Effective in-season weed management options are limited for organic cereal farmers. Two alternatives to current farmer practices are improving efficacy of physical weed control through use of interrow cultivation or increasing the competitive ability of the crop through elevated seeding rates and more uniform spatial planting patterns. It is unknown how these two methods affect yield, quality, and economic returns. Field experiments were conducted in the northeast United States to determine whether the yield gain from increased weed control from these contrasting weed management strategies resulted in increased net returns and how these different systems affected grain quality. Wheat was planted at two seeding rates (400 and 600 plants m−2), in three row spacings (11, 18, and 23 cm). A fourth crop arrangement that approaches a more uniform spatial distribution through a combination of drilling and broadcasting seed was included. For weed control, treatments received tine harrowing. Wheat sown in wide rows also received interrow cultivation. Each system was sown in the presence and absence of condiment mustard, which was sown as a surrogate weed. Increased seeding rate reduced weed density 64% compared to a crop-free check and 30% compared to regional farmers' practices of 18-cm rows and 400 plants m−2. Increased seeding rates lowered grain protein 5% compared to standard seeding rates. Wide rows, in combination with interrow cultivation, reduced weed density 62%, increased yield 16%, and net returns 19% compared to regional organic practices. Significant increases in grain N were limited to weed-free plots. While increased seeding rates improved weed suppression, the high input cost of organic seed make this an unsatisfactory alternative to interrow cultivation and current farmer practices, as yield would need to be. 15 t ha−1 higher at elevated density to offset the extra cost of seed.

Waterhemp is a major weed of field crops in the Midwestern United States. Its prevalence is at least partly due to its rapid evolution of resistance to many groups of herbicides over the last two decades. In light of its dioecy and anemophily, pollen movement in waterhemp is considered to be an important factor in the spread of herbicide resistance genes. Thus, the biology and dispersal profile of waterhemp pollen are critical determinants for understanding and predicting the spatial population dynamics of herbicide resistance in this species. In this study, pollen longevity was investigated with greenhouse experiments, and pollen dispersal and the effect of pollen competition were investigated in field plots. Pollen dispersal was determined by measuring the frequency of seeds produced on receptor plants positioned at various distances from a pollen source, which flowered in synchrony with the receptor plants. Results indicated that waterhemp pollen can remain viable up to 120 h, implying a low limitation of pollen dispersal by its longevity. Effective pollen dispersal declined exponentially with distance, with most pollen fertilizing recipient plants within 50 m of the pollen source. However, long distance pollen dispersal (800 m) was also observed. We also saw evidence for pollen swamping in this species. Under conditions of pollen competition among distinct genotypes, pollination success was inversely related to distance between pollen donors and receptors. However, relative pollen density may also play an important role in determining the rate of long distance gene flow. This study confirmed the potential of waterhemp pollen to effect long distance gene flow and provides supporting data for quantitative spatial modeling of waterhemp resistance dynamics.

The need for practical revegetation strategies for managing medusahead-infested rangeland is substantial and unmet. Our objective was to test the potential for using the single-entry approach (simultaneous application of herbicide and seed with one entry) developed for other invasive-weed infested systems for revegetating medusahead-infested rangeland. Since areas infested with medusahead are frequently burned and burning may help control medusahead and provide a more suitable seedbed for desired species, we also tested the single-entry approach in conjunction with burning. Our primary hypotheses were that (1) the combination of imazapic and seeding would provide best establishment of desired species and (2) burning would enhance the efficacy of imazapic on medusahead and enhance seedling establishment. Treatments included three seeding rates (none, 13.2, or 25.0 kg ha−1 of an even mixture of all species), two herbicides (0 and 52 g a.i. ha−1 imazapic; with and without), and two burning regimes (burned, not burned) applied mid-October 2006 on two sites. In late July, 2008 through 2010, plant density and biomass were sampled. We found that the simultaneous application of imazapic and seeding resulted in establishment of desired species where they were absent. Imazapic application without seeding increased perennial bunchgrass density where enough residual plants existed before treatment. Burning tended to improve the control of medusahead with imazapic and promoted desired species. Even though follow-up treatments may be necessary, applying imazapic and seeding in a single-entry approach may provide a more practical initial revegetation strategy than multi-entry approaches on rangeland devoid of desired residual species. This procedure should reduce the costs of treatment application and make revegetating annual grass-infested rangeland more affordable.

