Many agricultural producers apply glyphosate to glyphosate-resistant crops to control weeds, including Palmer amaranth. Populations of this weed in Tennessee not completely controlled by glyphosate were examined. Field and greenhouse research confirmed that two separate populations had reduced biomass sensitivity (1.5× to 5.0×) to glyphosate compared to susceptible populations, although the level of resistance was higher based on plant mortality response (about 10×). Shikimate accumulated in both resistant and susceptible plants, indicating that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was inhibited in both biotypes. These results suggest that an altered target site is not responsible for glyphosate resistance in these Palmer amaranth biotypes.

Cut-leaved teasel is an invasive weed along highway corridors and is classified noxious in four states, including Missouri. Few herbicides have been examined for cut-leaved teasel control. Herbicides were evaluated for efficacy on established plants and residual activity for suppressing seedling emergence. Various growth regulator herbicides, amino acid inhibitors, and paraquat were applied on established teasel at two locations in central Missouri in fall 2003 and spring 2004, and two additional locations in fall 2004 and spring 2005. At 2 wk after treatment (WAT), paraquat resulted in the highest injury of teasel (85%), but injury declined over time. At 4 WAT, teasel control was most consistent with dicamba + diflufenzopyr applied in spring, ranging from 75 to 94% control. At 8 wk, glyphosate, dicamba + diflufenzopyr, metsulfuron-methyl, imazapyr, and combinations of growth regulator herbicides with 2,4-D were most effective, with teasel control from 86 to 100%. Control with 2,4-D alone was inconsistent; sulfosulfuron, sulfometuron-methyl, and paraquat were ineffective. Residual herbicides did not reduce teasel seedling emergence the following year. A number of herbicides were effective in managing emerged plants but reinfestations of treated sites is likely, even with the residual herbicides used in this research.

In-field surveys, which directly estimate weed population densities, typically utilize either random or nonrandom field selection methods. We used both methods to characterize the distribution and frequency of glyphosate-resistant (GR) horseweed populations and other late-season soybean weed escapes and to develop a database for tracking weed shifts, control failures, and the presence of other herbicide-resistant biotypes over time in Indiana. In-field surveys were conducted in a total of 978 Indiana soybean fields during September and October of 2003, 2004, and 2005. Information from fields with horseweed was obtained from 158 sites (19%) sampled through a systematic random site selection method and 128 fields through a nonrandom site selection method. When present, horseweed seed was collected and germinated in the greenhouse; rosettes 5 to 10 cm wide were sprayed with 1.72 kg ae/ha of glyphosate. Populations with less than 60% control at 28 d after treatment were determined to be glyphosate resistant. A selected subset of glyphosate-resistant populations was confirmed resistant by subsequent glyphosate dose response experiments. All populations in the subset with less than 60% control at the 1.72 kg ae/ha rate of glyphosate demonstrated 4- to 110-fold levels of resistance (R : S ratios). Glyphosate-resistant populations were found in all regions of Indiana; however, the highest frequencies were in the southeastern (SE) region with 38% of fields sampled and only 1, 2, and 2% of fields sampled in the northwestern (NW), northeastern (NE), and southwestern (SW) regions, respectively. Information gathered in this survey can assist in the development of applied research, as well as reactive glyphosate-resistant horseweed management education in the SE region of the state. Moreover, detecting resistance at low frequencies can direct proactive resistance education to farmers and practitioners in the other regions of the state as a means of providing an early warning system to address glyphosate resistance in weeds.

