To develop more effective pest-management strategies, it is essential to understand how different pests interact with each other and the crop. Field studies were conducted in 2003 and 2004 at two Nebraska locations to determine the effects of early-season crop defoliation on the critical time for weed removal (CTWR) in narrow-row soybean. Three soybean defoliation levels were selected to simulate 0, 30, and 60% leaf tissue removal by the bean leaf beetle. Weeds were allowed to compete with the crop until V2, V4, V6, R3, and R5 growth stages. There were also season-long weedy and weed-free treatments. Results indicated that the CTWR in soybean occurred earlier as defoliation levels increased from 0 to 60%. The CTWR occurred at V3, V2, and V1 growth stage for 0, 30, and 60% defoliation levels, respectively. Overall, 60% defoliation resulted in earlier CTWR by at least 14 d. Yield losses from defoliation and weed interference were primarily associated with a reduction in number of pods per plant−1.

London rocket is a common winter annual weed in southern New Mexico that can host beet curly top virus and its insect vector, the beet leafhopper. Experiments were conducted in southern New Mexico to determine if London rocket could serve as a host for overwintering beet leafhopper. Field experiments were carried out from 2002 to 2003 and from 2003 to 2004 to compare the impact of three London rocket planting dates on plant emergence and life history and leafhopper survival. Emergence was highest in October-planted London rocket, low in January/February plantings, and did not occur for August plantings. The life cycle was 185 d and 125 d for October- and January-planted London rocket, respectively, and growth of the plant (including height and rosette base diameter) was greater for London rocket that was planted in October. October-planted London rocket survived from late October through mid to late April, the period of time needed to serve as an overwintering host for beet leafhoppers. Caging the plants to assess beet leafhopper survival did not affect rate of plant growth, but it reduced the time to flowering for October-planted London rocket and increased the height and weight of plants. Beet leafhoppers were able to survive for approximately 2 mo in early winter or spring on caged London rocket plants.

The dynamics of soil seed banks in crop rotations of corn, soybean, and winter wheat were investigated to determine whether weed seed inputs associated with the winter wheat phases of the rotation were present in the readily germinable fraction of the seed bank in subsequent phases. Two studies were conducted, each in chisel-plowed systems. In one study, we compared seed banks in plots after 8 yr of corn grown continuously and with rotation that included winter wheat and soybean. A second study followed seed bank composition and abundance for 3 yr in plots that were planted to corn and soybean in successive years after planting to winter wheat. Seed banks were measured by direct germination in a heated greenhouse. In the first study, seed banks in plots planted to continuous corn (never planted to winter wheat) and the corn phase of the rotation (winter wheat planted 3 yr earlier) did not differ in species composition or abundance despite significant differences in seed banks in plots that had been planted to winter wheat the previous season. In the second study, seed bank abundance and composition in plots planted to winter wheat in 2001 rapidly changed after planting of corn and soybean in 2002 and 2003, respectively. Data from the two experiments suggest that seed banks in annual row crops experience rapid change in composition and abundance and can be strongly influenced by the most recent crop. This could limit our ability to infer longer term trends associated with changes in management practices from studies of soil seed banks.

The influence of management practices at a system level is rarely studied in weed science, even though weed communities respond to the cumulative effect of farm management systems. On-farm visits and detailed grower surveys were used to objectively classify 59 Indiana tomato fields into management systems. Fields were chosen to represent a range of practices used to grow conventional and organic tomatoes. Multivariate statistical analyses identified five distinct management systems based primarily on differences in hours spent hand-weeding, use of plastic mulch, irrigation, row spacing, and whether tomatoes were staked. Farmers generally reported many more hours of hand-weeding for organically managed fields than for fields in the other groups. This finding may reflect a trade-off between the use of herbicides and the need for hand-weeding. However, some organically managed fields were grouped with conventional fresh market fields, suggesting that management practices besides herbicide inputs can be used to reduce hand-weeding. Although some fresh market fields used to produce organic or conventional tomatoes had similar management systems, there was little overlap between fields in fresh market or processing tomato production. Further research is needed to determine underlying relationships among management systems and weed control in Indiana tomato production.

