Fall planted cover crops can help suppress glyphosate-resistant Palmer amaranth in cotton. Cover crops reduced early season Palmer amaranth emergence from the soil seed bank compared to no cover crop. Postemergence Liberty-based herbicide systems combined with residual herbicides provided better Palmer amaranth control. Cotton yield was unaffected by cover crop or herbicide program. Using cover crops and aggressive herbicides together can help reduce Palmer amaranth pressure.

1. Cover Crops for Management of
Glyphosate-Resistant Palmer Amaranth
in Cotton
Mike Marshall and Colton Sanders
Clemson University

2. Palmer Amaranth (Amaranthus
palmeri)
• grows faster than other pigweeds (up to
1-2” per day!!)
• Very competitive
• prolific seed producer
– Up to 450,000 seeds/plant

3. Untreated
Glyphosate 22 oz/A
Glyphosate 44 oz/A
EPSP Synthase Inhibitor (Roundup) Herbicide
Normal Use Rate

4. Untreated
Thifensulfuron 0.33 oz/A
Thifensulfuron 0.66 oz/A
ALS-Inhibitor (Harmony) Herbicide
Normal Use Rate

5. Current Glyphosate and ALS-Resistant
Palmer Amaranth Distribution in SC
Newberry
Greenwood
Williamsburg
Georgetown
Clarendon
Bamberg
Allendale
Orangeburg
Dorchester
Barnwell
Lancaster Chesterfield
Marlboro
Dillon
Darlington
Hampton
Jasper
Beaufort
Edgefield
McCormick Sumter
Richland
Florence
Marion
Cherokee
Lee
Lexington
Charleston
Abbeville
Pickens
Greenville
Spartanburg
York
Oconee
Union Chester
Anderson Laurens
Kershaw
Fairfield
Horry
Saluda
Calhoun
Aiken
Berkeley
Colleton

6. Fall Planted Cover Crops for Weed
Suppression

7. Cover Crop for Weed Suppression
Cover Crop Rolled andTerminated Prior to Planting

8. Crop Emergence

9. Effect of Residue Cover on Weed
Seed Soil Bank
 Reduced light
reaching soil surface
 Reduced temperature
fluctuations in the
upper soil profile
 Release of chemicals
inhibit germination of
small seeded weeds

10. Cover Crop Residue Effects on Soil
Moisture on Summer Crops

11. Fall Planted Cover Crops for
Weed Suppression

12. Objectives
 To determine efficacy of fall planted
cover crops on glyphosate-resistant
Palmer amaranth populations in cotton.
 To determine impact of selected
herbicide programs in conjunction with
fall cover crops on cotton growth and
yield.

13. Materials and Methods
 Field experiments were conducted on
growers fields and at Edisto Research and
Education Center (EREC) located near
Blackville, SC.
 Cover crop mixture (rye, oats, turnip, vetch,
radish, and clover) was seeded at 70 lb/A
between October and December 2012, 2013,
and 2014 in half of the field. The rest was
left unplanted (weedy winter cover).
 Cover crops/weedy cover sections were
terminated with glyphosate and 2,4-D and
Valor (preplant) approximately 20 days
before planting.

14. Materials and Methods (cont)
 Cotton variety ‘Phytogen Widestrike 499’ was
planted at 3.8 seed/ft row mid-May of each
year.
 Preemergence (PRE) herbicides were applied
after planting followed by POST1 (APT1) at
2-3 lf cotton, POST2 (APT2) at 6-8 lf cotton,
and LAYBY at 18-20 inch cotton.
 Palmer amaranth populations were collected
using of 1.0 m2 quadrat at APT1, APT2, and
at LAYBY.
 Palmer amaranth population data and seed
cotton yield data were analyzed using
ANOVA.

