Soil samples were taken from 32 commercial cotton fields of the mid-south which had been treated with one or more practical applications of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron)2 every year for the past 4 to 8 years. These samples showed no phytotoxic residues in laboratory and greenhouse studies conducted approximately one year after the last application. Cotton seed samples from eight of these fields were analyzed, and none contained a detectable residue of diuron or any of its breakdown products. Diuron degradation in soil under field conditions was demonstrated by isolation and identification of the intermediate breakdown products: 3-(3,4-dichlorophenyl)-1-methylurea, 1-(3,4-dichlorophenyl)urea, and 3,4-dichloroaniline.

Collego™ is a selective postemergent mycoherbicide for the control of northern jointvetch [Aeschynomene virginica (L.) B.S.P. # 2 AESVI] in rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.]. Collego is a two-component product. Component A is a water-soluble spore rehydrating agent while Component B is a water-suspendible dried spore preparation of the fungus Colletotrichum gloesporioides (Penz) Sacc. f. sp. aeschynomene ATCC 20358. Both components are packaged in an 18.9-L plastic mixing container. A mixture containing 0.95 L of Component A and one package of Component B will treat 4 ha. The dried spore component is packaged by number of viable spores rather than by weight. Consequently, the weight of each package will vary from lot to lot, but each package contains a minimum of 75.7 × 1010 viable spores.

Translocation of phloem-mobile herbicides was inhibited by their phytotoxic action on processes that maintain assimilate translocation. Glyphosate lowered import into developing sink leaves soon after it was applied to exporting sugarbeet leaves. Later, photosynthesis slowed down and starch accumulation stopped, but export of both assimilate and glyphosate continued until it was limited by starch availability at night Experiments with field pennycress and Tartary buckwheat indicated that self-limitation of chlorsulfuron translocation probably occurred and that it resulted from lowered assimilate entry into phloem rather than from inhibition of photosynthesis or carbon allocation. Leakage of chlorsulfuron from the phloem when export was slowed down also may have contributed to its reduced translocation.

Imazamox-resistant hybrids resulted from a cross between jointed goatgrass and an imazamox-resistant wheat (cv. FS-4 IR wheat). Two imazamox-resistant hybrids were discovered in a research plot where FS-4 IR wheat seed had been replanted from the harvest of an imazamox efficacy study conducted the year before at a different location. These hybrid plants survived imazamox applied at 0.053 and 0.069 kg ai ha−1 in the field and produced seven viable seeds (BC1). This seed germinated, and chromosomes were counted from the roots (2N number ranged from 39 to 54). In the greenhouse, six of the seven plants survived an application of 0.072 kg ai ha−1imazamox, which confirmed that the resistance trait had been passed to these progeny. A large amount of phenotypic variation was observed in the mature BC1 plants. A genetic description of the movement of the resistant gene is proposed based on the case of the gene being located on the D and the A or B genomes. Management strategies to reduce the occurrence of herbicide-resistant hybrids are presented.

Laboratory studies were conducted to investigate the possibility that herbicide runoff from treated fields might be adversely affecting submersed aquatic plants in the Chesapeake Bay. In laboratory studies, I1 and I50 values (the concentrations inhibiting growth 1 and 50%, respectively) were calculated for three herbicides and several aquatic plants. Exposure periods varied from 3 to 6 weeks, with the 3-week exposure being most common. For atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], I1 values were usually a few parts per billion (ppb), whereas I50 values varied from ca. 80 ppb for elodea (Elodea canadensis Michx) to ca. 1040 ppb for Eurasian watermilfoil (Myriophyllum spicatum L.). The toxicity of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] was similar to that of atrazine. Glyphosate [N-(phosphonomethyl)glycine] was essentially non-toxic. Interactions between salinity and atrazine were studied using the brackish water species, vallisneria (Vallisneria americana Michx.). Increasing salinity did not affect atrazine toxicity. When atrazine was present in both the hydrosoil and water, the concentration in the water determined the toxicity. At concentrations below 1000 ppb, atrazine in the hydrosoil did not adversely affect the plants. Based on the results of these studies, and the known concentrations of herbicides in runoff water, these herbicides do not appear to pose any threat to the species tested.

