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On-site surveys of weed populations provide information on the relative occurrence and density of weeds that can be useful to growers in that region. Data generated by weed surveys can aid in the management of weed issues by monitoring the movement of problem weeds and forecasting areas susceptible to infestations. Currently, on-site surveys are often performed on a small scale, within single fields or counties. Questionnaire surveys are helpful for assessing relative abundance but do not always provide detailed information on weed distribution in time or space. A survey was conducted annually in Ohio from 2013 through 2017 in 49 counties with soybean [Glycine max (L.) Merr.] production to assess the late-season occurrence of horseweed [Conyza canadensis (L.) Cronquist]. The objectives of this research were to: (1) determine the frequency, level of infestation, and distribution of C. canadensis in soybean fields in the primary soybean-producing Ohio counties over 5 yr; and (2) identify significant spatial clusters or movement trends over time. Conyza canadensis was encountered in each county from 2013 through 2017. Spatial cores of interest, or counties identified as having significant levels of C. canadensis infestations or a lack thereof relative to surrounding counties, were identified in all years except 2017. The lowest frequency of C. canadensis encountered at all rating levels occurred in 2017, which coincided with second-highest frequency of infestations (highest density level) among years. There was no distinct distribution or pattern of C. canadensis movement within the state from year to year, but there was an increase in counties with infestations over time compared with the early years of the survey when many counties had few to no infestations. These results suggest that C. canadensis persists as a common and troublesome threat to Ohio soybean producers and that growers should continue making C. canadensis management a priority when developing weed control programs.

Current recommendations for the control of glyphosate-resistant horseweed [Conyza canadensis (L.) Cronquist var. canadensis] in soybeans [Glycine max (L.) Merr.] consist of comprehensive herbicide programs, which often include herbicide applications outside the soybean growing season. Integration of cover crops with herbicides could potentially improve C. canadensis control and allow for a reduction in herbicide inputs. Two separate field studies were conducted from 2016 through 2018 with the objectives of: (1) determining the effect of planting date and seeding rate of a cereal rye (Secale cereale L.) cover crop on C. canadensis population density and control in the subsequent soybean crop; and (2) determining whether the cover crop could replace a fall herbicide treatment or allow for a reduction in the use of spring-applied residual herbicides. There was no effect of rye planting date, late September versus late October, on C. canadensis density in either study. In 2016 to 2017, C. canadensis density was greater in the absence of a rye cover crop in both studies, but otherwise not affected by seeding rates of 50 versus 100 kg ha−1. In the 2017 to 2018 season, the presence of rye resulted in an increased C. canadensis density in the spring residual herbicide study (Study I), and had no effect in the fall herbicide study (Study II). Conyza canadensis densities were lowest in the treatments where a comprehensive spring residual or fall herbicide treatment had been applied, averaged over rye planting date and seeding rate. Earlier-planted rye at a higher seeding rate produced the most biomass but did not result in lower C. canadensis densities. These results suggest that cereal rye planted at a density of 50 kg ha−1 as a cover crop before no-till soybeans may be sufficient to reduce glyphosate-resistant C. canadensis plant density, but cannot be relied upon to reduce the need for fall herbicide treatments and spring residual programs.

Herbicide resistance within key driver weeds, such as common waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif ], constrains available management options for crop production. Routine surveillance for herbicide resistance provides a mechanism to monitor the development and spread of resistant populations over time. Furthermore, the identification and quantification of resistance mechanisms at the population level can provide information that helps growers develop effective management plans. Populations of Amaranthus spp., including A. tuberculatus, redroot pigweed (Amaranthus retroflexus L.), and Palmer amaranth (Amaranthus palmeri S. Watson), were collected from 51 fields in Ohio during the 2016 growing season. Twenty-four A. tuberculatus populations were screened for resistance to the herbicides lactofen, atrazine, and glyphosate. Phenotypically resistant plants were further investigated to determine the frequency of known resistance mechanisms. Resistance to lactofen was infrequently observed throughout the populations, with 8 of 22 populations exhibiting resistant plants. Within those eight resistant populations, the ΔG210 resistance mechanism was observed in 17 of 30 phenotypically resistant plants, and the remainder lacked all known resistance mechanisms. Resistance to atrazine was observed in 12 of 15 populations; however, a target-site resistance mechanism was not observed in these populations. Resistance to glyphosate was observed in all populations. Gene amplification was the predominant glyphosate-resistance mechanism (147 of 322 plants) in the evaluated populations. The Pro-106-Ser mutation was identified in 24 plants, half of which also possessed gene amplification. In this study, molecular screening generally underestimated the phenotypically observed resistance. Continued mechanism discovery and marker development is required for improved detection of herbicide resistance through molecular assays.

Knowledge of the effects of burial depth and burial duration on seed viability and, consequently, seedbank persistence of Palmer amaranth (Amaranthus palmeriS. Watson) and waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] ecotypes can be used for the development of efficient weed management programs. This is of particular interest, given the great fecundity of both species and, consequently, their high seedbank replenishment potential. Seeds of both species collected from five different locations across the United States were investigated in seven states (sites) with different soil and climatic conditions. Seeds were placed at two depths (0 and 15 cm) for 3 yr. Each year, seeds were retrieved, and seed damage (shrunken, malformed, or broken) plus losses (deteriorated and futile germination) and viability were evaluated. Greater seed damage plus loss averaged across seed origin, burial depth, and year was recorded for lots tested at Illinois (51.3% and 51.8%) followed by Tennessee (40.5% and 45.1%) and Missouri (39.2% and 42%) forA. palmeriandA. tuberculatus, respectively. The site differences for seed persistence were probably due to higher volumetric water content at these sites. Rates of seed demise were directly proportional to burial depth (α=0.001), whereas the percentage of viable seeds recovered after 36 mo on the soil surface ranged from 4.1% to 4.3% compared with 5% to 5.3% at the 15-cm depth forA. palmeriandA. tuberculatus, respectively. Seed viability loss was greater in the seeds placed on the soil surface compared with the buried seeds. The greatest influences on seed viability were burial conditions and time and site-specific soil conditions, more so than geographical location. Thus, management of these weed species should focus on reducing seed shattering, enhancing seed removal from the soil surface, or adjusting tillage systems.

