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.

Root colonization by soil microorganisms has been shown to increase the activity of glyphosate in resistant and susceptible biotypes of giant ragweed and a susceptible common lambsquarters biotype, but not in horseweed biotypes. The objective of this study was to investigate the colonization of roots in glyphosate-resistant and -susceptible giant ragweed and horseweed biotypes, and glyphosate-tolerant and -susceptible biotypes of common lambsquarters after a sublethal glyphosate application. The three weed species were grown separately in sterile and unsterile field soil and treated with glyphosate at two sublethal rates. Soil microbes were isolated from the roots onto sterile media 3 d after the glyphosate treatment. The susceptible biotypes of giant ragweed and horseweed grown in unsterile soil were colonized by more soil microbes at the higher rate of glyphosate, compared to the resistant biotype grown in unsterile soil. Oomycetes were isolated separately on a selective media and they were also more prevalent in the roots of the susceptible biotypes of each weed species grown in the unsterile soil when glyphosate was applied at the highest rate. Therefore, the ability of these three weed species to tolerate a glyphosate application may involve differences in the susceptibility to soil microbial colonization, especially oomycetes.

In 2006 and 2007, farmers from two counties in Illinois reported failure to control waterhemp with glyphosate. Subsequent onsite field experiments revealed that the populations might be resistant to multiple herbicides. Greenhouse experiments therefore were conducted to confirm glyphosate resistance, and to test for multiple resistance to other herbicides, including atrazine, acifluorfen, lactofen, and imazamox. In glyphosate dose-response experiments, both populations responded similarly to a previously characterized glyphosate-resistant population (MO1). Both Illinois populations also demonstrated high frequencies of resistance to the acetolactate synthase (ALS) inhibitor, imazamox. Additionally, one of the populations demonstrated high frequencies of resistance to both atrazine and the protoporphyrinogen oxidase (PPO) inhibitor, lactofen. Furthermore, using combinations of sequential and tank-mix herbicide applications, individual plants resistant to herbicides spanning all four site-of-action groups were identified from one population. Molecular experiments were performed to provide an initial characterization of the resistance mechanisms and to provide confirmation of the presence of multiple resistance traits within the two populations. Both populations contained the W574L ALS mutation and the ΔG210 PPO mutation, previously shown to confer resistance to ALS and PPO inhibitors, respectively. Atrazine resistance in both populations is suspected to be metabolism-based, because a triazine target-site mutation was not identified. A P106S EPSPS mutation, previously reported to confer glyphosate resistance, was identified in one population. This mutation was identified in both resistant and sensitive plants from the population; however, and so more research is needed to determine the glyphosate-resistance mechanism(s). This is the first known case of a weed population in the United States possessing multiple resistance to herbicides from four site-of-action groups.

Interest in weed seed predation as an ecological weed management tactic has led to a growing number of investigations of agronomic and environmental effects on predation rates. Whereas the measurements in most of these studies have taken place at very short timescales, from days to weeks, measurements at longer timescales (from several months to a year) have greater relevance to the demographic impact of weed seed predation and potential contributions from this process to ecological weed management. Our aim was to quantify the impact of crop phase, within a corn–soybean–wheat crop sequence, on quarterly and annual seed predation rates of giant foxtail, giant ragweed, and velvetleaf. The study took place in areas of the northern U.S. Corn Belt contrasting in dominant land use: Savoy, IL (2005–2007), where corn and soybean production predominates, and East Lansing, MI (2005–2008), where crop production occurs within an old field/forest landscape matrix. Mean annual rates of weed seed predation by the combined action of invertebrate and vertebrate predators were 31 ± 1.6% for giant ragweed, 37 ± 1.4% for velvetleaf, and 53 ± 1.4% for giant foxtail. Crop phase had negligible effects upon long-term seed predation rates, accounting for less than 2% of observed variation. Weed species and site-year, in contrast, contributed 35% and 40%, respectively, of the variation in cumulative annual seed predation. These results are consistent with the spatial variability in best management practices seen at spatial scales greater than the county level: weed seed predation appears to be an inherently site-specific phenomenon. New developments in managing weed seed predation as an ecosystem service are therefore likely to have local recommendation domains or to be driven by stochastic annual variation related to weather or granivore demography.

