Palmer amaranth influences selection of crop production practices such as irrigation, nitrogen (N) application, and weed control. The objectives of this research were to determine if Palmer amaranth was more responsive to applied N than corn and if this differed under dryland and irrigated conditions in Kansas. Field experiments were conducted near Manhattan, KS, in 2005 and 2006 to evaluate the influence of N rate and Palmer amaranth densities when grown with corn in two soil moisture environments. A very drought-stressed environment and a well-watered environment occurred in 2006, while both environments in 2005 were intermediate. Dryland weed-free corn yields were 46.5% of irrigated corn yields at the high N rate across years. Irrigated corn yields responded to increasing N rates. In the presence of Palmer amaranth, parameter estimates I and A for the yield loss relationship were not different across N rates for each environment and year except 2006 where 100% yield loss was estimated in dryland compared to 62.5% loss in irrigated environment at high N rates. In three of four environment-years, N rate did not affect the corn yield loss relationship with weed density. In 2006 irrigated environment, greater N rates had less corn yield loss caused by Palmer amaranth. By corn anthesis, weed-free corn biomass was 167.5% greater in irrigated than dryland environments in 2006. Palmer amaranth with no corn increased its biomass by 373 and 361% as N rate increased in 2005 and 2006, respectively. Nitrogen concentrations in plant tissues of corn or weed increased similarly as N rates increased from 0 to 224 kg N ha−1, thus highlighting that both corn and Palmer amaranth responded similarly to increasing N. In general, soil moisture environment was most critical when determining potential corn yield, followed by Palmer amaranth density and N rate.

Synthetic auxin herbicides are widely utilized in golf course settings for selective broadleaf weed control. Aminocyclopyrachlor (AMCP) is a newly registered pyrimidine carboxylic acid with similar chemical mode-of-action and structure to triclopyr (TRIC) and clopyralid (CLPY). Off-target injury on terrestrial plants has been documented following exposure to turfgrass clippings previously treated with TRIC and CLPY. Management practices on golf courses can distribute turfgrass clippings into water bodies; however, research has not evaluated the bioavailability of synthetic auxin residues from turfgrass clippings to aquatic and riparian plants within these environments. A bioassay study was conducted to determine the response of alligatorweed and parrotfeather to tall fescue clippings previously treated with synthetic auxin herbicides. Previously treated AMCP and TRIC + CLPY clippings were placed into growth containers mimicking a lentic system containing both alligatorweed and parrotfeather. Results indicated all herbicide treated clippings induced significant growth responses to alligatorweed and parrotfeather growth compared to a nontreated mulch and nontreated control. Alligatorweed control was greater from AMCP clippings treated 14, 7, 3, and 1 DBCC (49, 60, 90, and 80%, respectively) than comparative TRIC + CLPY clippings (33, 25, 37, and 64%, respectively) at 10 weeks after treatment (WAT). Parrotfeather control was greater from AMCP clippings (57 to 87%) than TRIC + CLPY clippings (9 to 63%) collected from all days before clipping collection (DBCC) timings when evaluated 6 WAT. At 10 WAT, greater parrotfeather control and shoot reduction was observed from AMCP than TRIC + CLPY clippings when treated 14, 7, and 3 DBCC. Based on these data, synthetic auxin residues can become bioavailable to aquatic and riparian plants within aqueous environments.

Alligatorweed is well known for its potassium (K+)-accumulating capabilities and its strong resistance to undesired growth conditions. The results of this study revealed properties of K+ accumulation and its contribution to drought stress in alligatorweed. In addition, we attempted to characterize the molecular mechanisms of K+ accumulation in this plant. Alligatorweed plants showed a consistent increase in biomass in response to external K+ concentrations, ranging from micromolar levels up to 50 mmol L−1; K+ was also accumulated accordingly in the plants. The stem was the most K+-accumulating organ, accumulating up to 13% of the K+. Moreover, this K+ superaccumulation caused improved resistance to drought stress. The apparent K+ uptake by the roots showed a typical high-affinity property, and the Michaelis constant increased at higher rates of plant K+ in the starting materials. Furthermore, three putative, K+-uptake transporter complementary DNAs (cDNAs) were isolated from alligatorweed (ApKUP1, ApKUP2, and ApKUP3, respectively) using degenerated primers and rapid amplification of cDNA end techniques. The expression of ApKUP1 and ApKUP3 was predominately localized to the leaves, whereas ApKUP2 was expressed throughout the entire plant. The expression of ApKUP1 and ApKUP3 was stimulated in the stems and roots when K+ was depleted from the external medium. Moreover, ApKUP3 expression was enhanced in the stem in response to abscisic acid treatment and drought stress. In conclusion, our findings provide further insight into the mechanisms of K+ accumulation linked to K+ uptake in alligatorweed.

