Fall Herbicides to Control Marestail (Horseweed)

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.