© 2010 Plant Management Network. Accepted for publication 13 August 2010. Published 20 September 2010.
Weed Control in Dicamba-Resistant Soybeans Bill Johnson, Professor, Purdue University, West Lafayette, IN 47907; Bryan Young, Professor, and Joe Matthews, Researcher, Southern Illinois University, Carbondale, IL 62901; Paul Marquardt, Research Associate, Purdue University, West Lafayette, IN 47907; Charlie Slack, Research Specialist, University of Kentucky, Lexington, KY 40546; Kevin Bradley, Associate Professor, University of Missouri, Columbia, MO 65211; Alan York, William Neal Reynolds Professor Emeritus, North Carolina State University, Raleigh, NC 27695; Stanley Culpepper, Associate Professor, University of Georgia, Tifton, GA 31797; Aaron Hager, Associate Professor, University of Illinois, Urbana, IL 61801; Kassim Al-Khatib, Professor, Kansas State University, Manhattan, KS 66506; Larry Steckel, Associate Professor, University of Tennessee, Jackson, TN 38301; Mike Moechnig, Assistant Professor, South Dakota State University, Brookings, SD 57007; Mark Loux, Professor, Ohio State University, Columbus, OH 43210; Mark Bernards, Assistant Professor, University of Nebraska, Lincoln, NE 68583; and Reid Smeda, Associate Professor, University of Missouri, Columbia, MO 65211 Corresponding author: Bill Johnson.
[email protected] Johnson, B., Young, B., Matthews, J., Marquardt, P., Slack, C., Bradley, K., York, A., Culpepper, S., Hager, A., Al-Khatib, K., Steckel, L., Moechnig, M., Loux, M., Bernards, M., and Smeda, R. 2010. Weed control in dicamba-resistant soybeans. Online. Crop Management doi:10.1094/CM-2010-0920-01-RS.
Abstract Field experiments were conducted in 11 states to evaluate broadleaf weed management programs in dicamba-resistant soybeans which involved the use of preemergence and postemergence dicamba. Preemergence (PRE) dicamba at 0.25 lb ae/acre provided less than 60% control of smooth pigweed, giant ragweed, velvetleaf, palmer amaranth, waterhemp, and morningglory spp., but 97% control of common lambsquarters and horseweed at 3 weeks after treatment (WAT). Preemergence flumioxazin plus chlorimuron or sulfentrazone plus cloransulam provided 66 to 100% control of these weeds. Use of dicamba postemergence (POST) improved uniformity of control of velvetleaf, smooth pigweed, morningglory, and glyphosate-susceptible waterhemp. However, combining dicamba at 0.25 lb/acre with glyphosate resulted in 30% to 65% greater control of glyphosate-resistant palmer amaranth, glyphosate-resistant common waterhemp, glyphosate-resistant horseweed, and glyphosate-resistant giant ragweed compared to sequentially applied glyphosate.
