Evalutation of Weed Management in Glyphosate-Resistant Corn (Zea Mays) as Affected by Preemergence Herbicide, Timing of Postemergence Herbicide Application, and Glyphosate Products

PARKER, ROBERT GARY. Evaluation of W eed M anagement in Glyphosate-Resistant Corn (Zea mays) as Affected by Preemergence Herbicide, Timing of Postemergence Herbicide Application, and Glyphosate Products (Under the direction of Alan C. York.)

Cotton and soybean resistant to glyphosate are widely planted in the United States. Ninety-five percent of the cotton and 85% of the soybean in North Carolina is glyphosate-resistant (GR). Rapid adoption of the

technology is attributed to broad spectrum weed control, convenience of glyphosate-based weed management systems, and greater rotational crop flexibility. Corn resistant to glyphosate is less widely grown, but plantings are expected to increase.

W ide-spread planting of GR crops in the United States and glyphosate patent expiration has led to a

Broadleaf signalgrass, fall panicum, goosegrass, and large crabgrass were controlled at least 96% by all glyphosate treatments while control by nicosulfuron plus atrazine without PRE herbicide decreased as application timing was delayed. Sicklepod was controlled at least 94% when POST herbicides were applied timely, but control by both POST herbicides decreased with delayed application regardless of PRE herbicide. Entireleaf morningglory, ivyleaf morningglory, pitted morningglory, and tall morningglory were controlled 93% or greater when either POST herbicide was applied timely. Control by both POST herbicides decreased as application was delayed, with glyphosate being affected more by timing than nicosulfuron plus atrazine. Corn grain yield was similar with glyphosate and nicosulfuron plus atrazine following the PRE herbicide regardless of application timing while yield decreased as POST herbicide application was delayed in the absence of PRE herbicide.

as Affected by Preemergence Herbicide, Timing of Postemergence

Herbicide Application, and Glyphosate Products

by

ROBERT GARY PARKER

A thesis submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the Degree of Master of Science

Department of Crop Science

Raleigh 2004

Approved By:

_____________________________ ______________________________ Dr. D. L. Jordan Dr. J. R. Bradley

______________________________ Dr. Alan C. York

DEDICATIO N

BIO GRAPH Y

Robert Gary Parker (Robbie) was born in Jacksonville, North Carolina in 1973. He was raised on his family’s farm were his love for agriculture began. Robbie graduated from Swansboro High School in 1991. He attended Coastal Carolina Community College from 1992 through 1993. Robbie worked on several different farms over the next few years before continuing his education at North Carolina State University in 1999. W hile attending N.C. State University he worked for the USDA for two years. In the summer between his junior and senior years Robbie began to work with Dr. Alan York. After graduating with a B. S. in Agronomy in 2002, he accepted a graduate assistantship in W eed Science under Dr. Alan York. W hile working for Dr. York, Robbie conducted weed efficacy trials on cotton, corn, and soybean. Robbie’s graduate research focused on glyphosate-resistant corn and cotton, as well as comparing multiple glyphosate products for crop tolerance and weed efficacy. As a student, he participated in several extension field days and was a member of the NCSU W eed Team in 2002, were he his team finished first overall and he finished second place in the individual competition. In 2003 and 2004, Robbie placed first in the graduate student paper contests at both the Beltwide Cotton

ACKNOW LEDGM ENTS

I would like to begin by thanking my mother, Genelda R. Parker, for all of her love and support throughout my college career. She has always pushed me to be the best that I could be without forgetting where I come from. W ithout her behind me, I would never have been able to get through graduate school.

I would like to thank Dr. Alan C. York for giving me the opportunity to continue my education by obtaining my M asters of Science degree in Crop Science under his direction. I cannot thank him enough for this

opportunity. Dr. York is one of the nicest, most intelligent, hardest working individuals that I have ever had the privilege to work for. He has taught me a great deal about weed science and how to conduct research. I don’t think that I could have accomplished as much with any other advisor as I have under Dr. York. He has been more than just an advisor to me, he has been my friend.

I would like to also give thanks to the other members of my committee, Dr. D. L. Jordan and Dr. J. R. Bradley. I would like to thank both of them for their direction and input on my research. Their ideas about how to

conduct my tests gave me greater insight into how research should be conducted.

I would also like to thank Rick Seagroves, Jamie Hinton, Jamie Lanier, Sarah Hans, Andrew Gardner, and Josh Gaddy for all of their help with my field and greenhouse studies. If it were not for them, I would still be thinning pots in the greenhouse or spraying plots in the field. They also allowed me to vent my frustrations from time to time. I will never forget all of the laughs that we shared while backpacking and working ourselves ragged over the past three summers.

TAB LE OF CO NTENTS

LIST OF TABLES . . . vii

CHAPTER 1 LITERATURE REVIEW . . . . . . 1

LITERATURE Cited . . . .. . . 6

CHAPTER 2 COM PARISON OF GLYPHOSATE PRODUCTS IN GLYPHOSATE-RESISTANT COTTON AND CORN . . . 11

Abstract . . . . . . 11

Introduction . . . 12

M aterials and M ethods . . . 14

Results and Discussion . . . 16

Literature Cited . . . 20

CHAPTER 3 W EED CONTROL IN GLYPHOSATE-RESISTANT CORN AS AFFECTED BY PREEM ERGENCE HERBICIDE AND TIM ING OF POSTEM ERGENCE HERBICIDE APPLICAT ION . . . 31

Abstract . . . . . . 31

Introduction . . . 32

M aterials and M ethods . . . 34

Results and Discussion . . . 35

Literature Cited . . . . . . 39

CHAPTER 4 GLYPHOSATE-RESISTANT CORN HYBRID RESPONSE TO A GLYPHOSATE-ONLY AND A CONVENTIONAL HERBICIDE SYSTEM . . . . . . 55

Abstract . . . 55

Introduction . . . 56

M aterials and M ethods . . . 58

Results and Discussion . . . 59

CHAPTER 5

EVALUATION OF HERBICIDE SYSTEM S IN NO-TILL AND CONVENTIONAL TILLAGE CORN . . .

. . . 74

Abstract . . . 74

Introduction . . . 75

M aterials and M ethods . . . 77

Results and Discussion . . . 79

Literature Cited . . . 83

CHAPTER 2 APPENDICES . . . 103

CHAPTER 3 APPENDICES . . . 142

CHAPTER 4 APPENDICES . . . 161

LIST OF TAB LES

Chapter 2

Table 1. Description of soils at experiment sites . . . 24

Table 2. W eed species and densities at experiment sites . . . 25

Table 3. Glyphosate products evaluated in corn and cotton experiments . . . 27

Table 4. M ain effect of glyphosate rates on morningglory species control in corn . . . 28

Table 5. M ain effect of glyphosate rates on morningglory species control in cotton . . . 29

Table 6. Interaction of glyphosate products by application rate for cotton injury in 2003 . . . 30

Chapter 3 Table 1. Description of soils at experiment sites . . . 43

Table 2. W eed species and densities at experiment sites . . . 44

Table 3. Postemergence herbicide by postemergence herbicide application timing interaction for morningglory control in 2002 . . . 45

Table 4. Location by postemergence herbicide by postemergence herbicide application timing interaction for morningglory control 2 wk after postemergence herbicide application in 2003 . . . 46

Table 5. Control of morningglory species 2 wk after postemergence-directed herbicide application in 2003 . . . 47

Table 6. Preemergence herbicide by postemergence herbicide by postemergence herbicide application timing interaction for annual grass control 2 wk after postemergence herbicide application . . . 48

Table 7. Sicklepod control as affected by timing of postemergence herbicide application at Plymouth in 2003 . . . 49

