Of 15 hard red winter wheat (Triticum aestivum L.) cultivars grown on several soil types and treated in the spring with several rates of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one], ‘Lindon’ and ‘Vona’ were the most sensitive and ‘TAM W 101’ and ‘Osage’ the most tolerant. Differences in cultivar response to metribuzin occurred in 9 of 13 locations, but were not observed when precipitation was very light or when the soil was saturated at the time of treatment. TAM W 101 was also more tolerant than Vona in the greenhouse.

Downy brome is a grassy weed that infests millions of acres of winter wheat in the Western United States and costs farmers millions of dollars in lost yields. It can not be controlled consistently using chemicals or cultural practices. Isolates of naturally occurring rhizobacteria have shown potential as biological control agents for downy brome. Centrifuged supernatant from aerobic shake cultures of Pseudomonas fluorescens strain D7 suppressed germination of seeds and reduced root and shoot growth of downy brome in agar diffusion assays. The present article relates to the isolation and characterization of active compounds from strain D7. The active compounds were denatured and lost activity with commonly used techniques of extraction, concentration, and purification of microbial products. Two unusual techniques for isolation and partial purification of the active compounds from supernatant of strain D7 were developed. With one technique, supernatant was concentrated at 30 to 35 C under vacuum to 10% of the initial volume. Subsequently, the concentrate was chromatographed on sephadex gels to separate the active fraction. With the second technique, cell-free supernatant was frozen, with or without prior concentration, at −12 C for 12 to 24 h. The frozen supernatant was then thawed at 2 to 4 C, resulting in precipitation and crystallization of the active fraction. The active fraction isolated with either method inhibited downy brome. Inhibition was complete at concentrations as low as 1 mg total dry matter L-1 agar. Also, the active fraction inhibited the plant pathogenic fungus Gaeumannomyces graminis var. tritici. Chemical analysis of the active fraction complex revealed the presence of chromopeptides and other peptides, fatty acid esters, and a lipopolysaccharide matrix. Separation of any of the components from the complex resulted in nearly complete loss of activity against downy brome.

Plant canopy reflectance over the 0.45- to 1.25-μm wavelength (WL) of weed species and crops was recorded with a field spectroradiometer to evaluate the possible use of remote sensing to distinguish weeds from crops. Weed and weed-crop species reflectance differences were generally greater at the 0.85 μm WL in the near-infrared spectral region than at the 0.55 μm WL in the visible region, indicating that color infrared (CIR) aerial photography may be useful to detect weed populations in crops. Canopy reflectance data were more directly related to photographic differences in weed-crop images than were single leaf or inflorescence reflectance data. Aerial photography at altitudes of 610 to 3050 m distinguished climbing milkweed (Sarcostemma cyancboides ♯ SAZCY) in orange [Citrus sinensis (L.) Osbeck. ‘Valencia’) trees; ragweed parthenium (Parthenium hysterophorus L. ♯ PTNHY) in carrot (Daucus carota L., var. sativa ‘Long Imperator’); johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA) in cotton (Gossypium hirsutum L. ‘CP 3774’) and in sorghum (Sorghum bicolor L. Moench. ‘Oro’); London rocket (Sisymbrium irio L. ♯ SSYIR) in cabbage; and Palmer amaranth (Amaranthus palmeri S. Wats. ♯ AMAPA) in cotton. Johnsongrass was also detectable with CIR film in maturing grain sorghum from 18 290 m. Detection of weed species in crops was aided by differential stages of inflorescence and senescence, and by the chlorophyll content, color, area, intercellular space, and surface characteristics of the leaves. Discrete plant community areas were determined by computer-based image analyses from a 1:8000-scale positive transparency with the efficiency of 82, 81, 68, and 100% for Palmer amaranth, johnsongrass, sorghum, and cotton, respectively. The computer analyses should permit discrete aerial surveys of weed-crop communities that are necessary for integrated crop management systems.

A model for determination of economic thresholds, or minimum weed population densities justifying the use of postemergence herbicide treatment, for five weed species in soybeans [Glycine max (L.) Merr.] is presented. Sensitivity analysis was performed on the model with respect to economic, statistical, and agronomic variables. The model was refined to include uncertainty about lost field days during the spraying period. Predictions from both the simple and refined models were consistent with economic theory. It was also determined that the economic threshold is sensitive to choice of data-collection ranges and functional form in weed-interference studies.

