Horseweed (Marestail)
Horseweed (marestail) is an annual weed that often becomes a problem in
continuous notill fields, although it has also been a problem in some
tilled fields in recent years. Horseweed can follow a winter annual or
summer annual life cycle. While the majority of the horseweed emerges in
the fall, it can also emerge in spring and early summer. Horseweed does
not mature until late summer, so unlike many other winter annuals that
mature in late spring, horseweed competes directly with corn and soybean
growth during the growing season and interferes with harvest also.
Horseweed is more easily controlled when small in the late fall or early
spring. As the horseweed becomes larger in the spring and early summer,
it becomes more difficult to control. Consider including herbicides with
residual activity in spring burndown treatments to prevent emergence of
horseweed after crop planting. Many populations of horseweed in Ohio
and Indiana appear to be resistant to ALS inhibitors (Classic,
FirstRate, Beacon, etc) and herbicides with this site of action will be
ineffective for horseweed control. Producers should assume most
horseweed populations are ALS-resistant, so use of Roundup Ready
soybeans is suggested to ensure that an effective postemergence option
is available. However, glyphosate-resistant horseweed has been found in
the southern half of Ohio and Indiana, and appears to be spreading
northward.
All crops - preplant control. The most effective and economical
treatments are 2,4D ester (0.5 to 1.0 lb ai/A), or a combination of
2,4-D ester plus glyphosate (0.56 to 1.125 lb ae/A). Application of
2,4-D alone in the in the fall will greatly reduce horseweed populations
the following spring. When applied in early spring to small (less than 2
inches) horseweed, 2,4-D ester (1.0 lb ai/A) is usually effective.
Combinations of glyphosate plus 2,4-D ester should be used on larger
horseweed. Horseweed should be prevented from producing seed in the
summer/fall after wheat harvest with application of 2,4-D or glyphosate
plus 2,4-D in late July or early August.
Corn. Preemergence treatments containing atrazine should control
horseweed that emerge after planting. Most postemergence treatments
containing a growth regulator herbicide (2,4-D, Marksman, dicamba) will
suppress or control horseweed sufficiently until the corn is
wellestablished. Liberty (24 to 34 ounces/A) or Liberty ATZ (32 to 40
oz) will control small horseweed when applied postemergence in Liberty
Link Corn. Glyphosate applied postemergence in Roundup Ready corn
will also control marestail, but this strategy should be avoided if
Roundup Ready soybeans are in the crop rotation.
Soybeans. The presence of ALS and glyphosate resistance in
horseweed populations in Ohio can limit herbicide options for burndown
and postemergencetreatments. The strategy for management of horseweed
should focus on: 1) herbicide application in late fall or early spring
(early April) to control horseweed when small - 2,4-D can adequately
control horseweed in such an application; 2) including herbicides with
residual activity to control later-emerging horseweed (through early
June if possible); and/or 3) use of appropriate herbicides/rates when
trying to control large emerged horseweed later in spring if steps 1 and
2 have not been followed.
A number of soybean herbicides provide residual control of horseweed,
including CanopyXL, FirstRate/Amplify, Valor, Backdraft/Scepter, and
Sencor. These herbicides are more likely to control horseweed through
early June when applied in early April, compared to application the
previous fall. The longevity of control with a fall application will
vary with the density of horseweed population and its emergence pattern
as affected by environmental conditions in spring. CanopyXL is among the
most effective fall-applied herbicides for residual control of
horseweed, but will not adequately control ALS-resistant horseweed when
applied in fall. For residual control of ALS-resistant horseweed in
spring applications, use Valor, Sencor, or high rates of CanopyXL (the
Authority in CanopyXL can provide residual control at high enough
rates). Longevity of control with Valor and Sencor has been somewhat
variable when applied in fall.
In no-till soybean fields where marestail is not controlled with
herbicides in fall or early spring, marestail plants can bolt (i.e. stem
elongates) and become difficult to control by early May. Use of an
effective burndown treatment is essential, because plants that are not
completely controlled will regrow and be less susceptible to later
herbicide applications. The presence of glyphosate or ALS resistance in
some marestail populations results in fewer effective burndown options,
especially when plants become larger than 4 to 6 inches tall. One key to
effective control of emerged marestail is treatment when plants are less
than 2 inches tall, because the activity of 2,4-D and other herbicides
is maximized on small plants. OSU weed scientists conducted a number of
studies in 2002 and 2003 to determine effective treatments for control
of marestail that were 4 to 6 inches tall. This usually corresponded to
application in early to mid May. Marestail at the 2002 research site
were ALS-resistant in areas, and were glyphosate-resistant at two out of
three research sites in 2003. At the other 2003 site, marestail were
sensitive to glyphosate and ALS inhibitors. Some results of this
research (from evaluations taken 30 days after treatment):
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