Robust predictions of weed seedling emergence from the soil seedbank are needed to aid weed management. A common seed accession (Illinois) of giant ragweed was buried in replicate experimental gardens over 18 site years in Illinois, Michigan, Kansas, Nebraska, Ohio, and South Dakota to examine the importance of site and climate variability by year on seedling emergence. In a nonlinear mixed-effects modeling approach, we used a flexible sigmoidal function (Weibull) to model giant ragweed cumulative seedling emergence in relation to hydrothermal time accumulated in each site-year. An iterative search method across a range of base temperature (Tb ) and base and ceiling soil matric potentials (ψb and ψc) for accumulation of hydrothermal time identified optima (Tb = 4.4 C, ψb = −2,500 kPa, ψc = 0 kPa) that resulted in a parsimonious regional model. Deviations between the fits for individual site-years and the fixed effects regional model were characterized by a negative relationship between random effects for the shape parameter lrc (natural log of the rate constant, indicating the speed at which emergence progressed) and thermal time (base 10 C) during the seed burial period October through March (r = −0.51, P = 0.03). One possible implication of this result is that cold winter temperatures are required to break dormancy in giant ragweed seeds. By taking advantage of advances in statistical computing approaches, development of robust regional models now is possible for explaining arable weed seedling emergence progress across wide regions.

A segment of the debate surrounding the commercialization of genetically engineered (GE) crops, such as glyphosate-resistant (GR) crops, focuses on the theory that implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was conducted in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR corn, cotton, or soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on emerged weed density and diversity was analyzed. Species richness, evenness, Shannon's H′, proportion of forbs, erect growth habit, and C3 species diversity were all greater in agricultural sites that lacked crop rotation or were in a continuous GR crop system. Rotating between two GR crops (e.g., corn and soybean) or rotating to a non-GR crop resulted in less weed diversity than a continuous GR crop. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. The finding that fields with continuous GR crops demonstrated greater weed diversity is contrary to arguments opposing the use of GE crops. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits, or more broadly, GE crops, to provide a more complete characterization of their culture and local adaptation.

Interference for 40 d after emergence (DAE) of corn, cotton, peanut, and snap bean by four glyphosate-resistant (GR) and four glyphosate-susceptible (GS) Palmer amaranth populations from Georgia and North Carolina was compared in the greenhouse. Greater interference from Palmer amaranth, measured as crop height and fresh weight reduction, was noted in cotton and peanut compared with corn or snap bean. Crop height 15 to 40 DAE was reduced similarly by GR and GS populations. Crop fresh weight, however, was reduced 25 and 19% in the presence of GS and GR populations, respectively. Measured as percent reduction in fresh weight, GR and GS populations of Palmer amaranth were controlled similarly by glufosinate, lactofen, paraquat, and trifloxysulfuron applied POST. Atrazine and dicamba controlled GR populations more effectively than GS populations.

Spreading dogbane is a common perennial weed in wild blueberry fields. It is highly competitive and spreads rapidly once established. Herbicides can provide effective control of spreading dogbane, but application timing is important. The emergence pattern, ramet height, and flowering time of spreading dogbane were observed in 2008 and 2009, and thermal-based emergence, growth, and development models were developed and used to estimate optimum herbicide application timing. Spreading dogbane emergence and height were described with a three-parameter, sigmoid, nonlinear regression model, whereas flowering was described with a four-parameter, Weibull, nonlinear regression model. Spreading dogbane ramets initiated emergence soon after the biofix date of April 1. Peak emergence tended to occur at 420 growing degree days (GDD). Spreading dogbane reached its peak height by about 558 GDD. The maximum number of flowers per plant was reached at approximately 750 GDD. This study suggested that POST herbicides should be applied between 486 and 535 GDD to maximize efficacy. This time frame occurs after peak emergence and during early floral bud development.