Table 8. Reduction in crop vigor 2 wk after postemergence herbicide application in 2002 . . . 50

Table 9. Reduction in crop vigor 2 wk after postemergence herbicide application in 2003 . . . 51

Table 10. Reduction in crop vigor 2 wk after postemergence-directed herbicide application in 2002 . . . 52

Table 11. Reduction in crop vigor 2 wk after postemergence-directed herbicide application in 2002 . . . 53

Chapter 4

Table 1. Description of soils at experiment sites . . . 66

Table 2. W eed species and densities at experiment sites . . . 67

Table 3. Corn hybrids evaluated . . . . . . 68

Table 4. M ain effects of herbicide systems on morningglory control . . . 70

Table 5. M ain effect of herbicide systems on corn grain yield . . . 71

Table 6. Grain yield of glyphosate-resistant hybrids compared with two conventional hybrids . . . 72

Chapter 5 Table 1. Treatment by location interaction for morningglory species control in 2002 . . . 88

Table 2. Treatment by location interaction for annual grass control in 2002 . . . 90

Table 3. Percent sicklepod control in 2002 . . . . . . 92

Table 4. Yield and net return by treatment in 2002 . . . . . 94

Table 5. Location by herbicide treatment interaction for morningglory species control pooled over tillage systems in 2003 . . . . . . 95

Table 6. Location by herbicide treatment interaction for annual grass control pooled over tillage systems in 2003 . . . 97

Table 7. Herbicide systems main effect for weed control in 2003 . . . 99

Table 8. Location by tillage interaction for yield in 2003 . . . 100

Chapter 1 Literature Review

Corn development and grain yield are influenced by duration of weed competition, weed species, density, and

the environment in which corn grows (Hall et al. 1992; Knake and Slife 1961; Tapia et al. 1997; Vangessel et

al. 1995; Young et al. 1984). W eeds compete with corn for sunlight, water, nutrients, and space. Season-long

interference from weeds can reduce corn grain yields up to 50% (Hall et al. 1992).

Aside from cultivation, preemergence (PRE) herbicides have been used to control weeds in corn for many

years. However, soil-applied herbicides require adequate precipitation after application in order to perform

correctly. Growers in dryland cropping areas do not always receive rainfall necessary to activate a PRE

herbicide and herbicide failure is the result. Atrazine {6-chloro-N-ethyl-N

'-(1-methylethyl)-1,3,5-triazine-2,4-diamine} and metolachlor {2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide} are

widely used on the corn acreage in the USA (NASS 2003). W ide-spread use of these PRE herbicides is

indicative of their effectiveness in controlling a broad spectrum of grass and broadleaf weeds (York and

Culpepper 2003), with over 82% of the corn acreage in North Carolina being treated (NASS 2003). However,

both atrazine and metolachlor have been identified as contaminants in ground water in North Carolina as well as

other states (Cohen et al. 1986; Holden et al. 1992; W ade et al. 1998). To reduce the potential impact on

surface and groundwater, use restrictions have been put in place on products which contain atrazine.

Other herbicides have been developed to use in combination with or in place of PRE herbicides. Nicosulfuron

{2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino] carbonyl]amino] sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide}

is a postemergence (POST) herbicide in the sulfonylurea family that inhibits acetolactate synthase (ALS) (EC

4.1.3.18), the enzyme that catalyzes the first step of the biosynthetic pathway for the branched chain amino acids

leucine, isoleucine, and valine (Ferguson et al. 2001; Stidham 1991). Nicosulfuron plus atrazine or nicosulfuron

plus dicamba (3,6-dichloro-2-methoxybenzoic acid) applied POST can provide similar weed control as atrazine

plus metolachlor applied PRE (Dobbels and Kapusta 1993).

In the absence of PRE herbicides, there is concern that early season weed interference prior to POST

POST herbicide application increased weed control compared to early POST applications. Delaying POST

herbicide application allowed for a higher density of weeds to emerge before herbicide treatment. However, the

early POST herbicide application allowed for greater corn grain yields due to reduced early season weed

competition. Also, application timing is critical in order to obtain control of some weed species, and delaying

herbicide application may allow weeds to become to large for the herbicide to control. Nicosulfuron must be

applied to corn 50 cm or less in height or corn that has six or fewer collars (Anonymous 2004 ). Additionally,

nicosulfuron has varying size limits on hard-to-control weed species.

Another concern with using ALS-inhibitor herbicides is the possibility of interactions with organophosphate

pesticides, such as terbufos {S-[[(1,1-dimethylethyl)thio]methyl]O,O-diethyl phosphorodithioate} or

chlorpyrifos {O,O-diethyl O-(3,5,6-trichloro-2 pyridinyl) phosphorothioate}, which can lead to crop injury and

yield reductions if not used in conjunction with imidazilinone-resistant corn (Bailey and Kapusta 1994; M orton

et al. 1994; York and Culpepper 2003). Organophosphate pesticides can be important for the control of

nematodes (Koenning 2001; Koenning et al. 1999) and insects such as billbugs (Sphenophorus spp.), wireworms

(Conoderus spp.), and southern corn rootworm (Diabrotica undecimpunctata howardi Barber) (Van Duyn 1999; Van Duyn and Bacheler 2001).

In recent years, herbicide-resistant crop cultivars have been developed through both traditional breeding and

gene transfer. Examples of commercially available transgenic, herbicide-resistant crops include cotton

(Gossypium hirsutum L.) resistant to glyphosate {N–(phosphonomethyl)glycine} and bromoxynil

(3,5-dibromo-4-hydroxybenzonitrile) herbicides and soybean [Glycine max (L.) Merr.] resistant to glyphosate. Concerns over

the development of genetically modified crops have been expressed (Burnside 1992), and the advantages and

disadvantages have been debated in the press. However, herbicide-resistant crops provide several benefits, such

increased weed management options, economic advantages for growers, improved weed control, increased use

of more environmentally friendly herbicides, and increased conservation tillage (Burnside 1992; Culpepper and

York 1999a; W ilcut et al. 1996). Furthermore, with restrictions being imposed on herbicides, such as atrazine,

and the reduction in the number of agrochemical companies along with fewer new herbicides being developed,

Corn hybrids have been developed that are resistant to glufosinate {2-amino-4-(hydroxymethylphosphinyl)

butanoic acid}, glyphosate, and imidazolinone herbicides. Some of these hybrids were developed through

conventional plant breeding methods while others were transgenically developed. The first herbicide-resistant

corn hybrids, which were imidazolinone-resistant, were commercialized in 1992. This tolerance was not

produced through genetic modification but through screening of callus tissue arrays. Imazethapyr

{2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid} and other

imidazolinone herbicides inhibit ALS, thus depriving susceptible plants of essential amino acids and interfering

with DNA synthesis and cell growth. Resistance is due to the corn having an altered form of ALS, which can

metabolize imidazilinone herbicides more efficiently than corn without this trait (Shaner and O’Connor 1991).

A prepackeged mixture of imazethapyr plus imazapyr

{(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid} applied POST is suggested for use on imidazolinone-resistant

corn (York and Culpepper 2003). Imidazilinone-resistant corn has not been readily adopted in part because of

problems with ALS-resistant weeds (Foes et al. 1998; Franssen et al. 2001) and because other products are

available that can control the same weeds.

Glufosinate-resistant corn hybrids were commercialized in 1997. This corn was genetically modified by the

insertion of a gene found in Streptomyces viridochromogenes which encodes for phophinothricin

acetyltransferase (E.C.2.3.1.), an enzyme that catalyzes the conversion of lethal L-phosphinothricin into

non-lethal N-acetyl-L-phosphinothricin (Devin et el. 1993). Corn containing this gene is resistant to glufosinate at

recommended use rates (York and Coble 1997). Research has shown that glufosinate is generally not adequate

as a single POST treatment due to continued emergence of weeds. However, a PRE herbicide followed by

glufosinate applied POST followed by glufosinate postemergence-directed, gives comparable weed control to

conventional methods (Culpepper and York 1999b).

