Studies were conducted to determine the effect of interference between tropic croton (Croton glandulosus) and cotton (Gossypium hirsutum) on plant growth and productivity. Tropic croton height was not affected by weed density, but cotton height decreased with increased weed density 10 wk after planting. Tropic croton biomass per plant was not affected by weed density, but total weed biomass per meter of crop row increased with weed density. Cotton lint yield decreased linearly 2 kg ha−1 with each gram increase in weed dry biomass per meter of row. Percent yield loss–density relationship was described by the rectangular hyperbola model. Estimated coefficients A (maximum yield loss) and I (yield loss per unit density as density approaches zero) were 129.6 ± 42.2 and 35.6 ± 8.0%, respectively, when asymptotic iterations were based on least sums of squares. When A was constrained to 100% yield loss, I was 42.5 ± 5.1%. Results indicated that tropic croton was less competitive with cotton than many weeds but represents an economic threat to cotton growers.

Striga hermonthica is a major threat to cereal production in western Kenya and other parts of the world. In laboratory experiments, maize and sorghum showed some resistance to late Striga attachment and parasitism. Field experiments were conducted in 1996 and 1997 in western Kenya to assess the effect of transplanting maize and sorghum, under rain-fed field conditions, on grain yield and Striga parasitism. In transplanted sorghum, Striga emergence was not reduced and sorghum failed to produce grain yield in three out of four seasons. Transplanting maize in all experiments significantly increased grain yield compared to direct seeding. The improved productivity was largely associated with less Striga attachment. Striga densities were considerably lower if maize seedlings were more than 17 d old at transplanting, with decreasing levels of Striga with increasing age of the maize seedlings. Seedlings transplanted before they were 15 d old did not reduce Striga attachment. Transplanting maize under rain-fed conditions is probably only suitable for small areas that are highly infested with Striga, due to its high labor requirements. Under these conditions, crop yield can be more than doubled. An incentive to using this method by small-scale farmers would be that the main input at risk is their own labor. However, the establishment of nurseries and the timing of the transplanting operation require a certain level of farm management that could constrain the adoption of this technique.

Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO2]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO2 reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO2. In monoculture, elevated CO2 significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO2 did not adequately predict the CO2 response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cocklebur and decreased significantly for sorghum in competitive mixtures at elevated CO2. Data from this study indicate that vegetative growth, competition, and potential yield of economically important C4 crops could be reduced by co-occurring C3 weeds as atmospheric carbon dioxide increases.

Three models that empirically predict crop yield from crop and weed density were evaluated for their fit to 30 data sets from multistate, multiyear winter wheat–jointed goatgrass interference experiments. The purpose of the evaluation was to identify which model would generally perform best for the prediction of yield (damage function) in a bioeconomic model and which model would best fulfill criteria for hypothesis testing with limited amounts of data. Seven criteria were used to assess the fit of the models to the data. Overall, Model 2 provided the best statistical description of the data. Model 2 regressions were most often statistically significant, as indicated by approximate F tests, explained the largest proportion of total variation about the mean, gave the smallest residual sum of squares, and returned residuals with random distribution more often than Models 1 and 3. Model 2 performed less well based on the remaining criteria. Model 3 outperformed Models 1 and 2 in the number of parameters estimated that were statistically significant. Model 1 outperformed Models 2 and 3 in the proportion of regressions that converged on a solution and more readily exhibited an asymptotic relationship between winter wheat yield and both winter wheat and jointed goatgrass density under the constraint of limited data. In contrast, Model 2 exhibited a relatively linear relationship between yield and crop density and little effect of increasing jointed goatgrass density on yield, thus overpredicting yield at high weed densities when data were scarce. Model 2 had statistical properties that made it superior for hypothesis testing; however, Model 1's properties were determined superior for the damage function in the winter wheat–jointed goatgrass bioeconomic model because it was less likely to cause bias in yield predictions based on data sets of minimum size.

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.