Over 90% of Canadian kochia populations are resistant to acetolactate synthase (ALS)– inhibiting herbicides. We questioned whether the target site–based resistance could affect plant growth and competitiveness. Homozygous F2 herbicide-resistant (HR) kochia plants with an amino acid substitution at Trp574 (sources: Alberta [AB], Saskatchewan [SK], and Manitoba [MB]), or Pro197 (MB, AB with two populations) were grown in replacement series with homozygous F2 herbicide-susceptible (HS) plants from the corresponding heterogeneous population (total: six populations). In pure stands, growth of HR plants from AB and SK was similar to that of HS plants, regardless of mutation; conversely, MB2-HR plants (Trp574Leu) developed more slowly and were taller than MB2-HS plants. Final dry weight of HR plants in pure stands was similar across all six populations, whereas that for HS plants in pure stands and HR–HS plants in mixed stands (50–50%) varied with population. Results for AB and SK populations suggest little impact of either ALS mutation on kochia growth, whereas those for MB lines would suggest an unidentified factor (or factors) affecting the HS, HR, or both biotypes. The variable response within and between lines, and across HS biotypes highlights the importance of including populations of various origins and multiple susceptible controls in HR biotype studies.

Greenhouse studies were conducted to evaluate the growth response of itchgrass to water stress. Itchgrass plants produced the greatest aboveground biomass and seeds at 75% of field capacity and these parameters at 50 and 100% of field capacity were similar. With further increase in water stress, seed production was sharply reduced, but itchgrass was still able to produce an average of 63 and 9 seeds plant−1 at 25 and 12.5% of field capacity, respectively. Itchgrass plants responded to increasing water stress with increased leaf weight ratio; it was 2.5 times greater at 12.5% of field capacity than at 100% of field capacity. In another study, compared with daily irrigation, intervals of 9 d between irrigations reduced aboveground biomass of itchgrass by 27% and 12-d intervals reduced aboveground biomass by 67%. Compared with the daily irrigation regime, itchgrass seed production was reduced by 61% at intervals of 12 d between irrigations; however, the weed plants produced a considerable number of seeds (153 seeds plant−1) at the 12-d intervals. The ability of itchgrass to produce biomass and seeds under water stressed conditions necessitates strategies that minimize weed survival while maximizing irrigation efficiency for the crop at the same time.

Infection by Xanthomonas axonopodis pv. manihotis (Xam) of the perennial rangeland weed leafy spurge was tested to see whether Xam might serve a potential biological control agent for this invasive weed. Although leafy spurge was susceptible to Xam infection, it recovered within 21 d after inoculation (DAI). Microarray resources available for leafy spurge allowed us to follow the physiological and signaling pathways that were altered as leafy spurge was infected and then recovered from Xam infection. The first physiological effect of Xam infection was a down-regulation of photosynthetic processes within 1 DAI. By 7 DAI, numerous processes associated with well-documented pathogenesis responses of plants were observed. Although some pathogenesis responses were still detectable at 21 DAI, other processes associated with meristem development were noted. Ontological analysis of potential signaling systems indicated jasmonic acid plays a significant role in the recovery processes.

In the present field study, the capability of Canada thistle to develop shoots from intact roots and root fragments at different soil depths was studied. The experiments were performed on four sites with high-density Canada thistle, with three or four replications per treatment. At each site, the soil in the plots was removed layer by layer (to 30 or 40 cm, depending on the site), within a 1 by 1-m quadrat, and spread out on a plastic sheet. All roots and other plant parts were removed, and the soil was either replaced without any root material (two sites), or the roots of the thistles were cut into 10-cm-long fragments and replaced into the source holes (two sites). The measured variables were shoot number and biomass. The number of shoots of Canada thistle decreased with increasing depth (P < 0.001) and increased with time. Additionally, the two factors interacted (P < 0.001) such that shoot development was slower from greater depths. Roots from ≤ 20 cm depth produced higher biomasses than did roots from below 20 cm depth. Replacement of root fragments did not affect the amount of biomass produced. It was concluded that the intact root system contributed considerably more to the total biomass produced by Canada thistle than did the root fragments in the upper soil layers.