Glyphosate-resistant has been conferred to corn by the incorporation of a glyphosate-resistant

5-enolpyruvylshikimate-3-phophate synthase (CP4-EPSPS) gene cloned from Agrobacterium sp. strain CP4. The

expression of the CP4-EPSPS gene produces a glyphosate-resistant EPSPS enzyme, which can bypass the

acids and secondary metabolites (Barry et al. 1992; Nida et al. 1996).

Glyphosate is a non-selective POST herbicide that can be applied POST or postemergence-directed to

glyphosate-resistant corn (York and Culpepper 2003b). Glyphosate inhibits

phophate synthase (EPSPS) (E.C.2.5.1.19), the enzyme involved in the conversion of

5-enolpyruvylshikimate-3-phophate into the aromatic amino acids tryptophan, tyrosine, and phenylalanine (Devine et al. 1993; Franz

1985).

Glyphosate and its salts were first described as a herbicide in 1971(Franz 1985). Due to the limited solubility

of the acid in water, it was also disclosed that the more soluble salts of the isopropylamine formulation were

preferred. Today there are numerous glyphosate products being marketed in several different formulations,

which include the isopropylamine , diammonium , potassium , and trimethylsulfonium salt formulations. M any1 2 3 4

of these products are packaged and sold with adequate surfactant included in the formulated product while

others do not contain adequate surfactant. M ost tests which have been conducted to evaluate the efficacy of

glyphosate used the isopropylamine salt formulation. Culpepper and York (1999a) found that glyphosate

controls all annual grasses found in North Carolina, also johnsongrass [Sorghum halepense (L.) Pers.] and

bermudagrass [Cynodon dactylon (L.) Pers.]. Glyphosate also controls common broadleaf weeds, such as

Amaranthus spp., common cocklebur (Xanthium strumarium L.), and sicklepod [Senna obtusifolia (L.) Irwin &

Barneby]. M orningglory (Ipomoea spp.) control by glyphosate can be marginal (Culpepper and York 1999a).

However, the study conducted by York and Culpepper (1999a), did not evaluate different salt formulations for

weed control and crop tolerance.

Several studies have been conducted to compare different formulations of glyphosate for weed control and

formulated as isopropylamine salts, with only one containing its own adjuvent, and one formulated as a

trimethylsulfonium salt). They found no differences between the products applied alone for control of

barnyardgrass [Echinochloa crus-galli (L.) Beauv.], prickly sida (Sida spinosa L.), pitted morningglory

(Ipomoea lacunosa L.), and hemp sesbania [Sesbania exaltata (Raf.) Rydb. ex A.W .Hill]. Richardson et al. (2003) conducted studies to evaluate crop response and weed control using isopropylamine and diammonium

salts of glyphosate in glyphosate-resistant cotton and soybean. They found no differences between the products

for crop tolerance, yield, or control of ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.] or large crabgrass

[Digitaria sanguinalis (L.) Scop.]. However, they did find that the diammonium salt formulation controlled

common ragweed (Ambrosia artemisiifolia L.) greater at 28 d after treatment than did the isoproplyamine salt

formulation.

This research was conducted to evaluate weed control and crop tolerance in glyphosate-resistant corn. Field

experiments were conducted to compare glyphosate herbicide systems and a conventional herbicide system

when POST application timing was delayed in the presence or absence of a PRE herbicide. A hybrid trial was

conducted using multiple glyphosate-resistant corn hybrids to compare corn grain yield when treated with a

conventional herbicide system or with a glyphosate herbicide system. Field experiments were also conducted to

evaluate a conventional herbicide system to a glyphosate-based system in conventional and no-tillage plantings.

No-till corn may be a better fit for a total POST herbicide program as weed emergence is delayed in a no-till

system compared to a conventional system (Halford et al. 2001). Field experiments were also conducted to

evaluate multiple glyphosate products in glyphosate-resistant corn and cotton for crop tolerance, weed efficacy,

and yield. The glyphosate products used in these studies were comprised of isopropylamine, diammonium, and

LITERATURE CITED

Anonymous. 2004. Accent Herbicide label. E. I. du Pont de Nemours. W ilmington, DE. 19898.

http://www.cdms.net. Accessed April 4, 2004.

Bailey, J. A. and G. Kapusta. 1994. Soil insecticide and placement influence corn (Zea mays) tolerance to

nicosulfuron. W eed Technol. 8:598-606.

Barry, G., G. Kishore, S. Padgette, M . Taylor, K. Kolacz, M . W eldon, D. Re, K. Fincher, and L. Hallas. 1992.

Inhibitors of amino acid biosynthesis: strategies for impairing glyphosate tolerance to crop plants. p.

139-145. In B.K. Singh et al. (eds.) Biosynthesis and M olecular Regulation of Amino Acids in Plants.

American Society of Plant Physiologist, Rockville, M D.

Burnside, O.C. 1992. Rationale for developing herbicide-resistant crops. W eed Technol. 6:621-625.

Cohen, S. Z., C. Eiden, and M . N. Lorber. 1986. Monitoring ground water for pesticides. In R. C. Garner and

H. N. Higg, eds. Evaluation of Pesticides in Ground W ater. Symposium Series 315. W ashington, DC:

American Chemical Society. pp. 170-197.

Culpepper, A. S. and A. C. York. 1999a. W eed management and net returns with transgenic,

herbicide-resistant, and nontransgenic cotton (Gossypium hirsutum) W eed Technol. 13:411-420.

Culpepper, A. S. and A. C. York. 1999b. W eed management in glufosinate-resistant corn W eed Technol.

Devine, M . D., S. O. Duke, and C. Fedtke. 1993. Inhibition of amino acid biosynthesis. p. 251-291. In

Physiology of Herbicide Action. Prentice-Hall, Inc. Englewood Cliffs, N.J.

Dobbels, A. F. and G. Kapusta. 1993. Postemergence weed control in corn (Zea mays) with

nicosulfuron combinations. W eed Technol. 7:844-850.

Ferguson, G. M ., A. S. Hamill, and F. J. Tardif. 2001. ALS inhibitor resistance in populations of powell

amaranth and redroot pigweed. W eed Sci. Soc. 51:26-28.

Foes, M . J., L. Liu, P. J. Tranel, L. M . Loyd, and E. W . Stoller. 1998. A biotype of common waterhemp

(Amaranthus rudis) resistant to triazine and ALS herbicides. W eed Sci. 46:514-520.

Frannsen, A. S., D. Z. Skinner, K. Al-Khatib, M . J. Horak, and P. A. Kulakow. 2001. Interspecific

hybridization and gene flow of ALS resistance in Amaranthus species. W eed Sci. 49:598-606.

Franz, J. E. 1985. Discovery, development and chemistry of glyphosate. In E. Grossbard and D. Atkinson, eds.

The Herbicide Glyphosate. London: Butterworth and Co. pp. 3-17.

Halford, C., A. S. Hamill, J. Zhang, and C. Doucet. 2001. Critical period of weed control in no-till soybean

(Glycine max) and corn (Zea mays). W eed technol. 15:737-744.

Hall, M . R., C. J. Swanton, and G. W . Anderson. 1992. The critical period of weed control

Holden, L. R., J. A. Graham, R. W . W hitmore, W . J. Alexander, R. W . Pratt, S. K. Liddle,

and L. L. Piper. 1992. Results of the national alachlor well water survey. Environ. Sci.

