This document summarizes a presentation on weed management in corn, soybean and wheat. It addresses several questions from farmers on controlling specific weeds like atriplex, giant ragweed, wild carrot, Canada fleabane. For question 1, it shows corn yield decreases as weed size increases at the time of herbicide application. For question 2, it explains the rate of Roundup needs to be adjusted based on weed species and size. It also shows adding 2,4-D to Roundup can cause corn injury and yield loss. It provides options for controlling field horsetail in corn, soybean and wheat. And for question 5, increasing the Roundup rate alone provides better weed control than adding AMS.

1. Peter H. Sikkema
Eastern Ontario
Crop Conference
Kemptville, ON
February 18, 2016
Weed Management in Corn, Soybean & Wheat
Questions from Ontario Farmers
Atriplex in corn
Giant ragweed in cornRagweed in soybean
Wild Carrot in corn
Canada fleabane in wheat
Sikkema, UG
Sikkema, UG Sikkema, UG
Sikkema, UGSikkema, UG

2. Peter, I had planned to spray my corn before
the weeds were 10 cm (4 inches) in height
but we have had three rain events and the
weeds are now 20 cm (8 inches) tall. Will
there be a yield loss?
Question # 1

3. 203
0
50
100
150
200
250
Weed Free 10 cm 20 cm 30 cm
Weed size at the time of herbicide application
Average of 5 experiments
Yield(bu/ac)
Nurse, AAFC
Sikkema, UG
Corn yield decreases as weed size
increases at time application

4. 203
195
0
50
100
150
200
250
Weed Free 10 cm 20 cm 30 cm
Weed size at the time of herbicide application
Average of 5 experiments
Yield(bu/ac)
Nurse, AAFC
Sikkema, UG
Corn yield decreases as weed size
increases at time application
8 bu/ac
Yield loss
2 bu/ac
loss
for every
2.5 cm

5. 203
195
181
0
50
100
150
200
250
Weed Free 10 cm 20 cm 30 cm
Weed size at the time of herbicide application
Average of 5 experiments
Yield(bu/ac)
Nurse, AAFC
Sikkema, UG
Corn yield decreases as weed size
increases at time application
8 bu/ac
Yield loss
22 bu/ac
Yield loss
3.5 bu/ac
loss
for every
2.5 cm in
weed
height
2 bu/ac
loss
for every
2.5 cm in
weed
height

6. 203
195
181
159
0
50
100
150
200
250
Weed Free 10 cm 20 cm 30 cm
Weed size at the time of herbicide application
Average of 5 experiments
Yield(bu/ac)
Nurse, AAFC
Sikkema, UG
Corn yield decreases as weed size
increases at time application
8 bu/ac
Yield loss
22 bu/ac
Yield loss
44 bu/ac
Yield loss
5.5 bu/ac
loss
for every
2.5 cm in
weed
height
3.5 bu/ac
loss
for every
2.5 cm in
weed
height
2 bu/ac
loss
for every
2.5 cm in
weed
height

7. 1. Corn yield decreases as weed size at the time of application
increases
a. 10 cm weeds – reduction in corn yield of 8 bu/ac
b. 20 cm weeds – reduction in corn yield of 22 bu/ac
c. 30 cm weeds – reduction in corn yield of 44 bu/ac
Summary
Question # 1

8. Peter, I had planned to spray my corn before the
weeds were 10 cm (4 inches) in height but we
have had three rain events and the weeds are now
20 cm (8 inches) tall. Will there be a yield loss?
Question # 1
Answer
Most likely.
Corn is very sensitive to early weed competition.
I think all corn producers should consider a two-pass
weed control program of a PRE herbicide followed by a
POST herbicide.