A field study was conducted in 2014 and 2015 in Arkansas, Illinois, Indiana, Ohio, Tennessee, Wisconsin, and Missouri to determine the effects of tillage system and herbicide program on season-long emergence of Amaranthus species in glufosinate-resistant soybean. The tillage systems evaluated were deep tillage (fall moldboard plow followed by (fb) one pass with a field cultivator in the spring), conventional tillage (fall chisel plow fb one pass with a field cultivator in the spring), minimum tillage (one pass of a vertical tillage tool in the spring), and no-tillage (PRE application of paraquat). Each tillage system also received one of two herbicide programs; PRE application of flumioxazin (0.09 kg ai ha−1) fb a POST application of glufosinate (0.59 kg ai ha−1) plus S-metolachlor (1.39 kg ai ha−1), or POST-only applications of glufosinate (0.59 kg ha−1). The deep tillage system resulted in a 62, 67, and 73% reduction in Amaranthus emergence when compared to the conventional, minimum, and no-tillage systems, respectively. The residual herbicide program also resulted in an 87% reduction in Amaranthus species emergence compared to the POST-only program. The deep tillage system, combined with the residual program, resulted in a 97% reduction in Amaranthus species emergence when compared to the minimum tillage system combined with the POST-only program, which had the highest Amaranthus emergence. Soil cores taken prior to planting and herbicide application revealed that only 28% of the Amaranthus seed in the deep tillage system was placed within the top 5-cm of the soil profile compared to 79, 81, and 77% in the conventional, minimum, and no-tillage systems. Overall, the use of deep tillage with a residual herbicide program provided the greatest reduction in Amaranthus species emergence, thus providing a useful tool in managing herbicide-resistant Amaranthus species where appropriate.Nomenclature: Flumioxazin; glufosinate; metolachlor; paraquat; Amaranthus; soybean, Glycine max (L.) Merr.En 2014 y 2015, se realizo un estudio de campo en Arkansas, Illinois, Indiana, Ohio, Tennessee, Wisconsin, y Missouri para determinar los efectos del sistema de labranza y el programa de herbicidas sobre la emergencia de especies de Amaranthus a lo largo de la temporada de crecimiento en soja resistente a glufosinate. Los sistemas de labranza evaluados fueron labranza profunda (arado de vertedera en el otono seguido por (fb) un pase de cultivador de campo en la primavera), labranza convencional (arado de cincel en el otono seguido de un pase de cultivador de campo en la primavera), labranza minima (un pase de una herramienta de labranza vertical en la primavera), y cero labranza (aplicacion PRE de paraquat). Cada sistema de labranza tambien recibio uno de dos programas de herbicidas; aplicacion PRE de flumioxazin (0.09 kg ai ha−1) fb glufosinate POST (0.59 kg ai ha−1) mas S-metolachlor (1.39 kg ai ha−1), o solo aplicaciones POST de glufosinate (0.59 kg ha−1). El sistema de labranza profunda resulto en una reduccion de 62, 67, y 73% en la emergencia de Amaranthus cuando se comparo con los sistemas de labranza convencional, minima, y cero, respectivamente. El programa con un herbicida residual tambien resulto en una reduccion de 87% en la emergencia de especies Amaranthus al compararse con el programa de solo herbicidas POST. El sistema de labranza profunda, combinado con el programa residual, resulto en una reduccion de 97% en la emergencia de especies de Amaranthus cuando se comparo con el sistema de labranza minima combinado con el programa de solo herbicidas POST, el cual tuvo la mayor emergencia de Amaranthus. Muestras de suelo tomadas antes de la siembra y la aplicacion de herbicidas revelaron que en el sistema de labranza profunda solamente 28% de las semillas de Amaranthus fueron localizadas en los 5 cm de suelo superiores del perfil del suelo comparado con 79, 81, y 77% en los sistemas de labranza convencional, minima, y cero. En general

Pigweeds are among the most abundant and troublesome weed species across Midwest and mid-South soybean production systems because of their prolific growth characteristics and ability to rapidly evolve resistance to several herbicide sites of action. This has renewed interest in diversifying weed management strategies by implementing integrated weed management (IWM) programs to efficiently manage weeds, increase soybean light interception, and increase grain yield. Field studies were conducted across 16 site-years to determine the effectiveness of soybean row width, seeding rate, and herbicide strategy as components of IWM in glufosinate-resistant soybean. Sites were grouped according to optimum adaptation zones for soybean maturity groups (MGs). Across all MG regions, pigweed density and height at the POST herbicide timing, and end-of-season pigweed density, height, and fecundity were reduced in IWM programs using a PRE followed by (fb) POST herbicide strategy. Furthermore, a PRE fb POST herbicide strategy treatment increased soybean cumulative intercepted photosynthetically active radiation (CIPAR) and subsequently, soybean grain yield across all MG regions. Soybean row width and seeding rate manipulation effects were highly variable. Narrow row width (≤ 38 cm) and a high seeding rate (470,000 seeds ha−1) reduced end-of-season height and fecundity variably across MG regions compared with wide row width (≥ 76 cm) and moderate to low (322,000 to 173,000 seeds ha−1) seeding rates. However, narrow row widths and high seeding rates did not reduce pigweed density at the POST herbicide application timing or at soybean harvest. Across all MG regions, soybean CIPAR increased as soybean row width decreased and seeding rate increased; however, row width and seeding rate had variable effects on soybean yield. Furthermore, soybean CIPAR was not associated with end-of-season pigweed growth and fecundity. A PRE fb POST herbicide strategy was a necessary component for an IWM program as it simultaneously managed pigweeds, increased soybean CIPAR, and increased grain yield.

Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed.

