Stevan Z. Knezevic

Critical period for weed control: the concept and data analysis

Abstract The critical period for weed control (CPWC) is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Knowing the CPWC is useful in making decisions on the need for and timing of weed control and in achieving efficient herbicide use from both biological and economic perspectives. An increase in the use of herbicide-tolerant crops, especially soybean resistant to glyphosate, has stimulated interest in the concept of CPWC. Recently, several studies examined this concept in glyphosate-resistant corn and soybean across the midwestern United States. However, these studies presented various methods for data analysis and reported CPWC on the basis of a variety of crop- or weed-related parameters. The objectives of this study are (1) to provide a review of the concept and studies of the CPWC, (2) to suggest a common method to standardize the process of data analysis, and (3) to invite additional discussions for further debate on the subject. Wide adoption of the suggested method of data analysis will allow easier comparison of the results among sites and between researchers. Nomenclature: Glyphosate; corn, Zea mays L.; soybean, Glycine max (L.) Merr.

Utilizing R Software Package for Dose-Response Studies: The Concept and Data Analysis

Advances in statistical software allow statistical methods for nonlinear regression analysis of dose-response curves to be carried out conveniently by non-statisticians. One such statistical software is the program R with the drc extension package. The drc package can: (1) simultaneously fit multiple dose-response curves; (2) compare curve parameters for significant differences; (3) calculate any point along the curve at the response level of interest, commonly known as an effective dose (e.g., ED30, ED50, ED90), and determine its significance; and (4) generate graphs for publications or presentations. We believe that the drc package has advantages that include: the ability to relatively simply and quickly compare multiple curves and select ED-levels easily along the curve with relevant statistics; the package is free of charge and does not require licensing fees, and the size of the package is only 70 MB. Therefore, our objectives are to: (1) provide a review of a few common issues in dose-response-curve fitting, and (2) facilitate the use of up-to-date statistical techniques for analysis of dose-response curves with this software. The methods described can be utilized to evaluate chemical and non-chemical weed control options. Benefits to the practitioners and academics are also presented.

Nitrogen application influences the critical period for weed control in corn

Abstract The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. Field studies were conducted in 1999 and 2000 in eastern Nebraska to evaluate the influence of nitrogen application on the CPWC in dryland corn in competition with a naturally occurring weed population. Nitrogen fertilizer was applied at rates equivalent to 0, 60, and 120 kg N ha−1. A quantitative series of treatments of both increasing duration of weed interference and length of weed-free period were imposed within each nitrogen main plot. The beginning and end of the CPWC based on an arbitrarily 5% acceptable yield loss level were determined by fitting the logistic and Gompertz equations to relative yield data representing increasing duration of weed interference and weed-free period, respectively. Despite an inconsistent response of corn grain yield to applied nitrogen, there was a noticeable influence on the CPWC. The addition of 120 kg N ha−1 delayed the beginning of the CPWC for all site–years when compared with the 0-kg N ha−1 rate and for three of the four site–years when compared with the 60-kg N ha−1 rate. The addition of 120 kg N ha−1 also hastened the end of the CPWC at three of the four site–years when compared with both reduced rates. The yield component most sensitive to both nitrogen and interference from weeds was seed number per ear. Practical implications of this study are that reductions in nitrogen use may create the need for more intensive weed management. Nomenclature: Glyphosate; corn, Zea mays L. ‘Dekalb DK589RR’.

Row Spacing Influences the Critical Timing for Weed Removal in Soybean (Glycine max)1

Row spacing affects the time of canopy closure, thus influencing the growth and development of both crop and weeds. Field studies were conducted in 1999, 2000, and 2001 at Mead, NE, and 2000 and 2001 at Concord in eastern Nebraska to determine the effects of three row spacings (19, 38, and 76 cm) on the critical time for weed removal (CTWR) in dryland soybean. A three-parameter logistic equation was fit to data relating relative crop yield to increasing duration of weed presence. In general, earliest CTWR occurred in the 76-cm rows, and coincided with the first trifoliate stage of soybean. Latest CTWR occurred in the 19-cm rows and coincided with the third trifoliate. The CTWR in 38-cm rows occurred at the second trifoliate. Practical implications are that planting soybean in wide rows reduces early-season crop tolerance to weeds requiring earlier weed management programs than in narrower rows. Nomenclature: Glyphosate-resistant soybean, Glycine max (L.) Merr. Additional index words: Integrated weed management, row spacing, timing of removal, weed interference. Abbreviations: CPWC, critical period of weed control; CTWR, critical time for weed removal; DAE, days after emergence; DM, dry matter; GDD, growing degree days; HTCs, herbicide tolerant crops; IWM, integrated weed management; POST, postemergence.