Introduction Glyphosate-resistant soybean was commercialized in 1996 and as of 2007, 91% of soybean hectares in the United States were genetically engineered, herbicide-resistant varieties (9). Soybean producers have changed management practices during this time, relying more on conservation and no-tillage practices and use of glyphosate for weed control (10). Use of glyphosate in soybean production from 1995, the year prior to the introduction of glyphosate-resistant soybean when it was used as a burndown herbicide before planting, to 2006, the 10th year of use as a preplant or postemergence herbicide, in soybean increased by ten-fold in the United States (8) at the exclusion of other herbicide modes of action (10). As a result, several agronomically important broadleaf weeds have evolved resistance to glyphosate in the United States including: giant ragweed (Ambrosia trifida), common ragweed (Ambrosia artemisiifolia), waterhemp (Amaranthus rudis), palmer amaranth (Amaranthus palmeri), horseweed (Conyza canadensis) (4). Other species that are difficult to control with glyphosate include morningglory species (Ipomoea spp.), common
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lambsquarters (Chenopodium album), and dandelion (Taraxacum officinale). The aforementioned weed species have been identified by growers in several recent surveys as being the most difficult to manage in current soybean production systems (3,5,6). Although, glyphosate-resistant corn was introduced in 1997, many other herbicides in addition to glyphosate are used for postemergence weed control. Among those are plant growth regulators such as 2,4-D and dicamba. Dicamba has been used for broadleaf weed control in corn for several decades. Dicamba provides effective control of most of the common dicot weeds found in corn production (7) and to date, there are no weeds commonly found in corn production that have evolved resistance to dicamba (4). Dicamba-resistant soybean is currently being developed to assist farmers in controlling glyphosateresistant and hard-to-control broadleaf weeds. The dicamba tolerance trait (2) will be stacked with glyphosate resistance and will provide the option of using dicamba preemergence or postemergence in soybean for weed control. There is little research published on dicamba use as a preplant burndown herbicide applied within 14 days of soybean planting, as a soil residual herbicide in soybean, or as a postemergence tankmix partner with glyphosate for control of problematic weed species often faced by farmers in the United States. The objective of this research was to evaluate control of several problematic annual broadleaf weeds commonly found in soybean production in the United States with dicamba and dicamba + glyphosate weed control programs. Treatment programs consisted of preemergence or preplant application timings alone and followed by single or sequential postemergence applications. Evaluating Weed Management Programs in Dicamba-Resistant Soybeans Field experiments were conducted in Georgia, Kentucky, and Missouri in 2007 and in Georgia, Illinois, Indiana, Kansas, Kentucky, Missouri, Nebraska, North Carolina, Ohio, South Dakota, and Tennessee 2008 and 2009. Locations, soil types, and predominate broadleaf weeds at each site are shown in Table 1. Standard field research techniques were used to establish the experiments and apply preplant/preemergence and postemergence treatments. Treatments were applied with backpack sprayers at carrier volumes ranging from 15 to 20 gal/acre (GPA). Preemergence/preplant treatments were applied within 4 days before or after planting at 20 out of 23 site-years. Early postemergence (EPOST), postemergence, and late postemergence (LPOST) treatments were applied on weeds 3 to 5, 3 to 8, and 8 to 16 inches in height, respectively. Two separate research trials were conducted. The first trial, hereafter referred to as the “non-glyphosate trial” was conducted in Indiana and Ohio in 2008 and 2009 (4 site years). Treatments evaluated in this protocol are listed in Table 2. The second trial, hereafter referred to as the “glyphosate trial,” was conducted in all other states in 2007, 2008, and 2009 (19 site years) and treatments are shown in Table 3. The main difference between the treatments in the two trials is the fact that no glyphosate was applied postemergence with treatments 2 through 10 in the non-glyphosate trial. The preemergence residual herbicide used in the glyphosate trial was sulfentrazone plus cloransulam and the one used in the non-glyphosate trial was flumioxazin plus chlorimuron.
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Table 1. Year, location, soil characteristics, tillage system, planting dates, and herbicide application dates for dicamba-resistant soybean field trials. Herbicide application dates
Year Location
Soil class
Preplant or prePlanting emergence Tillage date (PRE)
Early postemergence (EPOST) 3-5 inch weeds
Mid postemergence (MPOST) 3-8 inch weeds
Late postemergence (LPOST) 8-16 inch weeds
Weeds
2007 Fayette Co., KY
Silty clay loam
Conventional
Jun 18
Jun 18
Jul 9
Jul 13
Aug 3
Velvetleaf, smooth pigweed, giant ragweed, morningglory species.