Knowledge of how plants will partition their new biomass will aid in understanding competition between crops and weeds. This study determined if the amount of biomass partitioned to the root versus the shoot can be predicted from tissue carbon [C] and nitrogen [N] concentrations and the daily gain in C (GC) and N (GN) for each unit shoot and root biomass, respectively. Pots measuring 28 cm diameter and 60 cm deep were embedded in the ground, and each contained one plant of either corn or velvetleaf. Each plant received one of three nitrogen treatments: 0, 1, or 3 g of nitrogen applied as ammonium nitrate in 2001 and 0, 2, or 6 g of nitrogen in 2002. Measurements of total above- and belowground biomass and tissue [C] and [N] were made at 10 different sample dates during the growing season. Fraction of biomass partitioned to roots (Pr) was predicted from [C], [N], GC, and GN. Accurate prediction of the fraction of biomass partitioned to roots versus shoots was evaluated by comparing observed and predicted Pr across all treatments. The coordination model has potential as a reliable tool for predicting plant biomass partitioning. Normalized error values were close to zero for corn in 2001 and 2002 and for velvetleaf in 2001, indicating that biomass partitioning was correctly predicted.

The critical period for weed control (CPWC) identifies the phase of the crop growth cycle when weed interference results in unacceptable yield losses; however, the effect of planting date on CPWC is not well understood. Field studies were conducted in 2004 and 2005 at Urbana, IL, to determine CPWC in sweet corn for early May (EARLY) and late-June (LATE) planting dates. A quantitative series of treatments of both increasing duration of interference and length of weed-free period were imposed within each planting-date main plot. The beginning and end of the CPWC, based on 5% loss of marketable ear mass, was determined by fitting logistic and Gompertz equations to the relative yield data representing increasing duration of weed interference and weed-free periods, respectively. Weed interference stressed the crop more quickly and to a greater extent in EARLY, relative to LATE. At a 5% yield-loss level, duration of weed interference for 160 and 662 growing-degree days (GDD) from crop emergence marked the beginning of the CPWC for EARLY and LATE, respectively. When maintained weed-free for 320 and 134 GDD, weeds emerging later caused yield losses of less than 5% for EARLY and LATE, respectively. Weed densities exceeded 85 plants m−2 for the duration of the experiments and predominant species included barnyardgrass, common lambsquarters, common purslane, redroot pigweed, and velvetleaf. Weed canopy height and total aboveground weed biomass were 300% and 500% higher, respectively, for EARLY compared with LATE. Interactions between planting date and CPWC indicate the need to consider planting date in the optimization of integrated weed management systems for sweet corn. In this study, weed management in mid-June–planted sweet corn could have been less intensive than early May–planted corn, reducing herbicide use and risk of herbicide carryover to sensitive rotation crops.

An improved understanding of crop stress from multiple pests is needed for better implementation of integrated pest management (IPM) strategies. Field studies were conducted in 2003 and 2004 at two locations in eastern Nebraska to describe the effects of simulated early-season insect defoliation of soybean and duration of weed interference on soybean growth. Three levels of simulated defoliation (undefoliated, 30, and 60%) and seven durations of weed interference (weedy and weed free; weed removal at V2, V4, V6, R3, and R5) were evaluated in a split-plot design. Defoliation significantly reduced soybean leaf-area index (LAI), total dry matter (TDM), and crop height in season-long weedy treatments only. Biomass partitioning during vegetative and reproductive growth was affected by both defoliation and weed interference. Increase in soybean relative growth rate (RGR) and biomass production soon after defoliation occurred (e.g., V5 stage) indicated potential defense mechanism by which soybean is able to adjust its physiology in response to the loss of leaf area. Weed interference combined with defoliation caused the greatest yield losses up to 97%. Results from this study indicate the need for monitoring early-season insect density and weed growth to determine if simultaneous control of both pests may be needed.

