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Giant Ragweed: Old Weed, New Problem

Loyd Wax Loyd Wax
USDA-ARS; Dept.of Crop Sciences
(217) 333-4424; l-wax@illinois.edu
Kurt Maertens
  Kurt Maertens
Graduate Assistant
Department of Crop Sciences
(217) 333-4424; kmaerten@illinois.edu
Christy Sprague  

Christy Sprague
Assistant Professor
Department of Crop Sciences
(217) 333-4424; csprague@illinois.edu

Giant ragweed is an early germinating, summer annual weed species that is commonly found throughout Illinois and the Midwest. Surveys conducted in Illinois this year rank giant ragweed one of the most prevalent weeds in the state, rivaling only waterhemp, which holds the number one spot. This is quite different from surveys that were conducted over 20 years ago, when giant ragweed was not even ranked among the top 10. So what has happened? Several factors have contributed to the overall predominance of giant ragweed across the state.

One of these factors is giant ragweed’s competitive ability. Giant ragweed is potentially the most competitive weed species in Illinois. The competitive characteristics of giant ragweed include seed persistence, early seedling emergence, and rapid plant growth that results in competition for light, water, and nutrients. Competition research has shown that season-long competition from two and one giant ragweed plants per m2, respectively, can reduce corn yield 37 percent (Harrison et al. 2001) and soybean yield 52 percent (Baysinger and Sims et al. 1994).

So how does this compare with Illinois’ most prevalent weed, waterhemp? Recent research by University of Illinois scientists has shown the effects of waterhemp and giant ragweed densities on soybean yield. Season-long waterhemp competition at 89 plants per m2 can result in a yield loss of 31 percent (Hager et al. 2002), while just 15 giant ragweed plants per m2 can reduce yield 87 percent. These data demonstrate how much more competitive giant ragweed is compared to waterhemp, potentially making giant ragweed the most problematic weed that a grower could face.

In addition to its competitive ability, giant ragweed is moving from its primary habitat. Historically, giant ragweed was found mostly in undisturbed areas such as fencerows and drainage ditches, and it could occasionally be found in flood-plain fields. However, in the last decade, giant ragweed populations have dispersed from their primary habitats into many fertile fields across the state. The cause of this spread is unknown. However, it is clear that giant ragweed has adapted to survive new agronomic practices, such as earlier planting and less tillage.

Counties that ranked giant ragweed as the #1 most common weed

Figure 1. Survey results from 2002: Counties that ranked giant ragweed as the #1 most common weed.

One adaptation that giant ragweed has made has been a shift in the time of emergence. Historically, giant ragweed plants would emerge early in the growing season and normally would not be a problem in agronomic production systems. Results from University of Illinois research in the 1960s and 1970s showed that virtually all giant ragweed plants would emerge by May 1st (Stoller and Wax 1973). However, recent research has shown that giant ragweed emergence in Illinois production fields can start in March and continue into June (and sometimes into late July), making this species a management challenge due to its multiple emergence times.

Last fall, a multistate collaborative study with weed scientists from Illinois, Iowa, and Ohio was initiated to examine giant ragweed’s emergence patterns from four giant ragweed populations from each of the three states. Each state collected seed from three field locations and one undisturbed area. Seeds from each state were planted in the fall of 2001, and in the spring, newly emerged giant ragweed seedlings were counted bi-weekly. Results from these studies may provide more insight into why we observe differences in giant ragweed from state to state.

Currently, giant ragweed is a management challenge for many Illinois growers. These challenges can be attributed to giant ragweed’s extended emergence patterns and rapid growth rate. In addition, growers must not overlook the development of ALS-resistance that has limited the use of some very effective herbicides in some areas. The most consistent giant ragweed control programs are those that combine sequential management approaches. It also is important to keep giant ragweed populations in undisturbed areas in check. These areas often can be the source of seeds to spread into fields.


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