Technol. 26:935-943.

Knake, E. L., and F. W . Slife. 1961. Competition of Setaria faberi with corn and soybeans.

W eeds 10:26-29.

M orton, C. A., R. G. Harvey, J. L. W edberg, J. J. Kells, D. A. Landis, and W . E. Lueschen. 1994. Influence of

corn rootworm insecticides on the response of field corn (Zea mays) to nicosulfuron. W eed Technol.

8:289-295.

National Agricultural Statistics Service (NASS). 2003. Chemical usage. W eb page:

www.pestmanagement.info/nass/act_dsp_usage_multiple.cfm. Accessed: April 5, 2004.

Norris, J. L., D. R. Shaw, and C. E. Snipes. 2001. W eed control from herbicide combinations with three

formulations of glyphosate. W eed Technol. 15:552-558.

Richardson, R. J., W . A. Bailey, G. R. Armel, C. M . W haley, H. P. W ilson, and T. E. Hines.

2003. Responses of selected weeds and glyphosate-resistant cotton and soybean to two glyphosate salts.

W eed Technol. 17:560-564.

Shaner, D. L. and S. L. O’Connor. 1991. Absorption and translocation of imidazilinone herbicides. pp. 53-69.

Stidham, M . A. 1991. Herbicides that inhibit acetohydroxyacid synthase. W eed Sci. 39:428-434.

Tapia, L. S., T. T. Bauman, R. G. Harvey, J. J. Kells, G. Kapusta, M . M . Loux, W . E. Lueschen, M . D. Owen,

L. H. Hageman, and S. D. Strachan. 1997. Postemergence herbicide application timing effects on annual

grass control and corn (Zea mays) grain yield. W eed Sci. 45:138-143.

Tharp, B. E. and J. J. Kells. 1999. Influence of herbicide application rate, timing, and interrow cultivation on

weed control and corn (Zea mays) yield in glufosinate-resistant and glyphosate-resistant corn. W eed

Technol. 13:807-813.

Vangessel, M . J., E. E. Schweizer, K. A. Garrett, and P. W estra. 1995. Influence of weed density and

distribution on corn (Zea mays) yield. W eed Sci. 43:215-218.

Van Duyn, J. W . 2003. Insect management. In Corn Production Guide 2003. Raleigh, NC:

North Carolina Cooperative Extension Service. pp. 47-62.

Van Duyn, J. W . and J. S. Bacheler. 2003. Insect control in field corn. In 2003 North Carolina Agricultural

Chemicals M anual. Raleigh, NC: North Carolina Cooperative Extension Service. pp. 62-64.

W ade, H. F., A. C. York, A. E. M orey, J. M . Padmore, and K. M . Rudo. 1998. The empact of pesticide use of

groundwater in North Carolina. J. Environ. Qual. 27:1018-1026.

agriculture. In S. O. Duke, ed. Herbicide Resistant-Crops: Agricultural, Environmental, Economic, Regulatory, and Technical Aspects. Boca Raton, FL: CRC Press, Inc. pp. 213-230.

York, A. C. and H.D. Coble. 1997. W eed management in Liberty Link corn and soybeans. In J.A. Dusky ed.

Proceedings of the Beltwide Cotton Conferences. TN: National Cotton Council of America. pp. 2. W eb

page: http://www.cotton.org/beltwide/proceedings/2001/abstracts/542.cfm. Accessed: March 22, 2004.

York, A. C. and A. S. Culpepper. 2003. W eed management. In Corn Production Guide 2003. Raleigh, NC:

North Carolina Cooperative Extension Service. pp. 73-108.

Young, F. L., D. L. W yse, and R. J. Jones. 1984. Quackgrass (Agropyron repens) interference on corn (Zea

Received for publication Date, and in revised form Date.1

Graduate Research Assistant and W illiam Neal Reynolds Professor, respectively, Department of Crop2

Science, Box 7620, North Carolina State University, Raleigh, NC 27695-7620. Corresponding author’s E-mail:

robbie_parker@ncsu.edu.

Chapter 2

Comparison of Glyphosate Products in Glyphosate-Resistant

Cotton and Corn 1

ROBERT G. PARKER and ALAN C. YORK2

Abstract: W ide-spread planting of glyphosate-resistant (GR) crops in the United States of America and glyphosate patent expiration has led to a proliferation of glyphosate products. Growers have questioned their

advisors on efficacy and crop tolerance of the many products available. Field experiments were conducted to

evaluate 10 glyphosate products, representing isopropylamine, diammonium, and potassium salts, applied

postemergence (POST) and postemergence-directed (PDIR) at 630 and 1680 g ae/ha for GR corn and GR cotton

tolerance and weed control. There were no differences among products for control of six annual grass and 10

annual broadleaf weed species. No injury to corn from any glyphosate product at 630 or 1680 g/ha or to cotton

from 630 g/ha was noted at any of seven locations. ClearOut 41 Plus™ , an isopropylamine salt of glyphosate,

and Roundup W EATHERM AX™ , a potassium salt of glyphosate, applied POST at 1680 g/ha injured cotton 27

to 30% and 10 to 17%, respectively, at 3 of 7 locations. No cotton injury was noted with Glyfos , Glyfos X-® ®

TRA, Glyphomax™ , Gly Star™ Original, Roundup Original™ , Roundup UltraMAX , Touchdown , or® ®

Touchdown Total™ . No differences were noted among glyphosate products or between rates for corn or cotton

Nomenclature: Glyphosate; corn, Zea mays L. ‘DK687’, ‘DKC 69-71’; cotton, Gossypium hirsutum L. ‘DP 458 B/RR’, ‘ST 4892BR’.

Additional Index W ords: ClearOut 41 Plus™ , crop tolerance, diammonium salt of glyphosate, Glyfos ,®

Glyfos X-TRA, Glyphomax™ , Gly Star™ Original, isopropylamine salt of glyphosate, potassium salt of®

glyphosate, Roundup Original™ , Roundup UltraM AX , Roundup W EATHERM AX™ , Touchdown ,® ®

Touchdown Total™ , weed control.

Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; GR, glyphosate-resistant; PDIR, postemergence-directed; POST , postemergence; W AP, weeks after postemergence application; W AP1, weeks

after first postemergence application; W AP2, weeks after second postemergence application; W APD, weeks

after postemergence-directed application.

INTRO DUCTIO N

Glyphosate is an environmentally benign herbicide that controls a broad spectrum of weeds (Franz et al. 1997;

M alik et al. 1989). Glyphosate inhibits the biosynthesis of the aromatic amino acids tryptophan, tyrosine, and

phenylalanine in sensitive plants (Siehl 1997). Glyphosate competes with the substrate phosphoenolpyruvate

(PEP) for a binding site on the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzyme [EC 2.5.1.19].

EPSPS is encoded inthe nucleus and imported to plastids where it converts shkimate-3-phosphate and PEP into

5-enolpyruvylshikimate-3-phosphate (Devine et al. 1993; Siehl 1997). Technological advances in identifying

genes coding for specific traits and transferring those genes from unrelated organisms into crop plants led to

development of transgenic, herbicide-resistant crops (W ilcut et al. 1996). Crops resistant to glyphosate were

developed by insertion of a bacterial gene, along with a promoter, which encodes for a version of EPSPS having

greatly reduced affinity for glyphosate (Nida et al. 1996).

Cotton and soybean [Glycine max (L.) M err.] resistant to glyphosate are widely planted in the USA (NASS

2003). Ninety-five percent of the cotton (USDA-AM S 2003) and 85% of the soybean (E. J. Dunphy, N. C. State

broad spectrum weed control, convenience of glyphosate-based weed management systems, and greater

rotational crop flexibility (Ateh and Harvey 1999; Culpepper and York 1999, Culpepper et al. 2000). Corn

resistant to glyphosate is less widely grown (NASS 2003), but plantings are expected to increase.