Control of early-emerging weeds is essential to protect the yield potential of maize. An understanding of the physiological changes that occur as a result of weed interference is required to address variability in yield loss across sites and years. Field trials were conducted at the University of Guelph (UG), the Ohio State University (OSU), and Colorado State University (CSU) during 2009 and 2010. There were six treatments (season-long weedy and weed-free, and weed control at the 1st-, 3rd-, 5th-, and 10th-leaf-tip stages of maize development) and 20 individual plants per plot were harvested at maturity. We hypothesized that, as weed control was delayed, weed interference in the early stages of maize development would increase plant-to-plant variability in plant dry-matter accumulation, which would result in a reduction of grain yield at maturity. The onset of the critical period for weed control (CPWC) occurred on average between the third and fifth leaf tip stages of development (i.e., V1 to V3, respectively). Rate of yield loss following the onset of the CPWC ranged from 0.05 MG ha−1 d−1 at UG 2009 to 0.22 MG ha−1 d−1 at CSU 2010 (i.e., 0.5 and 1.6% d−1, respectively). On average, reductions in kernel number per plant accounted for approximately 65% of the decline in grain yield as weed control was delayed. Biomass partitioning to the grain was stable through early weed removal treatments, increased and peaked at the 10th-leaf-tip time of control, and decreased in the season-long weedy treatment. Plant-to-plant variability in dry matter at maturity and incidence of bareness increased as weed control was delayed. As weed control was delayed, the contribution of plant-to-plant variability at maturity to the overall yield loss was small, relative to the decline of mean plant dry matter.

Canada thistle poses a particular threat to organic producers in temperate agriculture due to its ability to reproduce through an extensive system of underground roots. The Canada thistle life cycle, growth, and development are seasonally affected, and exploiting this biology may be useful for weed management. The objective of this study was to evaluate smother crop mixtures, seeded at different times, for Canada thistle control. Field trials were established in 2009 and 2010 to evaluate the ability of smother crop mixtures to suppress Canada thistle growth and development. Canada thistle aboveground biomass was suppressed 50% in 2009 and 87% in 2010 by the sorghum–sudangrass mixture, averaged over planting times. The oat mixture suppressed annual weed biomass more than 58% in 2009 and 67% in 2010 in all planting dates. Percent cover of Canada thistle was affected by crop mixture in 2009 and 2010, with sorghum–sudangrass being the most suppressive. The sorghum–sudangrass mixture was more suppressive of Canada thistle, probably because it included soybean and sunflower, all high-biomass, competitive crops. Planting date affected smother crop suppression of Canada thistle growth, but the effect was not consistent between 2009 and 2010 due to differences in weather conditions.

Glyphosate is regularly used to control annual bluegrass populations in dormant bermudagrass turf. A population of annual bluegrass not controlled by glyphosate at 840 g ha−1 (glyphosate resistant, GR) was identified on a golf course in Humboldt, TN in 2010. Mature tillers of GR plants were established in a greenhouse and treated with glyphosate at 0, 210, 420, 840, 1,680, 3,360, and 6,720 g ha−1. Mature tillers of a biotype known to be susceptible to glyphosate (SS) were also established in the greenhouse and subjected to the same treatments. At 14 d after treatment (DAT), glyphosate controlled the SS biotype > 95% at rates > 420 g ha−1. Comparatively, the GR biotype was only controlled 76% with glyphosate at 6,720 g ha−1. The rates required to provide 50% control (I50 values) for SS and GR biotypes were 236 and 2,812 g ha−1 respectively, resulting in a resistance factor of 12. Photochemical efficiency (Fv/Fm) values on SS plants treated with glyphosate at > 210 g ha−1 measured 0.000 at 14 DAT, whereas Fv/Fm values on GR plants were not significantly different from the untreated control with glyphosate rates ≤ 840 g ha−1 on the same date. In laboratory experiments, the SS biotype accumulated greater shikimate concentrations than the GR biotype 3 to 6 DAT. Future research should evaluate strategies for managing GR and SS annual bluegrass with alternative modes of action.