According to climate models, drier summers must be expected more frequently in Central Europe during the next decades, which may influence plant performance and competition in grassland. The overall source–sink relations in plants, especially allocation of solutes to above- and below-ground parts, may be affected by drought. To investigate solute export from a given leaf of broadleaf dock, a solution containing 57Co and 65Zn was introduced through a leaf flap. The export from this leaf was detected by analysing radionuclide contents in various plant parts. Less label was allocated to new leaves and more to roots under drought. The observed alterations of source–sink relations in broadleaf dock were reversible during a subsequent short period of rewatering. These findings suggest an increased resource allocation to roots under drought improving the functionality of the plants.

Sweet corn is seeded under a wide range of population densities; however, the extent to which variable population density influences weed suppression is unknown. Therefore, field studies were undertaken to quantify the influence of sweet corn seeding level on growth, seed production, and post-harvest seed germination of wild-proso millet, one of the most problematic weeds in the crop. As crop seeding level increased, path analysis results indicated the crop canopy became taller and thicker, resulting in less wild-proso millet biomass, seed production, and germinability. However, at the level of individual fields, reductions in wild-proso millet growth and seed production were modest, at best, between a crop population currently used by growers and a higher crop population known to optimize yield of certain hybrids. These results indicate near-future increases in sweet corn seeding levels may play a minor role in improving weed management in individual sweet corn fields. Nonetheless, a reduction in crop populations, via weather- or management-driven phenomenon, increases risk of greater wild-proso millet seed production.

Smutgrass is an invasive, well-rooted perennial that has long been recognized as an aggressive weed throughout Florida and in the subtropical regions of the United States. Small smutgrass and giant smutgrass are the two predominant smutgrass varieties found in Florida. The native soil pH of Florida flatwoods is 4.5 to 5.0 which is lower than the level of soil pH recommended for optimum bahiagrass growth. Therefore, replacement series experiments were conducted in a greenhouse in 2010 and 2011 to compare the competitive ability of bahiagrass with each of the two varieties of smutgrass at three levels of soil pH (4.5, 5.5, and 6.5), two densities; 4 (low) and 8 (high) plants pot−1, and at five planting ratios of 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100. Relative competitive ability and aggressivity of giant smutgrass was greater than bahiagrass across all pH levels and densities, whereas relative competitive ability and aggressivity of bahiagrass was greater than small smutgrass in all pH levels and densities, except at pH 6.5. At pH 5.5, biomass accumulation of giant smutgrass was at least 73% higher than bahiagrass, whereas small smutgrass biomass was at least 46% lower than bahiagrass at equal planting ratios of both low and high densities. Differential responses were observed on bahiagrass competitive ability with small and giant smutgrass. Amending soil pH is not a likely option to increase the growth and competitive ability of bahiagrass over giant smutgrass. However, for small smutgrass, it is likely to increase the aggressivity of bahiagrass in bahiagrass–small smutgrass mixture, unless the soil pH is raised above 5.5.

Weed emergence models require biological parameters such as base temperature for germination, determination of which is costly and time-consuming. Transferability of these parameters across different populations may therefore represent one of the main constraints for the development and practical use of emergence models at a large scale. A collaborative project was undertaken to assess the interpopulation variability of base temperature for germination in different European populations of velvetleaf and jimsonweed and evaluate possible applicative consequences in terms of weed control. Seeds were collected in Italy, Portugal, and Spain, and each population was then sown in every country, obtaining nine seed batches named as experimental lots. Base temperature for germination was estimated for each experimental lot to calculate lot-specific accumulation of growing degree days (GDD) under three dissimilar climatic scenarios. Threshold date (TD50) was calculated as the date when GDD accumulation of a given experimental lot surpassed the values corresponding to 50% of cumulative field emergence of seedlings. GDD accumulation and TD50 were then used as indicators to identify differences among experimental lots within each climatic scenario. No significant differences were detected among base temperatures estimated for velvetleaf experimental lots or among their patterns of accumulation of GDD and TD50 values within climatic scenarios. Each value of base temperature determined for a single experimental lot could therefore be adopted to model germination for all the lots regardless of the population of origin or cultivation site. In contrast, the population of origin affected the base temperature for jimsonweed, with significantly higher values for experimental lots of the Portuguese population. From an applicative perspective, differences among patterns of accumulation of GDD and TD50 of several experimental lots within each climatic scenario suggest the need to use population-specific values as base temperature for germination and emergence modeling of jimsonweed.

