Urbana, Illinois, USA
October 9, 2013

Widespread and often very dense populations of marestail (horseweed) in soybean fields last spring caught the attention of farmers and other weed management practitioners, with many coming to the difficult realization that marestail is a problem weed species in more than just the southern portions of Illinois, said a University of Illinois associate professor of weed science.

“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,” said Aaron Hager.

Earlier this year, Hager said many growers 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,” he said.
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,” Hager explained.

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 or as late as early summer in some instances.

Following harvest, when is the best time to apply herbicides to control emerged marestail plants?

“Fall-applied herbicides often provide more effective and consistent control of emerged marestail as compared with spring-applied (that is, burndown) herbicides. Apply 2,4-D (1.0 lb. acid equivalent per acre) any time 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,” Hager said.

“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,” he added.

Hager also recommends scouting fall-treated fields before spring planting and taking appropriate measures (that is, supplemental herbicides, tillage, etc.) to control any remaining marestail and emerged summer annual species, including spring-emerging marestail.

“Do not simply assume that fields treated with fall-applied herbicides will be free of marestail next spring,” he said.

Hager also cautioned that growers should not rely on 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, select an application rate that will provide control of winter annuals throughout the remainder of 2013, and avoid increasing the application rate in hopes of obtaining control of summer annual species next spring,” he said.

In northern areas of Illinois, most marestail demonstrates a winter annual life cycle, whereas a substantially higher proportion of spring emergence occurs in areas approximately south of Interstate 70. Both winter and summer annual life cycles can be found across central Illinois.

Fall-emerging plants form a basal rosette representing 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 most common.

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 important when considering the spread of herbicide-resistant biotypes, Hager said.

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.
 

Website: http://www.uiuc.edu

Published: October 9, 2013