The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2 s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3 levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Common ragweed is a native annual that colonizes disturbed habitats including agricultural fields and roadsides. It is especially abundant along roadways receiving regular applications of deicing salt. Anecdotal evidence has suggested that the emergence of common ragweed seedlings often occurs before the emergence of other roadside species and at salinity concentrations as high as 400 mM L−1, a level that can be found in roadside soils in early spring. However, the extent of this tolerance to salinity in common ragweed populations has not been quantified. The objective of this study was to assess the germination behavior of common ragweed seeds collected from three roadside and two agricultural populations across a salinity gradient. Seed germination of these five populations was monitored daily for 21 d across a sodium chloride gradient [0, 100, 200, 300, and 400 mM L−1] under controlled conditions. Seeds from roadside populations showed consistently greater total germination and rate of germination than seeds from agricultural populations. Germination differences were most evident at the 300 and 400 mM L−1 salinity concentrations. Average germination at the 400 mM L−1 sodium chloride concentration was 31% for two roadside populations and only 3% for two agricultural populations. Germination of seeds placed in distilled water after the 21-d salinity exposure treatments (i.e., recovery rates) was also greater for the roadside vs. agricultural populations. Findings indicate that the germination behavior of common ragweed seeds to salinity for roadside populations may be locally adaptive and allows common ragweed to emerge relatively early in spring thus providing a competitive advantage over later emerging roadside plants.

Experiments were conducted to examine the use of spectral reflectance curves for discriminating between plant species across moisture levels. Weed species and soybean were grown at three moisture levels, and spectral reflectance data and leaf water potential were collected every other day after the imposition of moisture stress at 8 wk after planting. Moisture stress did not reduce the ability to discriminate between species. As moisture stress increased, it became easier to distinguish between species, regardless of analysis technique. Signature amplitudes of the top five bands, discrete wavelet transforms, and multiple indices were promising analysis techniques. Discriminant models created from data set of 1 yr and validated on additional data sets provided, on average, approximately 80% accurate classification among weeds and crop. This suggests that these models are relatively robust and could potentially be used across environmental conditions in field scenarios.

Experiments were conducted in environmental chambers to the evaluate effects of photoperiod and temperature on Florida betony growth and development. Plants were exposed to two photoperiods, short day (9 h) and long day (9 + 3 h night interruption), and three day/night temperature regimes, 18/14, 22/18, and 26/22 C. After 10 wk of growth, shoot length and weight were 3.4 and 3.5 times greater, respectively, in the long-day photoperiod and with the 26 and 22 than with the 22 and 18 C day and night temperature regime, respectively. Shoot number, however, was greatest in the short-day photoperiod and at a lower temperature of 22/18 C. Shoot number in long day 22/18 C and 26/22 C environments increased asymptotically. No difference in root weight was observed between long- and short-day environments, but root weight increased with increasing temperature. Flowering and tuber production only occurred in long-day environments, with greater production of both at higher temperatures. Results provide a general framework for understanding Florida betony growth and development characteristics in the field and provide insights that should be considered in developing control strategies.

Field trials were conducted to investigate the influence of P application method on the critical period of smooth pigweed and common purslane interference in lettuce. Studies were carried out in low-P histosols, where supplemental P fertilization is needed for lettuce production. Phosphorus was either broadcast or banded 5 cm beneath the lettuce rows at rates of 250 or 125 kg ha−1, respectively. Seedlings of either smooth pigweed or common purslane were transplanted at a density of 16 plants per 5.4 m2 (6-m row by 0.9 m wide). Weed interference duration was achieved by manual removal 2, 4, 6, or 8 wk after lettuce emergence and subsequently keeping the plot weed free until harvest. A weed-free control within each P regimen was also established. Marketable head number, head fresh yield, and head diameter were measured at harvest. Weed-free lettuce fresh yield was 20% higher with banded P than broadcast applications. In the weed–lettuce mixtures, the P regimen by weed removal interaction affected lettuce fresh yield and head diameter but not head number. Compared with broadcast P application, banded P extended the time needed to cause significant weed interference in lettuce by 10 d: from 24 to 34 d for smooth pigweed and from 37 to 47 d for common purslane.

