Giant ragweed (Ambrosia trifida L.) is a summer annual native weed in the Asteraceae family that now inhabits most of the United States. Giant ragweed used to be found primarily as a weed of stream banks, roadsides, right-of-ways, and other sites with disturbed, moist soils, but in recent decades has become a primary weed of corn and soybean production throughout the Midwest. Giant ragweed is one of the earliest summer annual weeds to emerge in the spring. In central Missouri, we observed some giant ragweed seedlings emerging in corn and soybean fields last week. Giant ragweed will continue to emerge over the next several weeks and months, which is one of the reasons why this weed is a troublesome weed of corn and soybean production systems. Several studies have shown that giant ragweed emergence can be observed as early as March and as late as July. Giant ragweed seedlings can be identified by their large round or spatulate-shaped cotyledons which are 3/4 to 1 ½ inches long and ½ to ¾ of an inch wide (Figure 1). The first pair of true leaves are ovate to lanceolate in shape, unlobed, and contain entire to slightly toothed leaf margins. Young leaves are arranged oppositely and contain rough hairs. After the second pair of true leaves emerge, subsequent leaves take on a 3- to 5-lobed palmate leaf arrangement (Figure 2).
Giant ragweed is very competitive with both corn and soybean. In one study conducted in Ohio, the authors found that an infestation level of one giant ragweed plant per 10 m2 resulted in a 1% corn yield loss. Other studies have shown that giant ragweed is even more competitive in soybeans. In another study conducted in Missouri in 1991, season-long interference of giant ragweed at a density of 2 plants per 9 m of row resulted in a 50% soybean yield reduction.
Due to the nature of giant ragweed emergence, this species cannot and should not be managed with any one single herbicide application. In order to effectively manage this weed, you must consider a variety of herbicides with multiple modes of action, applied at different times throughout the growing season. Since approximately half of the total giant ragweed population that is going to emerge in any given year does so by the typical time of soybean planting in Missouri, the selection of an appropriate burndown herbicide program is critical in the management of giant ragweed. For the selection of an appropriate burndown herbicide in either corn or soybean, visit the 2011 Pest Management Guide at http://weedscience.missouri.edu/publications/m00171.pdf.
REVISED: October 2, 2015