Lauren M. Schwartz

Tall Waterhemp (Amaranthus tuberculatus) and Palmer amaranth (Amaranthus palmeri) Seed Production and Retention at Soybean Maturity

Two of the most problematic Amaranthus species in soybean production today are tall waterhemp and Palmer amaranth. This study determined the percentage of tall waterhemp and Palmer amaranth seed that was retained by the weed at soybean maturity to assess the likelihood of using at-harvest weed seed control tactics for soil seedbank management. Palmer amaranth plants were collected from fields in Arkansas, Tennessee, Illinois, Missouri, and Nebraska, and tall waterhemp plants were collected from fields in Nebraska, Missouri, Wisconsin, and Illinois. Collected plants were assessed for at-harvest weed seed retention in 2013 and 2014. Within 1 wk of soybean maturity, Amaranthus plants were harvested and the loose soil and debris beneath the plants were swept into a pan with a hand broom to collect any shattered seed. Percent seed retention ranged from 95 to 100% for all states both years, regardless of species. There was a strong correlation between weed biomass (g) and total seed production (no. plant−1) in that the larger the plant, the more seeds it produced. However, there was no correlation between percent seed retention and weed biomass, which indicates that regardless of plant size and likely time of emergence, seed retention is high at the time of crop maturity. Overall, this study demonstrated that there is great opportunity for Palmer amaranth and tall waterhemp seed capture or destruction at soybean harvest. It is likely that nearly all of the seeds produced for both Amaranthus species passes through the combine during harvest to be returned to the soil seedbank. Thus, there is continued need for research focused on developing and testing harvest weed seed control tactics that aim at reducing the soil seedbank and lowering risks for evolution of herbicide resistance. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats.; tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer; soybean, Glycine max (L.) Merr. Dos de las especies de Amaranthus más problemáticas en la producción de soja, hoy en día, son Amaranthus tuberculatus y Amaranthus palmeri. Este estudio determinó el porcentaje de semilla de A. tuberculatus y A. palmeri que fue retenido por la maleza al momento de la madurez de la soja, para evaluar la probabilidad de usar tácticas para el control de semilla de malezas durante la cosecha para el manejo del banco de semillas. Plantas de A. palmeri fueron colectadas en campos en Arkansas, Tennessee, Illinois, Missouri, y Nebraska, y plantas de A. tuberculatus fueron colectadas en campos en Nebraska, Missouri, Wisconsin, e Illinois. Las plantas colectadas fueron evaluadas por su retención de semilla al momento de la cosecha en 2013 y 2014. A la semana de la madurez de la soja, las plantas de Amaranthus fueron cosechadas y el suelo suelto y los residuos vegetales debajo de las plantas fueron removidos con una escoba de mano y fueron depositados en un contenedor para colectar semilla que hubiera caído al suelo antes de la cosecha. El porcentaje de retención de semilla varió de 95 a 100% en todos los estados y en ambos años, sin importar la especie. Hubo una correlación alta entre la biomasa de la maleza (g) y el total de semilla producida (no. planta−1), así entre más grande la planta, más semilla produjo. Sin embargo, no hubo una correlación entre el porcentaje de retención de semilla y la biomasa de la maleza, lo que indica que sin importar el tamaño de la planta y el momento de emergencia, la retención de la semilla es alta al momento de la madurez del cultivo. En general, este estudio demostró que existe una gran oportunidad para capturar o destruir la semilla de A. palmeri y A. tuberculatus durante la cosecha de la soja. Es probable que casi toda la semilla producida por ambas especies de Amaranthus pase por la cosechadora al momento de la cosecha y que sea retornada al banco de semillas del suelo. Por esta razón, existe una necesidad de investigación que se enfoque en el desarrollo y evaluación de tácticas de control de semillas de malezas durante la cosecha con el objetivo de reducir el banco de semillas del suelo y a su vez disminuir el riesgo de evolución de resistencia a herbicidas.