Effects of the imidazolinone herbicide AC243 997 [2-(5-isopropyl)-5-methyl-4-oxo-2-imidazolin-2-yl) nicotinic acid] on the growth of corn (Zea mays L. ‘Black Mexican Sweet’) cell suspension cultures were characterized. The herbicide caused half maximal growth inhibition at concentrations in the range of 10 nM to 30 nM. Significant reductions in protein synthesis were observed following treatment of cells with 10 μM herbicide. Analysis of free amino acid pool sizes indicated that levels of the biosyn-thetically related amino acids leucine and valine were substantially decreased in cells exposed to 10 μM herbicide. Supplementation of suspension culture growth media with leucine and valine plus isoleucine (1 mM each) reversed growth inhibitory effects of AC243 997 at levels ranging from 10 nM to 1 mM. The results suggest a specific interaction of this imidazolinone herbicide with leucine, valine, and isoleucine metabolism.

It is becoming increasingly clear that prescriptions for rangeland weed control are not sustainable because they treat the symptoms of weeds rather than their cause. Future restoration of invasive plant–infested rangeland must be based on ecological principles and concepts that provide for predictable outcomes. A generalized objective for ecologically based weed management is to develop and maintain a healthy plant community that is largely invasion resistant. Successional management based on ecological principles involves modifying the processes controlling the three general causes of succession: disturbance, colonization, and species performance. The processes controlling plant community dynamics can be modified to allow predictable successional trajectories. Successional management can lead to biomass optimization models for grazing management, spread vector analysis, and using resource availability to direct weedy plant communities toward those that are desired. Our challenge is to develop ecological principles on which management can be based.

Greenhouse and laboratory studies were conducted to confirm and quantify glyphosate resistance, quantify pyrithiobac resistance, and investigate interaction between flumiclorac and glyphosate mixtures on control of Palmer amaranth from Mississippi. The GR50 (herbicide dose required to cause a 50% reduction in plant growth) values for two glyphosate-resistant biotypes, C1B1 and T4B1, and a glyphosate-susceptible (GS) biotype were 1.52, 1.3, and 0.09 kg ae ha−1 glyphosate, respectively. This indicated that the C1B1 and T4B1 biotypes were 17- and 14-fold resistant to glyphosate, respectively, compared with the GS biotype. The C1B1 and T4B1 biotypes were also resistant to pyrithiobac, an acetolactate synthase (ALS) inhibitor, with GR50 values of 0.06 and 0.07 kg ai ha−1, respectively. This indicated that the C1B1 and T4B1 biotypes were 7- and 8-fold, respectively, more resistant to pyrithiobac compared with the GS biotype, which had a GR50 value of 0.009 kg ha−1. Flumiclorac was antagonistic to glyphosate by reducing glyphosate translocation. The C1B1 and T4B1 absorbed less glyphosate 48 h after treatment (HAT) compared with the GS biotype. The majority of the translocated glyphosate accumulated in the shoot above the treated leaf (that contains the apical meristem) in the GS biotype and in the shoot below the treated leaf in the resistant biotypes, C1B1 and T4B1, by 48 HAT. The C1B1 biotype accumulated negligible shikimate levels, whereas the T4B1 and GS biotypes recorded elevated levels of shikimate. Metabolism of glyphosate to aminomethylphosphonic acid was not detected in either of the resistant biotypes or the susceptible GS biotype. The above results confirm multiple resistance to glyphosate and pyrithiobac in Palmer amaranth biotypes from Mississippi and indicate that resistance to glyphosate is partly due to reduced absorption and translocation of glyphosate.

The relationship of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] decomposition to microbial activity in two soils was determined. Microbial responses to temperature, added energy material, and time were measured by evolution of CO2 from herbicide-treated soil samples. Evaluation of herbicide decomposition was made by bioassay and radioassay. Herbicide inactivation was directly related to metabolism of soil organic carbon. Larger amounts of herbicide were inactivated per unit of soil carbon metabolized in Norfolk loamy sand than in Decatur clay loam. This was associated with less soil specific surface and consequently greater concentration of herbicide on soil particle surfaces in Norfolk compared to Decatur soil. Addition of microbial energy sources accelerated decomposition of atrazine and diuron in both soils. Herbicide decomposition was closely associated with repeated additions of energy material, indicating an incidental or nonpreferential involvement in microbial metabolism. Decomposition of atrazine doubled with each 10-degree rise in temperature from 10 to 30 C, paralleling the response of soil organic matter decomposition. Diuron decomposition approximately tripled with each 10-degree rise in temperature.