Soybean row spacing and weed emergence time influence weed competitiveness and competitive indices

Abstract Weed competitiveness can be quantified with the concept of competitive index (CI), a relative scale of weed competitiveness. Field studies were conducted in 2002 and 2003 in northeastern and southeastern Nebraska to evaluate the influence of soybean row spacing and relative weed emergence time on the competitiveness of major weed species in soybean. Ten weed species were seeded in soybean spaced 19 and 76 cm apart at the planting, emergence, and first trifoliate leaf stages of soybean. Total weed dry matter (TDM), weed plant volume, and percent soybean yield loss were arbitrarily selected as a base for determining the CI for each weed species. Soybean yield loss was the least variable parameter used to quantify weed competitiveness and rank their CIs. In general, weeds grown with soybean planted in 19-cm rows produced less TDM, plant volume, and reduced soybean yield less than weed species grown in 76-cm rows. Later-emerging weeds produced less TDM, plant volume, and reduced soybean yield less than the early-emerging ones. In general, broadleaf species were more competitive than grass weed species. Common sunflower was the most competitive weed species in this study. Nomenclature: Common sunflower, Helianthus annuus L. HELAN; soybean, Glycine max (L.) Merr. ‘Agripro 2502’, ‘Agripro 2703’.

Influence of nitrogen and duration of weed interference on corn growth and development

Abstract An improved understanding of the effects of nitrogen (N) on crop–weed interactions is needed for the development of integrated weed management systems where responsible use of N fertilizers is considered. Field experiments conducted in 1999 and 2000 at two locations in eastern Nebraska quantify the effects of N and increasing duration of weed interference on corn growth and development. A naturally occurring population of weeds was allowed to compete with the corn crop for increasing lengths of time and at three rates of N application (0, 60, and 120 kg N ha−1). Weed interference and withholding applied N increased the time to 50% silking by an average of 3.9 and 2.9 d, respectively. Regardless of treatments, relative growth rates of corn leaf area and biomass were maximized between the V1 and V2 growth stages of corn and increased linearly with N rate but were affected to a lesser extent by weed presence. The improvement in early season corn growth with addition of N resulted in greater leaf area, biomass, and height, which improved the competitive ability of corn against weeds. Reductions in maximum corn leaf area and height due to weed interference usually began earlier and were more extensive at reduced rates of N. Partitioning of biomass to reproductive structures increased with N during reproductive stages, likely contributing to greater harvest indices at the end of the season. Results from this study indicate that the effects of N fertilization on early-season crop growth provided a competitive advantage for corn relative to weeds, thereby increasing the length of time that weeds could compete with a crop before removal was required, but further research is needed to identify mechanisms regarding improved crop tolerance to weeds. Nomenclature: Corn, Zea mays L. ‘DK589RR’.

Influence of tillage type on vertical weed seedbank distribution in a sandy soil

The vertical distribution of weed seeds in the seedbank of a sandy soil under three tillage systems (moldboard plow, chisel plow, and no-till) was estimated by a seedling-emergence method. The vertical distribution of the weed seedbank differed with tillage type and depth of tillage. The no-till system had the largest portion (90%) of the seedbank in the 0- to 5-cm layer. Chisel plowing distributed most of the seeds (66%) in the 5- to 10-cm layer. Moldboard plowing concentrated 71% of the seeds at the 10- to 15-cm depth. Our results suggest that the vertical distribution of the weed seedbank will be influenced by tillage type, depth of tillage, and soil type. Key words: Soil structure, moldboard plow, chisel plow, no-till

