A study was established to evaluate interactions between imazamox at 0 and 44 g ai ha−1 mixed with propanil at 0, 1,120, 2,240, 3,360, and 4,480 g ai ha−1 for the control of red rice and barnyardgrass. Blouin's Modified Colby's procedure was used to test for interactions. At 7 d after treatment (DAT), a synergistic response occurred for red rice treated with imazamox at 44 g ha−1 mixed with propanil at 3,360 and 4,480 g ha−1 by increasing expected control of 62% to an observed control of 67 and 75%, respectively, and the synergistic response continued across all evaluations through 49 DAT. No antagonism occurred for any imazamox plus propanil mixture for red rice control. An antagonistic response was shown for barnyardgrass control with imazamox at 44 g ha−1 mixed with any rate of propanil, at 7 DAT. However, imazamox plus propanil at 4,480 g ha−1 resulted in a neutral response at 14 through 49 DAT. Rice treated with imazamox plus propanil at 4,480 g ha−1 plus imazamox resulted in a yield of 6,640 kg ha−1. The synergistic response observed for red rice control with a mixture of imazamox plus propanil can benefit producers by increasing control of red rice, and this mixture contains two different modes of action that can be part of an overall resistance management strategy.

Rice is one of the staple food crops of India, and Karnataka is one of the major rice-producing states. The primary method of rice establishment in Karnataka is transplanting, but farmers are opting to shift to direct-seeding of rice. Weed management is critical for realizing optimal yield of direct-seeded rice (DSR). The objective of this review was to synthesize the published literature on weeds and weed management in rice in Karnataka, identify improved weed-management technologies for delivery to farmers, and suggest research needs. Some 98 weed species are reported to be associated with rice in Karnataka. Weed control to date in Karnataka has mostly been based on herbicides. Hand-weeding was found to be effective in all methods of rice establishment. However, it is time-consuming, tedious, and costly because labor is becoming scarce and unavailable, and labor wages are higher. Several PRE and POST herbicides that were effective in other Asian countries were also found to be effective in managing weeds in rice established by different methods in Karnataka. Bensulfuron plus pretilachlor and pyrazosulfuron in aerobic rice and pendimethalin, thiobencarb, bispyribac-sodium, cyhalofop, fenoxaprop plus chlorimuron plus metsulfuron, and fenoxaprop plus ethoxysulfuron in dry-DSR were found effective in managing weeds. In wet-DSR, butachlor plus safener and pretilachlor plus safener were effective. Thiobencarb, pendimethalin, pretilachlor, azimsulfuron plus metsulfuron, bispyribac-sodium, butachlor, cinosulfuron, oxadiazon, and quinclorac were found promising for weed management in transplanted rice. Integration of herbicides with hand-weeding or intercultivation was found to be effective in rice established by different methods. Options that were found economical in managing weeds varied across the different rice-establishment methods. The need for developing location-specific, sustainable, integrated weed management and extension of available technologies for the farming community in Karnataka is emphasized.

Evaluation of turfgrass performance at low nitrogen fertility levels is important because many home lawns are fertilized below common recommendations. The objective of this study was to evaluate visible quality and weed susceptibility of common and alternative cool season grasses under multiple management regimes in Wisconsin. A split-split plot completely randomized block design was used to evaluate ‘Kingfisher' Kentucky bluegrass (Kentucky bluegrass), ‘Kenblue' Kentucky bluegrass, ‘Victory II' chewings fescue, ‘Grande II' tall fescue, and ‘Jiffe II' perennial ryegrass. Each species was mowed at 3.5, 6.0, or 8.5 cm, and fertilized with 0, 98, or 196 kg ha−1 yr−1 of nitrogen. Visible quality and weed cover were evaluated four times annually for 3 yr. Tall fescue had the greatest turf quality across all treatments. Kingfisher Kentucky bluegrass, an improved variety, responded most dramatically to nitrogen fertilization, with quality rating improved from 5.1 to 7.1 when annual nitrogen applications totaled 196 kg ha−1 compared to the nonfertilized control. Kenblue Kentucky bluegrass, a common variety, had the greatest weed cover at all mowing heights and fertilizer rates. Assessment of common dandelion flowers by digital image analysis revealed that improved and common Kentucky bluegrass had greater common dandelion cover than fine or tall fescue when herbicides were withheld for 2.5 yr. Background soil fertility was found to have a significant impact on visible quality and weed cover. In an area with eroded, low-fertility soil, improved Kentucky bluegrass required 196 kg N ha−1 yr−1 to maintain high quality and limit weed invasion. These results suggest that tall fescue is best suited to low and high input conditions, while improved varieties of Kentucky bluegrass performed acceptably only under high inputs.