9. Peter, I have always applied Roundup at 0.67
L/ac. Am I doing the right thing?
Question # 2

10. Weed Height = 30 cm tall
You have to adjust the rate of Roundup
depending on weed species for 90% control
Nurse, AAFC
Sikkema, UG
0.5
0.7
0.6
0
0.5
1
1.5
2
2.5
Green foxtail Barnyard grass Redroot pigweed Lamb's-quarters Common
ragweed
RoundupRate(L/ac)
Roundup Weathermax Rate (L/ac)

11. Weed Height = 30 cm tall
You have to adjust the rate of Roundup
depending on weed species for 90% control
Nurse, AAFC
Sikkema, UG
0.5
0.7
0.6
1.3
2.4
0
0.5
1
1.5
2
2.5
Green Foxtail Barnyard grass Redroot pigweed Lamb's-quarters Common
ragweed
RoundupRate(L/ac)
Roundup Weathermax Rate (L/ac)

12. 0.07
0.22
0.50
0.00
0.50
1.00
1.50
10 20 30
RoundupRate(L/ac)
Green foxtailNurse, AAFC
Sikkema, UG
Weed Height (cm)
You have to adjust the rate of Roundup
depending on weed height for 90% control

13. 0.46
0.67
1.28
0.00
0.50
1.00
1.50
10 20 30
RoundupRate(L/ac)
Lamb's-quartersNurse, AAFC
Sikkema, UG
Weed Height (cm)
You have to adjust the rate of Roundup
depending on weed height for 90% control

14. 1. The rate of Roundup needs to be adjusted depending on
weed species. In general, the weeds to control from easiest
to most difficult with Roundup are …
a. Easiest - Annual and perennial grasses
b. Intermediate - Annual broadleaf weeds
c. Most difficult - Perennial broadleaf weeds
i. There are species differences within each of the above
groups
Summary
Question # 2

15. 2. Weed control decreases as size at the time of application
increases
a. Generally, the increase in size can be overcome by
increasing the rate of Roundup
Summary
Question # 2

16. Peter, I have always applied Roundup at 0.67
L/ac. Am I doing the right thing?
Question # 2
Answer
Probably not

17. Question # 3
Peter, my corn is in the V5 stage and I noticed
some Canada fleabane escapes after my
POST application of Roundup. I have never
seen them on the farm before, so I assume
they are glyphosate resistant.
Is it OK to add some 2,4-D to my late in-crop
application of Roundup to control GR Canada
fleabane?

18. Corn Injury
Symptoms
Phenoxies
1. Lodging or goose
necking caused by …
a. Heavy rain and high
winds
i. Saturated soils
b. Late application of
Roundup + 2,4-D
c. Sensitive hybrid
Sensitive Hybrid – 25 bu/acre yield loss
189 bu/ac 164 bu/ac
Sikkema, UG

19. 2,4-D Injury in Corn
Lodging or goose-necking
Brace-root malformationOnion leafing
Sikkema, UG
Sikkema, UG
Brittle snap
Sikkema, UG
Sikkema, UG

20. 0
4
0
11
0
10
20
30
40
50
Roundup 2,4-D Roundup + 2,4-D
1X 2X
Sikkema, UG
CornInjury(%)–1WAA
V5 application - Mean of 2 hybrids at 2 sites = 4 data points
Can I add 2,4-D to Roundup for the
control of GR Canada fleabane?
Corn Injury (1 WAA)

21. 0
4
9
0
11
28
0
10
20
30
40
50
Roundup 2,4-D Roundup + 2,4-D
1X 2X
CornInjury(%)–1WAA
Sikkema, UG
Can I add 2,4-D to Roundup for the
control of GR Canada fleabane?
5-17%
Synergistic
Increase
in Injury
V5 application - Mean of 2 hybrids at 2 sites = 4 data points
Corn Injury (1 WAA)

22. 269
253
269
241
0
50
100
150
200
250
300
Roundup 2,4-D Roundup + 2,4-D
1X 2X
CornYield(bu/ac)
Sikkema, UG
Can I add 2,4-D to Roundup for the
control of GR Canada fleabane?
16-28 bu/ac
Yield loss
V1, V3, V5 application - Mean of 2 hybrids at 1 site = 6 data points

23. 269
253
241
269
241
223
0
50
100
150
200
250
300
Roundup 2,4-D Roundup + 2,4-D
1X 2X
CornYield(bu/ac)
Sikkema, UG
Can I add 2,4-D to Roundup for the
control of GR Canada fleabane?
16-28 bu/ac
Yield loss
28-46 bu/ac
Yield loss
V1, V3, V5 application - Mean of 2 hybrids at 1 site = 2 data points

24. Question # 3
Answer
No. There is potential for corn injury and yield loss.
It would be OK if you have Enlist corn
Peter, I noticed some Canada fleabane escapes in my
Roundup Ready corn.
Is it OK to add some 2,4-D to my late in-crop application
of Roundup to control the GR Canada fleabane?