Glyphosate is an important herbicide worldwide, but its efficacy has been compromised where weed species have evolved glyphosate resistance. To better understand evolutionary outcomes of continued and strong selection from glyphosate exposure, we characterized variation in resistance in self-pollinating Conyza canadensis (horseweed) in Ohio and Iowa, where glyphosate resistance was first reported in 2002 and 2011, respectively. In 2015, we collected seeds from a total of 74 maternal plants (biotypes) from no-till soybean fields vs. non-agricultural sites in each state, using one representative plant per site. Young plants from each biotype were sprayed with glyphosate rates of 0x, 1x (840 g ae ha−1), 8x, 20x, or 40x. Resistant biotypes with at least 80% survival at each dosage were designated as R1 (1x), R2 (8x), R3 (20x), or R4 (40x). Nearly all Ohio agricultural biotypes were R4, as were 62% of biotypes from the non-agricultural sites. In Iowa, R4 biotypes were clustered in the southeastern soybean fields, where no-till agriculture is more common, and 45% of non-agricultural biotypes were R1–R4. Our results show that resistance levels to glyphosate can be very high (at least 40x) in both states, and that non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.

Palmer amaranth and waterhemp have become increasingly troublesome weeds throughout the United States. Both species are highly adaptable and emerge continuously throughout the summer months, presenting the need for a residual PRE application in soybean. To improve season-long control ofAmaranthusspp., 19 PRE treatments were evaluated on glyphosate-resistant Palmer amaranth in 2013 and 2014 at locations in Arkansas, Indiana, Nebraska, Illinois, and Tennessee; and on glyphosate-resistant waterhemp at locations in Illinois, Missouri, and Nebraska. The twoAmaranthusspecies were analyzed separately; data for each species were pooled across site-years, and site-year was included as a random variable in the analyses. The dissipation of weed control throughout the course of the experiments was compared among treatments with the use of regression analysis where percent weed control was described as a function of time (the number of weeks after treatment [WAT]). At the mean (i.e., average) WAT (4.3 and 3.2 WAT for Palmer amaranth and waterhemp, respectively) isoxaflutole +S-metolachlor + metribuzin had the highest predicted control of Palmer amaranth (98%) and waterhemp (99%). Isoxaflutole +S-metolachlor + metribuzin,S-metolachlor + mesotrione, and flumioxazin + pyroxasulfone had a predicted control ≥ 97% and similar model parameter estimates, indicating control declined at similar rates for these treatments. Dicamba and 2,4-D provided some, short-lived residual control ofAmaranthusspp. When dicamba was added to metribuzin orS-metolachlor, control increased compared to dicamba alone. Flumioxazin + pyroxasulfone, a currently labeled PRE, performed similarly to treatments containing isoxaflutole or mesotrione. Additional sites of action will provide soybean growers more opportunities to control these weeds and reduce the potential for herbicide resistance.

Herbicide-resistantAmaranthusspp. continue to cause management difficulties in soybean. New soybean technologies under development, including resistance to various combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, and mesotrione, will make possible the use of additional herbicide sites of action in soybean than is currently available. When this research was conducted, these soybean traits were still regulated and testing herbicide programs with the appropriate soybean genetics in a single experiment was not feasible. Therefore, the effectiveness of various herbicide programs (PRE herbicides followed by POST herbicides) was evaluated in bare-ground experiments on glyphosate-resistant Palmer amaranth and glyphosate-resistant waterhemp (both tall and common) at locations in Arkansas, Illinois, Indiana, Missouri, Nebraska, and Tennessee. Twenty-five herbicide programs were evaluated; 5 of which were PRE herbicides only, 10 were PRE herbicides followed by POST herbicides 3 to 4 wks after (WA) the PRE application (EPOST), and 10 were PRE herbicides followed by POST herbicides 6 to 7 WA the PRE application (LPOST). Programs with EPOST herbicides provided 94% or greater control of Palmer amaranth and waterhemp at 3 to 4 WA the EPOST. Overall, programs with LPOST herbicides resulted in a period of weed emergence in which weeds would typically compete with a crop. Weeds were not completely controlled with the LPOST herbicides because weed sizes were larger (≥ 15 cm) compared with their sizes at the EPOST application (≤ 7 cm). Most programs with LPOST herbicides provided 80 to 95% control at 3 to 4 WA applied LPOST. Based on an orthogonal contrast, using a synthetic-auxin herbicide LPOST improves control of Palmer amaranth and waterhemp over programs not containing a synthetic-auxin LPOST. These results show herbicides that can be used in soybean and that contain auxinic- or HPPD-resistant traits will provide growers with an opportunity for better control of glyphosate-resistant Palmer amaranth and waterhemp over a wide range of geographies and environments.

The use of chemicals by man to control unwanted vegetation dates back several centuries, but modern chemical weed management essentially began with the discovery of the hormonelike herbicides in the 1940s. The first hormonelike herbicides developed were the substituted phenoxyacetic acids. The phenoxyacetic acid herbicides were discovered quite by accident as a result of research being conducted concurrently in the United States and England to develop synthetic organic compounds that mimicked the biological activity of auxins (Kirby, 1980). Development and use of 2,4-D, MCPA, and related hormonelike herbicides to control broadleaf weeds in cereal grains ushered in a new era of chemical weed management after World War II. It was during the 1950s and 1960s that the modern practice of using relatively low rates of synthetic herbicides for selective weed control in field crops was widely adopted in developed countries of the world.

Control of early-emerging weeds is essential to protect the yield potential of maize. An understanding of the physiological changes that occur as a result of weed interference is required to address variability in yield loss across sites and years. Field trials were conducted at the University of Guelph (UG), the Ohio State University (OSU), and Colorado State University (CSU) during 2009 and 2010. There were six treatments (season-long weedy and weed-free, and weed control at the 1st-, 3rd-, 5th-, and 10th-leaf-tip stages of maize development) and 20 individual plants per plot were harvested at maturity. We hypothesized that, as weed control was delayed, weed interference in the early stages of maize development would increase plant-to-plant variability in plant dry-matter accumulation, which would result in a reduction of grain yield at maturity. The onset of the critical period for weed control (CPWC) occurred on average between the third and fifth leaf tip stages of development (i.e., V1 to V3, respectively). Rate of yield loss following the onset of the CPWC ranged from 0.05 MG ha−1d−1at UG 2009 to 0.22 MG ha−1d−1at CSU 2010 (i.e., 0.5 and 1.6% d−1, respectively). On average, reductions in kernel number per plant accounted for approximately 65% of the decline in grain yield as weed control was delayed. Biomass partitioning to the grain was stable through early weed removal treatments, increased and peaked at the 10th-leaf-tip time of control, and decreased in the season-long weedy treatment. Plant-to-plant variability in dry matter at maturity and incidence of bareness increased as weed control was delayed. As weed control was delayed, the contribution of plant-to-plant variability at maturity to the overall yield loss was small, relative to the decline of mean plant dry matter.