Several graminicides were evaluated at various application timings for control and seedhead suppression of red rice (Oryza sativa) in soybean (Glycine max). One application of clethodim, fluazifop-P, quizalofop-P, or sethoxydim at any timing did not control red rice more than 86% 2 wk after treatment. Emergence of red rice seedlings subsequent to applications reduced control later in the season. At a naturally infested location, seedhead reduction was greatest, regardless of graminicide, when application was delayed until the four-leaf stage. At a second location, seedhead reductions were highest following graminicide applications at the two-leaf stage of growth. No single graminicide application completely eliminated red rice seedhead production. Soybean yields were reduced when graminicide application was delayed until the boot stage at both locations, and following two-leaf stage applications at the naturally infested location.

Greenhouse experiments were conducted in Greece to evaluate the level of propanil resistance in three barnyardgrass biotypes collected from rice fields where propanil had been used for around 25 yr. Suspected resistant biotypes were not controlled by propanil at 2.6, 5.2, 10.4, and 20.8 kg ai/ha, with the highest rate five times greater than the recommended rate. Control was reduced even more when propanil application was delayed until the three- to five-leaf stage. The susceptible barnyardgrass biotype collected from a nontreated propanil area was controlled with 2.6 kg/ha propanil. All barnyardgrass biotypes were effectively controlled by quinclorac (0.75 kg ai/ha) applied either alone at the two- to three-leaf stage or in mixture with propanil (3.5 kg/ha) at the three- to five-leaf stage. Only two of the resistant barnyardgrass biotypes were effectively controlled by azimsulfuron (0.02 kg ai/ha) applied alone at the two- to three-leaf stage, but all biotypes were controlled with the addition of propanil (3.5 kg/ha) at the three- to five-leaf stage. The resistant barnyardgrass biotype, which required the highest rate of propanil to reduce biomass by 50%, also tended to exhibit the most growth potential 7 to 22 d after emergence when compared with the other resistant biotypes or the susceptible biotype. Results clearly show that varying levels of barnyardgrass resistance to propanil exist but that effective control alternatives are available.

Field experiments were conducted to determine the effect of different rates and application times of MON 37500 on downy brome control in winter wheat and on rotational crop injury. In southeastern Washington, MON 37500 applied postemergence at 36 to 72 g ai/ha in the fall and spring controlled downy brome 90 to 97% and 45 to 71%, respectively. MON 37500 was applied postemergence to winter wheat at 18, 36, and 72 g/ha during fall 1995 and spring 1996 near Moscow, ID; Pendleton, OR; and Endicott, WA, to determine its effect on rotational crops planted during spring 1997. At Pendleton, OR, fall applications of MON 37500 at 18, 36, and 72 g/ha applied to actively growing wheat reduced seed yield of spring barley and pea 13 to 59% and 26 to 29%, respectively, when these crops followed winter wheat in the rotation. At this site, spring applications of 36 and 72 g/ha reduced seed yield of spring barley 17 to 34%; when applied in the fall or spring, MON 37500 reduced canola yield 31 to 73%. MON 37500 at 18 g/ha in the spring reduced pea biomass 32% at Endicott. Aboveground biomass and seed yield of barley, pea, lentil, and mustard at Moscow were not affected by MON 37500 carryover. However, using a grain sorghum bioassay, predicted final concentrations of MON 37500 in soil at all three locations 360 d after treatment (DAT) was similar, ranging from 0.5 to 2 ng ai/g. In growth chamber herbicide dose–response studies, grain sorghum shoot height was reduced more at lower doses (0.025 to 5 ng/g) of MON 37500 in Pendleton soil than in soils from the other field locations. Perhaps the greater rotational crop injury observed at the Pendleton location was partly related to greater soil activity of the herbicide.

