No-tillage corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed [Conyza canadensis (L.) Cronq.]. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences between the isopropylamine or diammonium salts of glyphosate.

Single-leaf CO2 assimilation rate under saturating light (CA) varies as a function of leaf nitrogen content per unit leaf area (NL). Measured CA for many crop species also declines with developmental stage. Because these relationships may have strong implications for crop–weed competition, a field experiment was conducted to quantify corn and velvetleaf CA as influenced by leaf NL and stage of development. Crop and weed CA were measured on the most recent fully expanded leaves of plants grown in four nitrogen (N) application treatments. Both corn and velvetleaf CA increased with increasing NL, up to about 1.5 g N m−2. Corn and velvetleaf NL values were rarely less than 0.75 g N m−2, indicating that both species may restrict leaf growth in order to maintain constant NL under conditions of limited soil N. Corn CA declined to half its maximum by physiological maturity, whereas velvetleaf CA only declined 18%. Although velvetleaf CA was considerably lower than that of corn, the difference decreased as the growing season progressed. Because corn leaf production is complete by anthesis and velvetleaf continues vegetative production throughout its life cycle, velvetleaf will produce relatively greater quantities of biomass late in the season, which may increase competition for light.

Several populations of smooth pigweed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine if differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption of foliar-applied cloransulam-methyl was rapid and similar in both populations of smooth pigweed. Translocation of the absorbed radioactivity out of the treated leaf was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. 14C-cloransulam-methyl metabolism was also similar in S and R2 populations. Differential tolerance to cloransulam-methyl in S and R2 populations in the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.

To be effective, postemergence herbicides must be absorbed and translocated to sites of action in proper form and quantity. Any factor that interferes in this process may account for differential sensitivity. Adjuvant effects on foliar absorption of BAY MKH 6561 by jointed goatgrass and downy brome were evaluated under growth chamber conditions. Absorption of BAY MKH 6561 by jointed goatgrass and downy brome without adjuvants was 41 and 30% of applied, respectively, 48 h after treatment (HAT). Herbicide absorption with methylated seed oil (MSO) was significantly higher than with nonionic surfactant (NIS) 24 and 48 HAT. The addition of urea ammonium nitrate (UAN) to MSO and NIS significantly increased absorption over MSO and NIS alone 24 HAT, but absorption was similar to that obtained with MSO 48 HAT. Averaged across adjuvant combinations, jointed goatgrass and downy brome absorbed 90 and 89% of applied BAY MKH 6561, respectively, 48 HAT. BAY MKH 6561 translocation and metabolism in jointed goatgrass, downy brome, and winter wheat were also evaluated. More 14C-BAY MKH 6561 translocated to shoot and root tissue in downy brome than in jointed goatgrass and winter wheat. Root exudation accounted for 26% of root-translocated BAY MKH 6561 in jointed goatgrass, 31% in downy brome, and 43% in winter wheat. Winter wheat, jointed goatgrass, and downy brome metabolized 82, 65, and 50% of absorbed 14C-BAY MKH 6561 12 HAT, respectively, and 97% metabolism occurred in all species 48 HAT. Exponential decay equations predicted a 7-h BAY MKH 6561 half-life in winter wheat, 10-h half-life in jointed goatgrass, and 13-h half-life in downy brome. Jointed goatgrass absorbed amounts of 14C-BAY MKH 6561 that were similar to those absorbed by downy brome, but jointed goatgrass was intermediate in translocation and metabolism compared to winter wheat and downy brome. Therefore, differential translocation and metabolism may explain differential field susceptibility observed between winter wheat, jointed goatgrass, and downy brome.

A mechanistic model was constructed for common ragweed growth and development based on the generic plant model CROPSIM. Adaptations were made to CROPSIM's growth and development subroutines to enable common ragweed growth to be simulated. Data from field studies using a single-source common ragweed grown in monoculture and from the literature were used to parameterize the model. The influences of varying environmental conditions across years, densities, and emergence timing on leaf number, leaf area, leaf weight, height, and biomass accumulation were taken into account by the model. Deviations between simulated and measured values generally fell within a relatively narrow range. Deviations outside this range tended to be associated with common ragweed growth shortly after emergence, particularly during temperature and moisture extremes. Future versions of the CROPSIM model may need to include more detailed algorithms for upper soil surface layer temperature and moisture conditions and improved germination and emergence algorithms to reduce these deviations.