Wheat (Triticum aestivum L.) grain yields were reduced an average of 4.2% for every 10 Italian ryegrass [Lolium multiflorum (Lam.) # LOLMU] plants/m2 within the range of 0 to 100 Italian ryegrass plants/m2. Yield reductions caused by Italian ryegrass were attributed primarily to decreased crop tillering. Italian ryegrass densities as high as 80 plants/m2 had little effect on wheat head or kernel weights. In greenhouse experiments, the growth response of Italian ryegrass to increasing concentrations of NO3– and K+ was greater than that of wheat. Net uptake rates for NO3– by both species growing in nutrient solution were 1.5 times greater than net uptake rates for K+. Nitrate and potassium Imax values for Italian ryegrass were approximately twice the corresponding values for wheat. Although Italian ryegrass responded more to changes in nutrients and had greater ion uptake rates compared to wheat, Italian ryegrass accumulated more biomass when grown in monoculture than when grown in association with wheat. This difference was probably due to the initial size of the seedlings. Wheat seedlings were much larger than Italian ryegrass seedlings during the first 20 days following emergence.

The degradation rates of 2,4-D [(2,4-dichlorophenoxy)acetic acid], dichlorprop [2-(2,4-dichlorophenoxy)propionic acid], 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], silvex [2,(2,4,5-trichlorophenoxy)propionic acid], dicamba (3,6-dichloro-o-anisic acid), and picloram (4-amino-3,5,6-trichloropicolinic acid) were determined in three soils. Herbicide breakdown was proportional to herbicide concentration, so half life of the various herbicides was calculated from linear regression of the logarithm transformed residue data. The average half life for 2,4-D, dichlorprop, silvex, 2,4,5-T, dicamba, and picloram were, respectively, 4 days, 10 days, 17 days, 20 days, 25 days, and greater than 100 days. The rate of degradation of 2,4-D was the same in all three soils, but for the other herbicides it was consistently faster in soil removed from under grass vegetation than from under trees.

Toxicity of nonradiolabeled and translocation of 14C-labeled glyphosate [N-(phosphonomethyl)glycine] in cotton (Gossypium hirsutum L.) were evaluated under different environmental conditions as affected by surfactant and by the maturity of the plant tissue at the place of herbicide application. Toxicity was affected primarily by temperature with 21 to 40% greater cotton injury at 25 C than at 35 C. The addition of surfactant often increased toxicity under environmental conditions where toxicity was initially low. Absorption and translocation of 14C was affected primarily by relative humidity (RH). At constant temperature and rate of surfactant, an increase in RH from 40 to 100% resulted in a 3- to 6-fold increase in translocation of 14C. Autoradiograms indicated that the movement of the 14C-label was primarily into leaves above the treated area. Translocation of 14C was significantly greater following application to the mature lower stem than to the mature lower leaves or to immature upper stem or leaves of cotton.

Tolerance of greenhouse-grown soybean (Glycine max (L.) Merr.) to preemergence applications of 4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)one (metribuzin) was greatly influenced by herbicide rate, soil organic matter, and simulated rainfall after treatment. Planting depth affected tolerance in cases where effects of the other variables were marginal. Severe injury was observed with 1.1% soil organic matter except at the 0.56 kg/ha herbicide rate with no rainfall for 10 days after treatment. Tolerance increased with increasing soil organic matter to the point where no injury occurred at 16.8 or 40.0% organic matter, even at herbicide rates up to 1.68 kg/ha with 1.25 cm simulated rainfall immediately after treatment.

Field and greenhouse trials have shown that ultra-low-volume applications (ULV) of glyphosate (N-(phosphonomethyl)glycine] exhibit a greater phytotoxic effect on barley (Hordeum vulgare L. ‘Bonanza′) at a given rate than do high-diluent-volume applications with a conventional hydraulic sprayer. Three greenhouse studies and one field trial were conducted to evaluate the influence of individual drop makeup on the effectiveness of ULV applications. Barley plants were treated at the four-leaf stage with one, three, or nine drops (1 μl each) of glyphosate and a non-ionic surfactant with the concentration of the drops adjusted so that the total amount of active ingredient applied to each plant was constant. Application of a single, concentrated drop was significantly more effective in reducing total shoot growth than were applications of more dilute drops in greater number. When the dilute drop was supplemented with additional surfactant, phytotoxicity was restored, provided there was a sufficiently high concentration of glyphosate in the drop. With the substitution of a commercial formulation of glyphosate, the response of barley to variations in drop number and drop concentrations followed a similar trend. The response of field-grown plants did not differ from plants grown under controlled conditions.