Field studies were carried out in Laingsburg, MI, from 2002 to 2004 on Houghton muck soil to assess the impacts of cover crops and soil fertility regimes on weed populations and celery yield. The cover crops were oilseed radish, cereal rye, hairy vetch, and a bare ground control. The fertility rates were full (180, 90, and 450 kg ha−1 nitrogen [N], phosphorus pentoxide [P2O5], and potassium oxide [K2O], respectively), half (90, 45, and 225 kg ha−1 N, P2O5, and K2O, respectively), and low (90 kg ha−1 N). Each cover crop treatment was combined with the low or half rate of fertilizer. An additional treatment with bare ground plus the full rate of fertilizer was added as standard practice. Treatments were maintained in the same location for the duration of the study. Major weed species were common chickweed, prostrate pigweed, shepherd's-purse, common purslane, and yellow nutsedge. Each year, oilseed radish consistently produced the greatest biomass and provided over 98% early season weed biomass suppression. Hairy vetch and cereal rye provided about 70% weed suppression in early spring. Soil fertility level affected weed populations during the 2004 growing season. In 2004, weed biomass in treatments without cover crops or with vetch increased when greater amounts of fertilizer were applied. Within individual fertility levels, higher celery yields were recorded in the oilseed radish plots. For example, in the low fertility rate, celery yield was 34.8, 29.2, 23.9, and 24.4 ton ha−1 in the oilseed radish, cereal rye, hairy vetch, and control plots, respectively in 2003. Overall, the results of this experiment indicate that when included in a system where hoeing and hand-weeding are the only weed control methods, cover crops can successfully improve weed management and celery yield on muck soils, allowing reduced fertilizer inputs.

Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r 2 = 0.21), air temperature (r 2 = 0.45) or rainfall (r 2 = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m−2 to a high of > 1,500 plants−2 at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations.

Integrated weed management (IWM) decision strategies in herbicide-resistant canola-production systems were assessed for net returns and relative risk. Data from two field experiments conducted during 1998 to 2000 at two locations in Alberta, Canada, were evaluated. A herbicide-based experiment included combinations of herbicide system (glufosinate-, glyphosate-, and imazethapyr-resistant canola varieties), herbicide rate (50 and 100% of recommended dose), and time of weed removal (two-, four-, and six-leaf stages of canola). A seed-based experiment included canola variety (hybrid and open-pollinated), seeding rate (100, 150, and 200 seeds m−2), and time of weed removal (two-, four-, and six-leaf stages of canola). For the herbicide-based experiment, strategies with glyphosate were profitable at Lacombe, but both imazethapyr and glyphosate strategies were profitable at Lethbridge. Weed control at the four-leaf stage was at least as profitable as the two-leaf stage at both sites. For the seed-based experiment, the hybrid was more profitable than the open-pollinated cultivar, seed rates of 100 and 150 seeds m−2 were more profitable than 200 seeds m−2, and weed control at the two- and four-leaf stages was more profitable than at the six-leaf stage. When risk of returns and statistical significance was considered, several strategies were included in the risk-efficient set for risk-averse and risk-neutral attitudes at each location. However, the glyphosate-resistant cultivar, the 50% herbicide rate, and weed control at four-leaf stage were more frequent in the risk-efficient IWM strategy set. The open-pollinated cultivar, 200 seeds m−2 rate, and weed control at the six-leaf stage were less frequent in the set. The risk-efficient sets of IWM strategies were consistent across a range of canola prices.

