Lonoke for herbicide programs including clomazone PRE followed by pethoxamid plus quinclorac or imazethapyr at three- to four-leaf stage rice. Considering the minimal injury and high levels of barnyardgrass control associated with pethoxamid-containing weed control programs, pethoxamid provides a unique and effective site of action for use in US rice production.

A field study was established to study symptoms, growth and yield of 2-year-old walnut trees exposure to simulated drift of several herbicides commonly used in rice production. Bispyribac-sodium, bensulfuron-methyl, and propanil were applied at four rates representing 0.5%, 1%, 3% and 10% of the normal use rate in rice (45, 70, and 6725 g ai ha−1, for the three herbicides respectively). Symptoms started to appear approximately 7 days after application (DAT) and peaked 28 DAT. At that time, bispyribac-sodium caused greater injury at low drift rates (6% and 15% visual injury for 0.5% and 1% rate, respectively) compared to bensulfuron-methyl and propanil. Bispyribac-sodium also appeared to slow walnut shoot elongation compared to nontreated trees; however, no yield reductions were observed either in the year after drift exposure. The effect of bispyribac-sodium simulated drift on the yield and nut quality in the year of drift exposure was evaluated in a separate study on 3-year-old walnut trees. While no yield or nut quality reductions were observed, a linear correlation between rate of bispyribac-sodium and color, an important quality factor, was found: higher herbicide rates tended to be associated with darker kernel color. Bispyribac-sodium may damage nearby walnut orchards if drifted at significant amounts. However, it is unlikely that in a field situation bispyribac-sodium would drift at high enough levels to cause the symptoms observed from the 10% use rates in this study.

Florpyrauxifen-benzyl is a new herbicide being developed for rice. Research is needed to understand its spectrum of control and optimal tank-mix partners. Multiple greenhouse and field experiments were conducted to evaluate florpyrauxifen-benzyl efficacy and tank-mix compatibility. In greenhouse experiments, florpyrauxifen-benzyl at 30 g ai ha–1 provided ≥75% control of all weed species evaluated (broadleaf signalgrass, barnyardgrass, Amazon sprangletop, large crabgrass, northern jointvetch, hemp sesbania, pitted morningglory, Palmer amaranth, yellow nutsedge, rice flatsedge, smallflower umbrellasedge), and control was similar to or better than other herbicide options currently available in rice. Barnyardgrass was controlled 97% with florpyrauxifen-benzyl at 30 g ha–1, ultimately reducing height (86%) and aboveground biomass (84%). In these field studies at 30 g ha–1, no antagonism was observed when florpyrauxifen-benzyl was tank-mixed with contact (acifluorfen, bentazon, carfentrazone, propanil, and saflufenacil) or systemic (2,4-D, bispyribac, cyhalofop, fenoxaprop, halosulfuron, imazethapyr, penoxsulam, quinclorac, and triclopyr) rice herbicides. Although not every tank-mix or weed species was evaluated, the lack of antagonistic interactions herein highlights the flexibility and versatility of this new herbicide. Once florpyrauxifen-benzyl becomes commercially available, it will be beneficial to tank-mix this new herbicide with others without sacrificing efficacy, so as to apply multiple sites of action together and thus lessen the risk for evolution of herbicide resistance.

Previous research has shown that some insecticide seed treatments provide safening effects in rice following exposure to low rates of the herbicides glyphosate and imazethapyr. However, no research has been conducted to determine whether a similar effect may be seen in soybean or grain sorghum, two important rotational crops across the Midsouth. To evaluate the potential safening effects of insecticide seed treatments in these two crops, field trials were conducted in Marianna, AR, in 2015 and 2016, and near Colt, AR, in 2016. In soybean, glyphosate, glufosinate, 2,4-D, dicamba, halosulfuron, mesotrione, tembotrione, and propanil were applied at low rates to simulate drift events, in combination with the insecticide seed treatments thiamethoxam and clothianidin at labeled rates. In grain sorghum, glyphosate, imazethapyr, and quizalofop were applied at low rates in combination with the insecticide seed treatments thiamethoxam, clothianidin, and imidacloprid at labeled rates. Injury reduction was seen at 1 site-year for glyphosate, glufosinate, 2,4-D, dicamba, mesotrione, and tembotrione, and at 2 of 3 site-years for halosulfuron. At 1 site-year, the safening in halosulfuron resulted in increases in both crop height and yield. In grain sorghum, reducing injury via seed treatments was generally more successful. All three herbicides applied in sorghum displayed instances of injury reduction when seed treatments were used at 1 or more site-years, including reducing injury upward of 40% in the case of quizalofop+clothianidin at Marianna in 2016. For 2 site-years, injury reduction through the use of insecticides resulted in increases in crop height and grain yield in grain sorghum compared with no insecticide use. Although the degree of safening seen varied depending on site-year in both crops, growers who use insecticide seed treatments on an annual basis may expect to see a safening effect from drift events of most herbicides evaluated in both soybean and grain sorghum.