The development of optimal weed management strategies that rely, in part, on crop interference will require an understanding of how weeds compensate for limitations in above- and belowground resources. Trade-offs in the leaf morphology and biomass partitioning of rice and late watergrass were investigated under glasshouse conditions in 1999 and 2000. Both species responded to shade with increased height, reduced biomass, greater partitioning of biomass to leaves, and greater leaf area ratios. At the lowest light level (18% sunlight), plants of both species showed little response to nitrogen (N). However, height, tillers, biomass, and leaf area increased for plants grown at 50% and full sunlight as N increased from 0 to 224 kg N ha−1. Late watergrass exhibited more plasticity in specific leaf area and root weight ratio than rice in response to shade. This plasticity contributed to the ability of late watergrass to maintain a higher percent of its tillers and total dry weight than rice when sunlight was reduced by 50%. These results support the hypothesis that except at low light levels, limited N further reduces the growth of shaded late watergrass plants. Thus, weed management strategies that limit the plasticity of late watergrass by manipulating light and N availability are likely to be more effective than strategies that rely on manipulating a single resource.

Canada thistle was grown under field conditions in 2000 and 2003 at ambient and elevated (∼ 350 μmol mol−1 above ambient) carbon dioxide [CO2] to assess how rising [CO2] alters growth, biomass allocation, and efficacy of the postemergent herbicide glyphosate. By the time of glyphosate application, approximately 2 mo after emergence, elevated CO2 had resulted in significant increases in both root and shoot biomass. However, the relative positive effect of [CO2] was much larger for root, relative to shoot growth, during this period (2.5- to 3.3-fold vs. 1.2- to 1.4-fold, respectively) with a subsequent increase in root to shoot ratio. Glyphosate was applied at 2.24 kg ae ha−1 in 2000 and 2003. Subjective classification of leaf damage in shoots after spraying indicated no significant difference in the extent of necrosis in aboveground tissue as a function of CO2 concentration. After a 6-wk regrowth period, significant reductions in shoot and root biomass relative to unsprayed plots were observed under ambient [CO2]. However, the decrease in the ratio of sprayed to unsprayed biomass was significantly less at elevated relative to ambient [CO2] conditions for roots in both years, and no difference in shoot biomass was observed between sprayed and unsprayed plots for Canada thistle grown at elevated [CO2] in either year. The observed reduction in glyphosate efficacy at the enriched [CO2] treatment did not appear to be associated with differential herbicide uptake, suggesting that tolerance was simply a dilution effect, related to the large stimulation of root relative to shoot biomass at elevated [CO2]. Overall, the study indicates that carbon dioxide–induced increases in root biomass could make Canada thistle and other perennial weeds that reproduce asexually from belowground organs harder to control in a higher [CO2] world.

Sphenoptera jugoslavica negatively influences diffuse knapweed populations, but the influence is inconsistent in space and time. In spring 1998, a 3-yr experiment was established to determine whether low rates of picloram or clopyralid would increase the percentage of plants infested with S. jugoslavica, thereby potentially enhancing its capacity to control diffuse knapweed. The experiment was conducted at three sites in Colorado, where S. jugoslavica was released in 1994. Picloram and clopyralid were applied separately to plots at 35, 70, or 140 g ha−1 during June or September, and a non–herbicide-treated control was added. Density and cover measurements were collected in permanent quadrats three times during each growing season to determine whether herbicides influenced diffuse knapweed growth and population dynamics. Diffuse knapweed plants were harvested outside the permanent quadrats to determine the percentage of plants bearing S. jugoslavica larvae. During the spring after herbicide application, all picloram rates and the 35-g ha−1 rate of clopyralid applied the previous June increased the percentage of plants infested by S. jugoslavica approximately 25% compared with the nonsprayed control. None of the herbicide treatments increased the percentage of plants infested by S. jugoslavica 2 yr after application, indicating that the herbicides' positive effects on the percentage of plants infested by S. jugoslavica lasted only 1 yr. Results indicate that combining S. jugoslavica with low rates of picloram or clopyralid applied in June can improve diffuse knapweed control compared with using S. jugoslavica alone.