Purple nutsedge is among the most troublesome weeds of vegetables in the Southeast US and a substantial impediment in the search for methyl bromide alternatives. Greater understanding of the environmental cues that regulate tuber sprouting may assist in improved nutsedge management. Experiments were conducted to evaluate the effect of diurnal temperature variation on sprouting of purple nutsedge tubers. Two temperature regimes were evaluated: the first averaged 28 C, with daily fluctuations ranging from 0 to 19.5 C; the second temperature regime averaged 16 C, with daily fluctuations ranging from 0 to 18.5 C. When average temperature was 28 C, cumulative tuber sprouting ranged from 88 to 92%, with no detectable differences among diurnal fluctuations. The high average temperature in the first study may have negated any type of enforced sprouting suppression. However, when average temperature was lowered to 16 C (simulating early spring diurnal fluctuations under polyethylene mulch), there was a positive linear correlation between maximum tuber sprouting and temperature variation. With an average temperature of 16 C, the absence of temperature variation resulted in 52% purple nutsedge sprouting, while 87% sprouting occurred when daily temperature varied 18.5 C at the same average temperature. The use of various types of mulching material can affect average soil temperatures and diurnal variations, potentially shifting nutsedge emergence. Further studies are needed to determine if these data on tuber sprouting in response to alternating temperatures can facilitate more efficient weed management.

Weed growth and N assimilation usually increase with N application rate. With the increasing price of N fertilizer, a better understanding N assimilation by weeds is necessary to maximize economic return. Total plant yield is generally independent of population density, except when plants are very small or at very low population density. If plant yield is independent of population density, weed N assimilation may also be independent of population density. However, the effect of weed population density on N assimilation has not been thoroughly investigated. A 2011 controlled-environment study was established in East Lansing, MI, to evaluate the effect of weed population density and N application rate on growth and N assimilation by common lambsquarters and redroot pigweed. Study factors included four weed densities (1, 2, 4, and 8 plants pot−1), three N application rates (0, 67, and 134 kg N ha−1), and two weed species (redroot pigweed and common lambsquarters). Weeds were destructively harvested 3 wk after emergence, and shoot height, biomass, total N concentration, N use efficiency, and N assimilation were measured. Redroot pigweed was taller, had greater shoot biomass, and a greater shoot N assimilation than did common lambsquarters. With similar environmental conditions, redroot pigweed is expected to be more competitive than common lambsquarters. Shoot N assimilation increased with increasing weed population density, indicating that N assimilation was not independent of population density 3 wk after emergence because weeds were small or at low population density.

Citral is a monoterpene commonly found as volatile component in many different aromatic plants. Although many studies have identified the presence of citral in phytotoxic essential oils, this work determines for the first time the potential herbicidal effect of citral on weeds. The use of citral against weeds and crops resulted in the potential for the management of barnyardgrass, redroot pigweed, and ribwort. Clear morphological differences were observed between adult thale cress plants exposed to citral in two different application methods: spraying and watering. Citral-sprayed and citral-watered thale cress plants showed completely different effects after treatment, suggesting that foliar or root absorption can determine the effectiveness of this compound. This work demonstrates that citral is effective not only on seedling metabolism but also on adult plants by inhibiting growth and development altering the plant oxidative status.

We developed two leafy spurge bacterial artificial chromosome (BAC) libraries that together represent approximately 5× coverage of the leafy spurge genome. The BAC libraries have an average insert size of approximately 143 kb, and copies of the library and filters for hybridization-based screening are publicly available through the Arizona Genomics Institute. These libraries were used to clone full-length genomic copies of an AP2/ERF transcription factor of the A4 subfamily of DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEINS (DREB) known to be differentially expressed in crown buds of leafy spurge during endodormancy, a DORMANCY ASSOCIATED MADS-BOX (DAM) gene, and several FLOWERING LOCUS T (FT) genes. Sequencing of these BAC clones revealed the presence of multiple FT genes in leafy spurge. Sequencing also provided evidence that two different DAM transcripts expressed in crown buds of leafy spurge during endo- and eco-dormancy result from alternate splicing of a single DAM gene. Sequence data from the FT promoters was used to identify several conserved elements previously recognized in Arabidopsis, as well as potential novel transcription factor binding sites that may regulate FT. These leafy spurge BAC libraries represent a new genomics-based tool that complements existing genomics resources for the study of plant growth and development in this model perennial weed. Furthermore, phylogenetic footprinting using genes identified with this resource demonstrate the usefulness of studying weedy species to further our general knowledge of agriculturally important genes.

