Brian J. Schutte

A Hydrothermal Seedling Emergence Model for Giant Ragweed (Ambrosia trifida)

Abstract Late-season giant ragweed emergence in Ohio crop fields complicates decisions concerning the optimum time to implement control measures. Our objectives were to develop a hydrothermal time emergence model for a late-emerging biotype and validate the model in a variety of locations and burial environments. To develop the model, giant ragweed seedlings were counted and removed weekly each growing season from 2000 to 2003 in a fallow field located in west central Ohio. Weather data, soil characteristics and geographic location were used to predict soil thermal and moisture conditions with the Soil Temperature and Moisture Model (STM2). Hydrothermal time (θHT) initiated March 1 and base values were extrapolated from the literature (Tb = 2 C, ψb = −10 MPa). Cumulative percent emergence initially increased rapidly and reached 60% of maximum by late April (approximately 400 θHT), leveled off for a period in May, and increased again at a lower rate before concluding in late July (approximately 2,300 θHT). The period in May when few seedlings emerged was not subject to soil temperatures or water potentials less than the θHT base values. The biphasic pattern of emergence was modeled with two successive Weibull models that were validated in 2005 in a tilled and a no-tillage environment and in 2006 at a separate location in a no-tillage environment. Root-mean-square values for comparing actual and model predicted cumulative emergence values ranged from 8.0 to 9.5%, indicating a high degree of accuracy. This experiment demonstrated an approach to emergence modeling that can be used to forecast emergence on a local basis according to weed biotype and easily obtainable soil and weather data. Nomenclature: Giant ragweed, Ambrosia trifida L.

Managing the evolution of herbicide resistance

Abstract BACKGROUND Understanding and managing the evolutionary responses of pests and pathogens to control efforts is essential to human health and survival. Herbicide‐resistant (HR) weeds undermine agricultural sustainability, productivity and profitability, yet the epidemiology of resistance evolution – particularly at landscape scales – is poorly understood. We studied glyphosate resistance in a major agricultural weed, Amaranthus tuberculatus (common waterhemp), using landscape, weed and management data from 105 central Illinois grain farms, including over 500 site‐years of herbicide application records. RESULTS Glyphosate‐resistant (GR) A. tuberculatus occurrence was greatest in fields with frequent glyphosate applications, high annual rates of herbicide mechanism of action (MOA) turnover and few MOAs field−1 year−1. Combining herbicide MOAs at the time of application by herbicide mixing reduced the likelihood of GR A. tuberculatus. CONCLUSIONS These findings illustrate the importance of examining large‐scale evolutionary processes at relevant spatial scales. Although measures such as herbicide mixing may delay GR or other HR weed traits, they are unlikely to prevent them. Long‐term weed management will require truly diversified management practices that minimize selection for herbicide resistance traits.

Seed Dormancy and Adaptive Seedling Emergence Timing in Giant Ragweed (Ambrosia trifida)

Abstract Giant ragweed germination is delayed by both a physiological dormancy of the embryo (embryo dormancy) and an inhibitory influence of embryo-covering structures (covering structure-enforced [CSE] dormancy). To clarify the roles of embryo and CSE dormancy in giant ragweed seedling emergence timing, we conducted two experiments to address the following objectives: (1) determine changes in germinability for giant ragweed dispersal units (hereafter “involucres”) and their components under natural burial conditions, and (2) compare embryo and CSE dormancy alleviation and emergence periodicity between successional and agricultural populations. In Experiment 1, involucres were buried in crop fields at Columbus, OH, periodically excavated, and brought to the laboratory for dissection. Involucres, achenes, and embryos were then subjected to germination assays at 20 C. In Experiment 2, temporal patterns of seedling emergence were determined at a common burial site. Reductions in embryo and CSE dormancy were compared with controlled-environment stratification followed by germination assays at 12 and 20 C, temperatures representative of soil conditions in spring and summer. Results indicated that overwinter dormancy loss involved sequential reductions in embryo and CSE dormancy. CSE dormancy, which may limit potential for fatal germination during fall, was caused by the pericarp and/or embryo-covering structures within the pericarp. In Experiment 2, successional populations emerged synchronously in early spring, whereas agricultural populations emerged throughout the growing season. Levels of embryo dormancy were greater in the agricultural populations than the successional populations, but CSE dormancy levels were similar among populations. In 12 C germination assays, embryo dormancy levels were positively correlated with time required to reach 95% cumulative emergence (run 1: r  =  0.81, P  =  0.03; run 2: r  =  0.76, P  =  0.05). These results suggest that late-season emergence in giant ragweed involves high levels of embryo dormancy that prevent germination at low temperatures in spring. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR.

