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Changes in Giant Ragweed Emergence

April 12, 2002
Once the snow melts and the temperature starts to warm, one of the first weed species that emerges through the cool, moist soil appears with a pair of large oval cotyledons. This species is giant ragweed. The emergence of this weed marks the beginning of spring. Over the last couple of years, giant ragweed has grown in prominence to become one of the top five troublesome weeds that corn and soybean producers face. The species has not always been a problem for corn and soybean producers in Illinois. In fact, in the 1960s and 1970s, most giant ragweed plants were found along drainage ditches and roadsides and in the occasional field along a floodplain. So, what has caused this weed to be one of the major challenges that Illinois growers face today?

One could speculate that something has changed in the biology of giant ragweed to make it more of a problem for Illinois corn and soybean farmers. Changes in the duration of giant ragweed emergence appear to be one such modification. In the late 1960s, Drs. Stoller and Wax, USDA-ARS scientists at the University of Illinois, conducted research on the emergence patterns of eight weed species, one of which was giant ragweed. In this study they demonstrated that virtually all giant ragweed plants emerged early in the growing season, usually before May 1. Because these populations emerged relatively early, giant ragweed management was not a problem because tillage operations prior to planting controlled giant ragweed. This does not appear to be the case for contemporary giant ragweed populations in Illinois. Over the years, these same researchers noticed that the species was becoming more of a problem and was showing up in fields much later in the season.

What was happening? Was giant ragweed emerging later in the growing season than it had in the past? To test this hypothesis, they initiated another emergence study in the fall of 1998 and repeated it in 1999. They collected giant ragweed seeds from two separate sites in northern Illinois. The first population came from a production field that was in a corn­soybean rotation, while the second population was collected from an undisturbed area along a railway, approximately 15 miles away from the production field. One hundred seeds from each population were planted at 1-, 2-, and 4-inch depths. Emergence was then monitored weekly the following spring. Giant ragweed from the undisturbed site started to emerge the second week of March, while plants from the field started one week later. Both populations then continued to emerge at the same rate until the first of April. Then something very interesting started to occur. Emergence rates of plants collected from the field slowed, while plants from the undisturbed site continued to emerge at the same rate. Giant ragweed collected from the undisturbed site reached total emergence by May 1, but field-collected giant ragweed continued to emerge well into June. The shift in giant ragweed's emergence duration from seeds collected from production fields, coupled with a shift in production practices to less tillage and earlier planting, has made giant ragweed a tremendous weed management problem for many producers in Illinois.

Even though we may never be able to pinpoint the exact cause for the shift in emergence patterns of giant ragweed, we are currently studying the weed's emergence patterns. The year 2002 is the second year of a research study examining giant ragweed emergence under different cropping systems. In addition, last fall, a multistate collaborative study with weed scientists from USDA-ARS, The Ohio State University, Iowa State University, and the University of Illinois was initiated to examine giant ragweed emergence patterns from four giant ragweed populations from each of the three states. Results from these studies may provide more insight into why we observe differences in giant ragweed from state to state.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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