Glyphosate-resistant Palmer amaranth is a serious problem in southern cropping systems. Much phenotypic variation is observed in Palmer amaranth populations with respect to plant growth and development and susceptibility to herbicides. This may be related to levels of genetic diversity existing in populations. Knowledge of genetic diversity in populations of Palmer amaranth may be useful in understanding distribution and development of herbicide resistance. Research was conducted to assess genetic diversity among and within eight Palmer amaranth populations collected from North Carolina and Georgia using amplified fragment length polymorphism (AFLP) markers. Pair-wise genetic similarity (GS) values were found to be relatively low, averaging 0.34. The highest and the lowest GS between populations were 0.49 and 0.24, respectively, while the highest and the lowest GS within populations were 0.56 and 0.36, respectively. Cluster and principal coordinate (PCO) analyses grouped individuals mostly by population (localized geographic region) irrespective of response to glyphosate or gender of individuals. Analysis of molecular variance (AMOVA) results when populations were nested within states revealed significant variation among and within populations within states while variation among states was not significant. Variation among and within populations within state accounted for 19 and 77% of the total variation, respectively, while variation among states accounted for only 3% of the total variation. The within population contribution towards total variation was always higher than among states and among populations within states irrespective of response to glyphosate or gender of individuals. These results are significant in terms of efficacy of similar management approaches both in terms of chemical and biological control in different areas infested with Palmer amaranth.

In summer, 2011, we investigated suspected glyphosate-resistant (GR) kochia in three chem-fallow fields (designated F1, F2, F3, each farmed by a different grower) in southern Alberta. This study characterizes glyphosate resistance in those populations, based on data from dose–response experiments. In a greenhouse experiment, the three populations exhibited a resistance factor ranging from 4 to 6 based on shoot biomass response (GR50 ratios), or 5 to 7 based on survival response (LD50 ratios). Similar results were found in a field dose–response experiment at Lethbridge, AB, in spring 2012 using the F2 kochia population. In fall 2011, we surveyed 46 fields within a 20-km radius of the three chem-fallow fields for GR kochia. In the greenhouse, populations were screened with glyphosate at 900 g ae ha−1. Seven populations were confirmed as GR, the farthest site located about 13 km from the three originally confirmed populations. An additional GR population more than 100 km away was later confirmed. Populations were screened for acetolactate synthase (ALS)–inhibitor (thifensulfuron : tribenuron) and dicamba resistance in the greenhouse, with molecular characterization of ALS-inhibitor resistance in the F1, F2, and F3 populations. All GR populations were resistant to the ALS-inhibiting herbicide, but susceptible to dicamba. ALS-inhibitor resistance in kochia was conferred by Pro197, Asp376, or Trp574 amino acid substitutions. Based upon a simple empirical model with a parameter for selection pressure, calculated from weed relative abundance and glyphosate efficacy, and a parameter for seedbank longevity, kochia, wild oat, and green foxtail were the top three weeds, respectively, predicted at risk of selection for glyphosate resistance in the semiarid Grassland region of the Canadian prairies; wild oat, green foxtail, and cleavers species were predicted at greatest risk in the subhumid Parkland region. This study confirms the first occurrence of a GR weed in western Canada. Future research on GR kochia will include monitoring, biology and ecology, fitness, mechanism of resistance, and best management practices.

Bermudagrass is a difficult perennial weed to manage in Louisiana sugarcane. Research was conducted to compare interrow tillage practice, postharvest residue management, and herbicide placement on bermudagrass proliferation and sugarcane yield. Tillage frequencies included conventional (four tillage operations per season), reduced (two tillage operations), and no-till. Residue management practices included removal by burning, sweeping from row top into the wheel furrow, and not removed. Spring herbicide placement treatments included broadcast, banded, or no herbicide application. With conventional tillage, broadcast and banded herbicide applications resulted in similar bermudagrass cover in the first and second ratoon crops, but bermudagrass cover was greater when using banded applications (22%) compared with broadcast application (15%) in the third-ratoon crop. Bermudagrass cover was greatest with no-till. When herbicides were banded, bermudagrass cover was greater in reduced tillage than conventional tillage in all three ratoon crops. Postharvest residue management did not affect bermudagrass ground cover. In plant cane, sugarcane yields were lowest when herbicide was not applied. In ratoon crops, sugarcane and sugar yield were reduced when herbicide was not applied regardless of tillage practice. Cane and sugar yield were generally equal when comparing reduced and conventional tillage. Total sugarcane yield (4 crop yr) for the no-till program was reduced 11, 15, and 25%, respectively, when herbicides were broadcast, banded, and when herbicide was not applied, compared with conventional tillage. Failure to remove residue reduced sugarcane yield by 5, 7, and 10% in first, second, and third ratoons, respectively, compared with burning. Eliminating unnecessary tillage practices can increase profitability of sugarcane through reduced costs, but it will be imperative that herbicide programs be included to provide adequate bermudagrass control and that postharvest residue is removed to promote maximum sugarpostane yield.