Certified Crop Advisors’ Perceptions of Giant Ragweed (Ambrosia trifida) Distribution, Herbicide Resistance, and Management in the Corn Belt

Abstract Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed. Nomenclature: Glyphosate; giant ragweed; Ambrosia trifida L. AMBTR; common earthworm; Lumbricus terrestris L.; corn; Zea mays L.; soybean, Glycine max (L.) Merr.

Age-dependent demographic rates of the bioenergy crop Miscanthus × giganteus in Illinois.

Abstract Some plants being considered as bioenergy crops share traits with invasive species and have histories of spreading outside of their native ranges, highlighting the importance of evaluating the invasive potential before the establishment of large-scale plantings. The Asian grass Miscanthus × giganteus is currently being planted as a bioenergy crop in the north central region of the United States. Our goal was to understand the demographic rates and vegetative spread of this species in unmanaged arable lands in Illinois to compare with those of large-statured invasive grasses (LSIGs). We collected data from 13 M. × giganteus plantings in Illinois, ranging in age from 1 to 7 yr, recording tiller number, plant spatial extent, spikelet production, and plant survival over 4 yr. Additionally, to understand recruitment potential, we conducted a greenhouse germination experiment, and, to estimate establishment from rhizome fragments, field trials were performed. Miscanthus × giganteus demographic rates were age dependent. Spikelet production was high, with 1- and 4-yr plants producing an annual average of more than 10,000 and 180,000 spikelets plant−1, respectively; however, data from our germination trial suggested that none of these spikelets had the potential to yield seedlings. On average, plants expanded vegetatively 0.15 m yr−1. Tiller density within the center of a clone decreased with age, possibly leading to a “dead center” found among some LSIGs. Rhizome establishment increased with weight, ranging from 0 to 42%. Survival was low, 24%, for first-year plants but quickly climbed to an asymptote of 98% survival for 4-yr-old plants. Our results suggest that efforts should be made to eradicate plants that escape biomass production fields within a year of establishment, before the onset of high survival. Future work is needed to determine what types of natural and anthropogenic disturbances can fragment rhizomes, leading to regeneration. Nomenclature: Miscanthus × giganteus Greef et Deu ex Hodkinson et Renvoize, Giant miscanthus. Management Implications: As demand for alternative fuels continues to grow, biofuels from herbaceous perennial biomass crops will be an important component of the U.S. energy portfolio. Before growing such crops across wide areas, it is important to understand both the risks and benefits of such an action. Because of similarities between many bioenergy crops and plant invaders, and the onset of new projects to scale-up biomass production rapidly in the Midwest, it is important to understand the potential behavior of candidate species for bioenergy production outside of cultivation. We quantified the age-specific demographic rates of the Illinois clone variety of Miscanthus × giganteus, a leading bioenergy crop in the Midwest. All M. × giganteus demographic rates were strongly dependent on plant age. Spikelet production was low in first-year plants and rose to more than 180,000 spikelets per plant by the fourth year, but none yielded seedlings. Vegetative expansion of M. × giganteus was moderate compared with other large grasses; 4-yr-old plants were observed to have an average vegetative creep radius of 0.15 m yr−1. Survival of first-year plants was low, 24%, but quickly rose to 97% by the fourth year. The lack of viable seed production and slow vegetative expansion in M. × giganteus observed in this study suggest that the sterile clonal cultivar for this bioenergy crop has low invasive potential in Illinois, assuming that the tough rhizomes are not fragmented by disturbance. However, several similarities between M. × giganteus and other large-statured invasive grasses suggest that caution must be taken when introducing this species, and the potential for rhizome establishment and dispersal must be explored in greater detail.