25. Sikkema, UG
Sikkema, UG
Question # 4
Peter, ever since I started growing RR corn and
soybean, field horsetail is an increasing problem on
my farm.
How can I control it?

26. Field Horsetail Control in Corn
37
50
66
57
73
0 20 40 60 80 100
Ultim
Broadstrike
MCPA
Ultim + Broadstrike
Ultim + MCPA
Broadstrike + MCPA
Ultim + Broadstrike + MCPA
Weed Control (%)
Summary of 6 experiments
Postemergence
Sikkema, UG

27. Field Horsetail Control in Corn
37
50
66
57
73
83
85
0 20 40 60 80 100
Ultim
Broadstrike
MCPA
Ultim + Broadstrike
Ultim + MCPA
Broadstrike + MCPA
Ultim + Broadstrike + MCPA
Weed Control (%)
Summary of 6 experiments
Postemergence
Sikkema, UG

28. Weedy Check
Field Horsetail - Corn - POST
MCPA (66%) Broadstrike + MCPA (83%)
Broadstrike (50%)
Sikkema, UG
Sikkema, UG Sikkema, UG
Sikkema, UG

29. Field Horsetail
Ultim + Broadstrike + MCPA
June 11
Litwin
June 24
Litwin
Litwin

30. What about MCPA injury in corn?
Lodging or goose-neckingBrace-root malformation
Onion leafing
Sikkema, UG Sikkema, UG
Brittle snap
Sikkema, UG Sikkema, UG

31. MCPA Tolerance in Corn
237
220
207
193
0 50 100 150 200 250
Control
1/4 X
1/2 X
1X
Yield (bu/ac)
V6 (8 leaf stage)
44 bu/ac yield loss when applied late!
Sikkema, UG

32. MCPA Tolerance in Corn
237
239
237
235
0 50 100 150 200 250
Control
1/4 X
1/2 X
1X
Yield (bu/ac)
V2 (4 leaf stage)
It is pretty safe applied at V2 (4 leaf stage)
Sikkema, UG

33. Field Horsetail in
Soybean
Roundup + Broadstrike + Dual
Applied: June 8
July 11
Litwin

34. Field Horsetail Control in Soybean
89
95
46
91
84
97
0 20 40 60 80 100
Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Weed Control (%)
Site 1 Site 2 Site 3 Site 4 Site 5 Site 6
Roundup + Broadstrike + Dual
Sikkema, UG

35. Field Horsetail in
Soybean
Roundup + Broadstrike + Dual
Applied: June 8
July 11 October 15
Litwin
Litwin Litwin

36. Field Horsetail
Roundup + Broadstrike + Dual
One month after application After harvest
LitwinLitwin

37. Field Horsetail in Wheat
MCPA is the product of choice
Sikkema, UG Sikkema, UG

38. Question # 4
Peter, ever since I started growing RR corn and
soybean, field horsetail is an increasing problem on
my farm.
How can I control it?
Answer
1. Corn: Broadstrike + MCPA
a. Apply early POST
b. Only spray the patches (can cause crop injury)
2. Soybean: Roundup + Dual + Broadstrike applied PP
3. Wheat: MCPA applied POST

39. Question # 5
Should I add AMS to Roundup in
my RR corn and soybean?

40. 1. The purpose of adding AMS is to tie up cations prior to
adding glyphosate to the tank
a. Calcium
b. Magnesium
To take note of …
Should I add AMS to Roundup?