One hundred and thirty-three fungal isolates, pathogenic to green foxtail, were evaluated for weed control potential under controlled conditions. To determine weed control efficacy, these pathogens were applied as spore or mycelial suspensions at approximately 105 propagules ml−1 to green foxtail at the three-leaf stage. One week after inoculation, most isolates caused only minor injury to the plants, but 15 isolates caused 50 to 100% disease. Among the most efficacious isolates, only those of Pyricularia setariae exhibited strong host specificity to the target weed, revealing no significant pathogenicity on 28 other plant species tested, including many important crops such as wheat, barley, and oat. On green foxtail leaves, conidia of this fungus germinated readily at 14, 20, and 26 C, but the process of germination and appressorial formation was more rapid at the higher temperatures. The fungus applied at the concentration of 105 spores ml−1 reduced weed fresh weight by 34% 7 d after the treatment when compared with controls, whereas a concentration of 107 spores ml−1 reduced fresh weight by 87%. This efficacy was comparable with that of the herbicide sethoxydim. When applied to the weed at the one- to four-leaf stages, the fungus reduced green foxtail fresh weight by more than 80%. Efficacy was slightly lower on plants at the five-leaf stage or older. On the green foxtail biotype resistant to the herbicide sethoxydim, P. setariae caused 80% fresh weight reduction compared with untreated controls, as opposed to 17% achieved with the herbicide. At 20 C, the fungus required a minimum of 6-h dew period to initiate infection, but a 10-h dew period was needed to cause severe damage to green foxtail.

Research during the past several decades on jointed goatgrass management has focused on individual cultural practices rather than on multi- or interdisciplinary components. Field studies were conducted at Hays, KS, from 1997 to 2003 to evaluate the interaction of crop rotation, fallow weed management, and winter wheat variety on jointed goatgrass density. Extending a wheat–fallow (W–F) rotation to include grain sorghum or grain sorghum and sunflower reduced jointed goatgrass populations more than other cultural practices tested. Fallow treatments were equal in most years, but mechanical fallow resulted in increased jointed goatgrass emergence compared with chemical fallow under drought conditions. Winter wheat cultivars had little effect on jointed goatgrass populations. However, taller, more competitive varieties are favorable for jointed goatgrass control in an integrated management program. No specific combination of crop rotation, fallow weed management, and wheat variety consistently reduced jointed goatgrass density more than other combinations during multiple years.

Adjuvant effects on disease severity caused by the bioherbicide P. papaveracea on opium poppy were evaluated. Tween 20, Tween 80, Triton X-100, Tactic, CelGard, and Keltrol inhibited appressorium formation but not conidial germination on detached leaves. The disease severity varied from 11 to 83% necrosis in field experiments involving eight adjuvants at various concentrations plus 1 × 106 conidia ml−1 or minus pathogen. The three best-performing adjuvants when combined with pathogen, Tactic (1%, v/v), Bond (1%, v/v), and Tween 20 (1%, v/v), were included along with Tween 20 (0.001%, v/v) in field experiments in 1998. Tween 20 (1%, v/v) plus pathogen (1 × 106 conidia ml−1) caused the most severe disease, averaging 68% necrosis within 2 wk of treatment. Overall, plots treated with adjuvant plus P. papaveracea had a 22% reduction in capsule weight per plot as compared to plots treated with the adjuvant alone. Tactic (1%, v/v), Silwet-L77 (0.1%, v/v), Tween 20 (1%, v/v), and Tween 20 (0.001%, v/v) were included in field experiments in 1999. The treatment with Tween 20 (1%, v/v) plus pathogen (2 × 106 conidia ml−1) caused severe disease, averaging 56% necrosis within 2 wk of treatment. In 1999 plots treated with adjuvant plus pathogen averaged a 27% reduction in capsule weight as compared with plots treated with the adjuvants alone. The inclusion of Tween 20 (1%, v/v) with P. papaveracea conidia greatly enhanced efficacy on opium poppy.