A model that describes the emergence of ripgut brome was developed using a two-season data set from a no-tilled field in northeastern Spain. The relationship between cumulative emergence and hydrothermal time (HTT) was described by a sigmoid growth function (Chapman). HTT was calculated with a set of water potentials and temperatures, iteratively used, to determine the base water potential and base temperature. Emergence of ripgut brome was well described with a Chapman function. The newly-developed function was validated with four sets of data, two of them belonging to a third season in the same field and the other two coming from independent data from Southern Spain. The model also successfully described the emergence in different field management and tillage systems. This model may be useful for predicting ripgut brome emergence in winter cereal fields of semiarid Mediterranean regions.

Competition between crops and weeds may be stronger at the root than at the shoot level, but belowground competition remains poorly understood, due to the lack of suitable methods for root discrimination. Using a transgenic maize line expressing green fluorescent protein (GFP), we nondestructively discriminated maize roots from weed roots. Interactions between GFP-expressing maize, common lambsquarters, and redroot pigweed were studied in two different experiments with plants arranged in rows at a higher plant density (using boxes with a surface area of 0.09 m2) and in single-plant arrangements (using boxes with a surface area of 0.48 m2). Root density was screened using minirhizotrons. Relative to maize that was grown alone, maize root density was reduced from 41 to 87% when it was grown with redroot pigweed and from 27 to 73% when it was grown with common lambsquarters compared to maize grown alone. The calculated root : shoot ratios as well as the results of shoot dry weight and root density showed that both weed species restricted root growth more than they restricted shoot growth of maize. The effect of maize on the root density of the weeds ranged from a reduction of 25% to an increase of 23% for common lambsquarters and a reduction of 42 to 6% for redroot pigweed. This study constitutes the first direct quantification of root growth and distribution of maize growing together with weeds. Here we demonstrate that the innovative use of transgenic GFP-expressing maize combined with the minirhizotron technique offers new insights on the nature of the response of major crops to belowground competition with weeds.

Infestations of Italian ryegrass are problematic in both conventional and organic wheat production systems. The development of wheat cultivars with superior competitive ability against Italian ryegrass could play a role in maintaining acceptable yields and suppressing weed populations. Research was conducted in North Carolina to identify indirect methods of selection for Italian ryegrass suppressive ability (hereafter referred to as weed suppressive ability) of winter wheat cultivars that correlate well with Italian ryegrass-to-wheat biomass ratios. Two winter wheat cultivars (Dyna-Gro Baldwin and Dyna-Gro Dominion) and one experimental wheat line (NC05-19684) with differing morphological traits were overseeded with varying densities of Italian ryegrass. Wheat height measured throughout the growing season in weed-free plots was strongly associated with weed suppressive ability, but high wheat tillering capacity had no significant effect on weed suppressive ability in the lines tested in this study. Italian ryegrass seed head density during grain fill was strongly correlated (r = 0.94) with Italian ryegrass-to-wheat biomass ratio, the generally accepted measure of weed suppressive ability. Visual estimates of percent Italian ryegrass biomass relative to the plot with the highest level of Italian ryegrass infestation in each replicate were also strongly correlated with weed suppressive ability at all growth stages, especially during heading (r = 0.87) (Zadoks growth stage [GS] 55). Measurements from nonimaging spectrophotometers and overhead photographs taken from tillering (Zadoks 23 to 25) to early dough development (Zadoks 80) were unreliable estimates of end-of-season Italian ryegrass-to-wheat biomass ratios because they failed to account for wheat cultivar differences in biomass, color, and growth habit. Italian ryegrass seed head density and visual estimates of Italian ryegrass biomass during grain fill are appropriate indirect methods of selection for weed suppressive ability in breeding programs.