Integrating Management of Soil Nitrogen and Weeds

Knowledge of the soil nitrogen (N) supply and the N mineralization potential of the soil combined with an understanding of weed-crop competition in response to soil nutrient levels may be used to optimize N fertilizer rates to increase the competitive advantage of crop species. A greenhouse study (2006) and field studies (2007 to 2008) in Illinois and Nebraska were conducted to quantify the growth and interference of maize and velvetleaf in response to varying synthetic N fertilizer rates in soils with high and low N mineralization potential. Natural soils were classified as having “low mineralization potential” (LMP), while soils amended with composted manure were classified as having “high mineralization potential” (HMP). Maize and velvetleaf were grown in monoculture or in mixture in both LMP and HMP soils and fertilized with zero, medium, or full locally recommended N rate. In the greenhouse, velvetleaf interference in maize with respect to plant biomass increased as N rate increased in the HMP soil, whereas increasing N rate in the LMP soil reduced velvetleaf interference. In contrast, velvetleaf interference in maize decreased as N rate increased regardless of soil class in the field experiment. With respect to grain yield, velvetleaf interference in maize was unaffected by N rate or soil class. In both greenhouse and field experiments, velvetleaf biomass was greater in the HMP soil class, whereas maize interference in velvetleaf was generally greater in the LMP soil class. While soil N levels influenced weed-crop interference in the greenhouse, the results of the field study demonstrate the difficulty of controlling soil nutrient dynamics in the field and support a maize fertilization strategy independent of weed N use considerations. Nomenclature: Velvetleaf, Abutilon theophrasti Medic. ABUTH; maize, Zea mays L

Maternal Corn Environment Influences Wild-Proso Millet (Panicum miliaceum) Seed Characteristics

Abstract Modification of the cropping environment to make weed seed more susceptible to fatal germination or decay processes is based, in part, on the premise that seed longevity is affected by the crop-influenced environment in which seed is produced, hereafter, called the maternal crop environment. The objective of this investigation was to determine the influence of maternal crop environment on wild-proso millet seed production, germinability, and seed coat tone (i.e., lightness), a trait previously associated with seed longevity in wild-proso millet. Maternal corn environments were established by growing wild-proso millet plants in four morphologically different sweet corn hybrids in four replicates over 2 yr. Wild-proso millet seed was collected at sweet corn harvest, enumerated, characterized for seed coat tone, and tested for germination. Principal component factor analysis reduced six sweet corn traits measured between silking and harvest into a single maternal corn environment factor that accounted for 84% of the variation among crop canopies. Functional relationships between maternal corn environment factor scores and wild-proso millet seed characteristics were clarified by fitting linear models. For each unit decrease in maternal environment factor score, wild-proso millet seed production increased 1,535 seed m−2, germination increased 2.2%, and seed coat tone was 1.8% lighter. These results show the size and germinability of wild-proso millet seed was highest in less-competitive maternal corn environments characterized by a short time to crop maturity and a small crop-canopy size. Nomenclature: Wild-proso millet, Panicum miliaceum L., PANMI; sweet corn, Zea mays L.

Palmer Amaranth (Amaranthus palmeri) Damage Niche in Illinois Soybean Is Seed Limited

Abstract Palmer amaranth, a dioecious summer annual forb, originating in Sonoran desert washes, compromises crop yields in much of the southern United States and its range is expanding northward. Appropriate tactics for managing this weed proactively in the Upper Midwest will depend on characterizing its damage niche, the geographic range in which it can reduce crop yields. We implemented a common garden study in 2011 and 2012, planting eight accessions of Palmer amaranth from the southern and midwestern United States, into soybean crops in southern, central, and northern Illinois, at a population density of 8 plants m−2 with a biocontainment protocol. Once Palmer amaranth plants initiated flowering, they were removed and burned. Weed survival, flowering, and weed biomass were measured, in addition to soybean yield and weather data. Analyses indicated that Palmer amaranth’s damage niche in Illinois soybean was independent of weed genotype or maternal environment. Despite competing only briefly, Palmer amaranth reduced soybean yields in all site–years, indicating its damage niche in Illinois, and much of the Midwest, is limited primarily by seed immigration rate. These results highlight the urgent need for weed managers to learn Palmer amaranth identification, prevent seed introduction, and maintain a policy of zero seed return. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.; soybean, Glycine max (L.) Merr.