Field studies were conducted near Blackville, SC, in 2002 and 2003 and near Pendleton, SC, in 2003 to compare the critical period for weed control (CPWC) in narrow- (48 cm) and wide-row (97 cm) corn. A series of treatments of increasing duration of weed interference and length of weed-free period were imposed within each row spacing. Diversity and density of the weed spectrum were greater at Blackville than at Pendleton. Weed interference duration and weed-free period curves were similar between row widths for each of the 3 site-years. Averaged over row width, the CPWC was 36 to 40 d longer at Blackville than at Pendleton. The CPWC began 5 to 9 d after corn emergence (DAE) (one- to two-leaf stage) and ended 45 to 53 DAE (eight- to 10-leaf stage) at Blackville. At Pendleton, the CPWC was only 4 d, beginning 21 DAE (five-leaf stage) and ending 25 DAE (five- to six-leaf stage). Light interception by corn at Blackville at the end of the CPWC averaged 78%, but light interception averaged only 31% at Pendleton at the end of the CPWC, implying that the weed density or weed spectrum may be more of a determinate of the CPWC than canopy formation. Light interception was similar between row widths throughout the growing season, resulting in similar late-season weed biomass between row widths. The CPWC and crop competitiveness with late-emerging weeds was similar between wide- and narrow-row corn when corn light interception did not differ between row widths. Therefore, other strategies, such as increasing the population of narrow-row corn, are likely needed to provide a competitive advantage over wider rows.

Few studies have examined the combined effect of herbicide-induced stress and arthropod herbivory to reduce weed fitness. The purpose of this study was to quantify the effect of arthropod herbivory on the herbicide dose–response of a perennial weed. Fluroxypyr dose–response bioassays using volunteer potato were conducted in the presence and absence of Colorado potato beetle (CPB) herbivory. Logistic model parameter estimates for leaf area, shoot biomass, tuber number, and tuber biomass were often lower with herbivory, compared with no herbivory. Greater variance of parameter estimates within herbivory plots was attributed largely to differential feeding because CPB density was not manipulated in the field. Results from short-season field studies (1,000 growing degree days [GDD] after postemergence [POST] herbicide application) indicated that herbivory had the most effect on potato during a period that coincided with high CPB density and optimal temperatures for CPB development. Season-long bioassays (> 3,100 GDD after POST) revealed that addition of herbivory reduced herbicide use 65 to > 85%, compared with the dose needed to achieve the same reduction in tuber production in the absence of herbivory. Integrated weed management systems targeting volunteer potato are more effective when fluroxypyr applications are made before periods of high herbivory. Moreover, this article describes an experimental approach contributing to optimization of combined effects of arthropod herbivory and reduced herbicide doses.

WeedSOFT® is a decision support system that was developed to help farmers and consultants in Nebraska with the selection of optimal weed management strategies. WeedSOFT® evolved from HERB, a bioeconomic model for soybean that was developed in North Carolina. The program is composed of four independent modules, namely, ADVISOR, EnviroFX, MapVIEW, and WeedVIEW. ADVISOR helps the user select a treatment based on maximum yield or maximum net gain. EnviroFX and MapVIEW provide environmentally relevant herbicide information and county soil maps that indicate vulnerability to groundwater contamination. WeedVIEW is a visual library of color images and line drawings of 46 common weed species. Over 500 farmers and consultants in Nebraska and adjacent states use WeedSOFT®. As a result of the current regionalization effort, the user base is expected to increase rapidly during the next 2 or 3 yr. This article explains the algorithms implemented in the current version of WeedSOFT®.