Tenuazonic acid (TeA), a naturally occurring product of Alternaria alternata, a pathogen to croftonweed, was discovered to be a novel natural photosystem II (PSII) inhibitor. However, herbicidal activity of AAC-toxin, a metabolite of this fungus containing TeA as the main active ingredient, has not been evaluated systematically. In this study, we conducted activity-evaluation experiments in the laboratory, greenhouse, and field trials to assess the herbicidal potential of this fungal metabolite. AAC-toxin had high herbicidal activity on all species tested: croftonweed, large crabgrass, barnyardgrass, redroot pigweed, and eclipta. The AAC-toxin caused brown, leaf spot symptoms and leaf necrosis, subsequently killing the seedlings. When AAC-toxin was applied POST at 83 ml ai/ha, more than 95% of large crabgrass, barnyardgrass, and redroot pigweed plants were controlled 2 d after treatment in field trials. It can be concluded that AAC-toxin has broad-spectrum, rapid, and high herbicidal activity similar to that of paraquat and may have the potential to be developed as a microbe-based herbicide.

Crabgrasses (Digitaria spp.), the first cultivated grains, were grown for food thousands of years before they were considered weeds. Digitaria, a genus of about 60 species, grows in the world's temperate and tropical regions. Many of the species are good forage grasses. Thirteen weedy Digitaria species infest the United States, but large crabgrass (D. sanguinalis # DIGSA) and smooth crabgrass (D. ischaemum # DIGIS) are the most common.

Members from all four Weed Science Society of America (WSSA) regions in the United States ranked the research need to “develop new methods for controlling the movement of herbicides and their metabolites into ground water, surface water, and air” first of six major weed science research needs. Canadian members ranked the need to “devise more efficient and less costly weed control technology for conservation-tillage crop-production systems” first; but they also gave high ratings to “improve base knowledge of weed science, improve applicator training, and transfer of information to Extension Service personnel, farm producers, and administrators” and to “discover new ecological, biological, and non-chemical methods of weed control.” The needs to “develop improved methods of increasing the tolerance of crops to herbicides” and to “develop new technology for control of perennial weeds of crops and rangeland” were ranked low. The WSSA Research Committee, at the request of WSSA Presidents J. D. Riggleman (1985) and O. C. Burnside (1986), asked 977 members to rank weed science research needs. The members (ca 50% of the active membership in North America) were selected at random from every other state, federal, industry, and “other” member of each state or province from the up-to-date list of the WSSA business office. Within the highest ranked priority research need, the 422 U.S. and Canadian respondents consistently ranked the research areas (a) to “develop new application techniques that minimize or eliminate herbicides and their residues in air and water”, and (b) to “conduct research to regulate movement of herbicides through the soil profile to avoid contamination of ground water” high, regardless of the type of employment. They emphasized increasing research on the morningglory (Ipomoea spp. # IPOXX) complex, yellow nutsedge (Cyperus esculentus L. # CYPES), quackgrass [Agropyron repens (L.) Beauv. # AGREE], and several other weeds. More members, regardless of region or type of employment, ranked conservation tillage the most important crop or situation that needed new and improved weed control technology.

A field experiment was conducted in 1986 and 1987 to determine the minimum effective rate (MER)3 of imazaquin and chlorimuron applied postemergence to common cocklebur. MER is defined as the quantity required to provide at least 90% control. Based on probit regression analyses, the mean calculated MERs of imazaquin and chlorimuron for 2-leaf common cocklebur were 27.5 and 4.5 g ai/ha, respectively. Under optimum growing conditions, the calculated MERs of imazaquin and chlorimuron for 6-leaf common cocklebur were 34 and 4.6 g/ha, respectively. When applied to 6-leaf plants under slight moisture stress, the calculated MERs for imazaquin and chlorimuron were 54 and 17.4 g/ha, respectively.

