Paul T. Marquardt

Competition of Transgenic Volunteer Corn with Soybean and the Effect on Western Corn Rootworm Emergence

Abstract Glyphosate-resistant (GR) volunteer corn has emerged as a problematic weed in corn∶soybean rotational systems, partly because of the rapid increase in adoption of corn hybrids that contain traits for both glyphosate and insect resistance. Volunteer GR corn can decrease soybean yields. The objectives of this study were to quantify the impact of volunteer corn on soybean growth and yield and determine how volunteer corn densities affect western corn rootworm (WCR) emergence. Volunteer corn seed was hand-planted at targeted densities of 0.5, 2, 4, 8, 12, and 16 seeds m−2 at soybean planting and 21 d after planting to evaluate both early- and late-emerging cohorts. WCR emergence was assessed with the use of field emergence traps placed over individual corn plants in the 0.5- and 16-plants-m−2 plots in 2008 and 2009. In 2010, WCR emergence traps were also placed over individual and clumped volunteer corn plants at densities of two and eight plants m−2. Soybean yield reductions ranged from 10 to 41% where early-emerging volunteer corn densities ranged from 0.5 to 16 plants m−2. No soybean yield loss occurred with the late-emerging cohort of volunteer corn. Twice as many adult WCRs emerged from a single volunteer corn plant growing at densities of 8 and 16 plants m−2, compared with plots containing 0.5 and 2 plants m−2. These results demonstrate that controlling volunteer corn will not only prevent soybean yield loss, but also may reduce the risk of WCR larval survival after exposure to Bt (Bacillus thuringiensis Berliner derived) corn. Nomenclature: Glyphosate; western corn rootworm, Diabrotica virgifera virgifera LeConte;corn, Zea mays L.; soybean, Glycine max L. Merr.

Dispersal and Mating Behavior of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) in Bt Cornfields

ABSTRACT Understanding of the mating and dispersal behavior of the western corn rootworm, Diabrotica virgifera virgifera (LeConte), is essential to predicting potential resistance to Bt corn technologies recently deployed to combat this pest. To quantify movement of male beetles, field studies were conducted during 2006 and 2007 in commercial Bt cornfields that included the Environmental Protection Agency-mandated 20% refuge acreage. Wing traps containing a single virgin female beetle were placed along transects throughout these fields. Male beetles were collected from the sticky bottoms of traps to compare the mean number of beetles captured at the different transect distances. Gut contents of each captured male were assayed with protein test strips to determine if the Bt protein (Cry3Bb1) was present. This provided an estimate of the distances traveled by males to reach virgin females. These data indicated that the mean number of males captured ≈200 m from refuge corn was not significantly different than the mean number of males captured close to the refuge (<22 m). Complementary field observations were conducted to determine how far newly emerged female beetles moved before mating and the time of day mating occurs. Results indicated that females do not move far from the site of emergence before mating and do not mate within the first 4 h of adult life. The implications of these data for movement and mating patterns of adult rootworms in Bt/refuge environments are discussed.

Competitive Effects of Volunteer Corn on Hybrid Corn Growth and Yield

Abstract Transgenic volunteer corn is a competitive weed in soybean that decreases soybean yield at densities as low as 0.5 plants m−2, yet the competitive effects of volunteer corn in corn have yet to be quantified in the peer-reviewed literature. In order to quantify competition between volunteer corn and hybrid corn, seed was harvested from transgenic hybrid corn. The seed was then hand-planted at two locations (Lafayette, IN and Wanatah, IN) into 3 by 9 m plots of hybrid corn at five densities: 0 (control), 0.5, 2, 4, and 8 plants m−2. Volunteer corn competition reduced leaf area and biomass of hybrid corn plants. Hybrid corn grain yield at Lafayette, IN, was reduced by 23 and 22% due to competition with volunteer corn growing in densities of 8 plants m−2 in 2010 and 2011, respectively, but when volunteer corn grain yield was combined with the hybrid corn grain yield, there was no reduction in total grain yield. This study demonstrates that the competitive effects on the grain yield of the hybrid corn will be offset by the grain yield of the volunteer plants. However, because the unpredictable locations and densities of volunteer corn plants present challenges to machine harvesting, future studies should examine what proportion of the volunteer crop is actually harvestable. Nomenclature: Glyphosate, corn, Zea mays L., Bt, Bacillus thuringiensis Berliner.

