Rodrigo Werle

Predicting Emergence of 23 Summer Annual Weed Species

Abstract First- and second-year seedbank emergence of 23 summer annual weed species common to U.S. corn production systems was studied. Field experiments were conducted between 1996 and 1999 at the Iowa State University Johnson Farm in Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for most species were planted in plastic crates. Seedling emergence was counted weekly for a 2-yr period following seed burial (starting in early spring). Soil temperature at 2 cm depth was estimated using soil temperature and moisture model software (STM2). The Weibull function was fit to cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time (HTT), and day of year (DOY). To identify optimum base temperature (Tbase) and base matric potential (&psgr;base) for calculating TT or HTT, Tbase and &psgr;base values ranging from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each species. The search for the optimal model for each species was based on the Akaikes Information Criterion (AIC), whereas an extra penalty cost was added to HTT models. In general, fewer seedlings emerged during the first year of the first experimental run (approximately 18% across all species) than during the second experimental run (approximately 30%). However, second-year seedbank emergence was similar for both experimental runs (approximately 6%). Environmental effects may be the cause of differences in total seedling emergence among years. Based on the AIC criterion, for 17 species, the best fit of the model occurred using Tbase ranging from 2 to 15 C with four species also responding to &psgr;base  =  −750 kPa. For six species, a simple model using DOY resulted in the best fit. Adding penalty costs to AIC calculation allowed us to compare TT and HTT when both models behaved similarly. Using a constant Tbase, species were plotted and classified as early-, middle-, and late-emerging species, resulting in a practical tool for forecasting time of emergence. The results of this research provide robust information on the prediction of the time of summer annual weed emergence, which can be used to schedule weed and crop management.

Environmental Triggers of Winter Annual Weed Emergence in the Midwestern United States

Abstract Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase] by six base water potential [&PSgr;base] by three base temperature fluctuation [Fbase] candidate threshold values  =  126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase, &PSgr;base, and Fbase) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and &PSgr;base resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherds-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, &PSgr;base and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherds-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions. Nomenclature: Carolina foxtail, Alopecurus carolinianus Walt. ALOCA; dandelion, Taraxacum officinale G.H. Weber ex Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; field pansy, Viola bicolor Pursh VIORA; field pennycress, Thlaspi arvense L. THLAR; henbit, Lamium amplexicaule L. LAMAM; pinnate tansymustard, Descurainia pinnata (Walt.) Britt. DESPI; purslane speedwell, Veronica peregrina L. VERPG; shepherds-purse, Capsella bursa-pastoris (L.) Medik. CAPBP; Virginia pepperweed, Lepidium virginicum L. LEPVI; Glycine max (L.) Merr.

Influence of Two Herbicides on Soybean Cyst Nematode (Heterodera glycines) Reproduction on Henbit (Lamium amplexicaule) Roots