Seedbank and Field Emergence of Weeds in Glyphosate-Resistant Cropping Systems in the United States

Abstract A segment of the debate surrounding the commercialization and use of glyphosate-resistant (GR) crops focuses on the theory that the implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was initiated in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR-corn, -cotton or -soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on seedbank weed population density and diversity was analyzed. The parameters of total weed population density of all species in the seedbank, species richness, Shannons H′ and evenness were not affected by any management treatment. The similarity between the seedbank and aboveground weed community was more strongly related to location than management; previous years crops and cropping systems were also important while GR trait rotation was not. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. Continuous GR crop, compared to fields with other cropping systems, only had greater species diversity (species richness) of some life forms, i.e., biennials, winter annuals, and prostrate weeds. Overall diversity was related to geography and not cropping system. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits to provide a more complete characterization of their culture and local adaptation to the weed seedbank. Nomenclature: Glyphosate; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

Life history of Achyranthes japonica (Amaranthaceae): an invasive species in southern Illinois1

Abstract Achyranthes japonica (Miq.) Nakai (Japanese chaff flower) is a relatively new, invasive species to the United States, with limited information on its life history characteristics. The purpose of this study was to assess the importance of seed survivorship in the soil of A. japonica and to compare survivorship, fecundity, and morphological characteristics within populations at two different sites, Chestnut Hills Nature Preserve (CH) and Bellrose Waterfowl Reserve (BWR), in southern Illinois. Plots were established at each site to determine seed viability by burying seed bags over each of three winters (2012 to 2014) to quantify seedling emergence in the latter 2 yr (2013 and 2014) and to monitor survival of three cohorts of 50 seedlings per site starting in 2012, 2013, and 2014. In addition, 20 reproductive adults were selected outside of the plots to determine average fecundity and to measure a suite of morphological characteristics. Environmental factors had a significant effect on seed viability, which decreased from 2012 to 2013 during a drought year and rebounded from 2013 to 2014 following flooding. On average, individuals at the CH site had higher performance and fecundity when compared with individuals at BWR, regardless of year. Additional differences among the sites, such as dryness, disturbance, and species composition, most likely affected plant performance (i.e. plant height, number of nodes and stems, and inflorescence length). Regardless of high between-population variability, this invasive species has high fecundity, high seed viability, and high propagule pressure that allow rapid spread and expansion of its invasive range. More research is needed on the soil seedbank, seed dormancy, and the effects of stress, which will allow more-informed methods of control.

Do plant traits predict the competitive abilities of closely related species

There is a strong incentive to predict which species will become invasive before they become too widespread and unmanageable. In this study, we conducted a multi-year, temporally replicated, greenhouse and field experiment based on plant functional traits, quantifying competitive ability, and phylogenetic comparison to determine the invasive potential of four closely related species varying in invasion status, life history and habitat. Our results suggest that these closely related species do exhibit similar competitive abilities and that the invasiveness and not the life history or habitat of these species appear to be the driving factor of competitiveness.

Use of Hppd-inhibiting Herbicides for Control of Troublesome Weeds inthe Midsouthern United States

Transgenic crops provide cotton and soybean producers additional weed control options for many of the most problematic weeds in mid southern US. production systems. The expected commercialization of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-resistant soybean in 2017 and cotton in 2020 will provide producers the option to apply HPPD-inhibiting herbicides that will offer an alternative mechanism of action for previously hardto- control weeds. Experiments were conducted in 2010 and 2011 to determine the efficacy of HPPD-inhibiting herbicides applied pre emergence (PRE) or post emergence (POST) for control of problematic weeds of cotton and soybean in the mid southern US. PRE experiments were conducted to understand the length and degree of control of Palmer amaranth and barn yard grass that could be expected with HPPD-inhibiting herbicides compared with current standards on silt loam and clay soil textures. The HPPD herbicides evaluated included mesotrione, tembotrione, and isoxaflutole compared to several standards currently labeled in soybean. In the POST experiment, applications of isoxaflutole, tembotrione, glyphosate, and two rates of glufosinate applied alone and both HPPD herbicides combined with glyphosate or glufosinate were evaluated for control of Palmer amaranth, barn yard grass, hemp sesbania, and yellow nutsedge. When herbicides were applied PRE, the HPPD-inhibiting herbicides and the current standard treatments all provided greater than 90% control of Palmer amaranth 4 weeks after treatment (WAT) on both soil textures. Barn yard grass control with HPPD-inhibitors was generally weaker than the current standards with the exception of mesotrione which proved to be comparable to the standards 4 WAT. In the POST experiment, all treatments, except for glyphosate alone, provided excellent (>85%) control of Palmer amaranth less than 10 cm in height. Barn yard grass, yellow nutsedge, and hemp sesbania were effectively controlled with HPPDinhibiting herbicides with and without glufosinate or glyphosate.