Field experiments were conducted in 2001 and 2002 to study the effect of date of planting, herbicide, and straw mulch on menthol mint yield and oil quality in northwest India. Menthol mint was planted at three dates (December 10, December 30, and January 20). Diuron was applied prior to emergence at rates of 0.0, 0.4, and 0.6 kg ai/ha, with and without rice straw mulch (6 t/ha). Menthol mint planted on December 30 and January 20 produced more plantlets and dry matter than the December 10 planting. Menthol mint planted on December 10 had higher weed density and weed biomass, and lower menthol mint herbage and oil yield than the later planting dates. Straw mulch application reduced weed density and weed biomass, and increased plantlet population, menthol mint dry matter accumulation, fresh herbage, and menthol mint oil yield. Preemergence (PRE) applications of diuron at 0.4 and 0.6 kg ai/ha increased menthol mint population and crop dry matter accumulation, and decreased weed density and weed biomass compared to the weedy check. Diuron at 0.6 kg/ha increased fresh herbage and menthol mint oil yield over the untreated weedy check, but there were no other differences between the two rates of diuron. Physicochemical properties of menthol mint oil were not affected by any of the three tested factors.

Sugarbeet varieties vary in their response to herbicides. s-Metolachlor and dimethenamid-P were recently registered for use in sugarbeet. Field trials were conducted in Michigan in 2004, 2005, and 2006 to evaluate the response of 12 sugarbeet varieties to s-metolachlor and dimethenamid-P applied PRE and POST to two-leaf and four-leaf stage sugarbeet. s-Metolachlor and dimethenamid-P reduced sugarbeet density when rainfall occurred within 7 d of the PRE applications. Dimethenamid-P PRE caused the most injury across all varieties followed by s-metolachlor PRE. Applying dimethenamid-P POST to two-leaf sugarbeet injured plants more than s-metolachlor applied POST to two- and four-leaf stage sugarbeet. The least amount of sugarbeet injury from dimethenamid-P was from POST applications at the four-leaf stage. Sugarbeet varietal differences were most pronounced from PRE applications of both herbicides and from the POST two-leaf application of dimethenamid-P. Of the 12 sugarbeet varieties evaluated, Hilleshog 2771RZ and Beta 5833R were the most tolerant, whereas Hilleshog 7172RZ was typically the most sensitive variety to these herbicides. Growers will probably not choose varieties based on herbicide tolerance alone, but instead base variety selection on sugar yield and disease resistance. However, if a grower has chosen a particular variety, this information could assist in assessing the risk of using s-metolachlor or dimethenamid-P for weed control.

Experts have long sought to understand the factors that underlie farmer decision making for weed management. The majority of this interest has been in relation to the weak adoption of integrated management approaches and more recently, herbicide resistance strategies. In order to increase adoption in these contexts there is a need to understand better the underlying drivers for weed management decisions. The objective of the research reported here was to probe farmers' understanding of weed management to establish a baseline understanding of these key drivers. Thirty Ohio farmers participated in an in-depth interview where they were asked to reflect on how weeds are introduced and spread, what risks and benefits weeds pose, and what management strategies farmers are familiar with and which they prefer. Their responses were mapped, coded, and analyzed for dominant beliefs and major decision-making influences. The results indicate that farmers largely attribute the introduction and movement of weeds to factors outside their control (e.g., the environment, plant characteristics). They frequently cite diverse and integrated management, but their focus is on control as opposed to prevention. In general, they tend to receive messages about integrated and preventive approaches, but do not always put them into practice because of underlying beliefs about the inevitability of new weed introductions and spread.

The majority of dry pea (pea) cultivars in current production have semileafless architecture. Although these cultivars tend to yield well and resist lodging, they may not be the best cultivars in terms of competition with weeds. Experiments were conducted at Lacombe and Lethbridge, Alberta, Canada, from 2003 to 2005 to compare semileafless and leafy pea cultivars under different seeding and herbicide regimes. During the study, in terms of weed emergence and biomass, oat (seeded) was usually more dominant than broadleaf weeds. Higher-disturbance sweep seeding led to slightly greater oat populations than lower disturbance seeding with knives. Leafy pea had lower yield potential in less weedy conditions than the semileafless pea, but the former were less susceptible to yield reduction as weed competition increased. Leafy pea often led to lower weed biomass and dockage than the semileafless pea. However, because semileafless pea yield at least as well as leafy pea in all of the environments encountered in this study, integrated weed management principles are currently best served by growing semileafless pea. Breeding to improve leafy cultivar yield potential may lead to greater opportunities for integrated weed management and herbicide input reductions in the future.