This study examined pollen morphological variation among Amaranthus species and interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and several putative hybrids resulting from interspecific mating between common waterhemp and Palmer amaranth were grown in a greenhouse. Mature pollen was collected, viewed, and photographed with a scanning electron microscope (SEM). The pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture arrangement. Differences were observed between the monoecious and dioecious Amaranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious species, Palmer amaranth, whose pollen was similar to that of the monoecious species spiny amaranth. However, pollen grain diameters did not differ between the monoecious and dioecious plants. Significant differences also were noted between the pollen from the putative common waterhemp × Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal parent but had an aperture number that was intermediate between parents. This indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought.

Giant ragweed exhibits a high degree of polymorphism among individual plants in seed size, shape, spininess, and color. These features may play an important role in giant ragweed seed survival and predation avoidance; however, they are difficult to evaluate because of lack of quantification methods. A computer imaging technique was developed for describing and classifying giant ragweed seeds using digital images of the seed top and side views. Seed samples collected from 20 different giant ragweed plants (classes) were mounted and digitally scanned. Quantitative features were extracted from the seed images, including color, width, height, area, and seed perimeter. A polygon (convex hull) of the seed image based on the seed outline was constructed, from which spininess indices were developed. Fisher's linear discriminant with normalized nearest neighbor classification was used to classify randomly selected images of individual seeds according to class (maternal origin), using the extracted features as a database. The best classification rate achieved was 99%, with 138 out of 140 seeds correctly matched using data from both the top and side views. Seed features were easily extracted and varied from 1.2- to 4.5-fold among classes. Area and perimeter measurements varied least within classes but varied most among classes, suggesting that these features discriminate effectively among seeds from different plants in giant ragweed. Convex hull area : seed area ratio, using the seed top view images, was the best index of seed spininess, aligning well with visual assessment and providing greatest discrimination among classes. This experiment shows that in the case of giant ragweed, seeds from different plants are distinguishable in an objective and quantitative manner. This imaging technique can be applied to identification of seeds from different species and to studies on variable seed morphology within a species.

Field experiments were conducted at Vegreville, Alberta, in 1997, 1998, and 1999 and in Lacombe, Alberta, in 1997 and 1998, to determine if barley row spacing (20 and 30 cm) and seeding rate (75, 125, and 175 kg ha−1) influenced the effects of variable tralkoxydim rates on barley seed yield, net economic returns, and wild oat seed production. In most cases, barley seed yield was unaffected by row spacing or seeding rate. Where no herbicide was applied, the presence of wild oat reduced barley yield at each location each year. When the herbicide was applied at 50, 75, or 100% of the recommended rate, barley yields were not affected by the presence of wild oat. Results were more variable at 25% of the recommended rate, especially at Lacombe, where yield losses occurred both years at this rate. The lowest net economic returns consistently occurred in the absence of herbicide application; however, the influence of herbicide rate on net returns varied among years and locations. Net returns were either higher at the lower herbicide rates or were unaffected by herbicide rate. Seeding rate and herbicide rate affected wild oat seed production at each location each year and also the amount of seeds in the soil seedbank at Vegreville in 1999. Row spacing had little or no effect on wild oat seed production. There was a consistent and highly significant seeding rate by herbicide rate interaction on wild oat seed production. The effects of tralkoxydim on wild oat seed production, especially at relatively low rates, were superior at the higher barley seeding rates. The results suggest that seeding barley at relatively high rates can result in optimum barley yields, undiminished economic returns, and effective wild oat management when tralkoxydim is used at lower than recommended rates.

This study was conducted to determine the effect of Canada thistle density and the direct and indirect effects of Canada thistle aboveground biomass and N concentration on wheat yield. A 4-yr experiment (1991–1995) with four Canada thistle densities (0, 4, 16, 64 plants m−2) was conducted. Initial statistical analysis showed a significant effect of Canada thistle density on wheat yield. Multiple regression and path analysis showed that the main factor causing wheat yield loss was Canada thistle N concentration. The second factor affecting wheat yield was Canada thistle biomass, and the last was Canada thistle density.

Vegetative propagules of an invasive riparian weed, giant reed, were collected monthly from two Southern California sites and planted in a greenhouse from August 1998 to July 1999. Rooting and emergence frequency of planted pieces and time to emergence, growth rate, and number of developing shoots were recorded; soluble carbohydrates were analyzed. Response variables were regressed against climatic, seasonal, and site effects using a stepwise model. Rhizomes established much more frequently than stems in all months. Time of year of collection was found to be the most important factor determining establishment of all propagule types. The interaction of maximum daily temperature and precipitation at the field sites had a lesser, but significant effect on rooting frequency. The lack of a consistent correlation between any of the response variables and climate or site may indicate broad environmental tolerance. Seasonal patterns in emergence, growth, and soluble carbohydrates suggest that control by shoot removal would be most effective in fall when rhizome carbohydrate reserves are the lowest, resulting in the greatest reduction in regrowth. Chemical control with phloem-mobile herbicides would be most effective in late summer or early fall, when carbohydrates are moving from leaves to belowground structures but prior to natural leaf senescence.