Polyethylene mulch is an effective component of weed management in vegetable production. However, nutsedges are persistent and proliferate in these systems. Greenhouse studies evaluated the growth and tuber production of purple nutsedge and yellow nutsedge grown in pots covered with black-opaque polyethylene mulch, clear-colorless polyethylene mulch, or nonmulched. Single, presprouted nutsedge tubers were planted and growth evaluated over 16 wk. Relative to the nonmulched, yellow nutsedge shoot production was reduced 46 and 72% by black and clear mulch, respectively. The number of yellow nutsedge shoots that pierced and emerged through black and clear mulches was reduced 96% relative to emerged shoots in the nonmulched control. Yellow nutsedge in the nonmulched produced 366 tubers per initial tuber, whereas tuber production was reduced 49 to 51% in black and clear mulch. Growth of purple nutsedge shoots and tubers in black polyethylene mulch was similar to the nonmulched. Clear polyethylene reduced purple nutsedge tuber biomass relative to nonmulched, but clear mulch was similar to black mulch in nearly all measured variables. Without mulch, yellow nutsedge produced more shoots, shoot biomass, tubers, tuber biomass, and root biomass than purple nutsedge. However, growth of yellow nutsedge was hindered by polyethylene mulch, whereas differences in purple nutsedge growth among mulches could not be detected. The relative insensitivity of purple nutsedge and sensitivity of yellow nutsedge growth to the physical mulch barrier could lead to a shift in nutsedge species composition in mulched vegetable production.

Dwarf sorghum (C4) was grown at ambient and at projected levels of atmospheric carbon dioxide (250 mol mol−1 above ambient) with and without the presence of a C3 weed (velvetleaf) and a C4 weed (redroot pigweed), to quantify the potential effect of rising atmospheric carbon dioxide concentration [CO2] on weed–crop interactions and potential crop loss. In a weed-free environment, increased [CO2] resulted in a significant increase in leaf weight and leaf area of sorghum but no significant effect on seed yield or total aboveground biomass relative to the ambient CO2 condition. At ambient [CO2] the presence of velvetleaf had no significant effect on either sorghum seed yield or total aboveground biomass; however, at elevated [CO2], yield and biomass losses were significant. The additional loss in sorghum yield and biomass was associated with a significant (threefold) increase in velvetleaf biomass in response to increasing [CO2]. Redroot pigweed at ambient [CO2] resulted in significant losses in total aboveground biomass of sorghum but not in seed yield. However, as [CO2] increased, significant losses in both sorghum seed yield and total biomass were observed for sorghum–redroot pigweed competition. Increased [CO2] was not associated with a significant increase in redroot pigweed biomass (P = 0.17). These results indicate potentially greater yield loss in a widely grown C4 crop from weedy competition as atmospheric [CO2] increases.

Purple and yellow nutsedge are the most troublesome weeds of vegetable crops in the southeast United States. Elimination of methyl bromide use will require alternative management programs to suppress nutsedge growth and interference in vegetables. Polyethylene mulch is an effective barrier for most weeds; however, nutsedges can proliferate in beds covered with polyethylene mulch. The influence of polyethylene mulch on shoot production and lateral expansion patterns of single tubers of purple nutsedge and yellow nutsedge over time was evaluated in field studies. Purple nutsedge patch size was similar in the black mulch treatment and nonmulched control after 8 and 16 wk after planting (WAP). By the end of the growing season, purple nutsedge patch size in the black mulch treatment was nearly twice that in the nonmulched control. At 32 WAP, there were 1,550 shoots in the 16.1 m2 patch in the black mulch treatment and 790 shoots in the 8.1 m2 patch in the nonmulched control. In contrast, yellow nutsedge growth was suppressed in the black mulch treatment, relative to the nonmulched control. Compared with the black mulch treatment at 16 and 24 WAP, the nonmulched control produced nearly three times as many yellow nutsedge shoots (140 shoots at 16 WAP and 210 shoots at 24 WAP) and patches that were twice the size (0.10 m2 at 16 WAP and 0.18 m2 at 24 WAP). These data indicate that there are significant differences in the growth habits of the two nutsedges species in mulched vegetable systems. The differences in response to black mulch will likely lead to purple nutsedge becoming a greater problem, relative to yellow nutsedge, in vegetable systems. The rapid expansion of a single purple nutsedge shoot to form a patch that is 22.1 m2 and containing 3,440 shoots at 60 WAP illustrates the importance of managing this species.