Abstract Soybean cyst nematode (SCN) is the most yield-limiting pathogen of soybean in the United States. Henbit is a prevalent winter annual weed species in no-till fields and is reported to be an alternative host of SCN. A greenhouse study was conducted to evaluate how the development of SCN on henbit roots was affected by herbicide mode of action and time of herbicide application. Henbit plants were grown in watertight pots placed in a water bath bench that kept soil temperature constant (27 ± 1 C) during the study. Ten d after transplanting, pots were inoculated with approximately 1,000 SCN eggs. At 7, 14, or 21 d after inoculation (DAI), henbit plants were sprayed with recommended dose of either glyphosate (870 g ae ha−1) or 2,4-D (1,070 g ae ha−1). The experiment was arranged in a randomized complete block design with five replications per treatment, and two experimental runs separated in time. At 28 DAI, the total number of SCN cysts and eggs, and plant shoot and root dry weight per pot were determined. Henbit root and shoot biomass increased as the time of herbicide application was delayed. Glyphosate reduced root biomass more than 2,4-D, but no differences in shoot biomass were detected. The number of SCN cysts per henbit plant and eggs per cyst increased as the herbicide application was delayed from 7 to 21 DAI. Glyphosate reduced the number of cysts found on henbit roots more than 2,4-D, especially at earlier application times. On plants treated with glyphosate, SCN-females produced only half the number of eggs of SCN-females on henbit plants treated with 2,4-D, regardless of time of application. These results indicate that early control of henbit plants, especially with glyphosate, can reduce SCN reproduction potential in SCN infested fields. Nomenclature: Glyphosate; 2,4-D; henbit, Lamium amplexicaule L. LAMAM; soybean, Glycine max (L.) Merr. ‘ASGROW 3005′; soybean cyst nematode, Heterodera glycines Ichinohe. Resumen El nematodo quístico de la soya (SCN) es el patógeno que limita más los rendimientos de la soya en los Estados Unidos. Lamium amplexicaule es una maleza anual de invierno prevalente en campos con labranza cero y que ha sido reportada como un hospedero alternativo de SCN. Se realizó un estudio de invernadero para evaluar cómo el desarrollo de SCN en raíces de L. amplexicaule fue afectado según el modo de acción y el momento de aplicación de herbicidas. Las plantas de L. amplexicaule fueron crecidas en macetas impermeables colocadas en un baño de agua manteniendo el suelo a temperatura constante (27 ± 1 C) durante el estudio. Diez días después del trasplante, las macetas fueron inoculadas con aproximadamente 1,000 huevos de SCN. Las plantas de L. amplexicaule fueron asperjadas con las dosis recomendadas de glyphosate (870 g ae ha−1) o 2,4-D (1,070 g ae ha−1) a 7, 14 ó 21 días después de la inoculación (DAI). El experimento fue arreglado en un diseño de bloques completos al azar con cinco repeticiones por tratamiento, y dos corridas experimentales realizadas en diferentes momentos. A 28 DAI se determinó el total de números de quistes y huevos de SCN, y el peso seco de la parte aérea y de las raíces de las plantas en cada maceta. La biomasa de las raíces y la parte aérea incrementó conforme se retrasó el momento de aplicación. Glyphosate redujo la biomasa de las raíces más que el 2,4-D, pero no se detectaron diferencias en la biomasa de la parte aérea. El número de quistes de SCN por planta y de huevos por quiste incrementó al retrasarse la aplicación del herbicida de 7 a 21 DAI. Glyphosate redujo el número de quistes que se encontraron en las raíces de L. amplexicaule más que 2,4-D, especialmente en las aplicaciones más tempranas. En las plantas tratadas con glyphosate, las hembras de SCN produjeron la mitad del número de huevos que las hembras en plantas tratadas con 2,4-D sin importar el momento de aplicación. Estos resultados indican que el control temprano de plantas de L. amplexicaule, especialmente con glyphosate, pueden reducir el potencial de reproducción de SCN en campos infestados con este patógeno.

Influence of Tillage on Common Ragweed (Ambrosia artemisiifolia) Emergence Pattern in Nebraska

Spring tillage is a component of an integrated weed management strategy for control of early emerging glyphosate-resistant weeds such as common ragweed; however, the effect of tillage on common ragweed emergence pattern is unknown. The objectives of this study were to evaluate whether spring tillage during emergence would influence the emergence pattern or stimulate additional emergence of common ragweed and to characterize common ragweed emergence in southeast Nebraska. A field experiment was conducted for three years (2014 to 2016) in Gage County, Nebraska in a field naturally infested with glyphosate-resistant common ragweed. Treatments consisted of a no-tillage control and three spring tillage timings. The Soil Temperature and Moisture Model (STM2) software was used to estimate soil temperature and moisture at a 2-cm depth. The Weibull function was fit to total common ragweed emergence (%) with day of year (DOY), thermal time, and hydrothermal time as independent variables. Tillage treatments and year had no effect on total common ragweed emergence (P = 0.88 and 0.35, respectively) and time to 10, 25, 50, 75, and 90% emergence (P = 0.31). However, emergence pattern was affected by year (P = <0.001) with 50% total emergence reached on May 5 in 2014, April 20 in 2015, and April 2 in 2016 and 90% total emergence reached on May 12, 2014, May 8, 2015, and April 30, 2016. According to the corrected information-theoretic model comparison criterion (AICc), the Weibull function with thermal time and base temperature of 3 C best explained the emergence pattern over three years. This study concludes that spring tillage does not stimulate additional emergence; therefore, after the majority of the common ragweed has emerged and before the crop has been planted, tillage could be used as an effective component of an integrated glyphosate-resistant common ragweed management program in Nebraska. Nomenclature: Glyphosate; common ragweed, Ambrosia artemisiifolia L.