Interseeded cover crops have the potential to maintain and improve soil quality, reduce the incidence of insect pests, and suppress weeds in vegetable production systems. However, the successful use of interseeded cover crops has been limited by their tendency to either inadequately suppress weeds or suppress both weeds and the crop. We hypothesized that in irrigated broccoli production, winter rye could suppress annual weeds through rapid emergence and shading, without adversely affecting the taller transplanted broccoli crop. In field experiments conducted in New York from 1999–2001, broccoli was cultivated at 0, 10, or 10 and 20 d after broccoli transplanting (DAT), with or without rye at the final cultivation. Rye interseeded at 0 DAT suppressed weeds and improved yields relative to unweeded controls but resulted in broccoli yield losses relative to weed-free controls in 2 of 3 years. Rye seeded at either 10 or 20 DAT did not reduce broccoli yields but had little effect on weeds for a given level of cultivation and resulted in Powell amaranth seed production of up to 28,000 seeds m−2. Rye interseeded at 0 DAT reduced light availability to weeds in 2000 but not in 2001 when Powell amaranth avoided shading from rye through rapid emergence and vertical growth. In greenhouse pot experiments, low temperatures for 7 d after seeding delayed the emergence of Powell amaranth by 3 d relative to rye and increased the suppression of Powell amaranth by rye from 61 to 85%. Our results suggest that winter rye may be more successfully integrated into broccoli production (1) when sown at higher densities, (2) in locations or seasons (e.g., spring) with lower initial temperatures, and (3) in combination with other weed management tools.

Laboratory experiments were conducted to determine the effects of oxygen concentration (21, 10, 5, and 2.5%), exposure to light, and osmotic potential on the germination of wheat, canola, and various weed species. Germination of most species increased as osmotic potential was increased. Seed germination for some species like barnyardgrass was inhibited by the combination of exposure to normoxic (21% oxygen) conditions and light. This combination of conditions may function as a signal to prevent soil surface germination. Wild mustard germination increased with increasing oxygen concentration when seeds were not exposed to light, whereas green foxtail germination was relatively insensitive to oxygen concentration. Wild oat germination increased with increasing osmotic potential, and osmotic potential had a greater influence when the seeds were exposed to light. Dandelion, foxtail barley, curly dock, and perennial sowthistle germination was affected more by osmotic potential and light exposure than by oxygen concentration. A better understanding of the mechanisms of depth detection for specific species will lead to a better understanding of their recruitment biology. This information may help model the potential for invasion and proliferation of each species as well as devise improved management strategies.

Reliable mechanical weed control requires knowledge of the achievable levels of weed control and crop damage when using certain implements in specific conditions. Quantitative methods that use weed, crop, soil, and cultivator characteristics to predict weed control and crop damage need to be developed. To that end, the relative susceptibility of weeds and crop plants to mechanical weeding and the selective ability of cultivators need to be quantified separately. The method presented in this study uses measured plant anchorage forces to quantify crop and weed sensitivity to being uprooted by a weed harrow and predicts the relationship between weed and crop uprooting by mechanical weeding. Uprooting and anchorage force of young perennial ryegrass and garden cress plants were measured in laboratory harrowing experiments on sandy soil. A nonlinear equation was introduced to describe the relationship between weed uprooting and crop uprooting. The parameters representing the selective potential of the actual crop–weed condition (Kpot) and the implement selective ability (Kcult) did not depend on crop uprooting. The relationship between potential weed and crop uprooting that could theoretically be obtained by a perfectly selective implement (i.e., pulling each plant with equal force) was calculated from plant anchorage force distributions measured before harrowing. The observed uprooting percentages achieved by harrowing were lower than the potential uprooting percentages. With Kcult accounting for imperfect weeder selective ability, prediction accuracy was satisfactory. Field validation is required to confirm whether this method improves comparison and prediction of weeding performance of different weeding implements in different crop–weed situations.