A study was conducted in three locations in Louisiana to evaluate interactions of imazethapyr at 0 and 70 g ai ha−1 mixed with propanil at 0, 1,120, 2,240, 3,360, and 4,480 g ha−1 for the control of red rice, barnyardgrass, and hemp sesbania. According to Blouin’s modified Colby’s, a synergistic response occurred for red rice treated with imazethapyr mixed with propanil at 4,480 g ha−1 for all evaluations. Observed control was 93% to 95% compared with expected control of 81% to 87%. An antagonistic response occurred for barnyardgrass control with imazethapyr mixed with propanil at 1,120 g ha−1 at 35 and 49 d after treatment (DAT), with control of 75% and 64%, respectively, compared with expected control of 89% and 78%. However, a neutral response occurred for barnyardgrass control when treated with all other imazethapyr plus propanil combinations. An antagonistic interaction occurred for hemp sesbania when treated with imazethapyr plus propanil at 3,360 and 4,480 g ha−1 at 21 DAT with an observed control of 89% compared with an expected control of 96%; however, a neutral response occurred at all other evaluation dates. An increase in rice yield was observed with an imazethapyr plus propanil at 4,480 g ha−1 mixture compared with a single application of imazethapyr or propanil at any rate evaluated.

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

A study was conducted at three locations in Louisiana to evaluate the interactions of imazamox at 44 g ai ha−1 mixed with propanil, thiobencarb, or with a prepackaged mixture of propanil plus thiobencarb. A synergistic response was observed for red rice control for all treatments at 14 days after treatments (DAT); however, at 21 DAT a neutral response was observed for imazamox mixed with propanil or thiobencarb. A synergistic response occurred for red rice control when imazamox was mixed with propanil plus thiobencarb at 3360 g ai ha−1 for all evaluation dates; however, imazamox mixed with propanil plus thiobencarb at 1680 g ha−1 resulted in synergism at the same evaluations except at 35 DAT with neutral response. Synergism was observed for barnyardgrass control with imazamox mixed with propanil plus thiobencarb at 3360 g ha−1 at 35 and 49 DAT. The only antagonism observed for barnyardgrass control was at 42 DAT with imazamox plus propanil at 840 and 1680 g ha−1 or mixed with the prepackage mixture at 1680 g ha−1. Rice treated with imazamox mixed with propanil plus thiobencarb at 3360 g ha−1 yielded 5770 kg ha−1.

The response of giant burreed to bentazon, propanil, 2,4-D (amine salt), and 2,4-D plus crop oil (0.5% v/v) was evaluated at the 2-aerial-leaf stage of wild rice. Bentazon, 2,4-D, and 2,4-D plus crop oil at 1.1 kg/ha or more in 1984, and propanil and 2,4-D plus crop oil at 4.5 kg/ha in 1985 reduced giant burreed dry weight. Generally, herbicide rates above 1.1 kg/ha injured wild rice and reduced yields compared to weed-free controls. None of the study treatments resulted in effective giant burreed control without unacceptable injury to wild rice.

Preplant control of mannagrass was greater than 90% with clethodim, glyphosate, sethoxydim, and sulfosate. Glyphosate at 0.5 to 1.0 kg ai/ha controlled mannagrass greater than 97%, and was not increased with any adjuvant. Fenoxaprop applied POST controlled mannagrass 97% but injured rice 33%. Except for fenoxaprop-ethyl, all POST treatments increased rice yields at least 1560 kg/ha more than the non-treated check. Southern watergrass was controlled 79 and 89% with 1.0 kg ai/ha glyphosate and 4.5 kg ai/ha sulfosate, respectively, while control was less than 55% with other herbicides.

Four-year cropping sequences of continuous cotton, cotton-soybeans-soybeans-cotton, continuous soybeans, soybeans-grain sorghum-soybeans-grain sorghum, and cotton-rice-cotton-rice were treated each year with high and low levels of herbicides to control johnsongrass. High levels of herbicide were necessary to control johnsongrass in continuous cotton, cotton-soybeans-soybeans-cotton, continuous soybeans, and the soybeans-grain sorghum rotation. Johnsongrass was not eradicated, however, after 4 years of cropping sequences with high herbicide inputs. Both low and high levels of herbicide coupled with water management required for rice production controlled johnsongrass and prevented rice yield reductions.