2007 Platte Co., MO
Silty clay loam
Min
Jun 6
Jun 8
Jul 5
Jul 6
Jul 26
Glyphosateresistant common waterhemp
None
Jun 22
Jun 22
Jul 13
Jul 16
Aug 1
Glyphosateresistant Palmer amaranth
Conventional
Jun 2
Jun 2
Jun 19
Jun 27
Jul 25
Velvetleaf, smooth pigweed
Conventional
May 9
May 13
May 30
Jun 5
Jun 25
Glyphosateresistant Palmer amaranth
Min
Jun 18
Jun 18
Jul 4
Jul 11
Jul 25
Velvetleaf, common waterhemp, morningglory species
None
Jun 18
Jun 20
Jul 16
Jul 23
Aug 1
Glyphosateresistant common waterhemp
2007 Macon Co., Sandy GA loam 2008 Fayette Co., KY
Silty clay loam
2008 Wayne Co., Loamy NC sand 2008 St. Clair Co., IL
Silty loam
2008 Brown Co., Clay IL loam
2008 Riley Co., KS
Silty loam
Conventional
May 15
May 15
Jun 9
Jun 17
Jul 4
Velvetleaf, Palmer amaranth, morning-glory, ivyleaf morningglory
2008 Callaway Co., MO
Silty clay loam
Conventional
May 29
May 5
Jun 24
Jun 26
Jul 15
Glyphosateresistant common waterhemp
None
Jun 2
Jun 3
Jun 23
Jun 30
Jul 14
Smooth pigweed, pitted morning-glory, glyphosateresistant horseweed
2008 Gibson Co., Silty TN clay loam
2008 Brookings Co., SD
Clay loam
Conventional
May 28
May 28
Jun 20
Jul 15
Aug 8
Wild buckwheat
2008 Clark Co., OH
Silty clay loam
Min
May 25
May 25
Jun 17
Jun24
Jul17
Glyphosateresistant giant ragweed, recdroot pigweed, velvetleaf
2008 Tippecanoe Co., IN
Silt loam
Conventional
May 22
May 22
Jun 17
Jul 2
Jul 2
Common lambs-quarter, giant ragweed, velvetleaf
(continued)
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Table 1 (continued). Herbicide application dates
Year Location
Soil class
Preplant or prePlanting emergence Tillage date (PRE)
Early postemergence (EPOST) 3-5 inch weeds
Mid postemergence (MPOST) 3-8 inch weeds
Late postemergence (LPOST) 8-16 inch weeds
Weeds
2009 Wayne Co., Loamy NC sand
None
May 20
May 20
Jun 3
Jun 11
Jun 22
Glyphosateresistant Palmer amaranth
2009 Jackson Co., IL
Fine, sandy loam
None
May 19
May 4
May 29
Jun 1
Jun 20
Glyphosateresistant horseweed, common waterhemp
2009 Fayette Co., KY
Silty loam
Conventional
May 20
May 20
Jun 5
Jun 22
Jul 20
Smooth pigweed
2009 Brown Co., Clay IL loam
None
May 22
May 22
Jun 22
Jun 29
Jul 14
Glyphosateresistant common waterhemp
2009 Riley Co., KS
Silty clay loam
None
Jun 8
Jun 8
Jun 26
Jul 1
Jul 22
Palmer amaranth, giant ragweed
2009 Saunders Co., NE
Loamy sand
None
May 21
May 23
Jun 13
Jun 13
Jul 8
Common waterhemp, glyphosateresistant horseweed
Conventional
May 22
May 21
Jun 23
Jul 2
Jul 10
Wild buckwheat
2009 Brookings, Clay Co., SD loam 2009 Callaway Co., MO
Silty clay loam
None
Jun 30
Jun 30
Jul 16
Jul 16
Aug 18
Glyphosateresistant common waterhemp
2009 Pickaway Co., OH
Silty clay loam
Min
May 5
May 5
Jun 2
Jun 16
Jun 30
Common lambsquarter, glyphosate resistant giant ragweed
2009 Tippecanoe Co., IN
Silt loam
Conventional
Jun 9
Jun 9
Jun 23
Jun 30
Jul 16
Lambsquarter, giant ragweed, redroot pigweed
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Table 2. Core treatments in the non-glyphosate trial which was conducted in Indiana and Ohio in 2008 and 2009 . Treatment Herbicide number
Active ingredient
Formulation
Rate
Application timing
glyphosate ammonium sulfate glyphosate ammonium sulfate
4.5 lb ae/gal 100% 4.5 lb ae/gal 100%
0.75 lb ae/acre 5 %v/v 0.75 lb ae/acre 5 %v/v
EPOST EPOST LPOST LPOST
1
Roundup PowerMax N-Pak AMS Roundup PowerMax N-Pak AMS
2
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.25 lb ae/acre 0.25 lb ae/acre
EPOST LPOST
3
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.25 lb ae/acre 0.25 lb ae/acre