Inadequate corn stands due to extreme weather conditions may require producers to replant their corn fields. The use of GR corn, however, can result in difficulty in managing replanted corn without experiencing yield loss. Therefore, the objectives of this research were to evaluate the herbicide options for control of GR corn in a corn replant situation and to determine the effect of corn replanted into various initial corn stands on grain yield. Two field studies were conducted to accomplish the objectives. The first experiment was designed to identify the most efficacious herbicide treatment for GR corn removal in a corn replant situation. Clethodim (51 g ai ha−1) applied 6 d prior to replanting, paraquat (700 g ai ha−1) plus metribuzin (160 g ai ha−1) applied at replanting, and glufosinate (450 g ai ha−1) applied at replanting along with a sequential treatment 3 wk later provided 96 to 100% control of the initial corn stand and resulted in the highest yield. If corn from the first planting remains, the interaction between different sized plants can reduce yield of corn. Thus, a second field experiment was conducted to evaluate the influence on grain yield of corn replanted into various initial corn stands. Corn stands of 0, 20,000, 40,000, 60,000, 80,000, and 100,000 plants ha−1 were established and either followed by a corn replant at 80,000 seeds ha−1 or not replanted. Initial stands ≥ 60,000 plants ha−1 did not require a replant to maximize yield. Initial corn stands ≤ 40,000 plants ha−1 required a replant with initial stand control to maximize grain yield. The percent yield contribution from an initial stand of 20,000 plants ha−1 was 20% greater than the same population replanted a few weeks later, which showed the competitive advantage to earlier planting even at the lowest initial corn stand. Because of this competitive advantage, an initial stand must be removed to maximize corn yield in a replant situation.

Field studies were conducted in 2007 and 2008 at seven sites in Ohio, Indiana, and Illinois to determine the effect of PRE herbicide and POST application timing on weed control and yield of glyphosate-resistant corn. Levels of PRE herbicide included none; low—atrazine; medium—atrazine and metolachlor; and high—atrazine, mesotrione, and metolachlor. Glyphosate was applied POST when corn was 30 cm tall, or 1 or 2 wk later. Common lambsquarters, giant foxtail, and giant ragweed infested at least six of the seven sites, and other weed species occurred at two to three sites. Control of weeds at the time of POST application ranged from 48 to 91%, 58 to 99%, and 87 to 100% for the low, medium, and high levels of PRE herbicide, respectively, averaged over POST application timing. Control of giant foxtail and redroot pigweed decreased by about 20% between the second and third POST timing, averaged over PRE herbicide, but control of other weeds was similar among timings. Late-season control of common ragweed, velvetleaf, common lambsquarters, and Pennsylvania smartweed exceeded 90%, regardless of PRE herbicide or POST timing. Control of redroot pigweed, ivyleaf morningglory, and giant ragweed was as low as 74, 67, and 83%, respectively, but the high level of PRE herbicide resulted in 90 to 97% control of these weeds. An interaction between PRE herbicide and POST timing for late-season control of giant foxtail, tall waterhemp, and yellow nutsedge reflected the more effective control among POST timings from the higher levels of PRE herbicide. The overall trend in this study was for more effective weed control in PRE/POST herbicide programs with more comprehensive PRE herbicides that have substantial activity on both grass and broadleaf weeds. Highest yield occurred where the PRE treatment consisted of a two- or three-way combination of herbicides applied at 50% of the recommended rate or higher. Yield was reduced at all POST timings with atrazine alone or in the absence of PRE herbicide.

Field research was conducted to determine the effectiveness of glyphosate, fomesafen, and adjuvant combinations on 6-inch glyphosate-resistant (GR) and 12-inch glyphosate-sensitive (GS) giant ragweed. Glyphosate alone controlled GS giant ragweed greater than 93%, but control of GR giant ragweed was less than 47%. GS and GR giant ragweed responded similarly to fomesafen as control ranged from 42 to 86%. Control with reduced rates of fomesafen was enhanced with additional adjuvants. Regardless of adjuvant, the labeled use rate of fomesafen alone at 0.352 lb/acre provided 75 to 86% control of both GR and GS giant ragweed. The addition of glyphosate to fomesafen resulted in over 90% control of GS giant ragweed. The addition of glyphosate to fomesafen did not improve control of GR giant ragweed and when additional adjuvants were not used, control was reduced when glyphosate was added to 0.352 lb/acre of fomesafen.

Greenhouse studies were conducted to determine the prevalence of resistance to acetolactate synthase (ALS)-inhibiting herbicides in 266 Indiana horseweed populations, both glyphosate-susceptible and glyphosate-resistant, and to characterize the response of selected biotypes to combinations of glyphosate and cloransulam. Populations with individuals resistant to ALS inhibitors were more frequent in the northern half (38% of the populations in the NW and 50% of the populations in the NE) of Indiana than in the southern half (26% of the populations in the SW and 5% of the populations in the SE). Only 2% of the populations appeared to be resistant to both glyphosate and ALS inhibitors in an initial greenhouse study. Horseweed populations with resistance to ALS inhibitors exhibited herbicide doses required for 50% reduction in plant growth (GR50) values ranging from 14 to 255 g ai ha−1of cloransulam. The resistant : susceptible (R : S) ratio for four horseweed populations with suspected resistance to glyphosate and ALS inhibitors ranged from 0.3 to 50 and from 2.5 to 8.1 for cloransulam and glyphosate, respectively. The tank mixtures exhibited an antagonistic response to 3 of the 16 combinations of cloransulam and glyphosate on the susceptible population. The tank mixtures exhibited primarily an additive response to those same combinations in the multiple-resistant populations, but the response was occasionally synergistic for two of the four populations. The additive response between glyphosate and cloransulam indicates that, where the level of resistance is fairly low, combinations of these herbicides should be more effective for control of multiple-resistant populations compared with application of a single herbicide.