Laboratory and greenhouse experiments were conducted to determine the allelopathic potential of centipedegrass. Germination and growth of indicator species were evaluated in soil leachates, leaf debris, and aqueous leaf extracts of centipedegrass. Centipedegrass soil leachates did not inhibit annual bluegrass, goosegrass, henbit, large crabgrass, or perennial ryegrass germination compared with the nonfertilized control. Incorporated centipedegrass leaf debris did not reduce lettuce germination, shoot weight, or root weight compared with the control. However, shoot and root dry weights of radish were reduced with increasing rates of centipedegrass leaf debris. Six and 9 mg debris g−1 soil reduced radish shoot weight by 49 and 64%, respectively, compared with the control. Aqueous leaf extracts of centipedegrass reduced lettuce germination; however, radicle and hypocotyl length were similar to the control. These data do not conclusively suggest centipedegrass has widespread allelopathic activity; however, significant reductions in shoot and root weight of radish with increasing centipedegrass leaf debris demonstrate a pattern of inhibition of one species against another, which fulfills a requirement of allelopathic interactions.

Field trials were conducted to evaluate the season long population dynamics and location (in leaf or on leaf surface) of an antibiotic resistant strain of the bacterium Pseudomonas syringae pv. tagetis (PST) applied to Canada thistle leaves. An application preceding 2 to 3 d of hot dry weather was compared to an application preceding 2 to 3 d of cool wet weather. Leaf samples were taken weekly to assess the population of PST found inside the leaves and on the leaf surface. While PST populations initially differed, populations were similar for both treatments one week after application. While this suggests that environment did not have a major impact, weather conditions for testing this hypothesis were not ideal. Over the first 35 d of the experiment, little rainfall was observed. PST populations were low and stable. However, rain events over the 40 d that followed resulted in great oscillations in mean PST populations and in some cases significant population increases. During dry periods, internal and total PST populations differed significantly, suggesting the external populations played a major part in population composition. However, the two sampling periods that closely followed three consecutive days of rainfall indicated internal populations were not significantly different from the total, suggesting that internal populations played the primary role in population composition. The results of this research provide evidence that rain events lead to overall PST population increases and to greater proportions of PST inside Canada thistle leaves, suggesting that it is better to apply PST during wet periods than dry.

Defatted seedmeals from 15 glucosinolate-containing plant species were analyzed for herbicidal activity by determining inhibition of seedling emergence when added to a sandy loam soil containing wheat and sicklepod seeds at concentrations of 0.1, 0.5, and 1% (w/w). In general, the seedmeals were more phytotoxic to wheat than sicklepod. For wheat, all of the seedmeals significantly inhibited seedling emergence at the 1.0% concentration. At the 0.1% concentration three of the seedmeals (Indian mustard, money plant, and field pennycress) completely inhibited wheat emergence. For sicklepod emergence, eight of the seedmeals were completely inhibitory at the 1% level (Indian mustard, field pennycress, garden rocket, Siberian wallflower, English wallflower, garden cress, sweet alyssum, and evening stock) and four were completely inhibitory at the 0.5% level (brown mustard, garden rocket, English wallflower, and sweet alyssum). Intact glucosinolates and their corresponding hydrolysis products varied among the seedmeals with the highest activity. Major hydrolysis products produced by the seedmeals with the most phytotoxicity, respectively, included 2-propenyl (allyl) isothiocyanate (AITC) by brown mustard seedmeal, allyl thiocyanate and AITC by field pennycress seedmeal, erucin (4-methylthiobutyl isothiocyanate) by arugula seedmeal, 3-butenyl isothiocyanate and lesquerellin (6-methylthiohexyl isothiocyanate) by sweet alyssum seedmeal, and isopropyl isothiocyanate by money plant seedmeal. From our data it appears that both the type and concentration of glucosinolates and their hydrolysis products present in the seedmeals affect seed-emergence inhibition.