Studies were conducted to evaluate density-dependent effects of common ragweed on weed growth and peanut growth and yield. Common ragweed height was not affected by weed density and peanut canopy diameter. Weed height exceeded peanut height throughout the growing season, indicating that competition for light occurred between the two species. Common ragweed aboveground dry biomass per plant decreased as weed density increased, but total weed dry biomass per meter of crop row increased with weed density. The rectangular hyperbola model described the effect of weed density on percent peanut yield loss. With the asymptote constrained to 100% maximum yield loss, the I coefficient (yield loss per unit density as density approaches zero) was 68.3 ± 12.2%. Common ragweed did not influence the occurrence of tomato spotted wilt virus, early leaf spot (Cercospora arachidicola), southern stem rot (Sclerotium rolfsii), and Cylindrocladium black rot (Cylindrocladium crotalariae). However, as common ragweed density increased, the incidence of late leaf spot (Cercosporidium personatum) increased. Results indicate that common ragweed is one of the more competitive weeds in peanut and a potential economic threat to peanut growers.

Geostatistical techniques were used to describe and map the spatial distribution of crenate broomrape populations parasitizing broad bean over 6 yr (from 1985 to 1990). In the first year, the spatial distribution was random, but from 1986 to 1989, crenate broomrape populations were clearly aggregated. The crenate broomrape infection severity (IS: number of emerged broomrape m−2) increased every year, from an average of 0.45 in 1985 to 29.4 in 1989, with a slight decrease the following year (IS = 27.4). Spherical functions provided the best fit because the cross-validation criteria were accomplished in all study cases. Kriged estimates were used to draw contour maps of the populations. About 34.3, 43.3, and 74.3% of the field plot surface exhibited an IS ≥ 1 (economic threshold) in 1985, 1986, and 1987, respectively, and nearly 100% of the area exceeded the economic threshold from 1988 to 1990; 1985 and 1986 were key years for control of the parasitic weed population. The percentage of infested area at different IS intervals in each year's map obtained by kriging was used to estimate the percentage of yield losses in each infested area (YA) with the equation: YA = A ∗ Ymax ∗ (1 − IS ∗ 0.124), where A is the infested area at a given IS interval and Ymax is the expected broomrape-free broad bean yield. Yield losses under different IS intervals were compared with yield loss attributable to a uniform distribution of crenate broomrape. Results showed that yield loss assuming a uniform distribution of crenate broomrape was clearly overestimated, which is important to avoid overuse of herbicides.

The influence of tillage, crop rotation, and weed management regimes on the weed seedbank in land previously under the conservation reserve program (CRP) for 8 yr was determined from 1994 through 1997. The study was a split-plot design with four replications, two tillage systems, two crop rotations, and three weed management treatments. Eleven weed species were recorded in 1994 and 1995, and 13 in 1996 and 1997. The weed seedbank was dominated by broadleaf species. In 1994, the first year after CRP, the seed population density in the top 15 cm of the soil profile was 51,480 seeds m−2, of which 60 and 20% were pigweed and common lambsquarters. The population density of pigweed seeds in the seedbank increased over time and reached 51,670 seeds m−2 in 1996. In contrast, the seed population density for foxtail species was only 417 seeds m−2 in 1994, but it increased to 7,820 seeds m−2 in 1997. The large increase in foxtail species seed population density in the 4-yr period was mainly in the no-herbicide weed management treatment. The weed seedbank was reduced similarly by band and broadcast herbicide treatments. Tillage and crop rotation did not influence the weed seedbank or Shannon's diversity index, nor did they interact with the weed management treatments in any of the years. The weed seedbank population density varied with the years and time of soil sampling. Weed seed population densities tended to be greater in the fall but declined significantly by time of the spring sampling. The no-herbicide treatment had a more diverse weed seedbank compared with band and broadcast herbicide weed management treatments. An average of one grass and three broadleaf weed species were identified in the three weed management treatments. Band and broadcast herbicide treatments reduced the weed seedbank population density but did not affect the number of broadleaf weed species observed.