Cover crop systems were investigated in 2004 and 2005 for their effects on the activity-density (a function of movement and density) of a promising group of weed biocontrol organisms, the ground beetles collectively known as carabids, with particular emphasis on a beneficial carabid species Harpalus rufipes DeGeer. Marked H. rufipes released into pea/oat–rye/vetch cover crop plots were more than twice as likely to be recaptured within the same plots as beetles released in nonvegetated fallow plots (18 and 8%, respectively). Marked beetles released into fallow plots were more than twice as likely to leave their plots and be recaptured in pea/oat–rye/vetch plots as vice versa (13 vs. 5%), indicating a clear preference for habitat with vegetative cover. Overall recapture rates were not different between treatments. Unmarked H. rufipes activity-density was also higher in pea/oat–rye/vetch compared to fallow plots. Additionally, five cover crop systems, including the fallow and pea/oat–rye/vetch treatments, and two residue management methods (conventional and zone tillage) were investigated from June to August in 2005 for their effects on H. rufipes activity-density. Corn was planted in 2005 into residues of the five cover crop systems grown in 2004. H. rufipes activity-density was higher in zone and conventionally tilled corn planted in pea/oat–rye/vetch residues and conventionally tilled corn planted in red clover/oat residues than in any other cover crop and residue management combination. Pea/oat–rye/vetch cover crop systems are apparently beneficial for H. rufipes during the cover crop year as well as in subsequent crops planted into this cover crop's residues. This system was not the least disturbed system but, based on the number of tillage events, represented a medium level of disturbance among the various systems. Thus, some level of disturbance might be beneficial for H. rufipes, but how and when that soil disturbance occurs requires further research to determine the best means of conserving this species.

The timing of nitrogen (N) fertilizer application may influence germination, emergence, and competitiveness of weeds. Research was conducted to determine the influence of total inorganic soil N (Nit) on the germination, emergence, and growth of five weed species. In a greenhouse experiment, seed of five weed species were exposed to four levels of N, and seed germination was measured. In the field, urea ammonium nitrate (UAN 28%) was applied at multiple rates at three spring timings, and Nit, weed emergence, and growth were measured for 21 to 35 d after application (DAA). Germination of the four dicotyledonous and single grass species was not stimulated by 450 ppmw of N compared with the untreated control. In the field, Nit of 112 or 168 kg N ha−1, measured at 7 and 21 DAA, was always greater than Nit in the untreated control. The duration of the available N pulse in the upper 8 cm of soil was dependent on N application rate and timing. At 8 to 16 cm of soil depth, Nit was greater when 168 kg N ha−1 was applied compared with no N at 21 and 35 DAA in 2004. Emergence of common lambsquarters increased as N application rate increased for each application date in 2003, but not in 2004. Emergence of ladysthumb increased with N application rate for the April 15, 2003, date; emergence of giant foxtail increased with N application rate for the April 6, 2004, date. Weed biomass was always greater when 168 kg N ha−1 was applied compared with no N, and at four of six N application dates, when 112 kg N ha−1 was applied. This research shows that spring N fertilizer applications increase Nit and weed growth, but the influence of N on weed emergence is dependent on the weed species, seed source, and environmental conditions.

This article discusses the concept of relative potency of herbicides in bioassays where individual dose–response curves can be similar or nonsimilar, often denoted parallel and nonparallel curves, and have different upper and lower limits. The relative potency is constant for similar dose–response curves and measures the relative horizontal displacement of curves of a similar shape along the dose axis on a logarithmic scale. The concept of similar dose–response curves has been used extensively to assess results from herbicide experiments, for example, with the purpose of adjusting herbicide doses to environmental conditions, formulations, and adjuvants. However, deeming dose–response curves similar when they are not may greatly affect the calculation of the relative potency at response levels such as effective dosage (ED)90, which is relevant for effective weed control, or ED 10, which is used in crop tolerance studies. We present a method for calculating relative potencies between nonsimilar dose–response curves at any response level. It also is demonstrated that if the upper, lower, or both limits among response curves are substantially different, then the ED 50 or any other ED level cannot be used indiscriminately to compute the relative potency. Rather, the relative potency should be viewed as a function of the response level.