41. Should I add AMS to Roundup?
How did we answer this question?
These are the rates and prices used:
Roundup Rate 0.67 L/ac
Roundup Cost $8.00/L
Price/acre $5.36/ac
AMS Rate 1 L/ac
AMS Cost $3.00/L
Price/acre $3.00/ac

42. Should I add AMS to Roundup?
There are two options …
Roundup (0.67 L/ac) $5.36/ac
AMS (1.0 L/ac) $3.00/ac
Total cost/acre $8.36/ac
Roundup (0.67 L/ac) $5.36/ac
Roundup (0.38 L/ac) $3.00/ac
Roundup (1.05 L/ac) $8.36/ac
Farmer A
Add AMS
Farmer B
Increase the rate of Roundup

43. 72
81
74
84
0
20
40
60
80
100
1/2X 1X
Roundup Roundup + AMS
WeedControl(%)
Sikkema, UG
Weed Control (1 WAA)
Glyphosate Rate
Should I add AMS to Roundup?

44. 72
81
74
8481
86
0
20
40
60
80
100
1/2X 1X
Roundup Roundup + AMS Roundup + Roundup
WeedControl(%)
Weed Control (1 WAA)
Glyphosate Rate
Sikkema, UG
Should I add AMS to Roundup?

45. 83
90
84
92
88
92
0
20
40
60
80
100
1/2X 1X
Roundup Roundup + AMS Roundup + Roundup
WeedControl(%)
Sikkema, UG
Weed Control (8 WAA)
Glyphosate Rate
Should I add AMS to Roundup?

46. Question # 5
Answer
1. There is a greater increase in weed control by simply
increasing the rate of Roundup.
2. Corn and soybean yields were equivalent
Should I add AMS to glyphosate in
my RR corn and soybean?

47. 1. Results may vary depending on
a. Water source
b. Weed species
c. Environmental conditions
d. Glyphosate formulation
Summary Thoughts
Should I add AMS to Roundup?

48. Peter, I really would like to underseeded my
wheat to red clover. What are my herbicide
options?
Question # 6
Sikkema, UG Sikkema, UG

49. 61
26
52
71
69
30
0 20 40 60 80 100
2,4-D
Target
Estaprop
Buctril M
Refine
Trophy
Infinity
Red Clover Biomass (% of control) - 19 site years
Robinson, UG
Winter Wheat – Red Clover

50. 61
26
52
100
71
69
30
0 20 40 60 80 100
2,4-D
Target
Estaprop
Buctril M
Refine
Trophy
Infinity
2,4-D Target Estaprop Buctril M Refine Trophy Infinity
Robinson, UG
Winter Wheat – Red Clover
Red Clover biomass (% of control)

51. Check
Buctril M
Sikkema, UG
Sikkema, UG
Target
Infinity
Sikkema, UG
Sikkema, UG
Winter Wheat – Red Clover

52. Peter, I really would like to underseeded my
wheat to red clover. What are my herbicide
options?
Question # 6
Answer
I think Buctril M is your best option

53. Buctril M
Winter Wheat &
Red Clover
1. Apply when the red clover is in
the 1st to 3rd trifoliate and when
the wheat provides a protective
canopy over the clover
a. May burn clover when hot &
dry
i. May not come back
b. Most of the time at harvest
there is no difference
Bourassa, Dupont
Bourassa, Dupont
Buctril M

54. Question # 7
I have heard about glyphosate
resistant weeds so I want to add a
tankmix partner to my preplant
burndown. Is it safe to add 2,4-D
prior to seeding soybean?

55. Less than 1% injury with 2,4-D 600
ester at 0.5 L/acre applied 7 or 14 DPP
0.6
0.5
0.0 2.0 4.0 6.0 8.0 10.0
14 DPP
7 DPP
1 DPP
7 DAP
Injury (%) 7 DAE - Mean of 6 Studies
0.5 L/ac
Hamill, AAFC
Swanton, UG
Sikkema, UG

56. Less than 1% injury with 2,4-D 600
ester at 0.5 L/acre applied 7 or 14 DPP
0.6
0.5
3.4
3.2
0.0 2.0 4.0 6.0 8.0 10.0
14 DPP
7 DPP
1 DPP
7 DAP
Injury (%) 7 DAE - Mean of 6 Studies
0.5 L/ac
Hamill, AAFC
Swanton, UG
Sikkema, UG