Horseweed populations with mixtures of biotypes resistant to glyphosate and acetolactate synthase (ALS)–inhibiting herbicides as well as biotypes with multiple resistance to glyphosate + ALS-inhibiting herbicides have been documented in Indiana and Ohio. These biotypes are particularly problematic because ALS-inhibiting herbicides are commonly tank mixed with glyphosate to improve postemergence horseweed control in soybean. The objective of this research was to characterize the growth and seed production of horseweed populations with resistance to glyphosate or ALS-inhibiting herbicides, and multiple resistance to glyphosate + ALS-inhibiting herbicides. A four-herbicide by four-horseweed population factorial field experiment was conducted in the southeastern region of Indiana in 2007 and repeated in 2008. Four horseweed populations were collected from Indiana or Ohio and confirmed resistant to glyphosate, ALS inhibitors, both, or neither in greenhouse experiments. The four herbicide treatments were untreated, 0.84 kg ae ha−1glyphosate, 35 g ai ha−1cloransulam, and 0.84 kg ae ha−1glyphosate + 35 g ai ha−1cloransulam. Untreated plants from horseweed populations that were resistant to glyphosate, ALS-inhibiting, or multiple glyphosate + ALS-inhibiting herbicides produced similar amounts of biomass and seed compared to populations that were susceptible to those herbicides or combination of herbicides. Furthermore, aboveground shoot mass and seed production did not differ between treated and untreated plants.

Thirteen field experiments were conducted in Illinois, Indiana, Ohio, and Ontario from 2005 to 2007 to determine the effects of simulated glyphosate drift followed by in-crop applications of nicosulfuron/rimsulfuron plus dicamba/diflufenzopyr or foramsulfuron plus bromoxynil plus atrazine on nontransgenic corn injury, height, stand count, shoot dry weight, and yield. Simulated glyphosate drift at 100 and 200 g/ha, resulted in 11 to 61% visual crop injury and a 19 to 45% decrease in corn height. Simulated glyphosate drift at 200 g/ha caused a reduction in shoot dry weight by 46%, stand count by 28% and yield by 49 to 56%. Generally, simulated glyphosate drift followed by the in-crop herbicides resulted in an additive response with respect to visual crop injury, height, stand count, shoot dry weight, and yield. Nomenclature: Atrazine; bromoxynil; dicamba/diflufenzopyr; foramsulfuron; glyphosate; nicosulfuron/rimsulfuron; corn, Zea mays L

Field studies were conducted in 2006 and 2007 to evaluate strategies for management of four glyphosate-tolerant common lambsquarters populations in glyphosate-resistant soybeans. Treatments consisted of several different preplant herbicide combinations followed by one or two postemergence applications of 0.84 to 3.36 kg ae/ha of glyphosate. Preplant application of a combination of glyphosate, 2,4-D ester and residual herbicides resulted in the most effective control of all populations, and allowed use of a single postemergence glyphosate application at the lowest rate. Late-season control of common lambsquarters ranged from 66 to 95% where no preplant herbicides were applied, 72 to 97% for preplant application of glyphosate and 2,4-D, and 96 to 100% for the combination of glyphosate, 2,4-D, flumioxazin, and cloransulam-methyl. Individual plants survived and produced seed following single postemergence glyphosate applications of 3.36 kg ae/ha, and multiple glyphosate applications totaling 2.5 kg ae/ha. Multiple postemergence treatments were more effective than single postemergence treatments for reduction of common lambsquarters population density and seed production. The progeny of plants that survived high rates of glyphosate in the field were screened for their response to glyphosate in the greenhouse. Progeny from one of three populations exhibited increased glyphosate tolerance from 1 yr of selection pressure.

Biotypes of common lambsquarters with tolerance to glyphosate have been identified in a number of states, but little is known about their fitness characteristics. Field and greenhouse studies were conducted to characterize the response of selected glyphosate-tolerant common lambsquarters biotypes to glyphosate, and also their biological and reproductive characteristics. In a greenhouse dose-response study, GR50and GR90values for four tolerant biotypes ranged from 1.48 to 3.22 and 8.73 to 18.7 kg ae ha−1, respectively, compared to 0.57 and 2.39 kg ae ha−1, respectively, for a glyphosate-sensitive biotype. In a field dose-response study, the GR50and GR90values were 0.06 and 0.48 kg ae ha−1, respectively, for a tolerant biotype, compared to 0.036 and 0.19 kg ae ha−1, respectively, for the sensitive biotype. The growth rate, time until flowering, and seed production of eight tolerant and two sensitive biotypes was evaluated in a field study. The tolerant biotypes grew taller, amassed more leaf area and dry weight, and advanced through growth stages more rapidly than sensitive biotypes during the early portion of the growing season. The tolerant biotypes were taller than sensitive biotypes at 6 and 10 wk after transplanting, but had lower dry weight at maturity. Tolerant biotypes initiated flower primordia approximately 6 to 8 wk after transplanting, whereas sensitive biotypes required 12 wk. However, no apparent fitness penalties were observed in glyphosate-tolerant biotypes based on seed-production estimates.

Field trials were conducted in 2002 and 2003 at seven sites to determine the optimum rates of mesotrione and atrazine applied PRE for minimal crop injury and control of common lambsquarters, velvetleaf, Pennsylvania smartweed, common ragweed, giant ragweed, ivyleaf morningglory, and common cocklebur. All rates of each herbicide resulted in greater than 95% control of triazine-susceptible common lambsquarters. Mesotrione at 105 g ai/ha resulted in greater than 90% control of triazine-resistant common lambsquarters, velvetleaf, and Pennsylvania smartweed. Control of common ragweed was 90% or greater from mesotrione at 158 g/ha in combination with atrazine at 280 g/ha or greater. In addition, mesotrione at 210 g/ha combined with any rate of atrazine provided at least 92% control of common ragweed. Combinations of mesotrione and atrazine only suppressed, and did not effectively control, giant ragweed, common cocklebur, and ivyleaf morningglory. Nomenclature: Atrazine, mesotrione, s-metolachlor, common cockleb...

