Flumioxazin has recently (2010) been registered for aquatic use for controlof hydrilla and other noxious invasive aquatic plant species. Due to therapid degradation of flumioxazin, especially in high pH water, some hydrillaresearch trials have produced less than desirable results with rapid plantregrowth. Therefore, laboratory experiments were conducted to evaluate theinfluence of pH on flumioxazin's effect on photosynthesis. Flumioxazinapplied at concentrations ≥ 200 µg ai L−1 in high (9.0) pH waterand ≥ 100 µg L−1 in low (6.0) pH water required 68 to 123 h toreduce photosynthesis by 50% (ET50). The effect of 400 µg L−1 flumioxazin on photosynthesis of apical hydrilla tips wasalso compared at low (20 µmol m−2 s−1), medium (170µmol m−2 s−1), and high (400 µmol m−2 s−1) light levels at pH 9.0. Low light–treated tips were stillphotosynthetic at approximately 73% of the nontreated control plants 168 hafter treatment. Low light–treated hydrilla required an estimated 303 h toachieve a 50% reduction in photosynthesis, while high light plants onlyrequired 99 h. Chlorophyll content of hydrilla was reduced as flumioxazinconcentration was increased from 100 to 1,600 µg L−1. These dataindicate that flumioxazin activity on hydrilla photosynthesis is influencedby herbicide concentration, water pH, and light intensity.

Wild oat is the second-most abundant, but most economically important, weedacross the Canadian Prairies of western Canada. Despite the serious economiceffects of resistance to acetyl-CoA carboxylase (ACC) or acetolactatesynthase (ALS) inhibitors or both in this weed throughout the Northern GreatPlains of North America, little research has examined the basis forherbicide resistance. We investigated target-site and nontarget-sitemechanisms conferring ACC- and ALS-inhibitor resistance in 16 wild oatpopulations from across western Canada (four ACC-inhibitor resistant, fourALS-inhibitor resistant, and eight ACC- and ALS-inhibitor resistant). The ACC1 mutations were found in 8 of the 12 ACCinhibitor-resistant populations. The Ile1781Leu mutation was detected inthree populations, the Trp2027Cys and Asp2078Gly mutations were in twopopulations each, and the Trp1999Cys, Ile2041Asn, Cys2088Arg, and Gly2096Sersubstitutions were in one population each. Three populations had two ACC1 mutations. Only 2 of the 12 ALS inhibitor-resistantpopulations had an ALS target-site mutation—Ser653Thr andSer653Asn substitutions. This is the first global report of ALS target-site mutations in Avena spp.and four previously undocumented ACC1 mutations in wildoat. Based on these molecular analyses, seedlings of five ACC + ALSinhibitor-resistant populations (one with an ACC1 mutation;four with no ACC or ALS mutations) weretreated with malathion, a known cytochrome P450 monooxygenase inhibitor,followed by application of one of four ACC- or ALS-inhibiting herbicides.Malathion treatment often resulted in control or suppression of thesepopulations, suggesting involvement of this enzyme system in contributing toresistance to both ACC and ALS inhibitors.

Giant ragweed germination is delayed by both a physiological dormancy of theembryo (embryo dormancy) and an inhibitory influence of embryo-coveringstructures (covering structure-enforced [CSE] dormancy). To clarify theroles of embryo and CSE dormancy in giant ragweed seedling emergence timing,we conducted two experiments to address the following objectives: (1)determine changes in germinability for giant ragweed dispersal units(hereafter “involucres”) and their components under natural burialconditions, and (2) compare embryo and CSE dormancy alleviation andemergence periodicity between successional and agricultural populations. InExperiment 1, involucres were buried in crop fields at Columbus, OH,periodically excavated, and brought to the laboratory for dissection.Involucres, achenes, and embryos were then subjected to germination assaysat 20 C. In Experiment 2, temporal patterns of seedling emergence weredetermined at a common burial site. Reductions in embryo and CSE dormancywere compared with controlled-environment stratification followed bygermination assays at 12 and 20 C, temperatures representative of soilconditions in spring and summer. Results indicated that overwinter dormancyloss involved sequential reductions in embryo and CSE dormancy. CSEdormancy, which may limit potential for fatal germination during fall, wascaused by the pericarp and/or embryo-covering structures within thepericarp. In Experiment 2, successional populations emerged synchronously inearly spring, whereas agricultural populations emerged throughout thegrowing season. Levels of embryo dormancy were greater in the agriculturalpopulations than the successional populations, but CSE dormancy levels weresimilar among populations. In 12 C germination assays, embryo dormancylevels were positively correlated with time required to reach 95% cumulativeemergence (run 1: r = 0.81, P = 0.03; run 2: r = 0.76, P = 0.05). These results suggest thatlate-season emergence in giant ragweed involves high levels of embryodormancy that prevent germination at low temperatures in spring.