Hand-held hyperspectral reflectance data were collected in the summers of 2002, 2003, and 2004 to differentiate unique spectral characteristics of common turfgrass and weed species. Turfgrass species evaluated were: bermudagrass, ‘Tifway 419’; zoysiagrass, ‘Meyer’; St. Augustinegrass, ‘Raleigh’; common centipedegrass; and creeping bentgrass, ‘Crenshaw’. Weed species evaluated were: dallisgrass, southern crabgrass, eclipta, and Virginia buttonweed. Reflectance data were collected from greenhouse and field locations. An overall classification accuracy of 85% was achieved for all species in the field. A total of 21 spectral bands between 378 and 1,000 nm that were consistent over the three data collection periods were used for analysis. Only centipedegrass, zoysiagrass, and dallisgrass were correctly classified less than 80% of the time. An overall classification accuracy of 69% was achieved for the greenhouse species. Spectral bands used in this analysis ranged from 353 to 799 nm. Creeping bentgrass and Virginia buttonweed were classified correctly at 96 and 92%, respectively.

Postdispersal weed seed predation by animals during the summer fallow period may lead to a reduction in the number of weeds that grow in the following winter cropping season. In this study, we investigated the patterns of weed seed removal, the influence of crop residue cover on seed removal, the types of granivores present and their seed preferences in a 16-ha postharvest cropping field in Western Australia during the summer months over 2 yr. Seed removal from caches was extremely variable (from 0 to 100%). Removal rates were generally highest along the edges of the field near bordering vegetation and lowest in the center of the field and within the bordering vegetation. However, there were many deviations from this general pattern. There was no change in rates of predation with different levels of residue cover. Ants or other small invertebrates were found to remove the most seeds. However, seed removal by other animals, such as rodents, was also evident. Annual ryegrass seeds were preferred over wild oat seeds, followed by wild radish pod segments. Seed harvesting was lowest in late January, peaked in February, and decreased in March. Results from this study suggest seed harvesters could reduce the number of surface seeds in the field, reducing the weed seed bank. Management options that increase the activity of the seed harvesters may lead to less variability in seed predation and could, therefore, be incorporated into an integrated weed management program.

Wild radish is a major weed of field crops in southern Australia. The effects of various densities of wild radish and wheat on the growth and reproductive output of each other were investigated in field studies in 2003 and 2004. The experiments were established as a factorial combination of wheat (0, 100, 200, and 400 plants m−2) and wild radish (0, 15, 30, and 60 plants m−2) densities. The effect of wild radish density on wheat yield loss and wild radish seed production were described with a rectangular hyperbola model. The presence of wild radish in wheat reduced aboveground dry matter, leaf-area index (LAI), and grain yield of wheat, and the magnitude of this reduction was dependent on weed density. Increasing the density of wheat substantially reduced the adverse effects of wild radish on wheat. As crop density increased, wild radish dry matter, LAI, and seed production per unit area decreased. The maximum seed production of wild radish was achieved at its highest density (60 plants m−2), and was 43,300 and 61,200 seeds m−2 for the first and second year, respectively. The results indicated that higher densities of wheat were able to suppress seed production of this weed species. From a practical viewpoint, this study shows that increased wheat density in the range of 200 to 400 wheat plants m−2 can reduce wild radish seed production and also give some reduction in crop yield loss, and could be an important component of an integrated weed management program.

Weed species respond to the cumulative effect of multiple practices employed within weed management systems. However, this response is rarely studied at the system level, and the relationships between weed communities and management systems in crops are not well understood. We used multivariate analyses to assess relationships among tomato management systems and weed species identified through on-farm sampling of 59 fields. Giant foxtail, common lambsquarters, prickly sida, and carpetweed were common in all systems. Eastern black nightshade was common in the conventional processing and mixed fresh-market systems but largely absent from the organic system. Barnyardgrass and goosegrass were common in the organic fresh-market system but not in the other systems. Canonical correspondence analysis identified distinct associations between specific species and the management systems. Common purslane was strongly associated with the rain-fed, mixed fresh-market system, and barnyardgrass, goosegrass, yellow nutsedge, and green foxtail were associated with the irrigated, organic fresh-market system. Indicator species analysis identified five species that were significant indicators of the organic, fresh-market system and one species that was a significant indicator of the rain-fed, mixed fresh-market system. Weed populations persist or increase when a set of species-specific environmental conditions are met. The association of weed species in this study with particular systems supports the hypothesis that weed communities are strongly affected by management systems.