Distribution of glyphosate-resistant Amaranthus spp. in Nebraska

BACKGROUND Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus tuberculatus var. rudis), and redroot pigweed (Amaranthus retroflexus L.) are major weeds occurring in fields throughout Nebraska with recurrent grower complaints regarding control with glyphosate. The objective of this study was to investigate the frequency and distribution of glyphosate-resistant Palmer amaranth, common waterhemp, and redroot pigweed populations in Nebraska. The study also aimed to investigate how agronomic practices influence the occurrence of glyphosate resistance in the three Amaranthus species. RESULTS Glyphosate resistance was widespread in common waterhemp (81% of the screened populations), few Palmer amaranth populations were glyphosate-resistant (6% of the screened populations), whereas no glyphosate-resistant redroot pigweed populations were identified in Nebraska. Weed species, geographic region within the state, and current crop were the most important factors predicting the occurrence of glyphosate resistance in fields infested with Amaranthus species in Nebraska. CONCLUSION The intensive glyphosate selection pressure exerted in soybean (Glycine max) fields in eastern Nebraska is one of the major factors causing widespread occurrence of glyphosate resistance in common waterhemp in the state. The relatively low frequency of glyphosate-resistant Palmer amaranth in the state highlights the importance of the application timing and the adoption of multiple modes of action in weed management practices to delay the evolution of glyphosate resistance.

Likelihood of Soybean Cyst Nematode (Heterodera glycines) Reproduction on Henbit (Lamium amplexicaule) Roots in Nebraska

Abstract Soybean cyst nematode (SCN) is a major soybean yield–limiting disease in the United States. Henbit, a winter annual species common to no-till fields in the midwestern United States, is known to act as an alternative host for SCN. A simulation was performed to estimate how likely SCN was to reproduce on henbit roots during a 30-yr period in two important soybean production areas of Nebraska. Simulations were conducted using published information on henbit seedling emergence, SCN reproduction on henbit roots, and SCN response to soil temperature. Results indicate that SCN would be able to complete one generation on henbit roots under Nebraska conditions. The SCN reproductive cycle was not likely to be completed before the winter in south central Nebraska, but one SCN generation was predicted to be completed in the fall in 2 out of 30 simulation years (7% likelihood) in southeast Nebraska. Based on our predictions, to reduce the chances of SCN population build-up in the absence of its main host (soybean), weed management in fields infested with both henbit and SCN should be completed after crop harvest in the fall when most henbit seedlings have emerged and are growing but the SCN developing on henbit roots have not yet achieved full maturity in Nebraska. Nomenclature: Henbit, Lamium amplexicaule L. LAMAM; soybean cyst nematode, Heterodera glycines Ichinohe. Resumen El nematodo cístico de la soja (SCN) es la enfermedad que más limita el rendimiento de la soja en los Estados Unidos. Lamium amplexicaule es una especie anual de invierno común en campos con labranza cero, y que se conoce que actúa como hospedero alternativo de SCN. Se realizó una simulación para estimar qué tan probable fue la reproducción de SCN en raíces de L. amplexicaule durante un período de 30 años en dos áreas importantes de soja en Nebraska. Las simulaciones se realizaron usando información publicada acerca de la emergencia de plántulas de L. amplexicaule, reproducción de SCN en raíces de L. amplexicaule, y la respuesta de SCN a la temperatura del suelo. Los resultados indican que SCN podría ser capaz de completar una generación en raíces de L. amplexicaule en las condiciones de Nebraska. No fue probable que el ciclo reproductivo de SCN se completara antes del invierno en el sur-central de Nebraska, pero se predijo que se completaría una generación de SCN en el otoño en 2 de los 30 años de simulación (7% de probabilidad) en el sureste de Nebraska. Con base en nuestras predicciones, para reducir las oportunidades de aumentos en las poblaciones de SCN en ausencia de su hospedero principal (soja) en Nebraska, el manejo de malezas en campos infestados con L. amplexicaule y SCN debe ser completado después de la cosecha del cultivo en el otoño, cuando las plántulas de L. amplexicaule han emergido y están creciendo, pero el SCN que está desarrollándose en raíces de L. amplexicaule no ha alcanzado la madurez.