Herbicide-resistant corn hybrids offer additional options for POST weed control in corn, and growers may benefit from information on the consistency of these weed-control strategies. Studies were conducted in Indiana, Illinois, Michigan, and Ohio, in 2000 and 2001, to evaluate weed control among herbicide strategies for imidazolinone-resistant, glufosinate-resistant, glyphosate-resistant, and conventional corn. Isogenic hybrids were utilized to minimize variation in growth and yield potential among hybrids. The glyphosate-resistant corn postemergence (glyphosate-POST) treatment provided more consistent control of giant foxtail than the PRE, conventional corn postemergence (conventional-POST), glufosinate-resistant corn postemergence (glufosinate-POST), and imidazolinone-resistant corn postemergence (imi-POST) treatments. All four POST treatments were more consistent and provided greater control than the PRE treatment of the large-seeded broadleaf weeds velvetleaf, giant ragweed, common cocklebur, and morningglory species. Conventional-POST and imi-POST were more consistent than glufosinate-POST and glyphosate-POST treatments in controlling giant ragweed. There were no statistical differences in the variability of PRE or POST treatments for control of common lambsquarters, common ragweed, and redroot pigweed. Corn yield varied among locations and years. The glyphosate-POST treatment did not reduce yield relative to the weed-free treatment, the imi-POST and glufosinate-POST treatments each reduced yield in one of eight locations, and the conventional-POST treatment reduced yield in three of eight locations.

Studies were conducted from 2001 through 2003 to determine the extent of resistance to acetolactate synthase (ALS) inhibitors and glyphosate in Ohio horseweed biotypes. The response of 66 horseweed biotypes to cloransulam-methyl and glyphosate was determined in the greenhouse. Application of 0.07 kg ai cloransulam/ha reduced plant biomass by less than 60% for 38 of the 66 biotypes. Application of 3.4 kg ae glyphosate/ha reduced biomass by at least 80% for the 51 biotypes collected in 2001, but biomass was similar to that of nontreated plants for 11 of the 15 populations collected in 2002. A dose–response study was conducted with selected biotypes, and a nonlinear, logistic dose–response curve was fit to the data to calculate the herbicide dose required to reduce fresh weight 50% (GR50). On the basis of GR50values, the resistance ratio (R/S) for two ALS-resistant biotypes was 34 and 943 for chlorimuron-ethyl and 32 and 168 for cloransulam, respectively. The R/S ratio for two glyphosate-resistant biotypes was 33 and 39. Results of these studies indicate that, in 2002, ALS-resistant horseweed was widespread throughout Ohio, whereas resistance to glyphosate occurred primarily in several counties in southwestern Ohio.

Soybean (Glycine max L. Merr.) varieties containing multiple traits that impart resistance to stress factors (e.g., herbicides and diseases) need to be investigated to determine if all resistance responses remain fully functional under simultaneous stress exposure. Experiments were conducted in the greenhouse to investigate the effects of soybean cyst nematode (SCN) infection on glyphosate uptake, distribution, and activity in a soybean variety containing genes for SCN and glyphosate resistance (R). A glyphosate-, SCN-susceptible variety (S) was included for comparison. Glyphosate absorption was not affected by SCN infection in either variety. Glyphosate distribution in the R variety was altered slightly by SCN infection when the herbicide was applied 9 days after inoculation (DAI), but this effect diminished as herbicide application timing was delayed to 18 DAI or later, possibly due to increased expression of the SCN resistance response and lower levels of SCN infection. Glyphosate did not affe...

The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean.

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and...

Three studies were conducted in 1999 and 2000 to determine whether acetolactate synthase (ALS)–resistant common ragweed and giant ragweed biotypes were present in Ohio. Results of field studies indicated that biotypes of both species had cross-resistance to three chemical families of ALS-inhibiting herbicides. Cloransulam-methyl applied postemergence at 9, 18, and 36 g/ha controlled more than 85% of two susceptible populations of common and giant ragweed 28 d after treatment, whereas less than 35% control of resistant populations was achieved at the same rates. Fomesafen, lactofen, and glyphosate applied alone at the recommended rates provided the most effective control of ALS-resistant common and giant ragweed. Mixtures of cloransulam-methyl with either fomesafen or lactofen did not significantly increase ALS-resistant common and giant ragweed control compared with each diphenylether herbicide used alone. Dose–response bioassays conducted in the greenhouse indicated that susceptible common and ...

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss. Nomenclature: Glyphosate; giant foxtail, Setaria faberi Herrm. #3...

Glyphosate is often mixed with residual herbicides to control emerged weeds in no-till crop production systems. Field studies were conducted in Ohio from 1992 to 1994 to evaluate the weed control provided by residual herbicides and reduced rates of glyphosate in full-season, no-till soybean. Herbicide treatments were applied at two timings to examine the effect of weed size. At 4 wk after treatment, greater than 85% Pennsylvania smartweed control was obtained with metribuzin plus chlorimuron and linuron plus chlorimuron applied with 280 g ai/ha glyphosate and with imazethapyr and imazaquin applied with 560 g/ha glyphosate. All residual herbicides provided at least 85% common lambsquarters control when applied with 560 g/ha glyphosate. In 1992, the early application of residual herbicides provided this level of common lambsquarters control without glyphosate due to the small weed size at the time of application. Residual herbicides applied with 280 g/ha glyphosate controlled giant foxtail 85% or ...