POST LPOST
4
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.125 lb ae/acre 0.25 lb ae/acre
EPOST LPOST
5
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.125 lb ae/acre 0.25 lb ae/acre
POST LPOST
6
Clarity
dicamba
4 lb ae/gal
0.25 lb ae/acre
PREPLANT/ PREEMERGENCE
7
Clarity
dicamba
4 lb ae/gal
0.25 lb ae/acre
Clarity
dicamba
4 lb ae/gal
0.25 lb ae/acre
PREPLANT/ PREEMERGENCE POST
Clarity
dicamba
4 lb ae/gal
0.25 lb ae/acre
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.25 lb ae/acre 0.25 lb ae/acre
PREPLANT/ PREEMERGENCE POST LPOST
9
Clarity Clarity
dicamba dicamba
4 lb ae/gal 4 lb ae/gal
0.25 lb ae/acre 1.5 lb ae/acre
PRE LPOST
10
Valor SX Classic Clarity
flumioxazin chlorimuron-ethyl dicamba
51% 25% 4 lb ae/gal
0.056 lb ai/acre 0.019 lb ai/acre 0.25 lb ae/acre
PREPLANT/ PREEMERGENCE POST
8
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Table 3. Core treatments in the glyphosate trial which was conducted in Georgia, Illinois, Kansas, Kentucky, Missouri, Nebraska, North Carolina, South Dakota, and Tennessee. Treatment Herbicide number
Active ingredient
Formulation
Rate
Application timing
1
Roundup PowerMax N-Pak AMS Roundup PowerMax N-Pak AMS
glyphosate ammonium sulfate glyphosate ammonium sulfate
4.5 lb ae/gal 100% 4.5 lb ae/gal 100%
0.75 lb ae/acre 5 %v/v 0.75 lb ae/acre 5 %v/v
POST POST LPOST LPOST
2
Clarity Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
dicamba glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate
4 lb ae/gal 4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v
EPOST EPOST EPOST LPOST LPOST LPOST
3
Clarity Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
dicamba glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate
4 lb ae/gal 4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v
POST POST POST LPOST LPOST LPOST
4
Clarity Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
dicamba glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate
4 lb ae/gal 4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.125 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v
EPOST EPOST EPOST LPOST LPOST LPOST
5
Clarity Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
dicamba glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate
4 lb ae/gal 4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.125 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v
EPOST EPOST EPOST LPOST LPOST LPOST
6
Clarity
4 lb ae/gal
0.25 lb ae/acre
4.5 lb ae/gal 100% 4.5 lb ae/gal 100%
0.75 lb ae/acre 5 %v/v 0.75 lb ae/acre 5 %v/v
4 lb ae/gal
0.25 lb ae/acre
4 lb ae/gal 4.5 lb ae/gal 100% 4.5 lb ae/gal 100%
0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.75 lb ae/acre 5 %v/v
4 lb ae/gal
0.25 lb ae/acre
4 lb ae/gal 4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v
4 lb ae/gal
0.25 lb ae/acre
Roundup PowerMax N-Pak AMS Roundup PowerMax N-Pak AMS 7
Clarity Clarity Roundup PowerMax N-Pak AMS Roundup PowerMax N-Pak AMS
8
Clarity Clarity Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
9
10
Clarity
glyphosate ammonium sulfate glyphosate ammonium sulfate dicamba dicamba glyphosate ammonium sulfate glyphosate ammonium sulfate dicamba dicamba glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate dicamba
Roundup PowerMax N-Pak AMS Clarity Roundup PowerMax N-Pak AMS
glyphosate ammonium sulfate dicamba glyphosate ammonium sulfate
4.5 lb ae/gal 100% 4 lb ae/gal 4.5 lb ae/gal 100%
0.75 lb ae/acre 5 %v/v 1.5 lb ae/acre 1.5 lb ae/acre 5 %v/v
Authority First DF