An experiment compared 2 yr of mowing, mulching, spot treatment with glyphosate, or no maintenance for reed canarygrass control and measured their effect on the establishment of red alder and arroyo willow. At one site, pretransplant control of reed canarygrass was poor, and mulching or no maintenance gave 9 and 14% control, respectively, at 5 mo after planting (MAP), but glyphosate spot treatment and mowing resulted in 89% and 72% control, respectively. The highest leafiness percentage by 24 MAP for arroyo willow and red alder at that site occurred in spot-treated plots (59 and 6%, respectively). Tree protection resulted in 30% more arroyo willow saplings with leaves at 24 MAP, at an average height of 68 cm at 17 MAP. Over the 2-yr trial at that site, mowing required far more time (30.6 min/plot) than either mulching or spot spraying (18.6 and 13.5 min/plot, respectively). At a second site, pretransplant weed control was excellent and maintenance programs controlled reed canarygrass from 88 to 98% by 5 MAP. Tree leafiness of red alder was improved 20% at 24 MAP by tree protection, with protected trees being 107 cm tall compared with 64 cm when left bare. Over the 2-yr trial at that site, mulching required 14.8 min/plot, compared with 12.9 and 7.6 min/plot for mowing and spot treatment. Annual spot treatment of reed canarygrass regrowth with glyphosate may be the most cost-effective means of achieving successful reestablishment of native broadleaf trees in northwestern riparian systems.

A study was conducted to test the effect of water clarity, as related to water seeding system, on herbicide efficacy in water-seeded imidazolinone-resistant rice production. Red rice control was 88 to 89% when 140 g/ha imazethapyr was applied in a single application immediately prior to permanent flood in the clear-water seeding system. All herbicide applications in either clear- and muddy-water systems controlled red rice 90 to 95%. The benefits of reducing soil erosion and surface water contamination from muddy-water discharge at seedling establishment can be obtained in a clear-water system without experiencing a decrease in weed control or rice yield.

Studies were conducted in 2001 and 2002 to determine the effect of POST herbicides on the spectral reflectance of corn. POST corn herbicides evaluated included 2,4-D, atrazine, bromoxynil, dicamba + diflufenzopyr, nicosulfuron, and primisulfuron. Multispectral and hyperspectral data were collected and spectral properties were analyzed using SAS procedures and MultiSpec image analysis. Corn treated with POST applications of atrazine and primisulfuron could not be distinguished from nontreated corn regardless of data type or analysis method used. 2,4-D and dicamba + diflufenzopyr were the most readily distinguished from nontreated corn plots using both hyperspectral and multispectral data.

Currently there are no herbicides registered for direct application to buckwheat for broadleaf weed control. This 4-yr Lithuanian study examined weed control using several rates of clopyralid alone or combined with a single rate of desmedipham. Most applications were applied at the 1-leaf stage of crop growth, however, one rate of clopyralid was applied pre-emergence (PRE) in 2 of the 4 yr. Buckwheat injury was evident within a few days after application (or emergence) with all treatments, but by harvest, no symptoms were evident. In the 2 yr with greatest weed densities, densities were reduced with increasing clopyralid rates alone or with desmediphan. However, weed biomass and density were similar to the nontreated control at harvest. Common lambsquarters, scentless mayweed, wild buckwheat, and narrowleaf hawksbeard were species that were best (> 40% density reduction) controlled with clopyralid. With few exceptions, buckwheat yield in all herbicide-treated plots was similar to the nontreated control. Because yield was not increased with these herbicides, other weed control benefits, such as reduced interference with harvesting equipment or less dockage due to weed seed contamination, must be carefully weighed against the costs of herbicide and application and crop injury that reduced early-season vigor.

