Giant ragweed is usually the first summer annual weed to emerge that poses significant problems in Iowa corn and soybeans. Giant ragweed usually initiates emergence in early April in Iowa, typically 2 to 4 weeks earlier than giant foxtail. In ISU studies, giant ragweed also has had a very rapid emergence pattern, typically reaching 50% emergence in less than a week of initial emergence. In contrast, in Ohio and Illinois giant ragweed typically did not reach 50% emergence for 6 to 7 weeks after the first seedling emerged. Although there have been few studies on this topic, most weeds seem to display similar emergence characteristics across the corn belt, apparently making giant ragweed somewhat unique.
The magnitude of problems created by giant ragweed varies across the corn belt. In Ohio, giant ragweed is considered the most problematic weed in corn and soybean. Illinois weed scientists state that giant ragweed would fall in the top five weeds. Iowa appears to be on the western edge of the adapted region for giant ragweed. In eastern Iowa giant ragweed probably would be ranked in the lower half of Top 10 Weeds, whereas I suspect it wouldn't even make the Top 25 in western Iowa. The emergence pattern observed in Ohio and Illinois poses greater management problems than the rapid emergence seen in Iowa, and probably is largely responsible for giant ragweed being a greater problem in the east than in the western corn belt. Problems with giant ragweed in Iowa have increased over the past 5 to 10 years, possibly because earlier planting dates are allowing more giant ragweed to emerge after crop establishment.
A cooperative project involving The Ohio State University, University of Illinois, USDA/ARS and Iowa State University was initiated in the fall of 2001 to determine whether differences in emergence patterns of giant ragweed are controlled genetically or due to differences in soils and climate across the region. Seed was collected from four giant ragweed populations in each state. Three of the populations from each state came from agronomic fields, whereas one population was collected from a non-agricultural habitat (e.g. floodplain). Seed was exchanged among the cooperating states and 500 giant ragweed seeds were mixed in the upper 2" of soil contained in buried PVC pipe. Seedlings were counted and pulled twice a week when cotyledons were fully expanded.
Results: Since the giant ragweed biotypes behaved similarly at all three locations, only data from the Iowa experiment is presented. In most instances, biotypes from ag fields within each state similarly, so the data presented is the average of several giant ragweed biotypes. The three biotypes from Iowa ag fields behaved similarly, as did the three biotypes from Ohio ag fields, and they were combined. Finally, two Illinois ag biotypes were similar and thus combined, whereas one Illinois biotype differed from the other Illinois biotypes and is presented separately. In addition, the three biotypes from undisturbed habitats (one from each state) all acted similarly and were combined for analyses. In Figures 1 and 2 the number in parenthesis indicates how many biotypes are included in that group.
There were relatively small differences in the initial emergence date of the biotypes at the Iowa site. The single Illinois ag biotype was the first to emerge on April 13, and all other populations emerged within two days (Figure 1). The small differences in initial emergence date may have been due to the dry soils at this location. When soils reached temperatures favorable for germination in late March the soils were too dry for emergence. Approximately 0.2 inch of rain fell on April 7, and emergence for all populations began a week later. Bigger differences in initial emergence were seen at Ohio and Illinois, and emergence began in mid to late-March at those locations. The Ohio ag populations and two Illinois ag populations initiated emergence three to nine days later than the biotypes from Iowa and those collected in undisturbed habitats.
A prolonged emergence pattern is favorable for survival in agricultural fields, and the differences in emergence duration among biotypes were much greater than differences in the initiation ofemergence. All three Iowa ag biotypes, one of the Illinois ag biotypes, and the three biotypes from undisturbed habitats reached 90% emergence in less than 20 days. This means that once emergence initiated, most of the seedlings emerged in a fairly short time period. In contrast, all Ohio ag biotypes and the other two Illinois ag biotypes required approximately three times longer to reach 90% emergence. Similar emergence patterns were seen at the Ohio and Illinois locations.
The results of this research suggest that the differences in problems caused by giant ragweed across the corn belt may be due to the differences in emergence characteristics. In addition, the emergence patterns appear to be controlled genetically since the patterns were similar at all three locations. The prolonged emergence patterns found in the Ohio ag biotypes and two of the Illinois biotypes pose much greater problems for management in corn and soybean than the biotypes with a short duration of emergence. Figure 3 depicts the number of seedlings present at each sampling date for three of the biotypes. If a field was planted on April 20, the majority of the Iowa ag biotype would have emerged at this time. Weeds present at planting are much easier to kill than plants that emerge after the crop is in the ground. In contrast, the Ohio ag biotype had significant numbers of seedlings emerging into June. The pattern demonstrated by the Ohio biotypes is very similar to how waterhemp behaves in Iowa. The emergence pattern of the biotypes collected in undisturbed habitats is very advantageous for survival in this setting. The early and rapid emergence of giant ragweed ensures that it is the first plant to capture resources and gives it an advantage over neighboring plants for the remainder of the growing season. The Iowa ag populations have maintained this emergence characteristic.
This research suggests that the prolonged emergence pattern of giant ragweed in Ohio and Illinois may be an adaptation favoring its survival in agronomic fields. Whereas giant ragweed has been a serious problem in Ohio for some time, it has only relatively recently become a major weed in Illinois. Giant ragweed also is increasing as a problem in eastern Iowa, but is only rarely found in fields in central or western Iowa. Research in the 1970's in Illinois reported a short emergence pattern for giant ragweed. The prolonged emergence seen in the Ohio and two Illinois biotypes is a tremendous ecological advantage, so it is very likely it will appear in Iowa. I suspect that the biotypes in eastern Iowa that have invaded corn and soybean fields already have this trait.
There is a tendency to blame all problems in weed management on a weed's ability to survive herbicides. This study illustrates that factors totally unrelated to herbicide tolerance can have a significant impact on the efficacy of weed management programs. It is critical to understand why weeds are able to survive management tactics. When this is determined, management practices can be implemented that will provide more effective control. In the case of giant ragweed biotypes possessing a prolonged emergence pattern, management programs must be designed that will provide effective control of this weed until the crop canopy is able to shade out the late-emerging individuals.
Kent Harrison - The Ohio State University
Loyd Wax, USDA/ARS, University of Illinois
Christy Sprague, University of Illinois
Bob Hartzler, Iowa State University
Prepared by Bob Hartzler, extension weed management specialist