sulfentrazone + cloransulam-methyl dicamba glyphosate ammonium sulfate glyphosate ammonium sulfate
70%
0.141 lb ai/acre
4 lb ae/gal 4.5 lb ae/gal 100% 4.5 lb ae/gal 100%
0.25 lb ae/acre 0.75 lb ae/acre 5 %v/v 0.75 lb ae/acre 5 %v/v
Clarity Roundup PowerMax N-Pak AMS Roundup PowerMax N-Pak AMS
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Visual evaluations of weed control were collected at 3 weeks after the PRE residual treatment and 3 to 5 weeks after the LPOST treatment on a 0 to 100 scale, with 0 = no control and 100 = control or death of all plants in the plot. Crop response and yield to the herbicide treatments were not collected for this research as the soybean cultivars used were not of commercial quality. Years were treated as a random variable and data were subject to analysis of variance using Proc Mixed in SAS. Data are presented as box and whisker plots and means are separated with Fisher’s Protected LSD at the 0.05 level of significance. Residual Control of Broadleaf Weeds with Soil-Applied Dicamba Soil activity of dicamba at 0.25 lbs ae/acre was variable depending on the target weed species. In the non-glyphosate trial, flumioxazin + chlorimuron resulted in 70% control of giant ragweed compared to less than 10% with dicamba (Fig. 1). However, control of common lambsquarters was 98% with dicamba, compared to 100% control with flumioxazin + chlorimuron. In the glyphosate trials, common lambsquarters control with dicamba PRE was excellent and similar to sulfentrazone + chloransulam (Fig. 3). In addition, PRE activity of dicamba resulted in greater than 90% control of horseweed across numerous states (Fig. 3). However, for a majority of the troublesome broadleaf weeds in soybean, residual activity of dicamba compared to sulfentrazone + chloransulam was unacceptable. For velvetleaf, smooth pigweed, Palmer amaranth, common waterhemp, giant ragweed, and morningglory, dicamba activity was less than 60% control (Fig. 3). Comparatively, sulfentrazone + cloransulam controlled velvetleaf 94%, smooth pigweed 90%, palmer amaranth 85%, common waterhemp 82%, giant ragweed 95%, and morningglory spp. 80%. These results indicate that soil-applied dicamba at 0.25 lb ae/acre is effective in suppressing horseweed and common lambsquarters, but is much less effective than common industry standards at suppressing other widespread, problematic weeds evaluated in this research.
Fig. 1. Box and whisker plots of percent control with preplant dicamba (0.25 lb ae/acre) or flumioxazin (0.056 lb ai/acre) + chlorimuron (0.019 lb ai/acre) at 3 WAT in the non-glyphosate trial conducted in Indiana and Ohio. Horizontal line in the box denotes the mean value, upper edge (hinge) denotes 75th percentile, lower hinge denotes 25th percentile, vertical lines extend to the highest and lowest values. Means followed by the same letters are not different at P = 0.05.
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Fig. 2. Box and whisker plots of percent control with preplant dicamba (0.25 lb ae/acre) or sulfentrazone + cloransulammethyl (0.141 lb ai/acre) at 3 WAT in the glyphosate trial conducted in Georgia, Illinois, Kansas, Kentucky, Missouri, Nebraska, North Carolina, South Dakota, and Tennessee. Horizontal line in the box denotes the mean value, upper edge (hinge) denotes 75th percentile, lower hinge denotes 25th percentile, vertical lines extend to the highest and lowest values. Means followed by same letters are not different at P = 0.05.