Virginia buttonweed is a difficult-to-control weed in southern turfgrass. Typically, auxin-type herbicides are considered to be the standard treatment for this species. Field experiments were conducted to compare the efficacy of flazasulfuron, a sulfonylurea herbicide, to pyridine herbicides on Virginia buttonweed. Treatments included single and sequential applications of flazasulfuron (0.05 or 0.08 kg ai/ha). The sequential applications were applied 3 and 6 wk after initial treatment (WAIT). Three applications of 0.05 and 0.08 kg/ha flazasulfuron controlled Virginia buttonweed greater than 93% at 10 WAIT. Both treatments were comparable to 0.6 kg ai/ha triclopyr + 0.2 kg ai/ha clopyralid (80% control). These studies indicated that increased application of flazasulfuron increased Virginia buttonweed control regardless of application rates evaluated.

Southern crabgrass is a major weed in turfgrass and it primarily is controlled through preemergence herbicide application. Separate studies were conducted to evaluate the efficacy of trifloxysulfuron and flazasulfuron alone and in tank mixtures for southern crabgrass control in bermudagrass as MSMA alternatives. In 2001, two applications of trifloxysulfuron at 0.03 kg ai/ha 6 wk apart controlled southern crabgrass equal to MSMA applied twice at 2.2 kg ai/ha 7 d apart (91% compared to 90%, respectively) 90 d after initial treatment (DAIT), but provided greater control than MSMA in 2002 (80 versus 74%, respectively) 90 DAIT. Two applications of trifloxysulfuron at 0.02 kg/ha tank mixed with MSMA at 1.1 kg/ha controlled southern crabgrass 90%, greater than either two applications of trifloxysulfuron or MSMA applied alone (80% and 74%, respectively). In the second study, flazasulfuron applied at 0.05 kg ai/ha tank mixed with prodiamine at 1.1 kg ai/ha, pendimethalin at 1.6 kg ai/ha, or quinclorac at 0.84 kg ai/ha controlled southern crabgrass greater than a single application of MSMA at 2.2 kg/ha 60 DAT. In both 2005 and 2006 flazasulfuron applied alone provided similar or greater southern crabgrass control compared to a single application of MSMA at 2.2 kg/ha 60 DAT. At the conclusion of these studies, trifloxysulfuron and flazasulfuron alone or in tank mixtures controlled southern crabgrass similar to or better than MSMA; therefore these would be effective alternatives to MSMA in bermudagrass.

Yellow nutsedge can readily puncture the plastic mulch used in plasticulture tomato production, compromising the benefits of the mulch and hastening its deterioration. Our objective was to identify a PRE-applied (i.e., under the plastic) treatment to minimize yellow nutsedge puncturing. In a greenhouse study a series of halosulfuron rates were PRE-applied to soil planted with yellow nutsedge tubers. These rates were also applied to established plants but with selective spray contact. Nonlinear regression revealed that the concentration of halosulfuron required to reduce dry weights by 90% (GR90) for PRE-applied halosulfuron was 11.6 g/ha. The GR90 for POST-applied halosulfuron was 17.1, 28.1, and 11.6 g/ha for foliar-only, soil-only and foliar plus soil spray contact, respectively. Thus halosulfuron was more effective as a POST-applied, foliar-contacting treatment. However, soil activity was deemed likely sufficient to suppress plastic puncturing. In a noncrop field study, suppression of puncturing was influenced (P < 0.05) by the rate of both PRE-applied halosulfuron and S-metolachlor. A field study with tomato was conducted to evaluate six selective treatments using plastic mulch, PRE-applied S-metolachlor, and the combination of PRE or PRE/POST-split applications of halosulfuron. Plastic alone increased tomato yield threefold compared with bare ground. The addition of various herbicide programs neither increased nor reduced yield compared with plastic alone. Selected herbicide treatments did reduce mulch puncturing but not to the extent or duration that would allow sequential crops to receive the full benefit of nonpunctured plastic.