Weed composition may vary because of natural environment, managementpractices, and their interactions. In this study we presented a systematicapproach for analyzing the relative importance of environmental andmanagement factors on weed composition of the most conspicuous species insugarcane. A data-mining approach represented by k-meanscluster and classification and regression trees (CART) were used foranalyzing the 11 most frequent weeds recorded in sugarcane cropping systemsof northern Argentina. Data of weed abundance and explanatory factorscontained records from 1976 sugarcane fields over 2 consecutive years. The k-means method selected five different weed clusters.One cluster contained 44% of the data and exhibited the lowest overall weedabundance. The other four clusters were dominated by three perennialspecies, bermudagrass, johnsongrass, and purple nutsedge, and the annualitchgrass. The CART model was able to explain 44% of the sugarcane's weedcomposition variability. Four of the five clusters were represented in theterminal nodes of the final CART model. Sugarcane burning before harvestingwas the first factor selected in the CART, and all nodes resulting from thissplit were characterized by low abundance of weeds. Regarding the predictivepower of the variables, rainfall and the genotype identity were the mostimportant predictors. These results have management implications as theyindicate that the genotype identity would be a more important factor thancrop age when designing sugarcane weed management. Moreover, the abioticcontrol of crop–weed interaction would be more related to rainfall than theenvironmental heterogeneity related to soil type, for example soilfertility. Although all these exploratory patterns resulting from the CARTdata-mining procedure should be refined, it became clear that thisinformation may be used to develop an experimental framework to study thefactors driving weed assembly.

Natural dissemination of johnsongrass seeds as well as the effect of combineharvesting on this process were studied in corn fields. The estimation ofnatural dispersal was carried out by two different methods, collecting seedsthroughout the season using seed traps and sampling soil–surface seedabundance before harvest using a vacuum device. Both methods showed the samedispersal pattern. A minimum of 84.6% was dispersed in the first 2 m fromthe focus and a maximum of 1.6% was dispersed beyond the first 5 m. Anaverage of 76.3% of these dispersed seeds were lost or buried after sheddingbut before harvest. Seed dispersal by the combine harvester was estimatedfrom the difference between soil–surface seed abundance in the same sitespre and postharvest. Although the quantity of seeds dispersed by the combinewas similar to those dispersed by natural factors, dispersal distances weresignificantly higher. Around 90% of the dispersed seeds were found in thefirst 5 m forward and backward of the combine direction from the infestationsource, and 1.6% of the seeds were found beyond 22 m forward and 10 mbackward of the combine direction from the infestation source. A largeproportion of the seeds dispersed were dormant or not viable. It isconcluded that the major role of sexual reproduction in johnsongrasspopulation dynamics may be to spread the risks, promoting dispersal in timeand space.

Silverleaf nightshade is a widespread, deep-rooted, summer-growing perennialthat significantly reduces production in Australian crop and pasturesystems. It has an extensive root system, which competes both directly andindirectly with summer and winter pastures and crops through depletion ofsoil moisture and nutrients. Long-distance dispersal of seeds is animportant mechanism for its spread and management. A range of experimentswas conducted to determine the factors influencing seed production andseedbank dynamics. Seed production ranged from 1,814 to 2,945m−2. Diurnally fluctuating temperatures of 25/15 C provided theoptimal thermal conditions for germination, with germination not affected bylight. Osmotic stress reduced germination, with no germination occurring at−1MPa. Germination was reduced to 5% at 160 mM NaCl, suggesting some saltsensitivity. Germination occurred over a pH range of 4 to 10, but declinedwith increasing acidity. Viability of buried seed declined to around 20%after 3 yr, with seed buried at 10 cm remaining the most viable. Theprolonged seed persistence in the soil indicates a long-term control programis necessary for depleting the soil seedbank.