Volunteer Corn in Northern Indiana Soybean Correlates to Glyphosate-Resistant Corn Adoption

Because glyphosate-resistant corn is rapidly increasing in cropping systems that already rely heavily on glyphosate for postemergence weed control, the authors tested the hypothesis that volunteer corn in soybean may increase in conjunction with increasing frequency of glyphosate-resistant corn.

Influence of Clethodim Application Timing on Control of Volunteer Corn in Soybean

Abstract Herbicide options for management of volunteer corn in soybean include a variety of acetyl CoA carboxylase-inhibiting herbicides, yet often, applications of acetyl CoA carboxylase herbicides are delayed until the weed is visible above the soybean canopy. Volunteer corn growing above the soybean canopy is a highly competitive weed, and herbicides applied at this point can kill the weed, yet soybean yield loss is still a concern. Our objective was to compare the effect of controlling various densities of volunteer corn growing in soybean EARLY (≤ 30 cm) versus LATE (≈ 90 cm) on percent control and soybean yield. Seven volunteer corn densities (0, 0.5, 2, 4, 8, 12, and 16 plants m−2) were hand planted into 19-cm row soybean. Clethodim 79 g ai ha−1 was tank-mixed with glyphosate at 840 g ae ha−1 and applied to the volunteer corn EARLY and LATE. The EARLY application provided higher and less variable control of volunteer corn 14 d after treatment (DAT) compared to LATE applications at all volunteer corn densities. There was no difference in control at 28 DAT for both the EARLY and LATE applications. Soybean yield was not affected by either application timing. Although no yield reduction was seen with the LATE treatments, later-season applications of clethodim to control volunteer corn may offer more variable control and could allow for additional Bt selection pressure on targeted insect pests. Nomenclature: Glyphosate; clethodim; corn; Zea mays L.; soybean; Glycine max (L.) Merr.; Bt; Bacillus thuringiensis Berliner. Resumen Las opciones de herbicidas para el manejo de maíz voluntario en soya incluyen una variedad de herbicidas inhibidores de acetyl CoA carboxilase, aunque a menudo, las aplicaciones de este tipo de herbicidas es retrasada hasta que las malezas son visibles por encima del dosel de la soya. El maíz voluntario que llega a crecer por encima del dosel de la soya es una maleza altamente competitiva, y los herbicidas que se aplican en este punto pueden matar a la maleza, pero las pérdidas de rendimiento de la soya continúan siendo una preocupación. Nuestro objetivo fue comparar el efecto de controlar varias densidades de maíz voluntario creciendo dentro de la soya, temprano (≤30 cm) versus tarde (≈90 cm), sobre el porcentaje de control y el rendimiento de la soya. Siete densidades de maíz voluntario (0, 0.5, 2, 4, 8, 12, y 19 plantas m−2) fueron plantadas en soya sembrada en hileras espaciadas a 19 cm. Se aplicó una mezcla en tanque de clethodim a 79 g ai ha−1 con glyphosate a 840 g ae ha−1 a maíz voluntario temprano y tardío. La aplicación temprana brindó mayor control y control menos variable del maíz voluntario 14 d después del tratamiento (DAT) al compararse con las aplicaciones tardías en todas las densidades de maíz voluntario. No hubo diferencias en control a 28 DAT en ninguna de las aplicaciones temprana y tardía. El rendimiento de la soya no fue afectado por ninguno de los momentos de aplicación. Aunque no se observaron reducciones en el rendimiento de la soya producto de las aplicaciones tardías, aplicaciones tardías con clethodim al maíz voluntario durante la temporada de crecimiento podrían favorecer un control más variable y podrían permitir mayor presión de selección de resistencia a Bt en insectos plaga.