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Fig. 3. Box and whisker plots of percent control with various postemergence treatments at 3 to 5 weeks after the last post treatment in the non-glyphosate trial conducted in Indiana and Ohio. Abbreviations: gly = glyphosate; dic = dicamba; pre = preplant; ep = early post; p = post; lp = late post; fb = followed by. Horizontal line in the box denotes the mean value, upper edge (hinge) denotes 75th percentile, lower hinge denotes 25th percentile, vertical lines extend to the highest and lowest values. Means followed by the same letter are not different at P = 0.05.
Control of Broadleaf Weeds with Postemergence Dicamba Alone Compared to Glyphosate Alone In the non-glyphosate trial, control of velvetleaf with dicamba was rate dependent (Fig. 3). Treatments which included dicamba at 0.125 lb ae/acre resulted in up to 5% lower control than treatments which included 0.25 lb ae/acre in the initial postemergence treatment, and more velvetleaf plants survived with the lower POST rates of dicamba. Redroot pigweed and common lambsquarters control was not as rate dependent as velvetleaf, but control was timing dependent. EPOST followed by LPOST sequential treatments provided slightly greater control (5 to 10%) than POST followed by LPOST sequential treatments, irrespective of the dicamba rate. Control of glyphosate-resistant giant ragweed was only 70% with glyphosate alone compared to 92% control of the glyphosate-susceptible biotype (Fig. 3). Application of sequential treatments of dicamba POST, irregardless of rate or timing, resulted in complete control of both biotypes of giant ragweed.
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Control of Broadleaf Weeds with Glyphosate + Dicamba vs. Glyphosate Alone In the glyphosate trial, control of velvetleaf, smooth pigweed, glyphosatesusceptible Palmer amaranth and waterhemp, giant ragweed, and wild buckwheat was 95% or higher with all treatments (Fig. 4). Control of smooth pigweed was timing dependent and treatments which include EPOST application timings provided slightly less variable control than treatments which were applied POST or LPOST, and the addition of dicamba improved control over glyphosate alone. At sites with glyphosate-resistant plants, inclusion of dicamba in the POST treatment greatly improved weed control versus glyphosate alone. For Palmer amaranth, waterhemp, and horseweed, control increased from 60 to 100%, 30 to 95%, and 85 to 98%, respectively. Common waterhemp control was variable with glyphosate alone at both glyphosate-resistant and -susceptible sites. This would indicate that despite the absence of glyphosate-resistant waterhemp at many sites, dicamba improved the consistency of control. Treatments which included glyphosate applied postemergence without dicamba provided lower levels of weed control than treatments which included POST dicamba. Horseweed control was higher and less variable in treatments that included dicamba or flumioxazin, sulfentrazone, chlorimuron, or cloransulam applied preemergence than with treatments that included only postemergence glyphosate or glyphosate + dicamba. With morningglory, treatments which included glyphosate applied postemergence without dicamba resulted in 90 to 93% control. This increased to 98 to 99% control when dicamba was included POST.
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Fig. 4. Box and whisker plots of percent control with various postemergence treatments at 3 to 5 weeks after the last postemergence treatment in the glyphosate trial conducted in Georgia, Illinois, Kansas, Kentucky, Missouri, Nebraska, North Carolina, South Dakota, and Tennessee. Abbreviations: gly = glyphosate; dic = dicamba; pre = preplant; ep = early post; p = post; lp = late post; fb = followed by. Horizontal line in the box denotes the mean value, upper edge (hinge) denotes 75th percentile, lower hinge denotes 25th percentile, vertical lines extend to the highest and lowest values. Means followed by the same letter are not different at P = 0.05.