Field experiments were conducted in Auburn, AL in 2008 and 2009 to assess new aryloxyphenoxypropionate (AOPP) herbicides applied alone and tank-mixed with triclopyr for Tifway bermudagrass control in Zorro zoysiagrass. Treatments included three sequential applications of clodinafop (0.07 kg ai ha−1); triclopyr (1.12 kg ae ha−1); clodinafop + triclopyr (0.07 + 1.12 kg ha−1); fenoxaprop (0.10 kg ai ha−1); fenoxaprop + triclopyr (0.10 + 1.12 kg ha−1); metamifop (0.40 kg ai ha−1); and metamifop + triclopyr (0.40 + 1.12 kg ha−1). Clodinafop, fenoxaprop, and metamifop applied alone controlled Tifway bermudagrass 32 to 65% when rated 3 wk after the final application; however, the addition of triclopyr to these AOPP herbicides increased bermudagrass control to ≥ 89%. All AOPP herbicides applied alone caused unacceptable injury to Zorro zoysiagrass (20 to 42%) 3 wk after the final application. Clodinafop resulted in the greatest zoysiagrass injury (42%), whereas metamifop caused moderate injury (20%). Zorro zoysiagrass exhibited less injury (< 3%) and greater turf coverage (≥ 95%) when AOPP herbicides were tank-mixed with triclopyr. Of the three AOPP herbicides evaluated in this study, only fenoxaprop is currently labeled for use on turfgrass. However, the nonlabeled herbicides (clodinafop and metamifop) provided bermudagrass control and zoysiagrass safety equal to the commercial standard (fenoxaprop) 3 wk after the final application when tank-mixed with triclopyr. Where zoysiagrass is contaminated with bermudagrass, turf managers will likely have to make applications of AOPP herbicides tank-mixed with triclopyr over multiple years to control bermudagrass as it will continue to regrow from deep rhizomes.

The objective of this research was to modify the senior author's previously developed method for predicting yield losses in maize crops attributable to johnsongrass. The new method is based on the following assumptions: a) relative leaf frequency determines relative biomass; b) total biomass is constant despite the crop/weed ratio; and c) biomass, and therefore leaf frequency, are related to grain yield. Experiments in Argentina from 1986 to 1989 supported the above hypotheses, and the new method was more accurate than the old for predicting relative species biomass in johnsongrass/maize mixtures. Maize grain yield reductions associated with weed interference were also more accurately predicted.

Thirty soybean cultivars were grown in a hydroponic culture to observe differential tolerance to AC 263,222. Root length was reduced with 1.1 mM AC 263,222 in 18 of the 30 cultivars. ‘Bedford’, ‘Asgrow 5403’, and ‘Asgrow 6297’ were cultivars with a root length reduction of 30% or more, whereas roots of ‘DPL 506’, ‘RA 606’, and ‘Coker 6727’ were reduced less than 7%. Root weight was reduced in only 10 of the 30 cultivars. Asgrow 6297, ‘Bragg’, and ‘Narow’ root weights were reduced more than 20%. Shoot weight was reduced in 23 cultivars, and ‘Coker 6955’, ‘Braxton’, ‘Coker 485’, ‘Epps', and Coker 6727 shoots were reduced more than 25%. The most tolerant cultivars were ‘Centennial’, RA 606, ‘Tracy M’, ‘Johnston’, and ‘Hartz 7126’. The most sensitive cultivars to AC 263,222 with all response variables measured were Bedford, Asgrow 6297, Asgrow 5403, Coker 6955, Narow, Bragg, and Braxton. These cultivars all have ‘Jackson’ as a common parent. This suggests a genetic basis for AC 263,222 sensitivity in soybean.

Field experiments on dry pea (Pisum sativum) were conducted at five locations across the United States in 1995 and 1996 to investigate the effects of low rates of chlorsulfuron, thifensulfuron, and dicamba applied postemergence and of chlorsulfuron, metsulfuron, and clopyralid applied preplant incorporated in the soil on pea plants. Although chlorsulfuron, thifensulfuron, and dicamba caused significant injury symptoms on pea plants, they had little effect on yield. The lowest rates of foliar applications that caused observable symptoms were 0.035, 0.086, and 1.56 g ai/ha for chlorsulfuron, thifensulfuron, and dicamba, respectively, whereas chlorsulfuron, thifensulfuron, and dicamba rates that reduced pea yield by 25% were 0.18, 1.36, and 25 g/ha, respectively. Clopyralid caused more injury symptoms than metsulfuron or chlorsulfuron with soil application. However, the lowest rates of chlorsulfuron, metsulfuron, and clopyralid that caused observable symptoms were lower than the rates that reduced yield. This study showed that pea plants can sustain some level of plant injury without a large reduction in yield.