A hydrothermal time model was developed to simulate field emergence for three weed species in maize (common lambsquarters, johnsongrass, and velvetleaf). Models predicting weed emergence facilitate well-timed and efficient POST weed control strategies (e.g., chemical and mechanical control methods). The model, called AlertInf, was created by monitoring seedling emergence from 2002 to 2008 in field experiments at three sites located in the Veneto region in northeastern Italy. Hydrothermal time was calculated using threshold parameters of temperature and water potential for germination estimated in previous laboratory studies with seeds of populations collected in Veneto. AlertInf was validated with datasets from independent field experiments conducted in Veneto and in Tuscany (west central Italy). Model validation resulted in both sites in efficiency index values ranging from 0.96 to 0.99. AlertInf, based on parameters estimated in a single region, was able to predict the timing of emergence in several sites located at the two extremes of the Italian maize growing area.

Canada thistle is difficult to manage in organic farming systems and others with reduced reliance on herbicides. Previous field studies found that defoliation or sudangrass interference suppressed Canada thistle. Our objective was to understand the factors causing suppression of Canada thistle observed in the field. Three greenhouse studies were conducted utilizing frequency of defoliation, sudangrass interference and defoliation, and interspecific phytotoxicity to discern mechanisms of Canada thistle suppression. Increased defoliation frequency (up to four defoliations) decreased Canada thistle shoot height, shoot and root mass, and root-to-shoot ratio. Plants with larger root mass had greater shoot mass and number (r = 0.87 and 0.73, respectively), indicating a probable interdependence of root size (carbohydrate reserves), bud density, and subsequent shoot growth. In the sudangrass interference and defoliation study, Canada thistle shoot dry mass was 38.7, 2.76, and 0.39 g pot−1 in the defoliation only, sudangrass interference only, and defoliation + interference + surface mulch treatments, respectively. Sudangrass interference by itself was effective in suppressing thistle growth; combining interference with defoliation did not further reduce growth (2.76 and 2.83 g pot−1, respectively). In the experiment minimizing interspecific competition, we found no evidence of sudangrass having a phytotoxic effect on Canada thistle. Overall results indicate that sudangrass competition or frequent shoot removal suppresses growth of Canada thistle.

A wild population of a plant species, especially a cross-pollinated species, can display considerable genetic variation. Genetic variability is evident in differential susceptibility to an herbicide because the population can show continuous phenotypic variation. Recent, recurrent selection studies have revealed that phenotypic variation in response to low herbicide rates is heritable and can result in rapid evolution of herbicide resistance in genetically variable cross-pollinated rigid ryegrass. In this study, the heritable genetic variation in an herbicide-susceptible rigid ryegrass population was exploited to shift the population toward greater herbicide susceptibility by recurrent selection. To enhance herbicide susceptibility, herbicide-susceptible rigid ryegrass plants were divided into two identical clones, and one series of cloned plants was treated with a low rate of herbicide (diclofop). The nontreated clones of individuals that did not survive the herbicide treatment were selected and bulk-crossed to obtain the susceptible progeny. After two cycles of selection, the overall susceptibility to diclofop was doubled. The results indicate that minor genes for resistance are present in an herbicide-susceptible rigid ryegrass population, and their exclusion can increase susceptibility to diclofop.