An experiment was conducted to determine the utility of multispectral imagery for identifying soybean, bare soil, and six weed species commonly found in Mississippi. Weed species evaluated were hemp sesbania, palmleaf morningglory, pitted morningglory, prickly sida, sicklepod, and smallflower morningglory. Multispectral imagery was analyzed using supervised classification techniques based upon 2-class, 3-class, and 8-class systems. The 2-class system was designed to differentiate bare soil and vegetation. The 3-class system was used to differentiate bare soil, soybean, and weed species. Finally, the 8-class system was designed to differentiate bare soil, soybean, and all weed species independently. Soybean classification accuracies classified as vegetation for the 2-class system were greater than 95%, and bare soil classification accuracies were greater than 90%. In the 3-class system, soybean classification accuracies were 70% or greater. Classification of soybean decreased slightly in the 3-class system when compared to the 2-class system because of the 3-class system separating soybean plots from the weed plots, which was not done in the 2-class system. Weed classification accuracies increased as weed density or weeks after emergence (WAE) increased. The greatest weed classification accuracies were obtained once weed species were allowed to grow for 10 wk. Palmleaf morningglory and pitted morningglory classification accuracies were greater than 90% for 10 WAE using the 3-class system. Palmleaf morningglory and pitted morningglory at the highest densities of 6 plants/m2 produced the highest classification accuracies for the 8-class system once allowed to grow for 10 wk. All other weed species generally produced classification accuracies less than 50%, regardless of planting density. Thus, multispectral imagery has the potential for weed detection, especially when being used in a management system when individual weed species differentiation is not essential, as in the 2-class or 3-class system. However, weed detection was not obtained until 8 to 10 WAE, which is unacceptable in production agriculture. Therefore, more refined imagery acquisition with higher spatial and/or spectral resolution and more sophisticated analyses need to be further explored for this technology to be used early-season when it would be most valuable.

Most management tactics used against leafy spurge are not economical, practical, or efficacious when used alone. Combinations of the biological control agent, Aphthona beetles, the herbicide imazapic (105 g/ha), and interseeded native grass species were evaluated for leafy spurge management at two sites: Sheyenne National Grassland and Ekre Grassland Preserve in North Dakota during 2001 to 2005. At the Sheyenne site, over a 5-yr study period, leafy spurge was reestablishing its stem density after a single application of imazapic, but stand suppression was maintained to < 11 stems/m2 when management combined imazapic with Aphthona or interseeding of native grasses. Aphthona beetles established at the Sheyenne site, but declined as leafy spurge density decreased. However, the remaining Aphthona population continued to suppress leafy spurge density. Leafy spurge stem control was successfully maintained for 3 yr by Aphthona and grass competition without repetition of the imazapic treatment. Leafy spurge root dry weights were reduced by 66% (< 111g/m2) in the insect plots during this period. At the Ekre site, similar results were observed for the first 3 yr. However, in the fourth yr, a failure of biological control agents to establish resulted in the resurgence of leafy spurge. During this study, lower Aphthona emergence was observed in imazapic-treated plots, possibly due to reduced leafy spurge density.

Weed resistance monitoring has been routinely conducted in the Northern Great Plains of Canada (Prairies) since the mid-1990s. Most recently, random surveys were conducted in Alberta in 2001, Manitoba in 2002, and Saskatchewan in 2003 totaling nearly 800 fields. In addition, nearly 1,300 weed seed samples were submitted by growers across the Prairies between 1996 and 2006 for resistance testing. Collected or submitted samples were screened for group 1 [acetyl-CoA carboxylase (ACCase) inhibitor] and/or group 2 [acetolactate synthase (ALS) inhibitor] resistance. Twenty percent of 565 sampled fields had an herbicide-resistant (HR) wild oat biotype. Most populations exhibited broad cross-resistance across various classes of group 1 or group 2 herbicides. In Manitoba, 22% of 59 fields had group 1–HR green foxtail. Group 2–HR biotypes of kochia were documented in Saskatchewan, common chickweed and spiny sowthistle in Alberta, and green foxtail and redroot pigweed in Manitoba. Across the Prairies, HR weeds are estimated to occur in fields covering an area of nearly 5 million ha. Of 1,067 wild oat seed samples submitted by growers and industry for testing between 1996 and 2006, 725 were group 1 HR, 34 group 2 HR, and 55 groups 1 and 2 HR. Of 80 submitted green foxtail samples, 26 were confirmed group 1 HR; most populations originated from southern Manitoba where the weed is most abundant. Similar to the field surveys, various group 2–HR biotypes were confirmed among submitted samples: kochia, wild mustard, field pennycress, Galium spp., common chickweed, and common hempnettle. Information from grower questionnaires indicates patterns of herbicide usage are related to location, changing with cropping system. Two herbicide modes of action most prone to select resistance, groups 1 and 2, continue to be widely and repeatedly used. There is little evidence that growers are aware of the level of resistance within their fields, but a majority have adopted herbicide rotations to proactively or reactively manage HR weeds.