Competitiveness of Herbicide-Resistant Waterhemp (Amaranthus tuberculatus) with Soybean

Abstract Waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of different A. tuberculatus populations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistant A. tuberculatus populations (2,4-D and atrazine resistant [2A-R], glyphosate and protoporphyrinogen oxidase [PPO]-inhibitor resistant [GP-R], and 2,4-D, atrazine, glyphosate, and PPO-inhibitor susceptible [2AGP-S]) with soybean [Glycine max (L.) Merr.]; and (2) the density-dependent response of each A. tuberculatus population within a constant soybean population in a greenhouse environment. Amaranthus tuberculatus competitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot − 1 (equivalent to 0, 20, 40, 80, and 160 plants m− 2) with 3 soybean plants pot − 1 (equivalent to 300,000 plants ha − 1). At the R1 soybean harvest time, no difference in soybean biomass was observed across A. tuberculatus populations. At A. tuberculatus densities <8 plants pot − 1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot − 1. The 2AGP-S population had greater earlyseason biomass accumulation and stem diameter compared with the other A. tuberculatus populations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance in A. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistant A. tuberculatus.

Prevalence and Mechanism of Atrazine Resistance in Waterhemp (Amaranthus tuberculatus) from Nebraska

Abstract Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathione S-transferase (GST) conjugation confers resistance in A. tuberculatus from Illinois. The distribution and mechanism of resistance to atrazine in A. tuberculatus populations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NE A. tuberculatus to soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NE A. tuberculatus. The chloroplastic psbA gene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations of A. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha-1) and sulfentrazone (0.28 kg ai ha-1) were effective on A. tuberculatus management. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha-1) was not effective on 39% and 73% of the A. tuberculatus populations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance in A. tuberculatus in NE. Sequence analysis of the psbA gene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptible A. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine in A. tuberculatus from NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies for A. tuberculatus is crucial for the control of this troublesome species.

Independent Evolution of Acetolactate Synthase-Inhibiting Herbicide Resistance in Weedy Sorghum Populations Across Common Geographic Regions

Traditional breeding has been used to develop grain sorghum germplasm that is tolerant to acetolactate synthase (ALS)-inhibiting herbicides (Inzen Technology, DuPont). Inzen sorghum carries a double mutation in the ALS gene (Val560Ile and Trp574Leu), which confers high level of tolerance to ALS-inhibiting herbicides. Overreliance on ALS-inhibiting herbicides for weed control during the 1990s resulted in the evolution of ALS inhibitor—resistant shattercane populations in Nebraska. According to a survey conducted in 2013, ALS inhibitor—resistant weedy Sorghum populations persist in Nebraska. The objectives of this research were to determine whether the ALS mutations present in Inzen sorghum were present in the ALS inhibitor—resistant shattercane and johnsongrass populations detected in Nebraska and northern Kansas, and whether these populations evolved ALS resistance independently. Primers specific to the Val560 and Trp574 region of the ALS gene were used to screen the populations with PCR. The Trp574Leu mutation was present in one ALS inhibitor—resistant johnsongrass population. The Val560Ile was detected in three ALS inhibitor—resistant shattercane, one susceptible shattercane, one ALS inhibitor—resistant johnsongrass, and one susceptible johnsongrass population. Moreover, Val560Ile was present in resistant and/or susceptible individuals within johnsongrass and shattercane populations that were segregating for ALS resistance, indicating that by itself the Val560Ile mutation does not confer resistance to ALS-inhibiting herbicides. None of the populations presented both mutations simultaneously, as does Inzen sorghum. A shattercane population containing the Ser653Thr mutation was also detected. This research indicates that the ALS mutations present in Inzen sorghum already exist individually in weedy sorghum populations. Moreover, our results present strong evidence that ALS resistance in these populations evolved independently. Thus, widespread overreliance on ALS-inhibiting herbicides prior to adoption of glyphosate-tolerant crops in the 1990s exerted sufficient selective pressure on shattercane and johnsongrass populations for resistance to evolve multiple times in the Midwest. Finally, a survey of the 5′ portion of the ALS gene in more diverse wild and weedy Sorghum species was hampered by limited coverage in genomic resequencing surveys, suggesting that refined PCR-based methods will be needed to assess SNP variation in this gene region, which includes the Ala122, Pro197, and Ala205 codons known to confer ALS resistance in other species. Nomenclature: Acetolactate synthase (ALS) -inhibiting herbicides, johnsongrass, Sorghum halepense (L.) Pers. SORHA, shattercane, Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse SORVLJ, sorghum, Sorfhum bicolor (L.) Moench ssp. Bicolor SORVU