Phenotypic plasticity and rapid evolution are two important strategies by which invasive species adapt to a wide range of environments and consequently are closely associated with plant invasion. To test their importance in invasion success of Crofton weed, we examined the phenotypic response and genetic variation of the weed by conducting a field investigation, common garden experiments, and intersimple sequence repeat (ISSR) marker analysis on 16 populations in China. Molecular markers revealed low genetic variation among and within the sampled populations. There were significant differences in leaf area (LA), specific leaf area (SLA), and seed number (SN) among field populations, and plasticity index (PIv) for LA, SLA, and SN were 0.62, 0.46 and 0.85, respectively. Regression analyses revealed a significant quadratic effect of latitude of population origin on LA, SLA, and SN based on field data but not on traits in the common garden experiments (greenhouse and open air). Plants from different populations showed similar reaction norms across the two common gardens for functional traits. LA, SLA, aboveground biomass, plant height at harvest, first flowering day, and life span were higher in the greenhouse than in the open-air garden, whereas SN was lower. Growth conditions (greenhouse vs. open air) and the interactions between growth condition and population origin significantly affect plant traits. The combined evidence suggests high phenotypic plasticity but low genetically based variation for functional traits of Crofton weed in the invaded range. Therefore, we suggest that phenotypic plasticity is the primary strategy for Crofton weed as an aggressive invader that can adapt to diverse environments in China.

Knowledge of weed emergence periodicity can inform the timing and choice of weed management tactics. We tested the effects of weed management system (conventional [CNV] and herbicide-free [HF]), timing of rye sowing (two dates), timing of soybean planting (5 planting dates, 3 in each system), and supplemental control (with and without) on weed suppression and weed community composition in soybean no-till planted into a cereal rye cover crop. Cereal rye was terminated with a roller-crimper and herbicide (CNV) or with a roller-crimper alone (HF), and supplemental weed control was achieved with a postemergence glyphosate application (CNV) or with interrow high-residue cultivation (HF). Supplemental control with glyphosate in CNV was more effective than high-residue cultivation in HF. When soybean was planted on the same date, CNV resulted in less weed biomass and a more even community composition, whereas HF resulted in greater weed biomass, dominated by common ragweed. When we controlled for cereal rye biomass and compared the effects of cereal rye sowing and termination timing within each system, earlier management reduced weed biomass in HF, but tended to increase weed biomass in CNV. Our results suggest the ability to control emerged weeds prior to soybean planting is an important factor that influences the optimal cereal rye cover crop management timing for weed suppression.

Field studies were used to examine the management strategies of mowing, herbicide, fertility, and all possible combinations on tall ironweed populations, weed biomass, and pasture yield at three Kentucky locations. Mowing was performed in July 2008 and 2009, herbicide was applied in August 2008, and fertilizer was applied in September 2008 and 2009 at all locations. Weed populations were measured in 2008, 2009, and 2010, and forage grass, clover, and weed biomass was collected in May or early June of 2009 and 2010. All treatments with herbicide reduced tall ironweed stems by 64% or greater in 2009 at all locations. Mowing alone, fertilizer alone, and mowing plus fertilizer did not reduce tall ironweed populations, except at one location where mowing alone reduced tall ironweed stems by 64% in 2009. Tall ironweed stems were not reduced in 2010 with any treatment at two locations, but herbicide combined with mowing or fertilizer reduced tall ironweed stems by 78% at the other location. Tall goldenrod population was reduced up to 100% by all treatments with herbicide or mowing alone, and mowing with fertilizer reduced tall goldenrod from 59 to 89%. Treatments did not reduce horsenettle populations. Herbicide-containing treatments reduced weed biomass at all locations. Weed biomass did not differ when comparing all treatments with and without mowing or treatments with or without fertilizer. Forage grass biomass was greatest with herbicide plus fertilizer and with the combination of mowing plus herbicide plus fertilizer at all locations in both years.