57. Control
2,4-D PP Injury in Soybean
Leaf Strapping
Leaf StrappingLeaf Strapping
Sikkema, UG
Sikkema, UG
Sikkema, UG
Sikkema, UG

58. 0 20 40 60 80 100
14 DPP
7 DPP
1 DPP
7 DAP
Yield (% of check) - Mean of 6 Studies
0.5 L/ac
Hamill, AAFC
Swanton, UG
Sikkema, UG
No yield loss with 2,4-D 600 ester at
0.5 L/acre applied 7 or 14 DPP

59. Question # 7
I have heard about glyphosate resistant weeds so
I want to add a tankmix partner to my preplant
burndown. Is it safe to add 2,4-D 600 ester prior
to seeding soybean?
Answer
Yes, when applied at 0.5 L/acre, 7 or 14 days
prior to seeding, there was …
Less than 1% injury
No yield loss

60. OK Peter, you convinced me that I
should use a PRE residual in
my RR corn.
Does it matter which one I use?
Question # 8

61. Converge
Engarde
Integrity
Lumax
Primextra
Which soil applied herbicide should
I use?
Lots to choose from…
Which one is the best?

62. 0
20
40
60
80
100
Pigweed, Green Lamb's-quarters Lady's Thumb Barnyard Grass Foxtail, Green
Integrity Converge Primextra Lumax
WeedControl(%)4WAE
Depending on the weed spectrum, sometimes it does not matter*
Which soil applied herbicide should
I use?
Nurse, AAFC
Gillard, UG
Sikkema, UG *Set up rates were used. Weed control ratings prior to Roundup applied POST

63. 64
74
65
77
44
64
76
83
0
20
40
60
80
100
Velvetleaf Ragweed, Common
Integrity Converge Primextra Lumax
WeedControl(%)4WAE
Which soil applied herbicide should
I use?
Depending on the weed spectrum, sometimes it does matter *
Nurse, AAFC
Gillard, UG
Sikkema, UG *Set up rates were used. Weed control ratings prior to Roundup applied POST

64. 107 d
205 abc 204 abc 202 abc
210 ab 207 ab
0
50
100
150
200
250
Weedy Integrity Converge Primextra Lumax Weed-free
Corn Yield (bu/ac)
CornYield(bu/ac)
Which soil applied herbicide should
I use?
No difference in corn yield*
Nurse, AAFC
Gillard, UG
Sikkema, UG
*Two-pass weed control – PRE residual fb Roundup POST

65. OK Peter, you convinced me that I should use a PRE residual
in my RR corn.
Does it matter which one I use?
Question # 8
Answer
1. Yes, it will depend on …
a. Weed control
i. Weed species composition and density
ii. Resistance management
b. Crop safety
i. Soil characteristics – texture, pH, OM, CEC
ii. Corn hybrid
c. Persistence - future crops in the rotation

66. I applied a PRE herbicide in corn and it was perfectly clean up to
the 6-leaf stage (V4), however, the last time I scouted the
field, I noticed some weeds. Will they reduce corn yield?
Is it worth it to apply a POST herbicide?
Question # 9
Weed free to 2-leaf Weed free to 10-leaf

67. Weed free to 4-leafWeed free to 2-leaf
Weed free to 6-leaf Weed free to 8-leaf
Weedy check
Experimental Design
Weed free to 10-leaf

68. 61
82
91
97 99
0
20
40
60
80
100
2 4 6 8 10
Weed free up to the above leaf stage
Average of 13 weed species in 11 experiments
WeedControl(%)–42DAAEnd of season weed control improves
the longer corn is kept weed free
Nurse, AAFC
Sikkema, UG
Everman, MSU
Sprague, MSU

69. Keep corn weed-free until at
least the 6-leaf stage
105
185
198
206 207 206 204
0
50
100
150
200
250
Weedy
check
2 4 6 8 10 Weed free
check
Weed free up to the above leaf stage
Average of 11 experiments
Yield(bu/ac)
Nurse, AAFC
Sikkema, UG
Everman, MSU
Sprague, MSU