Integration of new herbicide-tolerance traits such as dicamba results in the addition of novel modes of herbicide action and improves the consistency of POST broadleaf control programs versus glyphosate alone. Dicamba can also reduce the selection pressure for glyphosate-resistant weeds, preserving the technology of glyphosate-tolerant soybeans. In this research, residual activity of dicamba appears sufficient for early season control of horseweed and common lambsquarters. POST dicamba improved the control of the glyphosatesusceptible weeds evaluated, but improved control was most notable for the glyphosate-resistant weeds horseweed, giant ragweed, Palmer amaranth, and common waterhemp. Dicamba also improved the consistency of control of morningglory.
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Literature Cited 1. Delannay, X., Bauman, T. T., Beighley, D. H., Buettner, M. J., Coble, H. D., DeFelice, M. S., Derting, C. W., Diedrick, T. J., Griffin, J. L., Hagood, E. S., Hancock, F. G., Hart, S. E., LaVallee, B. J., Loux, M. M., Lueschen, W. E., Matson, K. W., Moots, C. K., Murdock, E., Nickell, A. D., Owen, M. D. K., Paschal, E. H., II, Prochaska, L. M., Raymond, P. J., Reynolds, D. B., Rhodes, W. K., Roeth, F. W., Sprankle, P. L., Tarochione, L. J., Tinius, C. N., Walker, R. H., Wax, L. M., Weigelt, H. D., and Padgette, S. R. 1995. Yield evaluation of a glyphosate-tolerant soybean line after treatment with glyphosate. Crop Sci. 35:1461-1467. 2. D'Ordine, R. L., Rydel, T. J., Storek, M. J., Sturman, E. J., Moshiri, F., Bartlett, R. K., Brown, G. R., Eilers, R. J., Dart, C., Qi, Y., Flasinski S., and Franklin, S. J. 2009. Dicamba monooxygenase: structural insights into a dynamic rieske oxygenase that catalyzes an exocyclic monooxygenation. J. Mol. Biol. 392:481497. 3. Gibson, K. D., Johnson, W. G., and Hilger, D. 2005. Farmer perceptions of problematic corn and soybean weeds in Indiana. Weed Technol. 19:1065-1070. 4. Heap, I. M. 2010. International survey of herbicide resistant weeds. Online. WeedScience.org, Corvallis, OR. 5. Johnson, B., Barnes, J., Gibson, K., and Weller, S. 2004. Late season weed escapes in Indiana soybean fields. Online. Crop Management doi:10.1094/CM-20040923-01-BR. 6. Kruger, G. K., Johnson, W. G., Weller, S. C., Owen, M. D. K., Shaw, D. R., Wilcut, J. W., Jordan, D. L., Wilson, R. G., Bernards, M. L., and Young, B. G. 2009. U.S. grower views on problematic weeds and changes in weed pressure in glyphosateresistant corn, cotton, and soybean cropping systems. Weed Technol. 23:162-166. 7. Loux, M., Doohan, D., Dobbels, A. F., Johnson, W. G., Nice, G. R. W., Jordan, T. N., and Bauman, T. T. 2010. Weed Control Guide for Ohio and Indiana. Joint publication, Ohio State Univ. Coop. Ext. Bul. 789 and Purdue Univ. Coop. Pub. WS16. Columbus, OH, and West Lafayette, IN. 8. USDA-NASS. 2009. Agricultural chemical use database. Online. NSF Center for Integrated Pest Management, USDA Regional Pest Mgt. Centers Info. System, Nat'l. Agric. Statistics Service (NASS), USDA, Washington, DC. 9. USDA-NASS. 2009. Acreage report, June 30, 2009. Agric. Statistics Board, Nat'l. Agric. Statistics Service (NASS), USDA, Washington, DC. 10. Young, B. G. 2006. Changes in herbicide use patterns and production practices resulting from glyphosate-resistant crops. Weed Technol. 20:301-307.
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