An isolate of Fusarium solani (Sud 96) obtained from infected Striga plants in Sudan and six other isolates from Japan were evaluated for their effects on Striga germination. Among all the isolates, only the one from Sudan demonstrated high inhibitory activity. Aqueous and organic solvent culture extracts, as well as fungus suspension, when mixed with GR24, a synthetic analog of the natural germination stimulant strigol, inhibited germination of conditioned Striga seeds. Fusarium solani (Sud 96) filtrates, from cultures grown on autoclaved rice, sorghum grains, and potato dextrose agar (PDA), were more effective in reducing Striga germination than those from cultures grown on wheat straw. A significant difference between rice compared to sorghum and PDA cultures only occurred at high dilutions (40-fold). Complete inhibition of germination occurred when F. solani (Sud 96) culture filtrates and GR24 were applied simultaneously. Filtrate treatments made 2, 4 and 6 h subsequent to treatment with GR24 were less inhibitory. Filtrate treatments applied 8 h or more following GR24 had negligible effects on germination. Chromatographic separation on a silica gel column indicated the presence of several compounds with high inhibitory activity.

The effect of shoot feeding by the biocontrol agents, Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) seed production and seed germination was quantified in two Minnesota wetlands. In a wet meadow where Galerucella spp. were present on isolated plants, feeding by adults and larvae during shoot elongation resulted in stunting and malformation of shoot tips. There was a subsequent reduction in purple loosestrife inflorescence length and number of flower buds and seed capsules. As Galerucella spp. larvae preferentially fed on shoot meristems, even low levels of feeding on a whole-plant basis (approximately 10% defoliation) reduced seed production. In a sedge meadow wetland with severe feeding damage (a minimum of 70% leaf defoliation), few to no flower buds formed on plants, and subsequently, few to no seed capsules were produced on purple loosestrife plants. Of the few capsules that were produced, number of seeds per capsule and percent germination of seeds did not differ from control plants. In both wetlands, feeding on a main shoot of purple loosestrife did not result in a compensatory increase in the number of axillary inflorescences. Feeding by Galerucella spp. and the subsequent reduction in number of seeds produced on purple loosestrife plants will decrease the number of seeds available for dissemination to new sites. Fewer seeds will enter the seedbank, and over time, feeding by Galerucella spp. will decrease the number of seeds available for seedling recruitment. The benefit of leaf defoliation on purple loosestrife plants caused by Galerucella spp. feeding has been reported. In this study, we have quantified the additional benefits of reduced seed production from Galerucella spp. feeding on purple loosestrife in North America.

A population of common ragweed not controlled by an acetolactate synthase (ALS)-inhibiting herbicide, cloransulam-methyl, was sampled near Dunkirk, IN, the first year of the herbicide's commercialization in 1998. Resistance in the Dunkirk population was confirmed by treating greenhouse-grown seedlings with cloransulam-methyl. ALS activity assays and DNA sequencing were used to identify the resistance mechanism. ALS isolated from plants of the Dunkirk population exhibited an R/S ratio for cloransulam-methyl of >5,000 when compared to ALS activity of populations from Claire City, SD, and V & J Seed Farms. R/S ratios of 4,100 and 110 were observed for two other ALS-inhibiting herbicides, chlorimuron and imazaquin, respectively. DNA sequencing revealed that an inferred leucine for tryptophan substitution at amino acid position 574 in ALS was responsible for the observed herbicide resistance. Additionally, DNA sequencing revealed significant variability among common ragweed ALS alleles. Two fragments of ALS were sequenced from three plants each of the Claire City and Dunkirk populations, totaling 688 nucleotide base pairs, of which 72 were polymorphic.

Greenhouse and laboratory studies were conducted to evaluate responses of ivyleaf morningglory, pitted morningglory, palmleaf morningglory, and smallflower morningglory to several herbicides in relation to leaf structure, epicuticular wax, and spray droplet behavior. Two- to four-leaf stage plants of each species were highly susceptible to acifluorfen, bentazon, bromoxynil, glufosinate, and glyphosate. However, at the five- to eight-leaf stage, these species were less susceptible, and control was herbicide specific. Spray droplets of these five herbicides had a higher contact angle on ivyleaf morningglory than the other three species with a few exceptions. Stomata and glands were present on both adaxial and abaxial leaf surfaces of all species, and palmleaf morningglory and smallflower morningglory had more of these than did the other two species. Trichomes were present on all species except palmleaf morningglory. Epicuticular wax mass was highest in ivyleaf morningglory (57 μg cm−2) and lowest in smallflower morningglory (14 μg cm−2). Wax consisted of homologous short-chain (< C18) or long-chain (> C20) hydrocarbons, alcohols, acids, and triterpenes. Smallflower morningglory waxes lacked short-chain length components. Triterpenes were absent in palmleaf morningglory and smallflower morningglory epicuticular waxes. Untriacontane (C31 hydrocarbon) and tridecanol (C30 alcohol) were common major long-chain components in waxes of all four species. Heptadecane (C17 hydrocarbon) and octanoic acid (C18) were common major short-chain length wax components in pitted, ivyleaf, and palmleaf morningglory. In spite of some differences in leaf surface structures, wax mass, and wax components among the four species, there was no clear relationship between these parameters and herbicide efficacy.