Trifluralin is widely used for control of rigid ryegrass in no-till grain crops in southern Australia. Trifluralin must be incorporated into soil to be effective. Several field studies were conducted to evaluate the effect of different tillage systems on vertical seed distribution and efficacy and rate of loss of bioavailable trifluralin. Experiments were conducted during the growing seasons of 2004 and 2005 at two sites in South Australia: on the Roseworthy Campus farm of the University of Adelaide and near Minlaton on the Yorke Peninsula. The tillage systems at the Roseworthy Campus were minimum tillage (MinTill) and no-till using narrow points, whereas four direct-drill-seeding systems were used at Minlaton. Plastic beads were spread on the soil surface to simulate weed seeds. MinTill sowing buried more plastic beads than no-till, indicating that greater soil disturbance was caused by the MinTill system. Trifluralin efficacy was lower under no-till as compared to MinTill. A bioassay, based on response of oat roots, was used to quantify the concentrations of bioavailable trifluralin under different tillage systems. In both years at the Roseworthy Campus, loss of bioavailable trifluralin was greater under no-till than under MinTill; however, the rate of loss under both systems was much faster in 2004 than in 2005. In 2004, bioavailable trifluralin at 12 d after sowing under MinTill and no-till was 55 and 33% of the initial concentration, respectively. In 2005, bioavailable trifluralin remaining at 23 d after sowing was 86% under MinTill and 54% under no-till. At Minlaton in both years, bioavailable trifluralin was greater under tillage systems that provided the highest level of soil disturbance. These results demonstrate that reduced bioavailability of trifluralin occurs in no-till seeding systems, which could result in reduced weed control.

Common lambsquarters is an important annual weed of many crops world-wide. Ascochyta caulina is a plant pathogenic fungus that, under natural conditions, causes necrotic spots on the leaves and stems of Chenopodium species. The objective of this study was to evaluate the effect of weed growth stage, relative humidity, dew period, and temperature on the infection of A. caulina isolates against common lambsquarters. In greenhouse experiments, replicated groups of common lambsquarters plants were sprayed with different isolates of A. caulina 2, 3, 4, 5, and 6 wk after emergence. Both disease severity and pathogen-induced dry weight reduction decreased with plant age. The efficacy of all isolates tested was reduced by high leaf-to-air vapor-pressure deficit. Disease severity was more responsive to relative humidity than temperature. However, a minimum dew period of 6 h was required to cause significant disease severity in common lambsquarters. Among all tested A. caulina isolates, W90–1 gave the highest disease scores under all conditions, with the exception of temperatures ≤15 C.

Residues of legume crops used to increase soil fertility may also serve as sources of phytotoxins that can suppress the germination and early growth of weed and crop species. To test the hypothesis that weed and crop susceptibility to extracts of red clover shoots would be inversely proportional to seed mass, we (1) identified 18 weeds and 44 crops whose 100-seed masses ranged from 20 to 26,250 mg; (2) exposed their seeds in petri dishes and filter paper to a 2% aqueous extract of ‘Marathon’ red clover shoots or distilled water; and (3) measured germination percentage and radicle length of germinated seeds after incubation for 4 days. In a second experiment, we assessed germination and radicle growth of four crop and four weed species after exposure to 1% extracts of Marathon or ‘Cherokee’ red clover or distilled water. Germination inhibition by red clover extracts was greatest for lighter seeds and least for heavier seeds in Experiment 1 (P = 0.0005), but was unrelated to seed mass in Experiment 2. Radicle inhibition by red clover extracts was inversely proportional to seed mass in both Experiment 1 (P < 0.0001) and Experiment 2 (P = 0.0047), and, in Experiment 1, was greater for monocots than dicots (P = 0.0002). Our findings corroborate the general relationship between seed mass and stress tolerance observed by other investigators and indicate that small-seeded monocots are most likely to be susceptible to phytotoxins contained in red clover shoots.