Field studies were conducted in Saskatchewan to evaluate the effect of herbicides on annual canarygrass (Phalaris canariensis L.) and associated weeds. Bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) at 0.35 kg ai/ha, tank mixes of bromoxynil at 0.28 kg/ha plus the ester of MCPA [(4-chloro-2-methylphenoxy)acetic acid] at 0.28 kg ae/ha, linuron [N′-(3,4-dichlorophenyl)-N-methoxy-N-methylurea] at 0.28 kg ai/ha plus MCPA amine at 0.56 kg/ha, or propanil [N-(3,4-dichlorophenyI)propanamide] at 1.0 kg ai/ha plus MCPA ester at 0.28 kg/ha resulted in annual canarygrass seed and dry matter yields equal to the unsprayed check and excellent wild mustard (Sinapis arvensis L. # SINAR) and cow cockle (Vaccaria pyramidata Medik. # VAAPY) control. Metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] at 0.21 kg ai/ha plus MCPA amine at 0.56 kg/ha reduced the crop stand in 1 out of 5 yr and seed yield every year. Postemergence application of difenzoquat (1,2-dimethyl-3,5-dipheny1-1H-pyrazolium) at 0.84 kg ai/ha or flamprop [N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL-alanine] at 0.53 kg ai/ha, or preplant-incorporated triallate [S-(2,3,3-trichloro-2-propenyl)bis(1-methylethyl)carbamothioate] at 1.40 kg ai/ha optimized wild oat (Avena fatua L. # AVEFA) control with seed yields. Diclofop {(±)-2-[4-(2,4-dichlorophenoxy)phenoxy)propanoic acid} at 0.70 kg ai/ha severely damaged annual canarygrass in all years. Triallate applied at 1.40 kg/ha preplant or preemergence incorporated marginally reduced the crop stand and seed yield compared to the untreated controls in tolerance tests.

Repeated use of propanil to control barnyardgrass (BYG) and other weeds in rice has led to the development of propanil-resistant barnyardgrass (R-BYG). R-BYG possesses elevated aryl acylamidase activity levels, which cause rapid metabolism of propanil analogous to propanil degradation in rice. The current screening method for determining propanil resistance in BYG requires about 10 mo. The present study examined the use of chlorophyll fluorescence as a more rapid method to identify propanil resistance in BYG soon after it is suspected. Chlorophyll fluorescence data from excised BYG leaf tissue (R-BYG and susceptible-BYG [S-BYG]; 13- to 41-d-old) exposed to 100 μM propanil for 2 h indicated a 95 to 100% inhibition of electron transport (photosynthesis inhibition) in both R- and S-BYG. However, when incubated in water in the dark for 22 h after the initial 2-h treatment, metabolism in R-BYG was sufficient to reduce levels of absorbed propanil and facilitate recovery. Lack of metabolism of propanil prevented recovery in S-BYG, thus allowing the two biotypes to be distinguished easily by the chlorophyll fluorescence assay. Further studies using this 2-h exposure to 100 μM propanil followed by a 22-h recovery period evaluated several assay parameters. A longer recovery time and the effects of various propanil concentrations were also evaluated. A herbicide dose-response curve showed the greatest difference in photosynthesis inhibition for both biotypes at about 100 μM propanil, but both biotypes were inhibited > 95% when treated with 400 μM propanil. Inhibition of photosynthesis in both biotypes was greatest when the recovery incubation temperature was 35 C compared to 20, 25, and 30 C. Fluorescence data from harvested tissue stored moist in plastic bags at 23 C (to simulate shipment) showed that biotypes could be differentiated even when received as late as 4 d after harvest. Thus, samples can be harvested from the field soon after propanil failure and resistance or susceptibility to propanil determined after only a few days. This technique can greatly reduce the time, space, and labor currently required to determine propanil resistance in BYG.

Resistance to propanil by a barnyardgrass (BYG) biotype has been reported, and its occurrence is becoming widespread in U.S. rice-producing regions. Interactions between propanil and the herbicides anilofos, molinate, pendimethalin, piperophos, quinclorac, and thiobencarb and the insecticide carbaryl were evaluated in laboratory studies for improved control of propanil-resistant barnyardgrass (R-BYG). Chlorophyll fluorescence measurements of BYG leaf segments were used to ascertain electron transport inhibition and to assess the synergy/antagonism of propanil (100 μM) with the various additives (50 μM). Synergistic effects on photosynthetic inhibition were found with anilofos, carbaryl, pendimethalin, and piperophos in combination with propanil. Such synergistic actions of chemicals with propanil may lead to chemical combinations useful for R-BYG control.