Seeds from five suspected acetyl-CoA carboxylase (ACCase) inhibitor–resistant wild oat biotypes (R1 to R5) were collected in wheat and lentil fields in the Pacific Northwest. Based on whole plant dose–response experiments, the five resistant biotypes were 2 to 24 times more resistant to the aryloxyphenoxypropionate (APP) herbicides (fenoxaprop, diclofop, and quizalofop) compared with the susceptible biotype. However, none of the resistant biotypes were resistant to the cyclohexanedione (CHD) herbicides, sethoxydim and clethodim. R2 was the only biotype resistant to tralkoxydim and pinoxaden, a phenylpyrazolin herbicide and an ACCase inhibitor. The R2 biotype was 35 and 16 times more resistant to tralkoxydim and pinoxaden, respectively, when compared with the susceptible biotype. The levels of resistance and cross-resistance patterns varied among biotypes indicating either more than one mechanism of resistance or different resistance mutations in these wild oat biotypes. The CHD herbicides, sethoxydim and clethodim, could be used to control these resistant biotypes. Except for the R2 biotype, pinoxaden could be used to control the resistant wild oat biotypes. The resistance patterns of these wild oat biotypes are an indication of the difficulty in predicting cross-resistance among the ACCase inhibitor herbicides.

Thermal weed control methods have been incorporated into weed control programs in organic and conventional production systems. Flaming is commonly used, but steaming has been proposed to increase efficiency of heat transfer to weeds and reduce the risk of fire. The objective of this research was to measure injury to leaves of plant species that differ in leaf morphology and to measure injury to plants at different stages of plant development. The study was conducted in a glasshouse and plants were exposed to steaming at 400 C for 0.36 s—equivalent to a steaming speed of 2 km/h. Overall, leaf thickness was the best morphological characteristic to predict injury ( = 0.51), with greater thickness resulting in less injury. For broadleaf species only, species with wider leaves were injured more than species with narrower leaves ( = 0.64). Injury was greatest when plants had fewer than six true leaves and when their shoots were less than 10 cm long. There was a wide range of injury across species, and the grass species bermudagrass and perennial ryegrass were injured (68 to 81%) more than other species such as common purslane and English daisy (23 to 34%). Biomass of all species tested was reduced by approximately 40%, indicating that leaf injury was not the sole effect of steaming on plant growth. These results indicated that considering both visual estimates of injury and morphological characteristics is important to properly assess thermal weed control effectiveness.

Two field studies were conducted in 2004 and 2005 to evaluate the response of glyphosate-resistant ‘TV52R42’ and ‘TV52R14’ soybean to glyphosate at 0, 458, 916, 1,375, 1,833, or 2,749 g ae/ha applied at the R4 reproductive growth stage and at R4 followed by the same rates at R6. A single glyphosate application at the R4 stage did not injure soybean or cause reductions in soybean height, nodes per plant, branch pods, main stem pods, or yield, regardless of application rate. Likewise, sequential applications of glyphosate at the R4 followed by R6 growth stages resulted in no adverse effects on growth parameters or yield. Preliminary findings indicate excellent tolerance to glyphosate application at reproductive growth stages beyond the currently labeled R2 growth stage restriction in varieties evaluated. Research findings warrant evaluation of an expanded range of germplasm under a variety of environmental conditions and potential effects on seed quality for establishment of tolerance levels and potential herbicide label amendments.