A study was initiated in 2001at four locations in western Canada to investigate an integrated approach to managing wild oat, the region's worst weed. The study examined the effects of combining semidwarf or tall barley cultivars with normal or twice-normal barley seeding rates in either continuous barley or a barley–canola–barley–field pea–barley rotation. Herbicides were applied at 25, 50, and 100% of recommended rates. The first phase of the study was completed in 2005. This paper reports on the second phase, which was continued for four more years at two of the locations, Beaverlodge and Fort Vermilion, AB, Canada. The objective was to determine the long-term impact of the treatments on wild oat seed in the soil seed bank. In 2009 (final year), the diverse rotation combined with the higher barley seeding rate (optimal cultural practice) resulted in higher barley yields and reduced wild oat biomass compared to continuous barley and lower barley seeding rate (suboptimal cultural practice). In contrast to the first phase, barley yield was higher with the semidwarf cultivar, and cultivar had no effect on wild oat management. Wild oat seed in the soil seed bank decreased with increasing herbicide rate, but amounts were often lower with the optimal cultural practice. For example, at the recommended herbicide rate at Beaverlodge, an approximate 40-fold reduction in wild oat seed occurred with the optimal compared to the suboptimal cultural practice. The results indicate that combining optimal cultural practices with herbicides will reduce the amount of wild oat seed in the soil seed bank, and result in higher barley yields. Optimal cultural practices may also compensate for reduced herbicidal effects in terms of reducing wild oat seed accumulation in the soil seed bank and increasing barley yield. The results have implications for mitigating the evolution of herbicide resistance in wild oat.

Glyphosate applied POST can provide a high level of efficacy on many weed species in soybean, but delayed application beyond optimal weed growth stages might fail to fully protect yield potential. Further, we do not have a good understanding of the extent to which delayed glyphosate application and its associated yield loss is occurring on-farm. Our goal was to characterize on-farm weed communities in glyphosate-resistant soybean just prior to glyphosate application and estimate potential yield loss associated with early-season soybean-weed competition. In field surveys conducted across 64 site-yr in southern Wisconsin in 2008 and 2009, common lambsquarters, velvetleaf, dandelion, Polygonum spp., and Amaranthus spp. were the five most abundant broadleaf weed species across site-years, present in 92, 69, 64, 42, and 50% of all fields, respectively, at average densities of 14, 5, 5, 14, and 10 plants m−2, respectively. Average height of these species was 21 cm or less at or near the time of glyphosate application. Grass and sedge species occurred in 95% of fields at an average density of 41 plants m−2 and height of 21 cm. The mean and median values of total weed density across site-years were 101 and 41 plants m−2, with heights of 19 and 17 cm, respectively. Recommended height for treatment is 15 cm. Glyphosate application occurred on average at V3 to V4 soybean growth stage, which is later than V2 soybean typically targeted to protect yield. Average yield loss predicted by WeedSOFT® was 5% with a mean economic loss of $47 ha−1. Predicted yield loss was greater than 5% on one-fourth of the site-years, all of which were treated at V4 soybean or later. The maximum predicted yield loss was 27%. These results suggest that glyphosate was applied at weed height and soybean growth stages that were greater than optimal to protect yield in many fields across southern Wisconsin. A soil-residual herbicide applied PRE, or a more timely POST application of glyphosate would alleviate the majority of these losses.

The use of eucalyptus leaves for weed control in maize-based cropping systems is proposed. Aqueous extracts of eucalyptus are known to exert phytotoxicity on many weeds and crops, but there is also experimental evidence of the relative tolerance of maize. Based on in vitro dose-response bioassays of leaf aqueous extracts, we conducted greenhouse pot experiments testing incorporated eucalyptus leaves as green manure. The phytotoxic effects were tested on the germination, establishment and growth of maize and some representative accompanying weeds, in comparison to the PRE herbicide metolachlor. Eucalyptus fresh leaves incorporated into the soil as green manure at 1 and 2% w/w reduced the emergence of the dicot weed species redroot pigweed and black nightshade. After one month of incorporation, both doses reduced aerial biomass >94% two monocot weed species (barnyardgrass + large crabgrass) with respect to the eucalyptus-free pots, and around 80% for the small seeded dicots. Although the aerial biomass of maize was reduced by 33%, the final relative yield of maize biomass with respect to the untreated control increased by 37%. On the assessment of the temporal phytotoxic effects, the reduction of aerial biomass in maize could be overcome by adopting a relay-planting of maize after 12 to 15 days from eucalyptus incorporation. Our results constitute evidence that the incorporation of E. globulus residues to soil could be a feasible practice to reduce the reliance on synthetic herbicides in maize-based cropping systems.