Leafy spurge seeds are responsive to alternating temperature rather thanconstant temperature for germination. Transcriptome changes of dry leafyspurge seeds and seeds imbibed for 1 and 3 d at 20 C constant (C) and 20 :30 C alternating (A) temperature were determined by microarray analysis toexamine temperature responsiveness. Principal component analysis revealeddifferences in the transcriptome of imbibed seeds based on the temperatureregime. Computational methods in bioinformatics parsed the data intooverrepresented AraCyc pathways and gene regulation subnetworks providingbiological context to temperature responses. After 1 d of imbibition, thedegradation of starch and sucrose leading to anaerobic respiration werecommon pathways at both temperature regimes. Several overrepresentedpathways unique to 1 d A were associated with generation of energy, reducingpower, and carbon substrates; several of these pathways remainedoverrepresented and up-regulated at 3 d A. At 1 d C, pathways for thephytohormones jasmonic acid and brassinosteroids were uniquelyoverrepresented. There was little similarity in overrepresented pathways at1 d C between leafy spurge and arabidopsis seeds,indicating species-specific effects upon imbibition of dry seeds.Overrepresented gene subnetworks at 1 d and 3 d at both temperature regimesrelated to signaling processes and stress responses. A major overrepresentedsubnetwork unique to 1 d C related to photomorphogenesis via the E3ubiquitin ligase COP1. At 1 d A, major overrepresented subnetworks involvedcircadian rhythm via LHY and TOC1 proteins and expression of stress-relatedgenes such as DREB1A, which is subject to circadianregulation. Collectively, substantial differences were observed in thetranscriptome of leafy spurge seeds imbibed under conditions that affect thecapacity to germinate.

The dynamics of the host–parasite relationship between tomato cv. Brigadeand Egyptian broomrape is temperature-related. This relationship wasutilized for the development of an equation on the basis of thermal time (asmeasured by growing degree days, GDD, C) to predict the parasitism dynamicsof Egyptian broomrape in tomato. To obtain a reliable prediction fromthermal time values, studies based on a wide range of temperatures areessential. Four temperature-regime treatments and five levels of infestationwith Egyptian broomrape seeds were tested in a multiclimate greenhouse(phytotron) and a temperature-controlled greenhouse, respectively. Theday/night temperature regimes were 20/12 C, 23/15 C, 26/18 C, and 29/21 Cand the infestation levels were 0 (noninfested control), 1, 5, 10, and 25 mgof Egyptian broomrape seeds per liter of soil. As expected, increasingtemperature or infestation levels resulted in faster appearance and higherrate of attachments, respectively. The relation between development ofattachments and GDD was described as a three-parameter logistic curve. Inboth temperature-regime and infestation-level experiments, the developmentof attachments began 200 GDD after planting and the maximal number ofattachments was recorded 800 GDD after planting. A significant reduction inthe aboveground biomass of the tomato plants due to increased Egyptianbroomrape biomass was recorded only for the 26/18 C and 29/21 C day/nighttreatments and the three highest infestation levels (5, 10, and 25 mg L−1 soil). The ability to predict the start of parasitism canbe used to develop a climate-based system for Egyptian broomrape controlwith herbicides.

Tall morningglory is an annual broadleaf vine and a problem weed in manyannual and perennial crops in several countries including the United States.A better understanding of the germination biology of tall morningglory wouldfacilitate the development of better control strategies for this weed.Experiments were conducted under greenhouse and laboratory conditions toevaluate the effects of various environmental factors, such as temperature,light, planting depth, pH, osmotic and salt stress, and flooding duration,on the germination of tall morningglory. The results suggested that theoptimum day/night temperature range for the germination of tall morningglorywas 20/12.5 to 35/25 C and maximum germination (89%) was observed at 30/20C. Temperature higher and lower than the optimum range significantly reducedgermination. Alternate light and dark did not have any adverse effect on thegermination of tall morningglory seeds. The germination was 10% at anosmotic stress of −0.3 and −0.4 MPa, and above that, no germination wasobserved. Tall morningglory showed some tolerance to salt stress. Thegermination was 40% and 12% at salt concentrations of 50 mM and 200 mM,respectively. Germination was affected by pH levels, and maximum germinationoccurred at pH 6, whereas above or below that level, germination wassignificantly reduced. Maximum germination of seeds was 83 and 94% when sownat 0 and 2 cm depth in soil, within a week of sowing; however, germinationwas significantly reduced to 76% when placed at a depth of 4 cm or deeper.Under no flooding treatment, 87% of seed germinated, but flooding delayedand inhibited the germination of tall morningglory seeds. It is concludedthat several environmental factors affected the germination of tallmorningglory, and this information could help to predict the spread of tallmorningglory in new areas such as Florida.