Weed suppression by cover crops grown during the winter fallow period in continuous corn may lead to a reduction in herbicide use. Rye, crimson clover, and subterranean clover cover crops were compared with corn stubble under a conventional management system (CS) that included plowing and use of preemergence residual herbicides and a low-input management system (LIS) that included no-tillage and use of a presowing nonresidual herbicide for three consecutive years (1994–1996). Cover crop and above-ground weed biomass prior to desiccation were not influenced by management system. Cover crop biomass ranged from 1,420 to 5,657 kg ha−1 for rye, from 563 to 4,217 kg ha−1 for crimson clover, and from 563 to 4,248 kg ha−1 for subterranean clover. At crop planting, rye reduced weed biomass from 54 to 99%, crimson clover from 22 to 46% (with a negative value in 1995), and subterranean clover from 21 to 67%. Weed growth suppression was usually higher in years when cover crop biomass was higher. There were no differences in weed suppression by cover crops later in the season (corn in the fourth leaf stage), while total weed density was higher in LIS than CS in 2 of 3 yr. Total weed cover at corn's ‘full dent’ stage ranged from 1 to 7% in CS and from 24 to 47% in LIS. Cover crops influenced weed composition only in years when cover crop growth was high; otherwise their effect was masked by that of the management system. Weed communities showed higher diversity under LIS than under CS. Consistency of associations between weed species and treatments over sampling dates and years was found especially for some of the species associated with LIS. After 3 yr, redroot pigweed, common lambsquarters, and black nightshade were regularly associated with rye-LIS at an early corn growth stage; this may indicate a species shift toward a more troublesome composition.

Cost-effective weed management requires accurate estimates of yield and the potential yield loss resulting from weed infestations. However, crop yield and the effects of weeds are highly variable across years and locations. Ecophysiological models may be useful for predicting the effects of environment and management on crop and weed growth and competitive ability. Ability of the model INTERCOM to predict corn (Zea mays) growth and yield, velvetleaf (Abutilon theophrasti) interference on corn yield loss, and single-year economic threshold velvetleaf density (Te) was evaluated using 13 data sets collected in four states. Predicted and observed monoculture corn total aboveground biomass and leaf area index were in close agreement for most of the growing season. Predicted and observed weed-free corn yields were in agreement for yields ranging from 8 to 13 Mg ha−1 but were over- and underpredicted under low-yielding and near-optimal production conditions, respectively. Predicted and observed corn yield loss agreed well across the full range of observed velvetleaf densities for five to nine location years, depending on the performance criterion used. Estimates of Te calculated from predicted weed-free yield and yield loss relationships were an average of 6% smaller than those calculated from observed data, indicating that the model predicts a conservative value of Te in most cases. Although results are encouraging, they indicate that further research is needed to improve the capacity of INTERCOM for predicting weed-free yield and corn–velvetleaf interference.

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.

Targeting weed patches for site-specific herbicide applications potentially represents cost savings for operators, reduction in environmental herbicide effects, and increased efficiency of weed control. An experiment was initiated in a no-till corn field in Ontario, Canada, in 1998 and was continued in rotation with no-till soybeans in 1999. Weeds were intensively scouted, and distribution maps of the most common weeds (field horsetail, spiny sowthistle, dandelion, and common lambsquarters) were generated for both years. A prescription map for each plot was made using the weed density maps. Treatment decisions were based on a weed threshold value of 1 shoot m−2. Four herbicide treatments were compared: a conventional broadcast, a site-specific application targeting weed patches only, and two combinations of broadcast and site-specific applications. Treatments were applied using a direct-injection sprayer. Efficacy of weed control and yield were compared among treatments. In 1998 and 1999 there were no differences in the level of weed control or yield among treatments. The average percent area sprayed was reduced as much as 26% in the site-specific treatment in 1998 and up to 59% in the site-specific and broadcast combination treatments in 1999. For those species present in the field, patches ranged from highly aggregated to completely random, and patch stability ranged from very stable to very unstable over the 2 yr.