W ide-spread planting of GR crops has increased market share for glyphosate and decreased usage of other

active ingredients. These factors, coupled with patent expiration on glyphosate, have lead to commercialization

of numerous glyphosate products. All glyphosate products are manufactured using the same parent acid

although different salts of glyphosate are sold. Additionally, each formulation contains proprietary adjuvants

which could affect product performance.

W eed control by glyphosate isopropylamine salt and glyphosate trimesium salt has been similar in most

studies, but injury to GR corn, cotton, and soybean has been observed with the trimesium salt (Buehring et al.

2003; Culpepper and York 2001; Etheridge et al. 2000; Krauz and Young 2001; MacDonald et al. 2000; Norris

et al. 2001; Scott et al. 2000). W ilson et al. (1985), however, reported greater control of horseweed [Conyza

canadensis (L.) Cronq.] and rye (Secale cereale L.) with the trimesium salt. Glyphosate trimesium salt is no longer recommended for use on GR crops. W eed control and GR crop tolerance have been similar with

glyphosate diammonium salt and various products containing glyphosate isopropylamine salt (Bloodworth et al.

2002; Culpepper and York 2001; Rankins et al. 2001; Richardson et al. 2003; Schraer et al. 2002; Smith et al.

2003). In limited research with glyphosate potassium salt, weed control and GR crop response have been

similar to that with glyphosate isopropylamine salt (Smith et al. 2003).

Field experiments were conducted to compare several commercially available glyphosate products for GR

corn and cotton tolerance and weed control. This research was initiated in response to growers’ questions

concerning performance of the many products available. A primary impetus also was observation of significant

GR cotton injury with a particular product, ClearOut 41 Plus, in greenhouse trials in North Carolina (A. C.

York, unpublished data) and other states (J. M . Chandler, Texas A & M Univ., personal communication; A. S.

Company, 6075 Poplar Avenue, M emphis, TN 38119.

M ATERIALS AND M ETHODS

The corn experiment was conducted on the Tidewater Research Station at Plymouth, NC, in 2002 and 2003, at

two sites on the Cherry Farm Unit at Goldsboro, NC, in 2002 and one site in 2003, and at two sites on the Upper

Coastal Plain Research Station at Rocky M ount, NC, in 2003. The cotton experiment was conducted at the

Tidewater Research Station, the Cherry Farm Unit, and the Upper Coastal Plain Research Station in 2002 and

2003 and also on a private farm near Beautancus, NC, in 2003. Soils at the sites are described in Table 1.

W eed species and densities at each site are listed in Table 2.

Conventional seedbeds were prepared at all sites. Corn GR hybrids <DK 687 RR’ and <DKC 69-71 RR’ were

planted during mid-April in 2002 and 2003, respectively. Cotton GR cultivars <ST 4892 BR’ and <DP 458

B/RR’ were planted in 2002 and 2003, respectively, during the second week in M ay. The experimental design

for both crops was a randomized complete block. Treatments in the corn experiment were replicated four times

at all sites. Treatments in the cotton experiment were replicated four times at Plymouth and three times at other

sites. Plots were four rows 9 m long with row spacing of 97 cm at Beautancus, Goldsboro, and Plymouth or 91

cm at Rocky M ount.

Treatments consisted of eight glyphosate products in 2002 or 10 products in 2003 (Table 3) each applied at

630 and 1680 g/ha. A nonionic surfactant at 0.5% (v/v) was included with Glyfos, Glyphomax, Gly Star3

Original, and Roundup Original as suggested on product labels. A non-treated check also was included. The

glyphosate products were applied twice to corn and three times to cotton. Corn received glyphosate POST at the

5- to 6-leaf stage followed by a PDIR application at the 10- to 11-collar stage. Cotton was treated POST in the

2- and 4-leaf stages and then PDIR at the 10- to 11-node stage. W eeds were typically 5 to 10 cm tall or less at

2

each application. Herbicides were applied POST to corn and cotton using a CO -pressurized backpack sprayer

equipped with flat-fan nozzles delivering 140 L/ha at 160 kPa. The PDIR herbicides were broadcast on cotton

2

2

deliver 140 L/ha at 140 kPa. In corn, PDIR herbicides were broadcast using a CO -pressurized backpack

sprayer equipped with one flood nozzle per row middle calibrated to deliver 140 L/ha at 310 kPa.

Corn injury was estimated visually 1 and 2 wk after the POST application (W AP) and 1, 2, and 8 wk after the

PDIR (W APD). Cotton injury was estimated visually 1 and 2 wk after the first POST application (W AP1), 1

and 2 wk after the second POST application (W AP2), and 1, 2, and 8 W APD. W eed control in corn was

estimated visually 2 W AP and 2 and 8 W APD; weed control in cotton was estimated 2 W AP1, 2 W AP2, and 2

and 8 W APD. Visual estimates were on a scale of 0 to 100, with 0 equal to no weed control or injury and 100

equal to complete weed control or crop death (Frans et al. 1986). Annual grass and morningglory species were

evaluated as a category; no attempt was made to evaluate control of grasses and morningglories by species.

The center two rows of each corn and cotton plot were mechanically harvested in mid-September and mid- to

late-October, respectively. Corn grain yields were adjusted to 15.5% moisture. A sample of mechanically

harvested seedcotton was collected from each plot to determine lint percentage and fiber properties. Seedcotton

was ginned on a laboratory gin without lint cleaning. Cotton grades are not presented as they would not be

representative of cotton ginned commercially. However, fiber upper half mean length, fiber length uniformity

index, fiber strength, and micronaire were determined by High Volume Instrumentation testing (Sasser, 1981) at

Cotton Incorporated in Raleigh, NC. In 2003, total nodes per plant, fruit production on monopodial branches,

fruit production on sympodial branches by node, and percentage of open bolls were determined on 30

consecutive plants per plot in cotton treated with ClearOut 41 Plus, Roundup UltraM AX, Roundup

W EATHERM AX, and Touchdown. For analysis and presentation, fruit production on sympodia arising from

the main stem were grouped into the following three node zone: nodes 4 to 8; nodes 9 to 13, and nodes greater

than 13.

Data were subjected to ANOVA with partitioning appropriate for the factorial treatment arrangements.

Non-transformed data for weed control are presented as arcsine square root transformation did not affect data

interpretation. Data were analyzed by year because of the additional glyphosate products included in 2003.

Data were averaged over sites within years except where mentioned otherwise. A separate ANOVA was

1989. Available only on computer disk from W SSA, 810 East 10 Street, Lawrence, KS 66044-8897.th

RESULTS AND DISCUSSIO N

Corn Experiment. Except for morningglory species, no differences in weed control were observed among glyphosate products or between rates of application (Appendix Tables A.2.1, A.2.2, A.2.3, A.2.4, A.2.5, A.2.6,

A.2.7, and A.2.8). Annual grasses, consisting of broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash #4

BRAPP], fall panicum (Panicum dichotomiflorum M ichx. # PANDI), goosegrass [Eleusine indica (L.) Gaertn. #

ELEIN], seedling johnsongrass [Sorghum halepense (L.) Pers. # SORHA], large crabgrass [Digitaria

sanguinalis (L.) Scop. # DIGSA], and Texas panicum (Panicum texanum Buck. # PANTE) were controlled 99 to 100% at each evaluation regardless of glyphosate product or rate of application (Appendix Tables A.2.9 and

A.2.10). Similarly, Palmer amaranth (Amaranthus palmeri S.W ats. # AM APA), sicklepod [Senna obtusifolia

(L.) Irwin & Barneby # CASOB], common lambsquarters (Chenopodium album L. # CHEAL), common

ragweed (Ambrosia artemisiifolia L. # AM BEL), smooth pigweed (Amaranthushybridus L. # AM ACH), and

prickly sida (Sida spinosa L. # SIDSP) were controlled at least 99% (Appendix Tables A.2.9, A.2.10, A.2.11,

A.2.12, and A.2.13).