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−1 glyphosate, 35 g ai ha−1 cloransulam, and 0.84 kg ae ha−1 glyphosate + 35 g ai ha−1 cloransulam. 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.

The widespread evolution of herbicide resistance in weed populations has become an increasing concern for no-tillage (NT) growers in semiarid regions of the U.S. Great Plains. Lack of cost-effective and alternative new herbicide sites of action further exacerbates the problem of herbicide-resistant (HR) weeds and threatens the long-term sustainability of prevailing cropping systems in the region. A recent decline in commodity prices and increasing herbicide costs to manage HR weeds has spurred research efforts to build a strong rationale for developing ecologically based integrated weed management (IWM) strategies in the U.S. Great Plains. Integration of cover crops (CCs) in NT dryland production systems potentially offers several ecosystem services, including weed control, soil health improvement, decline in selective pest pressure, and overall reduction in pest management inputs. This review article aims to document the role of CCs for IWM, with emphasis on exploring emerging weed issues; ecological, economic, and agronomic benefits of growing CCs; and constraints preventing adoption of CCs in NT cropping systems in the semiarid Great Plains. We attempt to focus on changes in weed management practices, their long-term impacts on weed seedbanks, weed shifts, and herbicide-resistance evolution in the most common weed species in the region. We also highlight current knowledge gaps and propose new research priorities based on an improved understanding of CC management strategies that will ultimately aid in achieving sustainable weed management goals and preserving natural resources in water-limited environments.

Downy brome, an obligately selfing winter annual, has invaded a variety of habitats in western North America. Seeds are at least conditionally dormant at dispersal in early summer and lose dormancy through dry after-ripening. In the field, patterns of germination response at dispersal vary among populations and across years within populations. Degree of dormancy at summer temperatures in recently harvested seeds, as well as rate of dormancy loss during dry storage, can be related to the risk of premature summer germination in different habitats. Patterns of dormancy loss are predictable and can be modeled using hydrothermal time concepts. To assess the relative contribution of genotype and maturation environment, multiple parental lines from contrasting populations were grown for three generations under manipulated greenhouse conditions. Significant germination response differences among populations were observed, as well as major differences among full-sib families. Among-population variation accounted for over 90% of the variance in germination traits, whereas within-family variance accounted for 1% or less. Populations from predictable extreme environments (subalpine meadow and warm desert margin) showed significantly less variation among families than did populations from less predictable environments (cold desert, foothill, and plains). Environmental conditions that shortened the seed ripening period (water stress and high temperature) resulted in reduced seed dormancy level at maturation, but there were strong inbred line–environment interactions. For fully after-ripened seeds, inbred line and environmental effects were no longer evident, indicating that differences in genotype and maturation environment function mainly to regulate dormancy and dormancy loss rather than to mediate response patterns of nondormant seeds.