Three sugarbeet selections differing in cross-resistance to three classes of acetolactate synthase (ALS)-inhibiting herbicides have been developed using somatic cell selection. Sugarbeet selections resistant to imidazolinone herbicides, Sir-13 and 93R30B, do not metabolize [14C]-imazethapyr any faster or differently than sensitive, wild-type sugarbeets or a sulfonylurea-resistant/imidazolinone-sensitive selection, Sur. ALS specific activity from the three herbicide-resistant selections ranged from 73 to 93% of the wild-type enzyme extracts in the absence of herbicide, indicating enzyme overexpression was not a factor in resistance. Acetolactate synthase from Sir-13 plants showed a 40-fold resistance to imazethapyr but no resistance to chlorsulfuron or flumetsulam. Polymerase chain reaction amplification and sequencing of two regions of the ALS gene spanning all known sites for ALS-based herbicide resistance in plants indicated a single nucleotide change in the Sir-13 gene (G337 to A337) resulting in a deduced substitution of threonine for alanine at position 113 in the sugarbeet amino acid sequence. Sur ALS was not significantly resistant to imazethapyr, but was 1,000- and 50-fold resistant to chlorsulfuron and flumetsulam, respectively. Sur gene sequencing indicated a single nucleotide change (C562 to T562) resulting in a serine for proline substitution at position 188 of the ALS primary structure. The 93R30B nucleotide sequence indicated two mutations resulting in two deduced amino acid substitutions: threonine for alanine at position 113 plus serine for proline at position 188. The 93R30B double mutant incorporated the changes observed in each of the single mutants above and correlated with higher resistance levels to imazethapyr (> 1,000-fold), chlorsulfuron (4,300-fold), and flumetsulam (200-fold) at the ALS level than observed in either of the single mutants. 93R30B represents the first double mutant derived by a two-step selection process that incorporates two class-specific ALS-inhibitor resistance mutations to form a single broad cross-resistance trait. The interaction of the two altered amino acids is synergistic with respect to enzyme resistance vs. the resistance afforded by each of the individual mutations.

The phytotoxicity of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl] benzenesulfonamide} was compared in six soils, and the relationship of activity to soil physical and chemical properties was evaluated. The influence of soil pH (4.2 to 7.8) on phytotoxicity and adsorption of chlorsulfuron incorporated into high-organic-matter soil was also studied. For the phytotoxicity studies, corn (Zea mays L. ‘Pioneer 3320’) was used as the bioassay plant. Organic matter was the soil variable most highly correlated with chlorsulfuron phytotoxicity. There was an inverse relationship between phytotoxicity and organic matter. No significant relationship between clay content and chlorsulfuron toxicity was observed. The adsorption of chlorsulfuron decreased with increasing soil pH while desorption was greater at alkaline pH. Phytotoxicity of chlorsulfuron increased with increasing soil pH and reached a maximum at pH 6.9.

The performance of a mechanistic simulation model of crop-weed competition was tested with data on the critical period of weed competition in sugarbeets and both seeded and transplanted tomatoes. In general, there was good agreement between simulated and observed yields for different periods of weed interference in each crop. The model was then used to evaluate the influence of weed density, weed height, and weather conditions on timing of the critical period. Simulations suggested that the greater the weed density, the shorter the period of time that the crop could tolerate early-season competition, and the longer the period of time that the crop must be kept weed free to prevent yield losses. Simulations also suggested that the length of time that a crop can tolerate early-season weed competition is related more to the availability of soil moisture, or possibly essential nutrients, than to light limitations.

Phytotoxicity of fluazifop-butyl {butyl 2-[4-[(5-trifluoromethyl)-2-pyridinyl)oxy] phenoxy] propanoate}, haloxyfop-methyl {methyl 2-[4-[(3-chloro-5-(trifluoromethyl)-2-pyridinyl)oxy)phenoxy] propanoate}, and sethoxydim {2-[1-ethoxyimino)-butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} increased as carrier volume was decreased from 570 to 24 L/ha. Addition of crop oil concentrate to the spray solution increased phytotoxicity, especially at carrier volumes of 190 L/ha and above. Sorghum plants were treated with the same amount of active ingredient in a single 1-, 2-, 4-, 8-, 16-, or 24-μl droplet. Smaller, more concentrated droplets were more phytotoxic than larger, more dilute droplets, with or without crop oil concentrate. Application site of single herbicide droplets also influenced phytotoxicity. In general, application of concentrated droplets to metabolically active leaf tissue resulted in maximum phytotoxicity.