Knowledge of environmental factors influencing demography of weed species will improve understanding of current and future weed invasions. The objective of this study was to quantify regional-scale variation in vital rates of giant ragweed and common sunflower. To accomplish this objective, a common field experiment was conducted across seven sites between 2006 and 2008 throughout the north central U.S. maize belt. Demographic parameters of both weed species were measured in intra- and interspecific competitive environments, and environmental data were collected within site-years. Site was the strongest predictor of belowground vital rates (summer and winter seed survival and seedling recruitment), indicating sensitivity to local abiotic conditions. However, biotic factors influenced aboveground vital rates (seedling survival and fecundity). Partial least squares regression (PLSR) indicated that demography of both species was most strongly influenced by thermal time and precipitation. The first PLSR components, both characterized by thermal time, explained 63.2% and 77.0% of variation in the demography of giant ragweed and common sunflower, respectively; the second PLSR components, both characterized by precipitation, explained 18.3% and 8.5% of variation, respectively. The influence of temperature and precipitation is important in understanding the population dynamics and potential distribution of these species in response to climate change.

Certain winter annual weeds have been documented as alternative hosts to soybean cyst nematode (SCN), and infestations by such species are common in no-till production fields in the midwestern United States of Indiana, Ohio, and Illinois. The objective of this research was to determine the influence of crop rotation and winter annual weed management on winter weed growth, SCN population density, and crop yield. Two crop rotations (SS and soybean–corn rotation) and six winter annual weed-management systems (autumn-applied herbicide, spring-applied herbicide, autumn + spring applied herbicides, autumn-seeded Italian ryegrass, autumn-seeded wheat, and a nontreated check) were evaluated in long-term, no-tillage systems at West Lafayette, IN, and Vincennes, IN. In the fourth and fifth years of these experiments, the 2-yr corn–soybean rotation generally resulted in increased soybean yield, decreased winter annual weed growth, and reduced SCN population density compared with SS. Autumn or spring herbicide applications or both were a more effective option than cover crops at reducing winter annual weed density. Cover-crop systems generally did not differ from the nontreated check in winter weed density. Between years three and five, winter annual weed SCN hosts in nontreated check plots increased approximately threefold to levels as high as 102 and 245 plants m−2 at West Lafayette, IN, and Vincennes, IN, respectively, which are infestation levels at or above those commonly observed in production fields. However, controlling winter annual weeds did not influence crop yields or SCN population density. The results of these studies suggest that winter weed management, even at the high levels of weed infestation present in these studies, appears to have little value as a tool for SCN management in corn and soybean production systems in the midwestern United States.

In previous research conducted on nonweed species, the efficacy of glyphosate was shown to be greater in unsterile soils compared to sterile soils and soil microorganisms were found to play an important role in glyphosate efficacy. Conducting greenhouse studies in microbe-free soil may therefore produce unreliable data, leading to erroneous conclusions. The objective of this study was to determine the effect of soil microorganisms on the response of glyphosate-resistant and -susceptible biotypes of three problematic weeds of the midwestern United States: giant ragweed, horseweed, and common lambsquarters. A greenhouse dose–response study was conducted on each of the three weed species grown in sterile and unsterile field soil, and the dry weight response of roots and shoots was measured. The three weed species responded differently to glyphosate when grown in the sterile and unsterile soil; that is, in the presence and absence of soil microbes. Soil microbes influenced the response of the susceptible and resistant giant ragweed biotypes and the susceptible common lambsquarters, but not the tolerant common lambsquarters or either horseweed biotype. The different responses of the three species to glyphosate in the presence and absence of soil microbes demonstrates that rhizosphere interactions are fundamental to the mode of action of glyphosate. These findings suggest that the range of tolerance to glyphosate observed in weeds and the evolution of resistance in weed biotypes may also be influenced by rhizosphere interactions. The soil media used in dose–response screenings to identify susceptible and resistant weed biotypes is very important. Unsterile field soil should be incorporated into growth media when conducting dose–response screenings to avoid false-positive results. In addition, researchers performing glyphosate dose–response assays should be aware of these findings.