A leaching test conducted in field lysimeters for the purpose of pesticide registration is evaluated, particularly in terms of factors such as the effects of soil type, variability in leaching between replicate lysimeters, and simulation of worst-case scenarios. Two herbicides, dichlorprop and bentazon, were chosen as test compounds due to their documented high mobility in laboratory tests. Four different soil types (sand, loam, clay, peat) and two irrigation treatments were included. Both herbicides were applied at rates representing normal doses (1.6 and 0.6 kg ai ha−1 of dichlorprop and bentazon, respectively). 36Cl was also applied to sand and clay lysimeters to follow the pattern of water movement. Leaching of dichlorprop for the varying soil type/treatment combinations ranged from 0.02 to 1.8% of the amount applied. Leaching losses of bentazon reached up to 0.07% of that applied. Leaching of both herbicides was greater mostly in clay monoliths than in sand monoliths, which was explained in terms of macropore flow. A more effective macropore flow was also suggested to be the main reason why more dichlorprop leached in clay and peat monoliths treated with a small water input. Detectable, and in some cases large, concentrations of dichlorprop were found in the first drainage water in early autumn in all soil/treatment combinations, indicating the occurrence of preferential flow in all soils tested, including sand. A rapid breakthrough of 36Cl was also found in clay and low-irrigation input sand, providing additional confirmation of the role of preferential flow processes in these soils. It is concluded that field mobility tests for pesticide registration are a necessary complement to measurements of physical/chemical properties of a compound and that these should be performed in a range of soil types, including at least one structured soil. Other factors identified to be of importance when evaluating lysimeter studies such as this were the analytical detection limits of the pesticides and the need for replication.

Rigid ryegrass and wild radish dominate and coexist throughout southern Australian dryland cropping regions. Widespread herbicide resistance in these species has led to adoption of diverse and complex integrated weed management practices, which require evaluation of their impact on farming systems. Therefore, a multispecies version of the bioeconomic model resistance and integrated management (RIM) has been developed to compare long-term economic and weed population outcomes of various integrated management scenarios. We have extended the original single-species ryegrass RIM model to include wild radish biology and additional weed management practices used to control this species. The multispecies model can be used to evaluate weed management scenarios for coexisting herbicide-resistant species by investigating the implications of different crop–pasture rotational sequences and of varying herbicide availability. Multispecies RIM shows that economic differences between the scenarios are not due to differences in weed densities but to differences in total weed control costs.

We measured some of the more important biologic and microclimatic parameters of the functional environment of downy brome (Bromus tectorum L.) communities during the establishment period of seedlings of intermediate wheatgrass (Agropyron intermedium (Host) Beauv., var. Amur). These measurements were made in relation to weed control and seeding treatments. Weed growth and establishment of replacement vegetation were directly related to available soil moisture and favorable temperatures in the immediate environment of growing plants. Lack of available soil moisture during critical periods of the growing season was the major deterrent to seeding success. The limited precipitation stored as soil moisture was available to perennial grass seedlings where weeds were controlled. Without weed control, growth of downy brome preempted the effective precipitation, and moisture became limiting for growth of perennial grass seedlings. Seeding in furrows, along with weed control, further improved moisture conditions for the seedlings. Also, use of furrows enhanced seedling growth by lowering maximum air and soil temperatures in the seedlings' immediate environment.

Weed seeds constitute one of the principal devices for invasion of weeds into new areas. Because of this fact seed laws require that the amount and kinds of weed seeds be indicated on the label attached to seeds offered for sale. In many states and provinces excessive amounts of noxious weed seeds per unit weight may be the basis for the prohibition of sale of otherwise acceptable seed.

Experiments were conducted to determine adsorption of methanearsonate by soils, soil separates, and reference clay minerals from solutions of disodium methanearsonate (DSMA), effects of soil type and pH on the leaching of surface-applied DSMA, and effects of soil type and added organic matter on the oxidation of the methyl carbon of methanearsonate.

Adsorption of methanearsonate by kaolinite and vermiculite increased with increasing concentration of DSMA in the equilibrium solution. Kaolinite and limonite adsorbed much more methanearsonate than vermiculite and montmorillonite. The clay fraction of Augusta silt loam adsorbed more methanearsonate than did the sand and silt fractions. Adsorption by whole soils was related generally to their clay contents.

Leaching with 20 successive 1-in increments of water removed 52% of surface-applied methanearsonate from Norfolk loamy sand. Rates of movement of DSMA in Norfolk loamy sand did not differ at three pH levels. About one-half of the applied methanearsonate remained in the surface inch of Decatur clay loam and none was leached below 6 in.

Oxidation of the methyl carbon of methanearsonate was associated with oxidation of soil organic matter in each soil. In three of the soils, there was no evidence of microbiological adaptation to methanearsonate. In Norfolk loamy sand, however, increasing decomposition of methanearsonate relative to soil organic matter occurred with time of incubation. Decomposition of added plant material in this soil enhanced the oxidation of methanearsonate.