There was not a glyphosate product by rate of application interaction for control of morningglory species,

which consisted of entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray # IPOHG), pitted

morningglory (Ipomoea lacunosa L. # IPOLA), and tall morningglory [Ipomoea purpurea (L.) Roth # PHBPU].

Additionally, there was no main effect of glyphosate products (Appendix Tables A.2.1). There was a small but

significant main effect of glyphosate rates 2 W AP in both years and 2 W APD in 2002. Glyphosate at 630 g/ha

controlled morningglory species 96 to 97% 2 W AP (Table 4). Control increased two percentage points when

the glyphosate rate was increased from 630 to 1680 g/ha. In 2002, glyphosate at 630 and 1680 g/ha controlled

morningglory species 97 and 99%, respectively, 2 W APD. Regardless of glyphosate rate, morningglory species

were controlled 98 to 99% 8 W APD in both years.

grain yields among the herbicide treatments in either year (Appendix Table A.2.14). Averaged over herbicide

treatments and locations within years, grain yields were 7320 and 8405 kg/ha in 2002 and 2003, respectively

(Appendix Table A.2.15). Yields with all herbicide treatments differed from that of the non-treated check,

which averaged 3620 and 6840 kg/ha in 2002 and 2003, respectively.

Cotton Experiment. Similar to results from the corn experiment, no differences among herbicide treatments in cotton were noted for control of weeds except morningglory species (Appendix Tables A.2.16, A.2.17, A.2.18,

A.2.19, A.2.20, A.2.21, and A.2.22). Palmer amaranth, annual grass species, common lambsquarters, sicklepod,

smooth pigweed, common cocklebur (Xanthium strumarium L. # XANST ), and common ragweed were

controlled 99 to 100% at each evaluation in 2003 (Appendix Tables A.2.23, A.2.24, A.2.25and A.2.26). Palmer

amaranth and smooth pigweed were controlled 99 to 100% at all evaluations in 2002 while annual grasses and

common lambsquarters were controlled 95 to 100% at all evaluations (Appendix Tables A.2.27 and A.2.28).

Prickly sida and sicklepod were controlled 99 to 100% 2 W AP1, 2 W AP2, and 2 W APD in 2002, but control

declined to 93 to 96% and 95 to 98%, respectively, 8 W APD due to a few weeds emerging after the PDIR

application (Appendix Table A.2.29).

There was not a glyphosate product by application rate interaction for control of morningglory species nor

were there differences in control of morningglory species among glyphosate products (Appendix Table A.2.30).

There were, however, effects of glyphosate rates. At 2 W AP1 and 2 W AP2 in both years, control of

morningglory was 3 to 16 percentage points greater with the higher glyphosate application rate (Table 5). No

differences in control between glyphosate rates were noted at 2 or 8 W APD. Control of morningglory by

glyphosate at both rates was greater at all evaluation dates in 2003 than in 2002. This was likely a reflection of

differing moisture conditions between the two years. In the 8 wk following cotton planting, the test sites

received over twice as much rainfall in 2003 as in 2002 (Appendix Table A.2.31).

There was no visible cotton injury from any treatment in 2002 or at Plymouth in 2003. However, injury was

noted with some treatments at Beautancus, Goldsboro, and Rocky M ount in 2003. W hen data were averaged

over these three locations, a glyphosate product by application rate interaction was observed for crop injury at 1

Plus and Roundup W EATHERM AX at 1680 g/ha (Table 6). This rate of glyphosate is twice the greatest

label-recommended application rate for GR cotton (Anonymous 2004a, 2004b). The injury was expressed as necrosis

on leaves contacted by the spray solution, and this injury appeared in less than 24 h after application. Some

stunting was also noted on cotton receiving ClearOut 41 Plus at 1680 g/ha. Similar symptoms have been noted

in greenhouse experiments (A. C. York, unpublished data). The rapid, contact-type injury caused by ClearOut

41 Plus and Roundup W EATHERM AX and the general lack of injury by other glyphosate products may have

been due to differences in formulation adjuvants among the glyphosate products. The cotton recovered and no

visible injury was noted 2 W APD or later in the season. Different cotton cultivars were used in the 2 yr of this

experiment, and one cannot eliminate differential cultivar tolerance as an explanation for injury by ClearOut 41

Plus in 2003 but not in 2002. However, greater injury by ClearOut 41 Plus in 2003 may also have been due to

greater rainfall and more succulent cotton. Below-normal and above-normal rainfall was received in 2002 and

2003, respectively. The Goldsboro and Rocky M ount locations received more than twice as much rainfall

during the first 8 wk after planting in 2003 compared with 2002 (Appendix Table A.2.31).

Crop injury observed following POST application of ClearOut 41 Plus and Roundup W EATHERM AX did

not adversely affect cotton yield or fiber properties (Appendix Tables A.2.33, A.2.34, and A.2.35). Averaged

across glyphosate products and rates and locations within years, cotton lint yield, fiber upper half mean length,

fiber length uniformity index, fiber strength, and micronaire reading were 1180 kg/ha, 27 mm, 84.1%, 247.2 kN

m/kg, and 4.4, respectively, in 2002 and 1120 kg/ha, 26 mm, 83%, 296.2 kN m/kg, and 3.9 respectively, in 2003

(Appendix Tables A.2.36 and A.2.37). Yield of the non-treated checks was assumed to be zero because

extremely heavy weed infestations prevented mechanical harvest. Essentially no fiber was produced in the

non-treated checks.

Plots treated with ClearOut 41 Plus, Roundup W EATHERM AX, Roundup UltraM AX, and Touchdown were

selected for plant mapping in 2003 because of injury with the first two and lack of injury with the second two.

Additionally, this selection of treatments represented each of the three commercially available salt formulations

of glyphosate registered for use in cotton. Plant mapping revealed no differences among these glyphosate

and greater than 13, total number of bolls, or maturity as measured by percentage of open bolls (Appendix

Tables A.2.38 and A.2.39).

Our results indicated no differences in weed control or GR corn tolerance among 10 commercially available

glyphosate products, including isopropylamine salt, diammonium salt, and potassium salt formulations, when

equivalent rates were applied. A considerable amount of cotton leaf necrosis was sometimes obtained with

Clearout 41 Plus and a lesser amount with Roundup W EATHERM AX when these products were applied at

1680 g/ha, or twice the labeled rate. This necrosis did not lead to yield reduction or adverse effects on fiber

properties or cotton maturity. No injury was observed with any glyphosate product applied at 630 g/ha. Our

results suggest that a grower’s choice among glyphosate products should be based upon product cost and upon

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Table 1. Description of soils at experiment sites.