Modification of the cropping environment to make weed seed more susceptibleto fatal germination or decay processes is based, in part, on the premisethat seed longevity is affected by the crop-influenced environment in whichseed is produced, hereafter, called the maternal crop environment. The objective of this investigation was todetermine the influence of maternal crop environment on wild-proso milletseed production, germinability, and seed coat tone (i.e., lightness), atrait previously associated with seed longevity in wild-proso millet.Maternal corn environments were established by growing wild-proso milletplants in four morphologically different sweet corn hybrids in fourreplicates over 2 yr. Wild-proso millet seed was collected at sweet cornharvest, enumerated, characterized for seed coat tone, and tested forgermination. Principal component factor analysis reduced six sweet corntraits measured between silking and harvest into a single maternal cornenvironment factor that accounted for 84% of the variation among cropcanopies. Functional relationships between maternal corn environment factorscores and wild-proso millet seed characteristics were clarified by fittinglinear models. For each unit decrease in maternal environment factor score,wild-proso millet seed production increased 1,535 seed m−2,germination increased 2.2%, and seed coat tone was 1.8% lighter. Theseresults show the size and germinability of wild-proso millet seed washighest in less-competitive maternal corn environments characterized by ashort time to crop maturity and a small crop-canopy size.

A perennial species in its native range of Asia and Africa, Benghaldayflower in North America establishes annually from seed. This species hasthe unique ability to produce aerial and subterranean flowers and seeds.Information on how various environmental factors affect Benghal dayfloweraerial and subterranean seed germination and emergence in the United Statesis lacking. Studies were conducted to determine the effect of temperature,planting depth, salt concentration, and pre-emergence herbicides ongermination or emergence of aerial and subterranean Benghal dayflower seed.Maximum aerial seed germination occurred at 30 C, whereas maximumsubterranean seed germination occurred at 30 and 35 C. Germination at 40 Cwas delayed relative to optimum temperatures. The seed coats in this studywere mechanically disrupted to evaluate the response of seeds to temperaturein the absence of physical dormancy. The physical dormancy imposed by theseed coat could require additional study. Benghal dayflower was not tolerantto ≥ 10 mM NaCl, indicating that this exotic species is not likely to becomeproblematic in brackish marshes and wetlands of coastal plain regions. Therewas an inverse linear response of Benghal dayflower emergence and plantingdepth, with no emergence occurring at a planting depth of 12 cm. A fieldsurvey of Benghal dayflower emergence revealed that 42% of plantsestablished from a depth of 1 cm in the soil profile, with 7 cm being themaximum depth from which seedlings plants could emerge. This suggests thatPRE herbicides must remain in the relatively shallow depths of the soilprofile to maximize control of germinating seedlings. Subterranean seedswere less sensitive than aerial seeds to S-metolachlor, theprimary means of controlling this species in cotton. There were nodifferences between the germination of aerial and subterranean seed inresponse to treatment with diclosulam.

The critical period of weed control (CPWC) for ‘Pardina’ and ‘Brewer’ lentilwas determined in field experiments near Pullman, WA, in 2008 and 2009.Trial treatments were kept either weed free for periods of 0, 14, 25, 35,45, 60, 75, or ∼90 d after emergence (DAE), or weeds were allowed to growbefore removal for periods of 0, 14, 25, 35, 45, 60, 75, or ∼90 DAE.Averaged across varieties, lentil with season-long weed interference had29.5 and 32% seed yield reduction compared to weed-free lentils in 2008 and2009, respectively. When measured at crop maturity, a 1% loss in lentil seedyield resulted from each 5.68 g m−2 of dry weed biomass. Based ona 5% yield loss threshold, the CPWC for lentil was estimated to be from 270to 999 growing degree days (GDD), 22 to 57 DAE, or crop growth stage (CGS) 7to the early pod stage during 2008. In 2009, the CPWC was 624 to 650 GDD,with no occurrence of a CPWC when estimated using DAE and CGS. Spinysowthistle emerged and competed with the lentil crop later in the growingseason than mayweed chamomile, indicating that mayweed chamomile may be anearlier and stronger competitor than spiny sowthistle.