Greenhouse studies determined dose responses of winter annual weeds and winter wheat to preemergence (PRE) and postemergence (POST) treatments of MKH 6561 and the residual effects on kochia. MKH 6561 at 11 to 45 g ha−1 did not affect wheat. MKH 6561 at 11, 34, and 45 g ha−1 reduced winter annual weed densities compared to an untreated control. Weed growth was inhibited as the rate of MKH 6561 was increased, but downy brome, cheat, and Japanese brome were three to six times more susceptible than jointed goatgrass. The GR70 values for cheat, downy brome, Japanese brome, and jointed goatgrass were 7, 11, 4, and >45 g ha−1, respectively. Dry weights of kochia seeded after removal of the winter annual grasses decreased as MKH 6561 rate increased, with average control being 77% compared to the nontreated control. Regression analysis indicated that kochia was controlled 80% with MKH 6561 at 28 g ha−1.

Pseudomonas fluorescens strain D7 (P. f. D7; NRRL B-18293) is a root-colonizing bacterium that inhibits downy brome (Bromus tectorum L. BROTE) growth. Before commercialization as a biological control agent, strain D7 must be tested for host plant specificity. Agar plate bioassays in the laboratory and plant–soil bioassays in a growth chamber were used to determine the influence of P. f. D7 on germination and root growth of 42 selected weed, cultivated or native plant species common in the western and midwestern United States. In the agar plate bioassay, all accessions of downy brome were inhibited by P. f. D7. Root growth of seven Bromus spp. was inhibited an average of 87% compared with that of controls in the agar plate bioassay. Root growth of non-Bromus monocots was reduced by 0 to 86%, and only 6 out of 17 plant species were inhibited 40% or greater. Among all plant species, only downy brome root growth from two accessions was significantly inhibited by P. f. D7 in plant–soil bioassays (42 and 64%). P. f. D7 inhibited root growth and germination in agar plate bioassays more than in plant–soil bioassays. Inhibition in plant–soil bioassays was limited to downy brome, indicating promise for P. f. D7 as a biocontrol agent that will not harm nontarget species.

Soil organic carbon (OC), clay content, water content, and pH often influence the bioactivity of soil-applied herbicides, and these soil properties can vary greatly within fields. The purpose of this work was to determine the influence of within-field soil heterogeneity on the efficacy of RPA-201772 where corn, shattercane, and velvetleaf were seeded as bioassays. An experimental approach was developed to quantify RPA-201772 dose–response across a range of soil conditions in an agricultural field. Based on a logistic model, crop injury was quantified with the I20 parameter, the dose eliciting 20% greenness reduction, using a series of photographic standards. Weed biomass was quantified with the I80 parameter, the dose eliciting 80% biomass reduction, relative to the untreated control. Crop and weed responses varied by two orders of magnitude. Significant correlation, as high as 0.76, was observed between measures of plant response and soil properties, namely particle size and OC. Furthermore, native velvetleaf spatial distribution at the study site was heterogeneous, and seedlings were observed in plots where seeded velvetleaf biomass was high. Spatial heterogeneity of soil affinity for herbicide results in differential weed fitness and contributes to weed “patchiness.”

Flumetsulam sorption and mobility studies were conducted on surface (0 to 15 cm) and subsurface (30 to 46 cm) soil of several southern soils. In a batch equilibrium study using a 1:1 ratio of soil–0.01 M CaCl2, flumetsulam adsorbed ranged from 2.9 to 48.7% and from 4.2 to 63.3% on surface and subsurface soils, respectively. Herbicide soil–solution distribution (K d) and organic carbon (K oc) coefficients ranged from 0.03 to 0.95 and from 5.1 to 77.1, respectively, in surface soils and from 0.04 to 1.72 and from 7.5 to 325.5, respectively, in subsurface soils. K d and K oc were correlated with humic and organic matter in surface soils. K d was correlated with extractable Fe, whereas K oc was inversely correlated to pH in subsurface soils. A desorption study using 0.01 M CaCl2 as an extractant on the Captina silt loam surface soil demonstrated that three to four washes were required to desorb more than 94% of the flumetsulam adsorbed over several equilibration times. Mobility studies on soil thin-layer chromatography plates demonstrated that flumetsulam and imazaquin had similar values, ranging from 0.50 to 0.90 and 0.59 to 0.90, respectively, in the surface soils, and both compounds had the same range of mobility in subsurface soils, with R f values between 0.60 and 0.93. At both soil depths, K d and K oc were inversely correlated with the R f of flumetsulam.