Weedy red rice is a troublesome weed problem in rice fields of the southern United States. Typically, red rice plants are much taller than rice cultivars, and most biotypes are either awnless with straw-colored hulls (strawhull) or have long awns with black-colored hulls (blackhull). Outcrossing between rice and red rice occurs at low rates, resulting in a broad array of plant types. Simple sequence repeat (SSR) markers were used to evaluate the genetic backgrounds of atypical red rice types obtained from rice farms in Arkansas, Louisiana, Missouri, and Mississippi, in comparison to standard red rice types and rice cultivars. Principal coordinates analysis (PCoA) and population structure analysis of atypical red rice accessions suggested that short-stature awnless (LhtsA−) and awned (LhtsA+) types, each representing a total of about 5% of a 460-accession collection, usually were closely genetically related to their normal-sized counterparts, and not with cultivated rice. A short-awned, intermediate height type, ‘Sawn’, representing about 4% of the accessions was genetically distinct from all of the other types. Key alleles in Sawn types appeared to be shared by both standard awnless (StdRRA−) and awned (StdRRA+) red rice, suggesting that Sawn types could have arisen from gene flow between awned and awnless red rice types.

While witchweed is nearing eradication in the United States, it continues to thrive in other parts of the world, especially in Africa, together with other witchweed species. The continuing problems from witchweeds and other parasitic weeds, the broomrapes, dodders and mistletoes, are outlined, including their extent, the degrees of damage caused, and the difficulties in their control. While a small minority are being successfully controlled by the use of immune varieties, most are currently controlled by existing techniques only partially, or on a local basis, and they may even be spreading or intensifying. The challenges they present are emphasised.

Previous greenhouse studies with a noncommercial glyphosate-resistant sugarbeet variety indicated that susceptibility to Rhizoctonia crown and root rot could increase after glyphosate was applied. Greenhouse and field experiments were conducted in 2008 and 2009 to determine if glyphosate influenced disease severity in potential commercially available varieties of glyphosate-resistant sugarbeet. In the first greenhouse experiment in 2008, Hilleshög 9027RR, the most tolerant variety to Rhizoctonia crown and root rot, exhibited an increase in disease severity when glyphosate was applied. There were no significant differences between herbicide treatments in Hilleshög 9028RR, and glyphosate decreased disease severity in Hilleshög 9032RR when compared with the no-herbicide treatment. Experiments conducted to determine if glyphosate influenced Rhizoctonia solani growth in vitro indicated that glyphosate did not increase the radial growth of R. solani, except at 10× (190 µg ae ml−1) the normal rate of glyphosate plus ammonium sulfate (AMS). Field and additional greenhouse experiments were conducted using four commercial varieties. Differences in disease severity were observed when comparing varieties, but glyphosate did not significantly influence the severity of Rhizoctonia crown and root rot when compared with the no-herbicide control. Choosing a glyphosate-resistant sugarbeet variety with the best demonstrated tolerance to Rhizoctonia crown and root rot is an important factor in reducing disease severity and maintaining sugarbeet yield.

Invasive species and acid rain cause global environmental problems. Creeping daisy, an invasive exotic allelopathic weed, has caused great damage in southern China, where acid rain is prevalent. The impact of the acidity of simulated acid rain (SAR) on soil nutrients, the decomposition of creeping daisy litter, and on the allelopathic potential of the surrounding soils was investigated. Litter was treated with SAR at different acidity (pH 2.5, 4.0, 5.6) or with water (pH 7.0) as a control. After 70 d, the remaining amount of creeping daisy litter, nutrient contents, and allelopathic potentials in the surrounding soil were determined. The litter decomposition was commensurate to the increase in the acidity of the SAR. Total C and N contents, NO3−-N and available P increased, levels of NH4+-N, the ratio of C/N and soil pH values decreased, water contents increased and then decreased, whereas available K did not significantly change in the soil surrounding the litters in response to the increase in the acidity of the SAR. Bioassays showed that SAR promoted the allelopathic activity in the soil surrounding the litter, as measured by seedling growth of turnip and radish. In conclusion, our results indicated that SAR influenced soil nutrient status, accelerated creeping daisy litter decomposition, and enhanced the allelopathic potential of its litter in the surrounding soil, suggesting that acid rain may enhance the invasiveness of creeping daisy plants.