The critical period for weed control (CPWC) is an integral component ofintegrated weed management strategies. Several studies have defined the CPWCin soybean under varying agronomic conditions, yet none have described themechanisms involved in crop yield losses caused by weed competition. Wehypothesized that under nonresource-limiting conditions, morphologicalchanges resulting from the expression of shade avoidance could be used todefine a period of developmental sensitivity to low red-to-far-red ratio (R: FR) that would overlap with the defined CPWC in soybean. Two experiments(a sequential harvest and a weed addition/removal series) were conducted in2008 and 2009 under controlled environmental conditions to address thishypothesis. Two light-quality treatments were used: (1) high R : FR ratio(i.e., weed-free), and (2) low R : FR ratio (i.e., weedy). The low R : FRratio treatment induced shade avoidance responses in soybean, which includedincreases in height, internode length, and the shoot : root ratio, as wellas a reduction in biomass accumulation and leaf number. Using themorphological changes in biomass and leaf number observed in the weedaddition/removal series, a period of developmental sensitivity to low R : FRwas defined between the first trifoliate (V1) and third trifoliate (V3)stages of soybean development. This period was found to be very similar tothe CPWC previously defined by field studies of soybean–weedcompetition.

Weeds can infest management-intensive grazed pastures and impact foragequantity, forage quality, and animal health. Common burdock, plumelessthistle, and Canada thistle are three common pasture weeds in the midwesternUnited States that are managed to avoid these impacts. Experiments wereestablished at two sites to determine if increasing grazing heights fromfall through summer would reduce emergence and survival of burdock,plumeless thistle, and Canada thistle seedlings. Five simulated grazingheights (5, 10, 15, and 20 cm and a not-clipped treatment) were implementedin October 2008 and repeated in May through August. Density of all specieswas reduced from May to September, with reductions ranging from 65 to 78%,regardless of treatment. Treatments that left at least 15 cm of residualgrass had reduced densities of burdock and Canada thistle compared to the10-cm treatment. Regression analysis demonstrated that reduction in burdockand summed planted weed density was related to increased interceptedphotosynthetically active radiation from forage in April. However, totalbiomass yield was reduced up to 60% when grazing heights were increased from5 to 20 cm, although differences were only observed at the fall and earlyspring grazing events. Relative forage quality (RFQ) was similar acrosstreatments, except at the third grazing event for which the 15 and 20-cmtreatments had reduced RFQ compared with other treatments. Results suggestthat increasing grazing heights can reduce emergence and survival of burdockand Canada thistle but can also result in a reduction in forage quantity inthe fall and early spring.

Glyphosate is regularly used to control annual bluegrass populations indormant bermudagrass turf. A population of annual bluegrass not controlledby glyphosate at 840 g ha−1 (glyphosate resistant, GR) wasidentified on a golf course in Humboldt, TN in 2010. Mature tillers of GRplants were established in a greenhouse and treated with glyphosate at 0,210, 420, 840, 1,680, 3,360, and 6,720 g ha−1. Mature tillers ofa biotype known to be susceptible to glyphosate (SS) were also establishedin the greenhouse and subjected to the same treatments. At 14 d aftertreatment (DAT), glyphosate controlled the SS biotype > 95% at rates >420 g ha−1. Comparatively, the GR biotype was only controlled 76%with glyphosate at 6,720 g ha−1. The rates required to provide50% control (I50 values) for SS and GR biotypes were 236 and 2,812 g ha−1 respectively, resulting in a resistance factor of 12.Photochemical efficiency (Fv/Fm) values on SS plants treated with glyphosate at > 210 g ha−1 measured 0.000 at 14 DAT, whereas Fv/Fm values on GR plants were not significantly different from theuntreated control with glyphosate rates ≤ 840 g ha−1 on the samedate. In laboratory experiments, the SS biotype accumulated greatershikimate concentrations than the GR biotype 3 to 6 DAT. Future researchshould evaluate strategies for managing GR and SS annual bluegrass withalternative modes of action.

A wild population of a plant species, especially a cross-pollinated species,can display considerable genetic variation. Genetic variability is evidentin differential susceptibility to an herbicide because the population canshow continuous phenotypic variation. Recent, recurrent selection studieshave revealed that phenotypic variation in response to low herbicide ratesis heritable and can result in rapid evolution of herbicide resistance ingenetically variable cross-pollinated rigid ryegrass. In this study, theheritable genetic variation in an herbicide-susceptible rigid ryegrasspopulation was exploited to shift the population toward greater herbicidesusceptibility by recurrent selection. To enhance herbicide susceptibility,herbicide-susceptible rigid ryegrass plants were divided into two identicalclones, and one series of cloned plants was treated with a low rate ofherbicide (diclofop). The nontreated clones of individuals that did notsurvive the herbicide treatment were selected and bulk-crossed to obtain thesusceptible progeny. After two cycles of selection, the overallsusceptibility to diclofop was doubled. The results indicate that minorgenes for resistance are present in an herbicide-susceptible rigid ryegrasspopulation, and their exclusion can increase susceptibility to diclofop.