The objectives of this study were to determine how the timing of weed management treatments in winter wheat stubble affects weed control the following season and to determine if spring herbicide rates in corn can be reduced with appropriately timed stubble management practices. Field studies were conducted at two sites in Ohio between 1993 and 1995. Wheat stubble treatments consisted of glyphosate (0.84 kg ae/ha) plus 2,4-D (0.48 kg ae/ha) applied in July, August, or September, or at all three timings, and a nontreated control. In the following season, spring herbicide treatments consisted of a full rate of atrazine (1.7 kg ai/ha) plus alachlor (2.8 kg ai/ha) preemergence, a half rate of these herbicides, or no spring herbicide treatment. Across all locations, a postharvest treatment of glyphosate plus 2,4-D followed by alachlor plus atrazine at half or full rates in the spring controlled all broadleaf weeds, except giant ragweed, at least 88%. Giant foxtail control at three locations was at least 83% when a postharvest glyphosate plus 2,4-D treatment was followed by spring applications of alachlor plus atrazine at half or full rates. Weed control in treatments without alachlor plus atrazine was variable, although broadleaf control from July and August glyphosate plus 2,4-D applications was greater than from September applications. Where alachlor and atrazine were not applied, August was generally the best timing of herbicide applications to wheat stubble for reducing weed populations the following season.

Giant foxtail, woolly cupgrass, and wild-proso millet infest millions of hectares of land devoted to corn production in the midwestern U.S. Control of these species and effects on corn grain yield were evaluated at various timings using POST applications of nicosulfuron vs. applications of various PRE herbicides at 17 locations across the midwestern U.S. in 1992 and 1993. Nicosulfuron applied to 5 to 10 cm giant foxtail and woolly cupgrass provided greater control than that observed with selected PRE herbicides. Giant foxtail control with nicosulfuron averaged 88%, and control of woolly cupgrass averaged 77% across all sites. Nicosulfuron, applied to 5 to 10 cm wild-proso millet, provided a level of control similar to that of selected PRE herbicides. Corn grain yield was greater when giant foxtail was controlled POST with nicosulfuron vs. PRE control with selected soil-applied herbicides. Corn grain yields were similar when nicosulfuron was applied POST to 5 to 10 cm woolly cupgrass or wild-proso millet vs. PRE control of these grass weeds. Across a broad range of geographical locations, nicosulfuron, applied POST to 5 to 10 cm tall grass, provided greater or similar levels of weed control vs. the selected PRE herbicides, with no deleterious effect on grain yield.

Transformation of soybean [Glycine max (L.) Merr.] with a gene encoding a glyphosate-tolerance 5-enolpyruvylshikimate-3-phosphate synthase enzyme from Agrobacterium sp. strain CP4 resulted in the development of glyphosate-tolerant line 40-3-2. Glyphosate (N-phosphonomethyl glycine) is the active ingredient of Roundup herbicide. Line 40-3-2 was yield tested at 17 locations in 1992, 23 locations in 1993, and 18 locations in 1994. At those locations, broadcast applications of glyphosate at various rates were made over 40-3-2 or its derivatives from early vegetative growth to pod fill. No significant yield reduction was observed as a result of the glyphosate treatment at any of the locations. Development of glyphosate-tolerant soybean promises to provide the farmer with access to a new weed control system that should result in lower production costs and reliable weed control under a wide range of conditions.

Field experiments were established at Columbus and near South Charleston, OH to determine the effects of giant ragweed population density on soybean yield and to characterize the development of giant ragweed grown in 76-cm soybean rows. An economic threshold was calculated for Ohio using a common treatment for giant ragweed control in soybean. A cost of $41/ha was estimated for a farmer to apply 0.56 kg/ha bentazon plus 0.28 kg/ha fomesafen plus COC (1.25% v/v). Assuming a soybean value of $0.22/kg, the cost of control was equivalent to 5.4 and 7.1% of the soybean yield in 1991 and 1992, respectively, which corresponded to the yield loss caused by 0.08 and 0.03 giant ragweed plants/m2. The competitiveness of giant ragweed can be at least partly attributed to its ability to initiate and maintain axillary leaves and branches within the shaded confines of the soybean canopy.

Laboratory experiments were conducted to determine the effect of three phenolic acids, acetone, and riboflavin on aqueous photolysis of chlorimuron and imazaquin. The phenolic acids investigated were caffeic acid (CA), ferulic acid (FA), and p-coumaric acid (PCA). Treatment solutions were contained in quartz vessels and irradiated with 300 to 400 nm UV light in a photoreactor. The extrapolated photolysis half-life of chlorimuron in pure solution was 107 h, compared to a half-life of 0.42 h for pure aqueous imazaquin. Chlorimuron in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA exhibited half-lives of 9, 57, 58, 67, and 146 h, respectively. Imazaquin in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA had half-lives of 0.70, 0.55, 0.55, 0.48, and 0.55 h, respectively. The presence of PCA in aqueous media delayed chlorimuron photolysis, whereas all other compounds, especially riboflavin, sensi- tized chlorimuron photolysis. In contrast, imazaquin photolysis was delayed in the presence of the test compounds, with riboflavin having the greatest effect and causing in a 68% increase in imazaquin half-life over that of imazaquin alone. Quantum yields for sensitized photolysis of chlorimuron by riboflavin and for riboflavin by imazaquin were 0.1134 and 0.0477, respectively. These results suggest that some soluble and naturally occurring organic compounds may enhance chlorimuron photolysis yet delay imazaquin photolysis in surface waters. Nomen- clature: Chlorimuron, 2-(((((4-chloro-6-methoxy-2- pyrimidinyl)amino)carbonyl)amino)sulfonyl)benzoic acid; imazaquin, 2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5- oxo-lH-imidazol-2-yl)-3-quinolinecarboxylic acid. Additional index words: Herbicide degradation, pho- todegradation.