A greater understanding of the factors that regulate weed seed return to and persistence in the soil seedbank is needed for the management of difficult-to-control herbicide-resistant weeds. Studies were conducted in Tifton, GA to (1) evaluate whether glyphosate resistance, burial depth, and burial duration affect the longevity of Palmer amaranth seeds and (2) estimate the potential postdispersal herbivory of seeds. Palmer amaranth seeds from glyphosate-resistant and glyphosate-susceptible populations were buried in nylon bags at four depths ranging from 1 to 40 cm for intervals ranging between 0 and 36 mo, after which the bags were exhumed and seeds evaluated for viability. There were no detectable differences in seed viability between glyphosate-resistant and glyphosate-susceptible Palmer amaranth seeds, but there was a significant burial time by burial depth interaction. Palmer amaranth seed viability for each of the burial depths declined over time and was described by exponential decay regression models. Seed viability at the initiation of the study was ≥ 96%; after 6 mo of burial, viability declined to 65 to 78%. As burial depth increased, so did Palmer amaranth seed viability. By 36 mo, seed viability ranged from 9% (1-cm depth) to 22% (40-cm depth). To evaluate potential herbivory, seed traps with three levels of exclusion were constructed: (1) no exclusion, (2) rodent exclusion, and (3) rodent and large arthropod exclusion. Each seed trap contained 100 Palmer amaranth seeds and were deployed for 7 d at irregular intervals throughout the year, totaling 27 sample times. There were seasonal differences in seed recovery and differences among type of seed trap exclusion, but no interactions. Seed recovery was lower in the summer and early autumn and higher in the late winter and early spring, which may reflect the seasonal fluctuations in herbivore populations or the availability of other food sources. Seed recovery was greatest (44%) from the most restrictive traps, which only allowed access by small arthropods, such as fire ants. Traps that excluded rodents, but allowed access by small and large arthropods, had 34% seed recovery. In the nonexclusion traps, only 25% of seed were recovered, with evidence of rodent activity around these traps. Despite the physically small seed size, Palmer amaranth is targeted for removal from seed traps by seed herbivores, which could signify a reduction in the overall seed density. To be successful, Palmer amaranth management programs will need to reduce soil seedbank population densities. Future studies need to address factors that enhance the depletion of the soil seedbank and evaluate how these interact with other weed control practices.

Exotic invasive species are nonnative species that thrive outside of their native habitat, and while it is difficult to determine which exotic plants will become invasive, successful invaders often share a wide range of traits including high growth rate and reproductive output, vegetative reproduction, high population growth rates, early reproductive age, phenotypic and physiological plasticity, and high resource use efficiency. Here we report on the response of pampasgrass, an important exotic invasive plant of the western United States, to experimental variations in soil nitrogen (N) and water availability. Given its ability to invade a wide variety of ecosystems in southern California, we hypothesized that pampasgrass would have higher water and N use efficiency under conditions of low water and N availability but rapid growth and resource use under conditions of high water and N availability. Our data support this hypothesis and indicate that pampasgrass exhibited large variations in growth, carbon allocation, morphology, and N and phosphorus (P) nutrition to variations in N availability and water table depth. Many of these traits are highly correlated with invasive performance, and the high N and P use efficiency observed under low soil N (control) and water table, coupled with the large increase in physiological performance and resource use under high N and water table, indicate that pampasgrass is highly flexible to soil resource levels that are typical for coastal sage scrub and riparian ecosystems of southern California. Such flexibility in resource use could allow pampasgrass to persist in low-resource environments and expand as resource levels increase.