Pesticide sorption by soil is among the most sensitive input parameters in many pesticide-leaching models. For many pesticides, organic matter is the most important soil constituent influencing pesticide sorption. Increased fertility, irrigation, and mowing associated with highly maintained turfgrass areas result in constant deposition of organic material, creating a soil system that can change drastically with time. Changes in soil characteristics could affect the environmental fate of pesticides applied to turfgrass systems of varying ages. Sorption characteristics of simazine and S-metolachlor were determined on five soils from bermudagrass systems of increasing ages (1, 4, 10, 21, and 99 yr) and compared to adjacent native pine and bare-ground areas. Surface soil (0 to 5 cm) and subsurface soil (5 to 15 cm) from all sites were air-dried and passed through a 4-mm sieve for separation from plant material. Using a batch-equilibrium method, sorption isotherms were determined for each soil. Data were fit to the Freundlich equation, and Kd (soil sorption coefficient) and Koc (organic carbon sorption coefficient) values were determined. Sorption and soil system age were directly related to organic matter content in the soil. Sorption of both herbicides increased with age of the soil system and was greatest on the surface soil from the oldest bermudagrass soil system. Herbicide sorption decreased at greater soil depths with lower organic matter. Greater amount of 14C–simazine sorbed to subsurface soil of the oldest turfgrass system compared to 14C–S-metolachlor. Results indicate that as bermudagrass systems age and accumulate higher organic matter levels increased herbicide sorption may decrease the leaching potential and bioavailability of simazine and S-metolachlor.

Weed residues can impact nitrogen (N) cycling in agro-ecosystems that primarily utilize POST weed control. Quantifying this potential N source or sink may influence weed control and fertilization practices. A laboratory experiment measured the rate and quantity of N release from common lambsquarters, common ragweed, and giant foxtail. Weeds were grown in the field at four N rates (0, 67, 134, or 202 kg N ha−1) and collected at two weed heights (10 or 20 cm) to give a range of residue chemical composition. Residue chemical composition parameters of carbon : N (C : N) ratio and total N, nitrate-N, acid detergent fiber, and neutral detergent fiber concentration were measured and correlated with N release. Nitrogen release from weed residue mixed with soil was determined over a 12-wk period. Nitrogen was released from all weed residues at 12 wk. Prior to 12 wk, N was immobilized by giant foxtail grown with no N application. Prior to 4 wk, N was immobilized by 20-cm weeds grown with no N application. Nitrogen release from weed residue was negatively correlated with C : N ratio. Weed residue with a C : N ratio of < 19 (weeds grown with N application and 10-cm weeds) released 25 to 45% total N concentration within 2 wk and may contribute N within the growing season. Weed residue with a C : N ratio > 19 (giant foxtail and 20-cm weeds grown with no N) initially immobilized N and may not contribute N within the growing season.

Cover crops can provide many benefits in agroecosystems, including the opportunity for improved weed control. However, the weed suppressive potential of cover crops may depend on the species (or mixture of species) chosen, and the method of cover crop termination and residue management. The objective of this study was to determine the effects of cover crop mixture and mechanical termination method on weed biomass and density, and relative crop yield in an organic cropping system. A field experiment was conducted from 2009 to 2011 near Mead, NE, where spring-sown mixtures of two, four, six, and eight cover crop species were included in a sunflower–soybean–corn crop rotation. Cover crops were planted in late March, terminated in late May using a field disk or sweep plow undercutter, and main crops were planted within 1 wk of termination. Terminating cover crops with the undercutter consistently reduced early-season grass weed biomass, whereas termination with the field disk typically stimulated grass weed biomass relative to a no cover crop control (NC). The effects of cover crop mixture were not evident in 2009, but the combination of the undercutter and the eight-species mixture reduced early-season weed biomass by 48% relative to the NC treatment in 2010. Cover crops provided less weed control in 2011, where only the combination of the undercutter and the two-species mixture reduced weed biomass (by 31%) relative to the NC treatment. Termination with the undercutter resulted in relative yield increases of 16.6 and 22.7% in corn and soybean, respectively. In contrast, termination with the field disk resulted in a relative yield reduction of 13.6% in soybean. The dominant influence of termination method highlights the importance of appropriate cover crop residue management in maximizing potential agronomic benefits associated with cover crops.