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3/24/2008 - 3/30/2008

Starting Clean in No-till

Bob Hartzler, Department of Agronomy

As more Iowa fields develop a history of no-till production, infestations of winter annual weeds are on the increase. The first step in managing winter annuals is proper identification of the species infesting the field. Many agronomists are relatively weak at identifying these weeds. A regional extension publication - Early spring weeds on no-till crop production  (NCR 614) - can assist in this process.

After the weeds have been identified, appropriate treatments and application timing can be selected. Applications in early to mid-April often have advantages compared to treatments made at planting. 

First, winter annuals will be in a vegetative stage of growth and frequently are much easier to kill than when they reach the reproductive stage around planting time. Secondly, research has shown that killing existing vegetation at least 10 days prior to planting minimizes the risk of negative effects on crop growth and yield. Finally, combining 2,4-D with glyphosate or other products improves the consistency of control on many common winter annuals and dandelion. Early spring applications avoid conflicts with the planting delays specified on the 2,4-D label for both corn and soybean, therefore minimizing risks of adverse crop responses.

The particular herbicide treatment used for controlling winter annuals is dictated by the weeds present and objectives of the treatment. Due to high frequency of glyphosate resistant horseweed (marestail) across the Midwest, applications of glyphosate alone are not recommended for fields infested with this weed. Including 2,4-D LVE will improve control of horseweed.

The use of preemergence (PRE) herbicides in glyphosate resistant (GR) crops is likely to increase due to the increased cost of glyphosate. Most PRE products can be applied early with burndown herbicides, but early applications will reduce their length of residual control. 

Reduced rates of PRE herbicides are popular with GR crops since the PRE herbicides are not expected to provide full season weed control. Too large of a reduction in the PRE rate combined with an application several weeks ahead of planting may result in failure to suppress weeds long enough to allow a single, in-season application of glyphosate to provide full season control and protect crop yields.

Bob Hartzler is a professor weed science with extension, teaching and research responsibilities.

Winter Annual Weeds and SCN – Is There Cause for Concern?

By Greg Tylka, Department of Plant Pathology

The frequency of occurrence of winter annual weeds in Iowa fields has increased as more fields are managed with no-till production practices. And in recent years, scientists have discovered that the winter annual weeds purple deadnettle, henbit and field pennycress are moderate to good hosts for the soybean cyst nematode (SCN).

Iowa growers and crop advisors are inquiring whether winter annual weeds could be supporting SCN reproduction leading to increases in SCN population densities. The answer depends on soil temperature.

SCN juveniles cannot develop in roots at temperatures below 50°F. But if purple deadnettle, henbit, and field pennycress are growing in SCN-infested fields and soil temperatures are greater than 50°F, SCN reproduction and increases in population densities can occur.

Female Soybean Cyst Nematode
Soybean cyst nematode female on root of purple deadnettle (E. Creech, Purdue University).

The SCN life cycle takes about 24 days to complete at ideal temperatures (76°F) and it takes 4 or more weeks at colder temperatures. So depending on the year, there may be a limited time when soil temperatures are warm enough for SCN reproduction to occur on winter annual weeds growing in Iowa fields.

Weed and Soybean Cyst Nematode Timeline
Timeline of winter annual weed and soybean growth and soybean cyst nematode activity (from “Winter annual weed and soybean cyst nematode management” available online at

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

This article was published originally on 3/31/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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