Fall Herbicides To Control Marestail (Horseweed)
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Dr. Aaron Hager
URBANA, ILL.
Widespread and often very dense populations of marestail in soybean
fields last spring caught the attention of farmers and other weed
management practitioners. Many came to the difficult realization that
marestail is not a problem weed species only in the more southern
portions of Illinois. It’s difficult to say with complete accuracy how
far north these infestations occurred, but mature marestail was easily
observed during recent travels through Kankakee and Will counties. As we
mentioned earlier this year, many reported poor marestail control from
herbicides applied prior to planting (primarily no-till soybean),
especially when burndown applications contained only glyphosate or
glyphosate plus 2,4-D. The increasing frequency of glyphosate-resistant
marestail populations, the rush to plant whenever field conditions were
conducive, and the less-than-ideal environmental conditions when many
burndown applications were made, contributed to a challenging situation
for which a good solution was not always readily available.
Marestail is native to North America and like many other plant species
completes its life cycle in one year. Unlike many other annual species,
however, marestail can exist as a winter or summer annual. Populations
of winter annual marestail typically emerge during the fall months,
within a few days or weeks after seed is dispersed from the parent
plant. Summer annual populations can emerge in early or late spring,
perhaps as late as early summer in some instances. In northern areas of
Illinois, most marestail demonstrates a winter annual life cycle,
whereas a substantially higher proportion of spring emergence occurs in
areas south of (approximately) Interstate 70. Both winter and summer
annual life cycles can be found across central Illinois.
Fall-emerging plants form a basal rosette that represents the plant’s
overwintering stage. In the spring, plants bolt by rapidly elongating
the main stem. Mature horseweed plants may reach heights in excess of 6
feet, but plants ranging from 3 to 5 feet are perhaps most common.
Flowers are produced in a panicle-type inflorescence at the top of the
plant. The seeds are known as achenes, and are produced with an
attached “parachute” (known as a pappus) to aid in wind-borne dispersal.
Research has demonstrated that mature marestail plants can produce in
excess of 200,000 seeds, with fall-emerging plants frequently producing
more seeds than spring-emerging plants. Marestail seed can travel long
distances with its dispersal mechanism, which becomes especially
important when considering the spread of herbicide-resistant biotypes.
Mature seeds do not demonstrate much dormancy, but rather germinate soon
after contact with the soil surface. Seeds do not remain viable in the
soil seedbank for very long.
We have received many questions about applying herbicides following
harvest to control emerged marestail plants. Fall-applied herbicides
often provide more effective and consistent control of emerged marestail
as compared with spring-applied (i.e., burndown) herbicides. We suggest
applying 2,4-D (1.0 lb acid equivalent per acre) anytime between
mid-October and late November to control emerged marestail. This
treatment should not be expected to provide much soil-residual activity,
so marestail plants that emerge after application will most likely not
be controlled. Do not rely solely on glyphosate (either in the fall or
spring) to control emerged marestail. Other herbicides (including
glyphosate) can be tankmixed with 2,4-D to broaden the spectrum of
winter annual species controlled.
Do not simply assume that fields treated with fall-applied herbicides
will be free of marestail next spring. Be sure to scout fall-treated
fields before spring planting and take appropriate measures (i.e.,
supplemental herbicides, tillage, etc.) to control any existing
marestail plants. Do not plant soybean into an existing marestail
population. Residual herbicides should be applied close to soybean
planting to control summer annual species, including spring-emerging
marestail.
We do not recommend fall herbicide applications as an avenue to provide
residual control of summer annual weed species. Control of summer annual
species, such as waterhemp, is often improved when soil-residual
herbicides are applied closer to planting compared with several weeks
(or months) prior to planting. If a soil-residual herbicide will be part
of a fall herbicide application, we suggest selecting an application
rate that will provide control of winter annuals throughout the
remainder of 2013, and recommend against increasing the application rate
in hopes of obtaining control of summer annual species next spring.∆
DR. AARON HAGER: Associate Professor, University of Illinois
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