Research in irrigated and nonirrigated corn production systems was conducted to evaluate the effect of leaf architecture of corn hybrids on weed management. The corn hybrids used in each study were ‘Pioneer 3394’ (upright leaf) and ‘Pioneer 3260’ (horizontal leaf). In the irrigated study, residual weed control treatments included two rates of prepackaged mixtures of metolachlor plus atrazine, encapsulated acetochlor plus atrazine, nonencapsulated acetochlor plus atrazine, or a tank mixture of simazine and metolachlor plus atrazine. In the irrigated experiments, horizontal leaf architecture reduced weed density (all three sites in 1 of 2 yr), weed biomass (five of six sites), solar radiation reaching the ground (all six sites), and weed seed production (one site each year) compared with upright leaf architecture. Weed density and weed biomass did not differ between herbicide rates or acetochlor formulation at any site. Corn hybrid was significant for yield at only one site. Reduced weed biomass did not translate into yield differences. The nonirrigated study evaluated two factors at four sites over 2 yr: leaf architecture (upright or horizontal leaf) and weed management program (preemergence residual and postemergence no residual) at two application rates. Neither weed density nor weed biomass was reduced because of corn leaf architecture or herbicide rates in the nonirrigated study. No interaction was detected in either irrigated or nonirrigated studies between leaf architecture and herbicide treatments, indicating that these factors are independent of one another. On the basis of these studies, it appears that horizontal leaf architecture of corn hybrids can assist in integrated weed management in irrigated corn production but may not be beneficial when corn is grown under drought-prone conditions.

Japanese honeysuckle presents a serious problem to the economically attractive natural regeneration of loblolly and shortleaf pine. This research investigated the potential allelopathic interference mechanisms of Japanese honeysuckle in relation to pine regeneration and growth, which may provide insight into overcoming this problem. The allelopathic potential of root exudates and leaf litter from Japanese honeysuckle was tested against loblolly and shortleaf pine seedlings. When Japanese honeysuckle and loblolly pine seedlings were grown using the same irrigation reservoir, there was no significant effect on the growth of either pine species. Exudates of Japanese honeysuckle grown as a pure culture in donor cups also produced no growth effects on pure-cultured pine seedlings grown in acceptor cups. In other assays, aqueous extracts of Japanese honeysuckle leaf tissue were toxic to duckweeds at concentrations well below levels where plasmolysis might cause effects. When Japanese honeysuckle leaf tissue was added to soil at a rate of 2 g tissue 100 g−1 soil, mean seedling height at 128 d after planting was reduced by as much as 40%. Moreover, pine seedlings grown in the presence of Japanese honeysuckle tissue exhibited significant chlorosis of the shoot and needles. Gas chromatography–mass spectroscopy analyses and high-performance liquid chromatography of Japanese honeysuckle leaf tissue aqueous extracts confirmed the presence of five compounds previously identified as possible allelochemicals: 4-hydroxycinnamic acid; 2-hydroxycinnamic acid; 3,4-dihydroxybenzoic acid; 3,4-dihydroxycinnamic acid; and chlorogenic acid. Results indicate that allelopathy plays at least a partial role in Japanese honeysuckle interference with loblolly and shortleaf pine.

No-tillage offers potential for improved soil quality, reduced erosion, and equal or increased crop yields. We hypothesized that, compared with conservation tillage (CT), no-tillage (NT) offers conditions more conducive to microbial decay of weed seed. In NT systems seed remain at or near the soil surface where crop residues, moisture, and lack of disturbance create an environment with greater soil microbial diversity. In late fall of 1998 and 1999, dormant seed of wild oat, either individually glued to plastic toothpicks or mixed with soil and placed in mesh bags, were buried (mean seed depth of 2.5 cm) in replicated field plots managed by NT or CT since 1982. Treatments including fungicide seed treatment (thiram + metalaxyl + captan) and soil fumigation (propylene oxide) provided estimates of the contribution of microorganisms to observed mortality. Seed were retrieved in May and August, 1999 and 2000. Contrary to our original hypothesis, the proportion of dead seed was generally similar in NT and CT systems. Lack of tillage system by seed or soil treatments affecting the proportion of dead or decayed seed suggests that the contribution of microorganisms to seed fate is similar in these tillage environments. However, the proportion of dormant seed was consistently lower in the NT compared with CT treatments; there was a corresponding increase in the proportion of germinated seed. Overall, more than half of the wild oat seed bank losses could be directly attributed to germination whereas losses due to decay were relatively minor by comparison. Despite favorable distribution of seed and improved quality of the surface-strata of soil in NT systems, this study fails to provide evidence that enhanced microbial decay will contribute to a “weed-suppressive” capacity in such cropping systems.