Field studies were conducted to determine how the spatial arrangement of weed populations influences interspecific competition. We studied the influence of regular, random, and clumped distributions of barnyardgrass on growth and yield of direct-seeded tomato planted at different densities. Increasing aggregation increased intraspecific competition in barnyardgrass. At the same time, interspecific competition experienced by tomato from barnyardgrass decreased. Differences in the amount of shading of the tomato canopy by barnyardgrass contributed to yield loss differences for the various spatial arrangements. Clumped barnyardgrass caused significantly less average shading than barnyardgrass in regular or random arrangements. At a typical planting density of 10 tomato plants m−1 of row, yield losses ranged from 10 to 35% (1993) or 8 to 50% (1994) when competing with a clumped arrangement of barnyardgrass. At the same tomato density, yields were reduced from 20 to 50% (1993) or 11 to 75% (1994) for the regular and random arrangements for the same barnyardgrass densities. Predicted single-season economic threshold densities for barnyardgrass at a typical tomato planting density of 10 plants m−1 would be one barnyardgrass plant per 25, 19, or 15 m of crop row, respectively, for regular, random, and clumped spatial distributions.

The goal of a mechanistic model is to determine the outcome of weed–crop interference. An understanding of weed phenology is essential for construction of such models because phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to determine the influence of temperature and photoperiod on the phenological development of common lambsquarters. Common lambsquarters is a short-day species adapted to a temperature range of 6.5 to 44.5 C. Phenological development of common lambsquarters grown under a constant temperature of 20 C and an 8-h photoperiod was described in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Three development phases of common lambsquarters were described as (1) a juvenile phase of 6.3 Bd, (2) a photoperiod-sensitive inductive phase of 8.2 Bd, and (3) a photoperiod-sensitive postinductive phase of 34.4 Bd. The photoperiod sensitivity of rate of development did not differ among phases of development across the life cycle. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

An understanding of environmental factors governing patchy weed distribution in fields could prove to be a valuable tool in weed management. The objectives of this research were to investigate the relationships between weed distribution patterns and environmental properties in two Mississippi soybean fields and to construct models based on those relationships to predict weed distribution. Two months before planting, fields were soil sampled on a 60- by 60-m coordinate grid, and samples were analyzed for calcium, magnesium, potassium, sodium, phosphorus, zinc, cation exchange capacity, percent organic matter, and soil pH. The relative elevation of each sample location was also recorded. Approximately 8 wk after planting, weed populations were estimated on a 30- by 30-m grid over the soil sample grid. Punctual kriging was used to estimate environmental values at each weed sample location. Discriminant analysis techniques were used to evaluate the associations between environmental characteristics on weed population densities of sample areas within each field. Generally, as sicklepod and pitted morningglory infestations increased, the prediction accuracy of the discriminant functions also increased; however, horsenettle infestations were not closely correlated to the environmental properties. Discriminant functions reasonably predicted presence or absence of sicklepod and pitted morningglory within the field. However, validation of the functions across years within the same field and with data collected from the other field resulted in poor classification of all species infestations. Prediction of weed infestations with environmental properties was specific for each field, year, and species.