15. Herbicide Programs with Cover/No
Cover
Treatment 1: (High Input RR program; Using Widestrike Cotton)
Burndown: Roundup (22 oz/A) + 2,4-D (1 qt/A) +Valor (2 oz/A) [30 preplant]
PRE: Reflex (1 pt/A) + Diuron (1 pt/A) + Paraquat (2 pt/A) [~4 weeks after preplant]
EPOST: Roundup (22 oz/A) + Warrant (3 pt/A) [2 weeks after PRE]
MPOST: Roundup (22 oz/A) + Warrant (3 pt/A) [2 weeks after EPOST]
Layby: MSMA (2.67 pt/A) + Diuron (1 pt/A) [2-3 weeks after MPOST]
Treatment 2: (Low Input LL program; Using Widestrike Cotton)
Burndown: Roundup (22 oz/A) + 2,4-D (1 qt/A) [30 preplant]
PRE: Reflex (1 pt/A) + Diuron (1 pt/A) [4 weeks after preplant]
EPOST: Liberty (29 oz/A) + Dual Magnum (1.3 pt/A) [2 weeks after PRE]
MPOST: Liberty (29 oz/A) [2 weeks after EPOST]
Layby: MSMA (2.67 pt/A) + Diuron (1 pt/A) [2-3 weeks after MPOST]
Treatment 3: (High Input LL program; Using Widestrike CottonVariety)
Burndown: Roundup (22 oz/A) + 2,4-D (1 qt/A) +Valor (2 oz/A) [30 preplant]
PRE: Reflex (1 pt/A) + Diuron (1 pt/A) + Paraquat (2 pt/A) [4 weeks after preplant]
EPOST: Liberty (29 oz/A) + Staple (2.5 oz/A) [2 weeks after PRE]
MPOST: Liberty (29 oz/A) + Dual Magnum (1.0 pt/A) [2 weeks after EPOST]
Layby: MSMA (2.67 pt/A) + Diuron (1 pt/A) [2-3 weeks after MPOST]

16. Effect of Cover Crops on Palmer
Amaranth Populations – 2012-2103
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
1 2 3 4 5 6
POST1 POST2 LAYBY
<-- No Cover Crop -->
PalmerAmaranthpopulations/Acre
Treatment
LSD (0.05) = 2024
<--- Cover Crop --->
RR LL LLRR LL LL

17. 0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
1 2 3 4 5 6
POST1 POST2 LAYBY
Treatment
<--- Cover Crop --->
<-- No Cover Crop -->
Effect of Cover Crops on Palmer
Amaranth Populations – 2013-14
LSD (0.05) = 3203
RR LL LLRR LL LL

18. CottonYield – 2012-2013
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
1 2 3 4 5 6
SeedCottonYield(lb/A)
LSD (0.05) = 396
Treatment

19. CottonYield – 2013-2014
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
1 2 3 4 5 6
LSD (0.05) = 452
SeedCottonYield(lb/A)
Treatment

20. Burndown:Roundup (22 oz/A) + 2,4-D (1 qt/A) +Valor (2 oz/A) [30 preplant]
PRE: Reflex (1 pt/A) + Diuron (1 pt/A) + Paraquat (2 pt/A) [4 weeks after preplant]
EPOST: Liberty (29 oz/A) + Staple (2.5 oz/A) [2 weeks after PRE]
MPOST: Liberty (29 oz/A) + Dual Magnum (1.0 pt/A) [2 weeks after EPOST]
Layby: MSMA (2.67 pt/A) + Diuron (1 pt/A) [2-3 weeks after MPOST]
EREC - Cover

21. Burndown:Roundup (22 oz/A) + 2,4-D (1 qt/A) +Valor (2 oz/A) [30 preplant]
PRE: Reflex (1 pt/A) + Diuron (1 pt/A) + Paraquat (2 pt/A) [~4 weeks after preplant]
EPOST: Roundup (22 oz/A) +Warrant (3 pt/A) [2 weeks after PRE]
MPOST: Roundup (22 oz/A) +Warrant (3 pt/A) [2 weeks after EPOST]
Layby: MSMA (2.67 pt/A) + Diuron (1 pt/A) [2-3 weeks after MPOST]
EREC - Cover

22. Glyphosate POST fb Glyphosate POST
EREC – No Cover

23. EREC Cotton Strip Plots
w and w/o cover crop
Cotton without rye
cover crop.
Cotton with rye
cover crop.

24. Summary
 Overall, fall seeded cover crops appeared
to reduce early season emergence of
Palmer amaranth from the soil seed bank.
 Postemergence Liberty based systems with
residual herbicides provided consistently
better Palmer amaranth control in these
studies.
 In-season cotton crop response was not
observed with the herbicide programs in
these studies.

25. Summary (cont)
 Seed cotton yields were not affected by
the cover crop or herbicide programs.
 A combination of cover crop and
aggressive herbicide program reduced
the Palmer amaranth emergence.

26. Interseeding System – Wheat/Cotton

27. Interseeding System
 System involves
planting skip row
wheat
 Blocking every
5th seed tube on
a grain drill (7.5
in spacing)
 Plant w/narrow
row unit planter
~3 wks before
wheat harvest

28. Interseeding System
 Wheat is
harvested
 Summer crop
has a 3 wk head
start over
double crop
practice
 Straw residue
left helps
suppress weeds

29. Interseeding Results
Cotton 2014

30. Weed Suppression in Interseeded
Cotton

31. Narrow row interseeded Soybeans

32. Interseeded Peanuts

33. Acknowledgements
 NRCS – National CIG grant program
 NRCS – SC CIG grant program
 SC Cotton Board
 SC Cotton and Soybean Growers

34. Questions?