Glyphosate-Resistant Palmer Amaranth Response

To Dicamba

R. D. Wallace, A. S. Culpepper, W. K. Vencill, A. C. York, and T. L. Grey

University of Georgia and NC StateDepartment of Crop and Soil Sciences

January 2008

Cotton In Georgia (2006)Most widely grown row crop1.4 million acres produced 2.3 million bales of cotton

National Agricultural Statistics Service, USDA

180

4,925

60

515

85

1,030

47

500

335

380

660

17551 860173

400

455

GA ranked second among cotton producing states in 2007

(thousands of acres)

National Agricultural Statistics Service, USDA

Glyphosate-Resistant Cotton

Glyphosate-resistant cotton was commercialized in 1997Rapidly adopted by growers

100

66

97

99

98

98

98

99

99

100

98

9883 9775

97

59

Percent of cotton acreage planted to RR varieties 2006

USDA, AMS. 2006. Publ. mp-cn833.

Advantages of Glyphosate-Resistant Cotton

Ease of systemProperties of glyphosateAgronomic value of the cotton– DP 555 BGR in GA

Control of Susceptible Palmer Amaranth by Glyphosate

Prior to 2005, glyphosate was an effective and economical tool

22 oz/A WeatherMax on 16” Palmer Amaranth

Glyphosate-Resistant Palmer Amaranth

Resistance to glyphosate in Palmer Amaranth was confirmed in 2005 in GAFactors influencing glyphosate-resistance in GA– Dependence on a single management tactic– Monocultural production system– Low use rates– Application in stressed situations

Glyphosate-Resistant Palmer Glyphosate-Resistant Palmer Amaranth in CottonAmaranth in Cotton

33 oz WeatherMax applied to 2” Palmer Amaranth

Georgia counties infested with glyphosate-resistant Palmer amaranth.

2005

2006

2007 (more sites being tested)

Confirmed in 83 of 149 fields sampled

Glyphosate-Resistant Palmer Amaranth

Current management tactics are not very successfulNew technology is neededDicamba-tolerant cotton (2014)

Objective

Determine glyphosate-resistant Palmer amaranth response to

dicamba programs.

Two Studies to Evaluate Palmer Amaranth Control by Clarity in

2007

Mount Olive

Macon Co.

*

*

Field Trial Macon Co. GA

4 Clarity rates: 4, 8, 12, or 16 oz/A3 Palmer amaranth heights: 3, 6, or 12”12 oz/A sequential application 21-30 days after initial Clarity applicationVisually evaluated for % control

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

Evaluations 26 d after initial application

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

3 inch

45c

86b

97 a

99 a

% C

on

trol

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

3 inch Sequential

Evaluations 23 d after sequential application

99a86

b

92ab

96a

% C

on

trol

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

6 inch

91 a

Evaluations 24 d after initial application

65 b60

b

35c

% C

on

trol

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

6 inch Sequential

98a91

a70 b

38c

Evaluations 23 d after sequential application

% C

on

trol

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

12 inch

78a

46 b

46 b43

b

Evaluations 19 d after initial application

% C

on

trol

Glyphosate-resistant Palmer amaranth control by Clarity. Macon

Co., GA 2007.

0

20

40

60

80

100

4 oz 8 oz 12 oz 16 oz

12 inch Sequential

80a

55 b

58 b

43 c

Evaluations 23 d after sequential application

% C

on

trol

Clarity 8 oz/A

3 inch 6 inch

Clarity 16 oz/A

3 inch 6 inch

Field Trial Mt. Olive, NC

1 Palmer amaranth height: 6-7”5 Treatments of Clarity:– 8 oz/A – 8 oz/A fb 8 oz/A– 16 oz/A– 16 oz/A fb 8 oz/A– 16 oz/A fb 16 oz/A

Visually evaluated for % control

Glyphosate-resistant Palmer amaranth control by Clarity. Mt.

Olive, NC 2007.97a90

a

53c

63b

50 c

0

20

40

60

80

100

8 oz/0 oz 8 oz/8 oz 16 oz/0 oz 16 oz/8 oz 16 oz/16 oz

Evaluations 42 d after sequential application

% C

on

trol

Conclusions – Macon Co.

Palmer amaranth control was affected by plant size, Clarity rate, and the use of sequential applications>90% control was achieved when:– 3” Palmer amaranth

12 oz/A, 16 oz/A, 8 oz/A fb 12 oz/A

– 6” Palmer amaranth16 oz/A, 12 oz/A fb 12 oz/A, 16 oz/A fb 12 oz/A

Conclusions – Mt. Olive

Palmer amaranth control was affected by Clarity rate and the use of sequential applications>90% control was achieved when 6-7” Palmer amaranth was treated with 16 oz/A fb 8 oz/A and 16 oz/A fb 16 oz/A

Future Studies

Evaluate control of glyphosate-resistant Palmer amaranth through herbicide mixturesEvaluate control of other problematic row-crop weeds through herbicide mixtures

Acknowledgements

Committee MembersDr. Alan York

University of Georgia Weed Science Unit