Field studies were used to examine the management strategies of mowing,herbicide, fertility, and all possible combinations on tall ironweedpopulations, weed biomass, and pasture yield at three Kentucky locations.Mowing was performed in July 2008 and 2009, herbicide was applied in August2008, and fertilizer was applied in September 2008 and 2009 at alllocations. Weed populations were measured in 2008, 2009, and 2010, andforage grass, clover, and weed biomass was collected in May or early June of2009 and 2010. All treatments with herbicide reduced tall ironweed stems by64% or greater in 2009 at all locations. Mowing alone, fertilizer alone, andmowing plus fertilizer did not reduce tall ironweed populations, except atone location where mowing alone reduced tall ironweed stems by 64% in 2009.Tall ironweed stems were not reduced in 2010 with any treatment at twolocations, but herbicide combined with mowing or fertilizer reduced tallironweed stems by 78% at the other location. Tall goldenrod population wasreduced up to 100% by all treatments with herbicide or mowing alone, andmowing with fertilizer reduced tall goldenrod from 59 to 89%. Treatments didnot reduce horsenettle populations. Herbicide-containing treatments reducedweed biomass at all locations. Weed biomass did not differ when comparingall treatments with and without mowing or treatments with or withoutfertilizer. Forage grass biomass was greatest with herbicide plus fertilizerand with the combination of mowing plus herbicide plus fertilizer at alllocations in both years.

Previous greenhouse studies with a noncommercial glyphosate-resistantsugarbeet variety indicated that susceptibility to Rhizoctonia crown androot rot could increase after glyphosate was applied. Greenhouse and fieldexperiments were conducted in 2008 and 2009 to determine if glyphosateinfluenced disease severity in potential commercially available varieties ofglyphosate-resistant sugarbeet. In the first greenhouse experiment in 2008,Hilleshög 9027RR, the most tolerant variety to Rhizoctonia crown and rootrot, exhibited an increase in disease severity when glyphosate was applied.There were no significant differences between herbicide treatments inHilleshög 9028RR, and glyphosate decreased disease severity in Hilleshög9032RR when compared with the no-herbicide treatment. Experiments conductedto determine if glyphosate influenced Rhizoctonia solanigrowth in vitro indicated that glyphosate did not increasethe radial growth of R. solani, except at 10× (190 µg ae ml−1) the normal rate of glyphosate plus ammonium sulfate(AMS). Field and additional greenhouse experiments were conducted using fourcommercial varieties. Differences in disease severity were observed whencomparing varieties, but glyphosate did not significantly influence theseverity of Rhizoctonia crown and root rot when compared with theno-herbicide control. Choosing a glyphosate-resistant sugarbeet variety withthe best demonstrated tolerance to Rhizoctonia crown and root rot is animportant factor in reducing disease severity and maintaining sugarbeetyield.

Rapid adoption of glyphosate-resistant (GR) corn hybrids has led to thereemergence of volunteer corn as a problematic weed in soybean and has madecontrolling the initial stand of corn in a replant situation more difficult.If volunteer corn in soybean or the initial corn stand in a replantsituation is not controlled, yield loss can occur. Clethodim and glufosinateare often used to control GR corn in corn replant situations and in soybean.The objectives of this research were to evaluate the response of two hybridcorn varieties and their F2 progeny to clethodim and glufosinateand to evaluate the effect of plant nitrogen (N) concentration on clethodimand glufosinate efficacy. First, a dose-response study was conducted withclethodim and glufosinate on DeKalb 60-18 and 60-18F2, and DeKalb63-42 and 63-42F2 to compare the response of the hybrids andtheir F2 progeny to the herbicides. DeKalb 63-42 was moretolerant to clethodim than 60-18 and 60-18F2. No differences werefound between the hybrids and their respective F2 progeny in theresponse to clethodim or glufosinate. In a second dose-response studyassessing the effect of N conditions on herbicide efficacy, both clethodimand glufosinate were less injurious to plants growing in low N than in highN availability.