Giant foxtail putatively resistant to acetolactate synthase (ALS) inhibitors has been reported widely in the upper Midwest, typically in fields with a history of ALS inhibitor use in continuous corn or corn–soybean rotation. However, it is not known whether these giant foxtail populations vary in their response to ALS inhibitors. Therefore, our objectives were to confirm and quantify resistance of giant foxtail accessions from Wisconsin, Minnesota, and Illinois to imidazolinone and sulfonylurea herbicides; to determine the mechanism of resistance; and to determine the mechanism of resistance inheritance. Dose–response experiments using three- to four-leaf stage giant foxtail plants in the greenhouse confirmed cross-resistance of the Wisconsin, Minnesota, and Illinois accessions to imazethapyr and nicosulfuron. Based on ED50 values (the effective dose that reduced shoot dry biomass by 50% compared to the nontreated plants), the Wisconsin, Minnesota, and Illinois accessions were 16-, 17-, and 15-fold resistant to imazethapyr, respectively, and 21-, 19-, and 9-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. In contrast, all accessions were susceptible and responded similarly to fluazifop-P. Based on an in vivo ALS assay, the Wisconsin, Minnesota, and Illinois accessions were > 750-, > 320-, and > 670-fold resistant to imazethapyr, respectively, and 1,900-, > 1,900-, and 80-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. To determine the inheritance of resistance traits, hybrid F1 families were generated from crosses between ALS inhibitor–susceptible and -resistant plants from Minnesota. Three distinct plant phenotypes—resistant (R), intermediate (I), and susceptible (S)—were identified in the F2 generation following exposure to imazethapyr. In repeated experiments, these phenotypes segregated in a 1:2:1 (R:I:S) ratio, indicative of a trait associated with a single, nuclear, semidominant allele.

Barnyardgrass was grown at densities of 0, 0.25, 0.5, 1, 2, 5, and more than 50 plants m−1 of tomato crop row in either a regular, random, or clumped pattern. Tomato was established at 0, 5, 10, or 20 plants m−1 of crop row in a regular pattern. Crop density and weed density or spatial arrangement had little effect on phenological development of barnyardgrass. In the absence of tomato, barnyardgrass was estimated to produce over 400,000 seeds plant−1 when not subjected to intraspecific competition (0.25 plants m−1 density), decreasing to about 10,000 seeds plant−1 when weed density exceeded 50 plants m−1 of row. Differences in seed production between plants in the regular and random spatial arrangements were minor, but the clumped distribution resulted in 30 to 50% reduction in seed production at weed densities between 1 and 5 plants m−1 of row. Tomato reduced barnyardgrass seed production. The magnitude of the reduction depended on both tomato density and barnyardgrass density. In the absence of tomato, barnyardgrass produced over 200,000 seeds m−2 in 1993 and over 500,000 seeds m−2 in 1994 at 5 plants m−1 of row. Production was almost 700,000 seeds m−2 when the weed density exceeded 50 plants m−1 of row. Barnyardgrass seed production at the single-season economic threshold density in tomato was sufficient to maintain the seedbank at a level that would mandate high levels of weed control in subsequent crops. Because of the high fecundity of barnyardgrass, our experiments suggest that stopping seed production is the best long-term management strategy for the weed. Spatial arrangement of the weed, at the scale used in these studies, would not be a factor in establishing long-term management guidelines based on weed population biology.

Palmer amaranth (Amaranthus palmeri) is a major weed in corn (Zea mays) fields in the southern Great Plains of the United States. Field studies were conducted in 1996, 1997, and 1998 near Garden City, KS, to evaluate the effects of Palmer amaranth density and time of emergence on grain yield of irrigated corn and on seed production of Palmer amaranth. Palmer amaranth was established at densities of 0.5, 1, 2, 4, and 8 plants m−1 of corn row both concurrently at corn planting and when corn was at the three- to six-leaf stage. The control plots were weed free. The Palmer amaranth planted with corn emerged with corn, whereas that planted later emerged at the four-, six-, and seven-leaf stages of corn. The Palmer amaranth emerging with corn reduced yield from 11 to 91% as density increased from 0.5 to 8 plants m−1 of row. In contrast, yield loss from Palmer amaranth emerging later than corn was observed only when the emergence occurred at the four- and six-leaf stages. The corn leaf area index (LAI) decreased as Palmer amaranth density increased. Reduction in corn LAI from Palmer amaranth interference was smaller for the second emergence date than for the first emergence date. Seed production per Palmer amaranth plant decreased with greater density, but seed per unit area increased from 140,000 to 514,000 seeds m−2 at densities of 0.5 and 8 plants m−1 of row, respectively, when Palmer amaranth emerged with corn and from 1,800 to 91,000 seeds m−2 at the same densities for later emergence dates. Although Palmer amaranth is highly competitive in corn, this study shows that yield loss is affected more by time of emergence than by 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.