Field experiments were conducted to determine if aldicarb, disulfoton, or phorate applied in-furrow at 0.84 or 1.12 kg ai ha–1 affected cotton response to clomazone applied PPI or PRE at rates ranging from 0 to 1.12 kg ai ha–1. Disulfoton and phorate greatly reduced clomazone-induced chlorosis, stunting, and death of cotton seedlings. Compared with no insecticide or with dimethoate applied POST, aldicarb did not affect cotton response to clomazone. A clomazone rate by insecticide interaction was not observed for cotton yield. Clomazone reduced cotton yield at one of five locations. Insecticides affected yield at three locations, where highest yields were from cotton treated with aldicarb. In a separate experiment with 0.56, 1.12, and 2.24 kg ha–1 of phorate applied in-furrow and 1.12 kg ha–1 of clomazone applied PPI, phorate at all rates protected cotton from clomazone phytotoxicity.

Palmer amaranth is one of the most problematic weeds in cropping systems of North America, especially in midsouthern United States, because of its competitive ability and propensity to evolve resistance to several herbicide sites of action. Previously, we confirmed and characterized the first case of nontarget site resistance (NTSR) to fomesafen in a Palmer amaranth accession from Randolph County, AR (RCA). The primary basis of the present study was to evaluate the cross- and multiple-resistance profile of the RCA accession. The fomesafen dose-response assay in the presence of malathion revealed a lower level of RCA resistance when compared with fomesafen alone. The resistance index of the RCA accession, based on 50% biomass reduction, ranged from 63-fold (fomesafen alone) to 22-fold (malathion plus fomesafen), when compared with a 2007 susceptible, and 476-fold and 167-fold, respectively, relative to a 1986 susceptible check. The RCA accession was resistant to other protoporphyrinogen oxidase (PPO) inhibitors (i.e., flumioxazin, acifluorfen, saflufenacil) as well as the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor tembotrione and acetolactate synthase (ALS) inhibitor pyrithiobac sodium. Sequencing of the ALS gene revealed no point mutations, indicating that a target-site mechanism is not involved in conferring ALS-inhibitor resistance in the RCA accession. Of the three PPO-inhibiting herbicides tested in combination with the malathion, saflufenacil resulted in the greatest biomass reduction (80%; P < 0.05) and lowest survival rate (23%; P < 0.05) relative to nontreated plants. The application of cytochrome P450 or glutathione S-transferase inhibitors with fomesafen did not lead to any adverse effects on soybean, suggesting a possible role for these compounds for management of NTSR under field conditions. These results shed light on the relative unpredictability of NTSR in conferring herbicide cross- and multiple resistance in Palmer amaranth.

Weed size can nfluence herbicide performance and herbicide interactions in mixtures. To control a broad range of species in soybean or cotton, POST herbicide mixtures will likely be commonplace in Roundup Ready® XtendFlex® and Enlist™ technologies. The impact of weed size on herbicide interactions that could occur in Roundup Ready XtendFlex or Enlist crops was assessed in two field experiments conducted in 2015 and 2016 at the Northeast Research and Extension Center in Keiser, AR. Combinations of glufosinate, glyphosate, dicamba, and 2,4-D were applied to either 10-cm or 30-cm weeds and evaluated for percent weed control, height reduction, and density reduction, collected 5 wk after treatment. Colby’s method was used to analyze treatments for herbicide interactions for control of barnyardgrass, Palmer amaranth, and pitted morningglory. Antagonism was identified with at least one treatment on all species. Almost all treatments were antagonistic for percent weed control, height reduction, and density reduction on barnyardgrass. When glyphosate in mixture with 2,4-D or dicamba was applied to 30-cm barnyardgrass, control declined 9% for both mixtures relative to glyphosate alone. Glufosinate plus glyphosate was antagonistic when applied to both 30-cm pitted morningglory and barnyardgrass. Glufosinate plus dicamba provided less control and density reduction of Palmer amaranth than what was expected from Colby’s equation. Overall, antagonism was more likely to be identified when applications were made to 30-cm weeds compared with 10-cm weeds. The utility of a given herbicide mixture will depend on the species present in the field and the size of those species at the time of application.