Field studies were conducted in 1988 through 1990 to determine the effect of soil pH, over a 4.5 to 6.5 range, on imazethapyr persistence and on imazaquin and imazethapyr carryover to ‘Pioneer 3377’ corn in a Crosby silt loam (1.6% organic matter) and a Hoytville clay (3.3% organic matter). Imazethapyr appeared to be more persistent in Hoytville clay than in Crosby silt loam. In Crosby silt loam, imazethapyr persistence increased as pH decreased, whereas soil pH did not affect persistence in Hoytville clay. Imazaquin or imazethapyr did not affect corn grown the year following application to Crosby silt loam. Imazethapyr applied to Hoytville clay did not cause significant injury or yield reduction to corn. Carryover of imazaquin in Hoytville clay caused corn injury and yield reduction, increased as pH decreased, and was more evident at 280 g ai ha−1than at 140 g ha−1.

Laboratory and field studies were conducted in 1988 through 1990 to determine the effect of soil pH on imazaquin adsorption and persistence in a Crosby silt loam (1.6% organic matter) and a Hoytville clay (3.3% organic matter). Above a pH of 6, 6 to 7% of the total imazaquin present was adsorbed to both soils. Adsorption increased as pH decreased from about 7 to 4.5, with a maximum of 16.5 and 31% adsorbed on Crosby and Hoytville soils, respectively. Imazaquin was more persistent in the Hoytville clay than in the Crosby silt loam. Herbicide dissipated more slowly in both soils in 1988 than in 1989 due to dry soil conditions immediately prior to and following herbicide application in 1988. In both soils, imazaquin persistence increased as soil pH decreased, over a pH range from 6.5 to 4.5. Increased persistence with decreasing pH was attributed in part to increased adsorption and decreased availability for microbial degradation.

The photolytic degradation of several imidazolinone herbicides in solution and on soil was investigated. Ultraviolet light caused 100% degradation of imazaquin, imazethapyr, and imazapyr, and 87 and 8% degradation of imazamethabenz and atrazine in aqueous solutions, respectively, after 48 h. The order of susceptibility to photolysis in decreasing order was imazaquin = imazethapyr > imazapyr > imazamethabenz > atrazine. In soil Studies, 45% of imazaquin and 52% of imazethapyr dissipated from moist sand after 48 h of exposure. Herbicide dissipation on air-dry sand and on field capacity and air-dry silty clay loam was less than 10% in most instances. Atrazine photolysis was not detected. This research indicates that photolysis of imidazolinone herbicides in solution is rapid. Photolysis on soil occurs readily on coarse-textured wet soils probably due to greater availability of the herbicide for photochemical alteration.

Adsorption of imazaquin and imazethapyr on soils, sediments, clays, and ion exchange resins was studied. Multivariate regression analysis of Freundlich K values and soil and sediment properties yielded a model for imazaquin adsorption which included soil pH, organic carbon, and clay as significant independent variables. Clay content and pH were significant independent variables for the imazethapyr adsorption model. Adsorp- tion of imazaquin and imazethapyr was greater at pH below 6 than at a higher pH. Adsorption of both herbicides on Ca- and HIAI-kaolinite and Ca-montmoril- lonite was low, but increased on H/AI-montmorillonite. The results of adsorption studies with ion exchange resins indicated that binding of protonated herbicide at low pH could be due to physical forces, hydrogen bonding, or cationic binding. At soil pH levels above the pKa's of imazaquin and imazethapyr, ionic bonding of herbicide anions with positively charged soil components could also occur. Nomenclature: Imaz- aquin, 2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo- 1H-imidazol-2-yl)-3-quinolinecarboxylic acid; im- azethapyr (?)-2-(4,5-dihydro-4-methyl-4-(1-methylethyl)- 5-oxo-lH-imidazol-2-yl)-5-ethyl-3-pyridinecarboxylic acid. Additional index words. Imidazolinone herbicides, herbi- cide adsorption, herbicide binding.

The availability and persistence of imazaquin, imazethapyr, and clomazone were studied in a Cisne silt loam (1.3% organic matter) and a Drummer silty clay loam (5.8% organic matter). Availability of all three herbicides to bioassay species was greater in the Cisne soil than in the Drummer soil. Corn root growth was more sensitive to imaza- quin and imazethapyr than corn shoot growth. Shoot and root growth of wheat was inhbited by similar clomazone concentrations. In field experiments conducted in 1984, 1985, and 1986, all three herbicides were more persistent in the Drummer silty clay loam than in the Cisne silt loam. Clomazone and imazethapyr were detected by liquid or gas chromatographic analysis in the Drummer soil 3 yr follow- ing application. Crop injury occurred 5 months after appli- cation of imazaquin and clomazone to the Drummer soil. In the Cisne soil, only imazethapyr caused crop injury 5 months after application. Herbicide residues found below 7.5 cm were greater in the Drummer soil than in the Cisne soil. Nomenclature: Imazaquin, 2- (4,5-dihydro-4-methyl- 4- (1- methylethyl)- 5 -oxo - 1H- imidazol- 2 -ylI -3 -quinolinecar- boxylic acid; imazethapyr, (?)-2- (4,5-dihydro-4-methyl-4- (1-methylethyl)-5-oxo-1H- imidazol- 2 -yl) -5-ethyl- 3 -pyridine- carboxylic acid; clomazone 2- ((2-chlorophenyl)methyl) - 4,4-dimethyl-3-isoxazolidinone; corn, Zea mays L. 'B73 x Mol7'; wheat, Triticum aestivum L. 'Caldwell'.

Regression analysis of clomazone adsorption on 19 soils and sediments yielded a positive correlation and linear relationship between distribution adsorption constants (Kd) and soil organic carbon content. The agreement of the experimental mean K0c with Koc values predicted from the octanol-water partition coefficient and water solubility of clomazone indicated hydrophobic bonding was one mecha- nism responsible for adsorption. Kaolinite and montmoril- lonite had a strong affinity for clomazone, and adsorption increased with the charge of the cation on the clay exchange sites. Adsorption was not correlated with the clay content of soils and sediments, indicating that organic coatings on clay in soils may block adsorption sites on clay surfaces. The adsorption of clomazone on some soils and sediments with high clay contents was greater than predicted based on organic carbon content alone, however, indicating clay contributed to adsorption on these samples. Nomenclature: Clomazone, 2- ((2-chlorophenyl)methyll -4,4-dimethyl- 3 -isoxazolidinone.