Organic

Site Soil matter

Experiment number Locations series Texture content pH

%

Corn 1 Goldsboro, 2002 Goldsboroa Fine sandy loam 0.5 6.1

2 Goldsboro, 2002 Pantegob Loam 6.4 5.2

3 Plymouth, 2002 Portsmouthc Fine sandy loam 4.6 5.6

4 Goldsboro, 2003 Goldsboro Fine sandy loam 0.9 6.2

5 Plymouth, 2003 Hyded Loam 4.0 6.2

6 Rocky M ount, 2003 Aycocke Fine sandy loam 0.5 5.7

7 Rocky M ount, 2003 Norfolkf Sandy loam 1.0 5.6

Cotton 1 Goldsboro, 2002 Goldsboro Fine sandy loam 0.5 5.8

2 Plymouth, 2002 Hyde Loam 2.8 6.0

3 Rocky M ount, 2002 Norfolk Sandy loam 0.4 6.0

4 Beautancus, 2003 Lynchburgg Fine sandy loam 1.0 5.6

5 Goldsboro, 2003 Goldsboro Fine sandy loam 0.6 6.1

6 Plymouth, 2003 Portsmouth Fine sandy loam 6.0 5.7

7 Rocky M ount, 2003 M arvynh Sandy loam 0.6 6.0

Fine-loamy, siliceous, semiactive, thermic Aquic Paleudults.a

Fine-loamy, siliceous, semiactive, thermic Umbric Paleudults.b

Fine-loamy over sandy or sandy-skeletal, mixed, semiactive, thermic Typic Umbraquults.c

Fine-silty, mixed, active, thermic Typic Umbraquults.d

Fine-silty, siliceous, subactive, thermic Typic Paleudults.e

Fine-loamy, kaolinitic, thermic Typic Kandiudults.f

Fine-loamy, siliceous, semiactive, thermic Aeric Paleaquults.g

Table 2. W eed species and densities at experiment sites.

Sitesa

Experiment Species 1 2 3 4 5 6 7

No./m

_____________________________________ 2 ____________________________________

Corn Broadleaf signalgrass 6 0 0 0 0 23 0

Common lambsquarters 0 0 13 0 11 49 25

Common ragweed 0 0 0 0 0 5 5

Entireleaf morningglory 0 11 0 1 2 0 1

Fall panicum 6 0 21 36 9 20 0

Goosegrass 0 0 5 0 5 0 0

Johnsongrass, seedling 0 4 0 0 0 0 0

Large crabgrass 0 0 0 0 0 0 180

Palmer amaranth 50 5 0 36 0 0 0

Pitted morningglory 2 0 4 1 1 0.7 25

Prickly sida 0 0 0 0 7 0 0

Sicklepod 5 5 4 2 0 0 0

Smooth pigweed 0 0 42 0 4 0 10

Tall morningglory 3 0 1 0 0 0 1

Texas panicum 0 0 0 0 0 0 19

Cotton Broadleaf signalgrass 25 0 0 0 2 0 3

Common cocklebur 0 0 0 3 0 0 0

Common lambsquarters 3 36 0 0 0 7 3

Common ragweed 0 0 26 0 0 0 0

Entireleaf morningglory 4 0.6 0.6 3 0 1 0

Fall panicum 0 12 13 0 0 4 0

Goosegrass 0 0 0 0 0 25 19

Table 2. Continued.

Sitesa

Experiment Species 1 2 3 4 5 6 7

No./m

_____________________________________ 2 _____________________________________

Large crabgrass 0 0 23 5 0 0 9

Palmer amaranth 52 0 0 9 45 0 2

Pitted morningglory 6 0 0.9 1 0 0 2

Prickly sida 0 2 0 0 0 0 0

Sicklepod 0.3 0.9 0 1 2 0 0

Smooth pigweed 0 3 0.4 0 0 10 2

Tall morningglory 2 2 0.6 1 0.2 1 3

See Table 1 for site descriptions.

Table 3. Glyphosate products evaluated in corn and cotton experiments.

Glyposate Years

Glyphosate product Type of salt concentration M anufacturer evaluated

g ae/L

ClearOut 41 Plus™ Isopropylamine 356 CPT, LLC, 2002, 2003

Cartersville, GA

Glyfos® Isopropylamine 356 Cheminova, Inc., 2002, 2003

W ayne, NJ

Glyfos X-TRA® Isopropylamine 356 Cheminova, Inc., 2002, 2003

W ayne, NJ

Glyphomax™ Isopropylamine 356 Dow AgroSciences, LLC, 2002, 2003

Indianapolis, IN

Gly Star™ Original Isopropylamine 356 Albaugh, Inc., 2002, 2003

Ankeny, IA

Roundup Original™ Isopropylamine 356 M onsanto Co., 2002, 2003

St. Louis, M O

Roundup UltraMAX® Isopropylamine 445 M onsanto Co., 2002, 2003

St. Louis, M O

Roundup W EATHERM AX™ Potassium 540 M onsanto Co., 2003

St. Louis, M O

Touchdown® Diammonium 356 Syngenta Crop Protection, 2002, 2003

Greensboro, NC

Touchdown Total™ Potassium 495 Syngenta Crop Protection, 2003

Table 4. M ain effect of glyphosate rates on morningglory species control in corn. a

Glyphosate 2002 2003

rate 2 W APb 2 W APDb 8 W APD 2 W AP 2 W APD 8 W APD

g/ha _____________________________________________________ % ____________________________________________________

630 97 97 98 96 99 99

1680 99 99 99 98 100 99

P > F 0.0403 0.0491 0.3016 0.0433 0.1142 0.3228

Data averaged over glyphosate products and locations within years. See Table 2 for description of

a

morningglory species.

Abbreviations: W AP, weeks after POST herbicide application; W APD weeks after

b

Table 5. M ain effect of glyphosate rates on morningglory species control in cotton. a

2002 2003

Glyphosate 2 2 2 8 2 2 2 8

rate W AP1b W AP2b W APDb W APD W AP1 W AP2 W APD W APD

g/ha __________________________________________________________ % _________________________________________________________

630 90 82 90 84 95 96 99 100

1680 96 98 98 94 98 100 99 100

P > F 0.0376 0.0001 0.1900 0.0948 0.0064 0.0419 0.1303 0.1358

Data averaged over glyphosate products and locations within years. See Table 2 for description of

a

of morningglory species.

Abbreviations: W AP1, weeks after first postemergence herbicide application; W AP2, weeks after second

b

Table 6. Interaction of glyphosate products by application rate for cotton injury in 2003.a

Application 1 2 1 2

Products rate W AP1b W AP1 W AP2b W AP2

g/ha ___________________________ ___________________________ %

ClearOut 41 Plus™ 630 6 bc 3 c 3 c 2 c

1680 27 a 28 a 29 a 30 a

Glyfos® 630 0 c 0 c 0 c 0 c

1680 2 c 2 c 3 c 2 c

Glyfos X-TRA® 630 0 c 0 c 0 c 0 c

1680 3 c 3 c 3 c 3 bc

Glyphomax™ 630 0 c 1 c 0 c 0 c

1680 1 c 1 c 1 c 1 c

Gly Star™ Original 630 0 c 0 c 1 c 0 c

1680 2 c 2 c 3 c 1 c

Roundup Original™ 630 0 c 0 c 0 c 0 c

1680 4 c 3 c 7 c 5 bc

Roundup UltraMAX® 630 0 c 0 c 0 c 1 c

1680 1 c 0 c 1 c 1 c

Roundup W EATHERM AX™ 630 0 c 0 c 0 c 1 c

1680 12 b 12 b 17 b 10 b

Touchdown® 630 0 c 0 c 0 c 1 c

1680 3 c 3 c 4 c 3 bc

Touchdown Total™ 630 0 c 0 c 1 c 0 c

1680 1 c 1 c 2 c 1 c

Data averaged over Beautancus, Goldsboro, and Rocky M ount locations; no injury was observed ata

Plymouth in 2003. M eans within a column followed by the same letter are not different according to

Fisher’s Protected LSD test at P = 0.05.

Abbreviations: W AP1, weeks after first postemergence application; W AP2, weeks after secondb

Department of Crop Science, North Carolina State University, Raleigh, NC 27695-7620.

Corresponding author’s E-mail: robbie_parker@ncsu.edu.