Spurred anoda [Anoda cristata (L.) Schlecht.], velvetleaf (Abutilon theophrasti Medic), prickly sida (Sida spinosa L.) and Venice mallow (Hibiscus trionum L.) competition in cotton (Gossypium hirsutum L. ‘Stoneville 213′) was studied during 1973 and 1974. Cotton height was reduced by full-season competition of all species at a density greater than 32 plants/12 m of crop row. Full-season cotton competition reduced the dry matter production of velvetleaf, spurred anoda, prickly sida, and Venice mallow at weed densities less than 8, 16, 32, or 64 plants/12 m of crop row, respectively. Full-season competition resulted in seed cotton yield reductions by spurred anoda, velvetleaf, and prickly sida at 8, 16, and 64, plants/12 m of crop row, respectively. Competition from 2 weeks after cotton emergence until harvest by spurred anoda at 16 plants/12 m, and velvetleaf or prickly sida at 64 plants/12 m, reduced yields. Spurred anoda, velvetleaf, or prickly sida competition 4 to 6 weeks after cotton emergence until harvest did not reduce the seed cotton yields. Venice mallow competition did not reduce yields at any density or competition period.

Remote sensing has been used to directly detect and map invasive plants, but has not been used for forest understory invaders because they are obscured by a canopy. However, if the invasive species has a leaf phenology distinct from native forest species, then temporal opportunities exist to detect the invasive. Amur honeysuckle, an Asian shrub that invades North American forests, expands leaves earlier and retains leaves later than native woody species. This research project explored whether Landsat 5 TM and Landsat 7 ETM+ imagery could predict Amur honeysuckle cover in woodlots across Darke and Preble Counties in southwestern Ohio and Wayne County in adjacent eastern Indiana. The predictive abilities of six spectral vegetation indices and six reflectance bands were evaluated to determine the best predictor or predictors of Amur honeysuckle cover. The use of image differencing in which a January 2001 image was subtracted from a November 2005 image provided better prediction of Amur honeysuckle cover than the use of the single November 2005 image. The Normalized Difference Vegetation Index (NDVI) was the best-performing predictor variable, compared to other spectral indices, with a quadratic function providing a better fit (R2 = 0.75) than a linear function (R2 = 0.65). This predictive model was verified with 15 other woodlots (R2 = 0.77). With refinement, this approach could map current and past understory invasion by Amur honeysuckle.

The predictability of the spring emergence period of kochia is a potentially exploitable attribute that may be applied to weed management. The emergence timing of kochia was monitored, during the spring and summer of 2005 and 2006, in 12 farmed fields in the northern region of the Northern Great Plains (NGP) of North America. Kochia emergence began at 50 cumulative growing-degree days (GDD Tbase 0 C, starting January 1) and continued into late summer. The onset of kochia emergence occurred well before the emergence onset of other weed species common to this region. Fall soil samples revealed few kochia seeds remained in the seedbank. In growth room studies, kochia seed placed at the soil surface had greater emergence compared with seed burial to a 10 mm or greater depth. Kochia did not emerge from seed planted 80 mm deep; however, radical emergence was evident indicating fatal germination. Kochia should be controlled early in the growing season, and burying seed will greatly reduce future kochia populations.

Seeds of common cocklebur (Xanthium pensylvanicum Wallr.), jimsonweed (Datura stramonium L.), ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.], giant ragweed (Ambrosia trifida L.), yellow foxtail [Setaria lutescens (Weigel) Hubb.], and velvetleaf (Abutilon theophrasti Medic.) were buried in the soil at depths down to 15.2 cm in November 1966. Seeds of jimsonweed, ivyleaf morningglory, giant ragweed, yellow foxtail, velvetleaf, common ragweed (Ambrosia artemisiifolia L.), and Pennsylvania smartweed (Polygonum pensylvanicum L.) were buried 2.5 and 10.2 cm below the surface in October 1968. Seeds were exhumed for periodic laboratory analyses of dormancy changes. All species except ivyleaf morningglory and common cocklebur germinated better in light than in darkness after at least one winter of burial in the soil. Seeds decayed faster at 2.5 cm below the soil surface than at 10.2 cm, but some viable seeds of all species were recovered from both depths after 3 years. The development or maintenance of hard seeds was considered the principle mechanism for seed survival for 3 years in these species.