COLLEGE OF AGRICULTURE AND NATURAL RESOURCES
COOPERATIVERESOURCES
EXTENSION
POSTEMERGENCE CONTROL OF HORSEWEED IN CORN.
Mark VanGessel, University of Delaware, Extension Weed Specialist.
Horseweed (or marestail) is a winter annual weed that also emerges in the early
spring. It has become a serious problem in no-till soybeans, but is seldom a problem in
no-till corn. Presumably, horseweed is less of an issue in corn because the crop is
planted earlier (thus, the horseweed plants are sprayed when they are smaller and
more susceptible) and herbicides available for corn are generally more effective for
horseweed control than soybean herbicides. In the past few years, the UD Weed
Science Program has only had one or two phone calls on control of horseweed in corn.
The week of May 12, 2008, UD Weed Science Program looked at a no-till corn field
near Greenwood, DE that had been treated with Gramoxone Inteon plus Bicep II
Magnum prior to planting corn. Initially, the horseweed plants showed signs of injury,
but they recovered and required an additional treatment.
We first tested horseweed plants for resistance to the herbicides used for
burndown. Since horseweed is a species known to develop resistance to a number of
herbicides, individual horseweed plants were transplanted into small pots, taken to the
UD, Research and Education Center, and tested with Gramoxone Inteon and atrazine.
In addition, horseweed plants from the UD-REC were transplanted and treated with the
same herbicides and combinations. There was no difference between plants collected
at the UD-REC and the field with initially poor horseweed control. Lack of control from
the pre-plant treatment was not due to herbicide resistance. Lack of control may have
been due to the environment or application method.
In order to provide better recommendations for horseweed control in emerged
corn, a field trial was established to determine the most effective treatment for
horseweed control in emerged corn. The plots were established in the field with poor
initial control and treated May 15, 2008 with a CO2 back pack sprayer with 11003
extended range tips. The herbicides were applied in a spray volume of 25 gpa with 25
psi and traveling 3 mph. The corn was about eight inches tall (3 to 4 visible collars) and
the horseweed was five inches tall and density was greater than 100 plants/m2. Table 1
shows the herbicides and rates used. The corn hybrid had the Liberty Link trait.
No corn injury was observed from any herbicide treatment. Liberty resulted in
extensive leaf burn on horseweed, but did not completely kill the plants. Horseweed
quickly recovered from the initial injury, and by 19 DAT, control was ≤75%. The highest
rate of Status (10 oz wt/A) plus atrazine provided a high level of horseweed control at
12 DAT, as well as 19 DAT. At 19 DAT, there was no difference between Status at 10
oz wt/A alone or with atrazine. Callisto plus atrazine provided a high level of horseweed
control by 19 DAT. Liberty, atrazine, NorthStar, and Status at 5 and 7 oz wt/A provided
less horseweed control than Status at 10 oz wt/A and Callisto plus atrazine.
Control of horseweed plants in emerged corn was best (88 to 92% control) with
Status at 10 oz wt/A or Callisto plus atrazine. Applications should be made as early as
possible, since horseweed plants grow rapidly in the spring and can quickly become too
large for effective control.
Mark VanGessel, Research and Education Center - Carvel Bldg,
16483 County Seat Highway, Georgetown, DE 19947
Fall, 2008
Table 1. Herbicide treatments and visual control ratings for horseweed taken 12 and 19
days after treatment (DAT).
Horseweed Control
5/27/2008
6/3/2008
12 DAT
19 DAT
0
0
87 ab
73 de
Treatment
Form Form
Rate
Product Product
Name
Conc Type Rate Unit
Rate
Rate Unit
Untreated Check
Liberty
1.67 SL
0.365 lb ai/A
28 fl oz/A
+Atrazine
4 L
1 lb ai/A
1 qt/A
+AMS
100 D
4.26 lb ai/A
17 lb/100 gal
Liberty
89 a
1.67 SL
0.417 lb ai/A
32 fl oz/A
+Atrazine
4 L
1 lb ai/A
1 qt/A
+AMS
100 D
4.26 lb ai/A
17 lb/100 gal
Liberty
88 ab
1.67 SL
0.417 lb ai/A
32 fl oz/A
+AMS
100 D
4.26 lb ai/A
17 lb/100 gal
Status
78 cd
56 WG
0.175 lb ai/A
5 oz wt/A
+Atrazine
4 L
1 lb ai/A
1 qt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
+30% Liquid N
100 L
1.25 % v/v
5 qt/100 gal
Status
77 cd
56 WG
0.245 lb ai/A
7 oz wt/A
+Atrazine
4 L
1 lb ai/A
1 qt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
+30% Liquid N
100 L
1.25 % v/v
5 qt/100 gal
Status
87 ab
56 WG
0.35 lb ai/A
10 oz wt/A
+Atrazine
4 L
1 lb ai/A
1 qt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
+30% Liquid N
100 L
1.25 % v/v
5 qt/100 gal
Status
82 bc
56 WG
0.35 lb ai/A
10 oz wt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
+30% Liquid N
100 L
1.25 % v/v
5 qt/100 gal
Callisto
80 cd
4 SC
0.094 lb ai/A
3 fl oz/A
+Atrazine
4 L
0.5 lb ai/A
1 pt/A
+COC
100 L
1 % v/v
1 qt/A
+30% Liquid N
100 L
2.5 % v/v
10 qt/100 gal
NorthStar
75 d
47.4 WG
0.148 lb ai/A
5 oz wt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
+30% Liquid N
100 L
2.5 % v/v
10 qt/100 gal
Atrazine
32 e
4 L
1 lb ai/A
1 qt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
Atrazine
37 e
4 L
1.5 lb ai/A
1.5 qt/A
+NIS
100 L
0.25 % v/v
32 oz/100 gal
Numbers followed by the same letter are not significantly different from one another.
NIS= non-ionic surfactant (80:20)
COC= Crop oil concentrate
69 e
75 d
87 b
85 bc
92 a
88 ab
90 ab
80 c
28 g
43 f