Effect of Plant Nitrogen Concentration on the Response of Glyphosate-Resistant Corn Hybrids and Their Progeny to Clethodim and Glufosinate

Abstract Rapid adoption of glyphosate-resistant (GR) corn hybrids has led to the reemergence of volunteer corn as a problematic weed in soybean and has made controlling the initial stand of corn in a replant situation more difficult. If volunteer corn in soybean or the initial corn stand in a replant situation is not controlled, yield loss can occur. Clethodim and glufosinate are often used to control GR corn in corn replant situations and in soybean. The objectives of this research were to evaluate the response of two hybrid corn varieties and their F2 progeny to clethodim and glufosinate and to evaluate the effect of plant nitrogen (N) concentration on clethodim and glufosinate efficacy. First, a dose-response study was conducted with clethodim and glufosinate on DeKalb 60-18 and 60-18F2, and DeKalb 63-42 and 63-42F2 to compare the response of the hybrids and their F2 progeny to the herbicides. DeKalb 63-42 was more tolerant to clethodim than 60-18 and 60-18F2. No differences were found between the hybrids and their respective F2 progeny in the response to clethodim or glufosinate. In a second dose-response study assessing the effect of N conditions on herbicide efficacy, both clethodim and glufosinate were less injurious to plants growing in low N than in high N availability. Nomenclature: Clethodim; glufosinate; corn Zea mays L.; soybean Glycine max (L.) Merr.

The effect of nitrogen rate on transgenic corn Cry3Bb1 protein expression.

BACKGROUND Combining herbicide-resistant and Bacillus thuringiensis (Bt) traits in corn (Zea mays L.) hybrids may affect insect resistance management owing to volunteer corn. Some Bt toxins may be expressed at lower levels by nitrogen-deficient corn roots. Corn plants with sublethal levels of Bt expression could accelerate the evolution of Bt resistance in target insects. The present objective was to quantify the concentration of Bt (Cry3Bb1) in corn root tissue with varying tissue nitrogen concentrations. RESULTS Expression of Cry3Bb1 toxin in root tissue was highly variable, but there were no differences in the overall concentration of Cry3Bb1 expressed between roots taken from Cry3Bb1-positive volunteer and hybrid corn plants. The nitrogen rate did affect Cry3Bb1 expression in the greenhouse, less nitrogen resulted in decreased Cry3Bb1 expression, yet this result was not documented in the field. CONCLUSION A positive linear relationship of plant nitrogen status on Cry3Bb1 toxin expression was documented. Also, high variability in Cry3Bb1 expression is potentially problematic from an insect resistance management perspective. This variability could create a mosaic of toxin doses in the field, which does not fit into the high-dose refuge strategy and could alter predictions about the speed of evolution of resistance to Cry3Bb1 in western corn rootworm Diabrotica virgifera virgifera LeConte.

The Influence of Nitrogen Application Timing and Rate on Volunteer Corn Interference in Hybrid Corn

Abstract Volunteer corn (VC) in hybrid corn has become more prevalent in recent years and can reduce grain yield. Nitrogen (N) management can influence VC interference in corn. Field experiments were established to determine the effects of N fertilizer management and VC interference on hybrid corn growth and grain yield. Treatments consisted of three VC densities (control, 0 plants m−2; low density, 1 plant m−2; high density, 4 plants m−2) and six N fertilizer treatments (0 kg N ha−1, 67 kg N ha−1 at planting, 67 kg N ha−1 at planting + 133 kg N ha−1 at V5 corn growth stage, 67 kg N ha−1 at planting + 133 kg N ha−1 at V10 corn growth stage, 200 kg N ha−1 at V5 corn growth stage, and 200 kg N ha−1 at V10 corn growth stage). The effect of VC on hybrid corn was dependent on N rate. When 200 kg N ha−1 was applied, regardless of application timing, hybrid corn dry weight, hybrid corn N content, and hybrid corn grain yield were reduced by the high VC density. However, when VC grain yield was added to hybrid corn grain yield, VC density did not affect total grain yield. When 0 and 67 kg N ha−1 were applied, neither hybrid corn dry weight nor hybrid corn N content was affected by either VC density, but the high VC density reduced hybrid corn grain yield for both N rates by 19% and total grain yield by 9 and 10%, respectively. Application timing of N fertilizer had no effect on hybrid corn dry weight, N content, or grain yield. However, late N fertilizer applications (200 kg N ha−1 at V10 and 67 kg N ha−1 at planting +133 kg N ha−1 at V10) resulted in greater VC N content, VC grain yield, and total yield. Assuming the harvestability of VC, the ability of a late N treatment (V10) to maximize total grain yield allows growers to use a late N application to reduce the competitive effects of VC in hybrid corn. Nomenclature: Corn, Zea mays L.