Weed Control in Dicamba-Resistant Soybeans

© 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


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


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


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


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


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


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



POST LPOST

4

Clarity Clarity

dicamba dicamba



EPOST LPOST

5

Clarity Clarity

dicamba dicamba



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



PREPLANT/ PREEMERGENCE POST LPOST

9

Clarity Clarity

dicamba dicamba



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%


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





POST POST POST LPOST LPOST LPOST

4






5






6

Clarity

4 lb ae/gal

0.25 lb ae/acre

4.5 lb ae/gal 100% 4.5 lb ae/gal 100%


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

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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PREPLANT/ PREEMERGENCE POST POST LPOST LPOST PREPLANT/ PREEMERGENCE POST POST POST LPOST LPOST PREPLANT/ PREEMERGENCE POST POST POST LPOST LPOST LPOST PREPLANT/ PREEMERGENCE POST POST LPOST LPOST LPOST PREPLANT/ PREEMERGENCE POST POST POST LPOST LPOST

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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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