Seeds were collected from an imidazolinone-resistant (R) population of smooth pigweed near Marion, MD, and from an imidazolinone-susceptible (S) population near Painter, VA, and grown in the greenhouse. Acetolactate synthase (ALS) enzyme was extracted from both biotypes and assayed in the presence of CGA 152005, chlorimuron, halosulfuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, pyrithiobac, rimsulfuron, and thifensulfuron to determine if an altered ALS was the mechanism of resistance in the R biotype and to determine if this biotype was cross-resistant to other ALS inhibitor herbicides. The inhibitor concentration required to cause a 50% reduction in ALS activity (I50) was calculated for each herbicide. ALS from the R biotype was approximately 71-, 109,000-, and 9-fold more resistant to imazaquin, imazethapyr, and rimsulfuron, respectively, than that from the S biotype. ALS from the R biotype was approximately threefold more sensitive to pyrithiobac and thifensulfuron than that from the S biotype. R ALS was also slightly more tolerant to CGA 152005 and nicosulfuron and slightly more sensitive to primisulfuron and chlorimuron. ALS from both biotypes generally responded similarly to halosulfuron. Resistance in the R biotype was due to an altered form of ALS that is insensitive to the imidazolinone herbicides and rimsulfuron.

Field experiments were conducted at Jay and Marianna, FL in 1988 and 1989 to determine the effects of sicklepod, Florida beggarweed, and common cocklebur density on chlorothalonil deposition to peanut foliage, peanut foliar disease incidence, and peanut yield. At a density of four weed plants per 8 m of row, Florida beggarweed and sicklepod reduced chlorothalonil deposition on peanut foliage by 20%, while common cocklebur reduced fungicide deposition by 34%. At the same density, incidence of the foliar diseases early leaf spot and late leaf spot increased 10% with Florida beggarweed, 14% with sicklepod, and 20% with common cocklebur compared with weed-free peanut. The predicted peanut yield loss from a weed density of four plants per 8 m was 16 to 19% for Florida beggarweed, 23 to 25% for sicklepod, and 31 to 39% for common cocklebur. Weed biomass increased with increasing weed density.

Field experiments were conducted in 2012 and 2013 across four locations for a total of 6 site-years in the midsouthern United States to determine the effect of growth stage at exposure on soybean sensitivity to sublethal rates of dicamba (8.8 g ae ha−1) and 2,4-D (140 g ae ha−1). Regression analysis revealed that soybean was most susceptible to injury from 2,4-D when exposed between 413 and 1,391 accumulated growing degree days (GDD) from planting, approximately between V1 and R2 growth stages. In terms of terminal plant height, soybean was most susceptible to 2,4-D between 448 and 1,719 GDD, or from V1 to R4. However, maximum susceptibility to 2,4-D was only between 624 and 1,001 GDD or from V3 to V5 for yield loss. As expected, soybean was sensitive to dicamba for longer spans of time, ranging from 0 to 1,162 GDD for visible injury or from emergence to R2. Likewise, soybean height was most affected when dicamba exposure occurred between 847 and 1,276 GDD or from V4 to R2. Regarding grain yield, soybean was most susceptible to dicamba between 820 and 1,339 GDD or from V4 to R2. Consequently, these data indicate that soybean response to 2,4-D and dicamba can be variable within vegetative or reproductive growth stages; therefore, specific growth stage at the time of exposure should be considered when evaluating injury from off-target movement. In addition, application of dicamba near susceptible soybean within the V4 to R2 growth stages should be avoided because this is the time of maximum susceptibility. Research regarding soybean sensitivity to 2,4-D and dicamba should focus on multiple exposure times and also avoid generalizing growth stages to vegetative or reproductive.