Dose–Response Curves of Kih-485 for Preemergence Weed Control in Corn

Abstract Field experiments were conducted in Nebraska with the experimental herbicide KIH-485 on soils with three different levels of organic matter (OM) to ascertain a dose response for weed control and corn tolerance. Dose–response curves based on the log-logistic model were used to determine the effective dose that provides 90% weed control (ED90 values) for three grasses (green foxtail, field sandbur, large crabgrass) and two broadleaf weeds (velvetleaf, tall waterhemp). The ED90 values for green foxtail control were 143, 165, and 202 g ai/ha for soils with 1, 2, and 3% OM, respectively at 28 d after treatment (DAT). The highest dose of 371 g ai/ha was needed for field sandbur control at 28 DAT, compared with 141 g ai/ha for large crabgrass, 152 g ai/ha for tall waterhemp, and 199 g ai/ha for velvetleaf. There was no significant corn injury observed. Grain yield increased with increasing doses of KIH-485; optimum yield was achieved at about 195 g ai/ha. From the dose–response curves it is clear that the proposed label rate of KIH-485 of 200 to 300 g ai/ha will provide excellent control of most grasses and certain broadleaf weeds in corn for at least the first 4 wk of the growing season on soils up to 3% OM in the state of Nebraska. Nomenclature: KIH-485 (proposed common name pyrasulfatole), 3-[(5-difluoromethoxy-1-methyl-3-trifluoromethylpyrazol-4-yl)-methylsulfonyl]-4,5-dihydro-5,5-dimethylisoxazole; green foxtail, Setaria viridis (L.) Beauv.; field sandbur, Cenchrus spinifex Cav.; large crabgrass, Digitaria sanguinalis (L.) Scop.; tall waterhemp, Amaranthus tuberculatus (Moq.); velvetleaf, Abutilon theophrasti Medicus; corn, Zea mays L

Confirmation and Control of Triazine and 4-Hydroxyphenylpyruvate Dioxygenase-Inhibiting Herbicide-Resistant Palmer Amaranth (Amaranthus palmeri) in Nebraska

Abstract Palmer amaranth is a difficult-to-control broadleaf weed that infests corn and soybean fields in south-central and southwestern Nebraska and several other states in the United States. The objectives of this research were to confirm triazine and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-resistant Palmer amaranth in Nebraska and to determine sensitivity and efficacy of POST-applied corn herbicides for control of resistant and susceptible Palmer amaranth biotypes. Seeds from a putative HPPD-resistant Palmer amaranth biotype from Fillmore County, NE were collected from a seed corn production field in fall 2010. The response of Palmer amaranth biotypes to 12 rates (0 to 12×) of mesotrione, tembotrione, topramezone, and atrazine was evaluated in a dose–response bioassay in a greenhouse. On the basis of the values at the 90% effective dose (ED90) level, the analysis showed a 4- to 23-fold resistance depending upon the type of HPPD-inhibiting herbicide being investigated and susceptible biotype used for comparison. This biotype also had a 9- to 14-fold level of resistance to atrazine applied POST. Results of a POST-applied herbicide efficacy study suggested a synergistic interaction between atrazine and HPPD-inhibiting herbicides that resulted in > 90% control of all Palmer amaranth biotypes. The resistant biotype had a reduced sensitivity to acetolactate synthase inhibiting herbicides (halosulfuron and primisulfuron), a photosystem-II inhibitor (bromoxynil), and a protoporphyrinogen oxidase inhibitor (fluthiacet-methyl). Palmer amaranth biotypes were effectively controlled (≥ 90%) with glyphosate, glufosinate, and dicamba, whereas 2,4-D ester provided 81 to 83% control of the resistant biotype and > 90% control of both susceptible biotypes. Nomenclature: 2,4-D; atrazine; bromoxynil; dicamba, fluthiacet-methyl; glufosinate; glyphosate; halosulfuron-methyl; lactofen; mesotrione; primisulfuron-methyl; pyrasulfotole; tembotrione; thiencarbazone-methyl; topramezone; Palmer amaranth, Amaranthus palmeri S. Wats; corn, Zea mays L; soybean, Glycine max (L.) Merr. Resumen Amaranthus palmeri es una maleza de hoja ancha difícil de controlar que infesta campos de maíz y soya en el centro y oeste del sur de Nebraska y en varios otros estados en los Estados Unidos. Los objetivos de esta investigación fueron confirmar la existencia de A. palmeri resistente a triazine y herbicidas inhibidores de 4-hydroxyphenylpyruvate dioxygenase (HPPD) en Nebraska y determinar la sensibilidad y la eficacia de herbicidas para maíz aplicados POST para el control de biotipos de A. palmeri susceptibles y resistentes. Semillas de A. palmeri con resistencia putativa a HPPD provenientes del condado Fillmore, NE fueron colectadas de un campo de producción de maíz en el otoño de 2010. La respuesta de los biotipos de A. palmeri a 12 dosis (0 a 12×) de mesotrione, tembotrione, topramezone, y atrazine fue evaluada en un bioensayo de respuesta a dosis en un invernadero. Con base en los valores del nivel de dosis efectiva de 90%, los análisis mostraron una resistencia de 4 a 23 veces mayor dependiendo del tipo de herbicida inhibidor de HPPD investigado y del biotipo susceptible usado como comparación. Este biotipo también tuvo un nivel de resistencia a atrazine POST de 9 a 14 veces mayor. Los resultados del estudio de eficacia de herbicidas aplicados POST sugirieron una interacción sinérgica entre atrazine y herbicidas inhibidores de HPPD que resultó en >90% de control de todos los biotipos de A. palmeri. El biotipo resistente tuvo una sensibilidad reducida a herbicidas inhibidores de acetolactate synthase (halosulfuron y primisulfuron), a un inhibidor del fotosistema II (bromoxynil) y a un inhibidor de protoporphyrinogen oxidase (fluthiacet-methyl). Los biotipos de A. palmeri fueron controlados efectivamente (≥90%) con glyphosate, glufosinate, y dicamba, mientras que 2,4-D ester brindó un control de 81 a 83% del biotipo resistente y >90% de los dos biotipos susceptibles.