70. WF to 4-leaf (-6 bu/ac)WF to 2-leaf (-19 bu/ac)
Weed free to 6-leaf Weed free to 8-leaf
Weedy (- 99 bu/ac)
Weed free to 10-leaf
Keep corn weed-free until at
least the 6-leaf stage

71. I applied a PRE herbicide in corn and it was perfectly
clean up to the 6-leaf stage (V4), however, the last
time I scouted the field, I noticed some weeds. Will
they reduce corn yield?
Is it worth it to apply a POST herbicide?
Question # 9
Answer
Probably not

72. I applied a PRE herbicide in soybean and it
was perfectly clean up to the V2 stage (2nd
trifoliate leaf), however, the last time I
scouted the field, I noticed some weeds. Will
they reduce soybean yield?
Is it worth it to apply a POST herbicide?
Question # 10

73. Weed free to VCWeedy check
Experimental Design
Weed free to V2Weed free to V1
Sikkema, UG Sikkema, UG
Sikkema, UG Sikkema, UG

74. 45
80
93
98 100 100
0
20
40
60
80
100
VC V1 V2 V3 V4 V5
Average of 13 weed species in 6 experiments
WeedControl(%)–42DAAEnd of season weed control improves
the longer soybean is kept weed free
Sikkema, UG
Weed free until the above leaf stage

75. Keep soybean weed-free until the V2 stage
86 89
99 99 100 100 99 100
0
20
40
60
80
100
Weedy
check
VC V1 V2 V3 V4 V5 Weed free
check
Average of 6 experiments
Yield(%ofweed-freecontrol)
Sikkema, UG
5 bu/ac yield
increase
Weed free until the above leaf stage

76. Weed free to VC (- 5 bu/ac)Weedy check
Weed free to V2Weed free to V1
Keep soybean weed-free until V2 stage
Sikkema, UG Sikkema, UG
Sikkema, UG Sikkema, UG

77. Question # 10
Answer
Probably not
I applied a PRE herbicide in soybean and it
was perfectly clean up to the V2 stage (2nd
trifoliate leaf), however, the last time I
scouted the field, I noticed some weeds. Will
they reduce soybean yield?
Is it worth it to apply a POST herbicide?

78. 1. To be conservative, and to avoid any yield loss …
a. Corn should be kept weed-free until the 6-leaf stage (V4)
b. Soybean should be kept weed-free until the 2nd trifoliate
leaf stage (V2)
2. There are other benefits to controlling late emerging weeds
a. Improved harvest efficiency
b. Less staining of beans (ie) nightshade
c. Reduced weed seed return to the soil
i. Reduced weed density in the following years
Further Thoughts
Late Emerging Weeds

79. What is the impact of weed competition on the yield of
field crops?
Question # 11
Weed competition in corn Weed competition in soybean

80. 56
49
34
12
3
0
10
20
30
40
50
60
Dry Bean Corn Soybean Spring Cereals Winter Wheat
Ridgetown & Exeter, ON (2007-2015) - 224 Field Studies
YieldLoss(%)
Sikkema, UG
Yield Loss due to Weeds

81. $ lost in Ontario if no weed control
Crop
Farm Gate
Value*
($ ‘000, 000)
Yield
Loss
(%)
Farm Gate
Value
($ ‘000, 000)
Corn 1397 49 680
Soybean 1939 34 663
Dry bean 118 56 66
Cereals, spring 380 12 11
Wheat, winter 92 3 11
Total 4653 1431
* 2014 - http://www.omafra.gov.on.ca/english/stats/crops/estimate_new.htm
Average 31% financial loss

82. Question # 11
Answer
1. If Ontario farmers did not employ any weed
management tactics the yield loss would be …
a. 56% in dry beans
b. 49% in corn
c. 34% in soybean
d. 12% in spring cereals
e. 3% in winter wheat
2. $1.43 billion dollars annually
What is the impact of weed competition on the yield of
field crops?

83. Acknowledgements
• Funding agencies
• Research technicians
• Graduate students
• Summer research assistants
Weed Management in Corn, Soybean & Wheat
Questions from Ontario Farmers