Chapter 3

W eed Control in Glyphosate-resistant Corn as Affected by Preemergence Herbicide and Timing of Postemergence Herbicide Application1

ROBERT G. PARKER and ALAN C. YORK2

Abstract: Field studies were conducted at six sites during 2002 and 2003 to evaluate weed control and glyphosate-resistant corn response to glyphosate and nicosulfuron plus atrazine applied postemergence (POST)

as affected by preemergence (PRE) herbicide and timing of POST herbicide application. Treatments included a

factorial arrangement of no herbicide or alachlor plus atrazine PRE at one-half manufacturer’s suggested use

rates, glyphosate or nicosulfuron plus atrazine POST, and POST herbicides applied timely or application

delayed 1 or 2 wks. All treatments were followed by a glyphosate postemergence-directed application.

Common lambsquarters, common ragweed, Palmer amaranth, prickly sida, and smooth pigweed were controlled

at least 94%, with no differences among treatments. Annual grasses were controlled at least 96% by all

glyphosate treatments while control by nicosulfuron plus atrazine without PRE herbicide decreased as

application timing was delayed. Sicklepod was controlled at least 94% when POST herbicides were applied

timely, but control by both POST herbicides decreased with delayed application regardless of PRE herbicide.

M orningglory species, consisting of entireleaf morningglory, ivyleaf morningglory, pitted morningglory, and tall

morningglory, were controlled 93% or greater when either POST herbicide was applied timely. Control by

both POST herbicides decreased as application was delayed, with glyphosate being affected more by timing than

Revised 1989. Available only on computer disk from W SSA, 810 East 10 Street Lawrence, Kansas 66044- th

8897.

following the PRE herbicide regardless of application timing while yield decreased as POST herbicide

application was delayed in the absence of PRE herbicide.

Nomenclature: Alachlor; atrazine; glyphosate; nicosulfuron; common lambsquarters, Chenopodium album L.

# CHEAL; common ragweed,3 Ambrosia artemisiifolia L. # AM BEL; entireleaf morningglory, Ipomoea

hederacea var. integriuscula Gray # IPOHG; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE;

Palmer amaranth, Amaranthus palmeri S.W ats. # AM APA; pitted morningglory, Ipomoea lacunosa L. #

IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin & Barneby # CASOB;

smooth pigweed, Amaranthus hybridus L. # AM ACH; tall morningglory, Ipomoea purpurea (L.) Roth #

PHBPU; corn, Zea mays L. ‘DKC 687 RR’, ‘DKC 69-71 RR’.

Additional Index W ords: Alachlor, atrazine, crop vigor, herbicide-resistant crops, nicosulfuron.

Abbreviations: EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; GR, glyphosate-resistant; PDIR, postemergence-directed; POST, postemergence; PRE, preemergence; W AP, weeks after postemergence

herbicide application; W APD, weeks after postemergence-directed herbicide application.

INTRO DUCTIO N

Corn development and grain yield are influenced by the duration of weed interference, weed species, weed

density, and the environment in which corn grows (Hall et al. 1992; Knake and Slife 1961; Staniforth 1957;

Tapia et al. 1997; Vangessel et al. 1995; Young et al. 1984). W eeds compete with corn for sunlight, water,

nutrients, and space. Season-long interference from weeds can reduce corn grain yields up to 50% (Hall et al.

1992).

Soil-applied herbicides, such as atrazine plus metolachlor or atrazine plus alachlor, have been used to control

2002). Postemergence herbicides have commonly been used as needed following soil-applied herbicides.

However, good weed control and corn yields have been obtained using POST herbicides only (Gower et al.

2003; Tapia et al. 1997).

W ith reductions in atrazine use rates, due to limitations imposed because of atrazine found in ground water

(Cohen et al. 1986; Holden et al. 1992; W ade et al. 1998), and commercialization of more effective POST

herbicides (Tharp and Kells 1999), growers are moving towards total POST weed management systems for corn.

Applied POST, nicosulfuron is effective on many annual grass species (Kapusta et al. 1995; Tapia et al.1997).

However, adverse interactions between organophosphate insecticides and nicosulfuron and other sulfonylurea

herbicides can limit use of these herbicides (Bailey and Kapusta 1994; Kapusta and Krausz 1992).

Glyphosate is an environmentally benign herbicide that controls a broad spectrum of weeds (Franz et al. 1997;

M alik et al. 1989). Glyphosate inhibits the biosynthesis of the aromatic amino acids tryptophan, tyrosine, and

phenylalanine in sensitive plants (Siehl 1997). Glyphosate competes with the substrate phosphoenolpyruvate for

a binding site on the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) enzyme [EC 2.5.1.19]. The

enzyme EPSPS is encoded in the nucleus and imported to plastids where it converts shikimate-3-phosphate and

PEP into 5-enolpyruvylshikimate-3-phosphate (Devine et al. 1993; Siehl 1997). Technological advances in

identifying genes coding for specific traits and transferring those genes from unrelated organisms into crop

plants led to development of transgenic, herbicide-resistant crops (W ilcut et al. 1996). Crops resistant to

glyphosate were developed by insertion of a bacterial gene, along with a promoter, which encodes for a version

of EPSPS having greatly reduced affinity for glyphosate (Nida et al. 1996). Glyphosate-resistant (GR) corn

would allow for an effective total POST herbicide program because of the broad spectrum of weed control by

glyphosate, lack of interactions between glyphosate and organophosphate insecticides, and rotational crop

flexibility (Ateh and Harvey 1999; Culpepper and York 1999; Culpepper et al. 2000; Johnson et al. 2000).

Lack of timely herbicide application is a concern in systems relying on POST herbicides. An experiment was

conducted to determine the effect of delayed POST application of glyphosate and nicosulfuron plus atrazine on

weed control and GR corn yield and to determine the benefit of PRE herbicides with timely and delayed POST

Helena Chemical Company, M emphis, TN.

M ATERIALS AND M ETHO DS

The experiment was conducted in North Carolina during the 2002 and 2003 growing seasons. Sites included

the following: the Central Crops Research Station near Clayton in each year, with two location in 2003

(hereafter referred to as north and south Clayton); the Tidewater Research Station near Plymouth in each year;

and the Cherry Farm Unit near Goldsboro in 2002. Soils at test sites are described in Table 1. W eed species

and densities at each site are listed in Table 2.

Conventional seedbeds were prepared at all locations and GR corn hybrids ‘Dekalb 687 RR’ and ‘Dekalb

69-71 RR’ were planted in 2002 and 2003, respectively, during the month of April. The experimental design was a

randomized complete block with treatments replicated four times, except at south Clayton in 2003 where

treatments were replicated six times. Plots were four rows by 9 m with row spacing of 97 cm apart at Goldsboro

and Clayton and 91 cm apart at Plymouth.

Treatments consisted of a factorial arrangement of two PRE herbicides, two POST herbicides, and three times

of POST herbicide application. Preemergence herbicides included no herbicide or alachlor plus atrazine applied

at (875 + 525 g ai/ha) at all locations except Plymouth; alachlor plus atrazine was applied at (1310 + 786 g/ha)

at Plymouth. These rates are one-half of normal use rates for the particular soils (Anonymous 2004).

Postemergence herbicides included glyphosate isopropylamine salt at 840 g ae/ha or nicosulfuron at 35 g ai/ha4

+ atrazine at 1120 g ai/ha plus crop oil concentrate at 1% (v/v). The POST herbicides were applied timely (5-5

to 9-cm weeds) or application was delayed 7 or 14 d. A non-treated check also was included. All treatments

except the non-treated check received a postemergence-directed (PDIR) application of glyphosate at 840 g/ha 2

wk after the 14-d delayed POST application.

2