Weed control in glufosinate-resistant corn (Zea mays).

Abstract: The development of glufosinate-resistant corn represents a new weed management system for corn growers. Field experiments were conducted from 1995 to 1997 at four locations in southwestern Ontario. The objective of this study was to determine the effect of timing of weed control relative to the growth stage of corn with glufosinate applied alone or in combination with residual herbicides. Control of all species tested improved with the addition of atrazine plus dicamba to glufosinate, applied from the two- to eight-leaf stage of corn growth. Based on a 90% weed dry matter reduction, glufosinate with atrazine plus dicamba controlled common ragweed, common lambsquarters, and pigweed species at the three-leaf stage of corn and yellow foxtail, barnyardgrass, and large crabgrass at the two-, four-, and eight-leaf stage of corn, respectively. Weed control with glufosinate alone was improved when applied at the later growth stages of corn. Glufosinate applied alone at the four-leaf stage of corn controlled common ragweed and common lambsquarters, whereas pigweed species were controlled effectively at the eight-leaf stage of corn growth. Corn grain yield was consistently higher when glufosinate was applied in combination with residual herbicides, compared to glufosinate alone. Glufosinate in combination with residual herbicides applied to corn at the three- to five-leaf stage may represent the best timing for weed control. Our data suggested that a tank mixture of glufosinate with other postemergence residual herbicides or a split application of glufosinate in combination with cultivation may be required for weed control in glufosinate-resistant corn. Nomenclature: Atrazine; dicamba; SAN 582 (proposed name, dimethenamid), 2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide; glufosinate; metolachlor; redroot pigweed, Amaranthus retroflexus L. #3 AMARE; common lambsquarters, Chenopodium album L. # CHEAL; yellow foxtail, Setaria glauca (L.) Beauv. # SETGL; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; barnyardgrass, Echinochloa crus-galli (L.) Beauv. # ECHCR; common ragweed, Ambrosia artemisiifolia L. # AMBEL; corn, Zea mays L. Additional index words: Integrated weed management. Abbreviations: DAE, days after emergence; DM, dry matter; HRC, herbicide-resistant crops; IWM, integrated weed management; POST, postemergence; PRE, preemergence.