Do Common Waterhemp (Amaranthus rudis) Seedling Emergence Patterns Meet Criteria for Herbicide Resistance Simulation Modeling

Abstract A study was conducted to quantify the magnitude and sources of variation in common waterhemp temporal patterns of emergence over 1 yr. In 2008 and 2010, emergence patterns in the absence of soil disturbance were determined for replicated samples of maternal families (progeny from one individual) separately harvested during the previous year from four plants within each of four agricultural fields (16 maternal families yr−1) at a university research farm near Urbana, IL. Combining data across years, variance partitioning indicated that seed sample within maternal family explained 48% of total variation in the percentage of viable, buried seeds that produced seedlings. Differences within, rather than among, maternal families also accounted for large fractions (60 to 99%) of total variation in cumulative percentage emergence at specific points during the growing season. Within years, seed samples characterized by delayed or accelerated emergence patterns did not originate from specific maternal plants. These results indicate that common waterhemp seed populations are without strong maternal plant effects that limit emergence to narrow intervals within the overall emergence period. Thus, results of this study support the use of contemporary approaches for modeling herbicide resistance evolution in common waterhemp, which assume seedling cohorts contain offspring from all individuals occurring within the maternal population. Nomenclature: Common waterhemp; Amaranthus rudis Sauer AMATA. Resumen Se realizó un estudio para cuantificar la magnitud y las fuentes de variación en los patrones temporales de emergencia de Amaranthus rudis durante un año. En 2008 y 2010, se determinaron los patrones de emergencia en ausencia de perturbación del suelo de muestras replicadas de familias maternas (progenie de un individuo) cosechadas separadamente durante el año previo a partir de cuatro plantas por campo, provenientes de cuatro campos agrícolas (16 familias maternas por año), en una finca experimental universitaria cerca de Urbana, Illinois. Combinando los años, la partición de la varianza indicó que la muestra de semilla dentro de la familia materna explicó el 48% del total de la variación del porcentaje de semilla viable que produjo plántulas. Diferencias dentro y no entre familias maternas también fue responsable de gran parte (60 a 99%) del total de la variación en el porcentaje de emergencia acumulado en momentos específicos durante la temporada de crecimiento. Dentro de los años, muestras de semillas caracterizadas por mostrar patrones de emergencia retrasados o acelerados no se originaron a partir de plantas maternas específicas. Estos resultados indican que las poblaciones de semillas de A. rudis no tienen fuertes efectos maternos que limiten la emergencia a intervalos cortos dentro del periodo de emergencia general. De esta forma, los resultados de este estudio apoyan el uso de métodos contemporáneos para el modelaje de la evolución de resistencia a herbicidas en A. rudis, los cuales asumen que los cohortes de plántulas contienen progenie proveniente de todos los individuos que están presentes dentro de la población materna.

Water-Deficit Stress Tolerance Differs between Two Locoweed Genera ( Astragalus and Oxytropis ) with Fungal Endophytes

Locoweeds are plants of the genera Astragalus and Oxytropis (Fabaceae family) and are toxic to cattle, sheep, and horses. The toxic property of locoweeds is due to the alkaloid swainsonine (SWA), which is synthesized by an endophytic fungus Alternaria spp. section Undifilum. Although the endophyte–locoweed complex is often considered mutualistic, empirical evidence for benefits to host plants is lacking. This study: 1) compared the growth, photosynthesis, and leaf pigment and antioxidant concentrations between endophyte-infected and endophyte-free plants under well-watered and water-deficit conditions; and 2) measured SWA to determine whether SWA concentrations are attenuated by water deficit and leaf age. Locoweed species in this study were woolly loco and silky crazyweed. Endophyte-infected and endophyte-free (by removal of seed coat) seedlings, as confirmed by DNA analyses, were grown under greenhouse conditions for 6 mo, after which plants were subjected to three 12- to 15-d water-deficit periods that created sublethal drought conditions. Results suggest that the endophyte did not influence photosynthetic gas exchange and leaf pigment concentrations. Under well-watered conditions only, endophyte-infected woolly loco plants had lower shoot and root biomass and higher concentrations of α-tocopherol than endophyte-free plants. SWA analyses revealed taxon-specific effects of water deficit, with water deficit increasing SWA concentrations in young leaves of woolly loco but not affecting SWA concentration in silky crazyweed. These results suggest that the endophyte behaves as a parasite in woolly loco plants grown under optimal but not under water-limited conditions. Further, results indicate that drought conditions elevate the toxicity of woolly loco plants. Improved understanding of endophyte-locoweed interactions and factors influencing SWA levels will contribute to the development of livestock management strategies to predict toxicity in particular locoweed populations. Nomenclature: Silky crazyweed, Oxytropis sericea Nutt.; woolly loco, Astragalus mollissimus Torr.