The giant ragweed, or Ambrosia trifida, emerges as a formidable adversary for soybeans.
Research indicates it can lead to yield losses of up to 52% in heavily infested fields.
Giant ragweed impacts different parts of the soybean plant. Its expansive leaves can overshadow soybean plants, hindering their ability to photosynthesize effectively. This can stunt growth and impede pod and seed development. Moreover, the weed's robust root system can outcompete soybean roots for water and nutrients, further affecting plant health and productivity.
The influence of giant ragweed on soybean extends beyond resource competition. The weed can disrupt soybean physiology by releasing chemicals into the soil that inhibit soybean growth, a process known as allelopathy.
How to Identify Giant Ragweed
The giant ragweed, living up to its name, is a towering plant, often stretching up to 12 feet. Its defining features include large, rough, three- to five-lobed leaves, a thick, hairy stem, and small, green flowers that cluster at the plant's apex.
Geographic Impact of Giant Ragweed
Giant ragweed is indigenous to North America but has expanded its reach to other continents, including Europe and Asia. In the United States, it is predominantly found in the Midwest, flourishing in the fertile soils of agricultural fields.
Timing of Giant Ragweed Emergence
Giant ragweed typically sprouts earlier than other weeds, often making its appearance in early spring. This early emergence, coupled with its swift growth rate, enables it to outcompete other plants for resources.
How to Manage Giant Ragweed in Soybeans
Herbicide resistance is a problem when managing giant ragweed in soybeans. Growers should assume resistance to glyphosate for management purposes. Some giant ragweed populations may be resistant to ALS-herbicides as well. Best practice is to spray 2,4-D choline post-emergence in tolerant soybeans for effective giant ragweed control.
With new rules, growers can't spray dicamba in-season for control. Conventional soybean post-spray management options are limited to glyphosate plus effective post-emergence broadleaf herbicides.
Pre-emergence
Herbicides containing chlorimuron (Authority Maxx) or cloransulam (Willowood Sulfen Cloran) applied pre-emergence at full rates are the most effective for limiting emergence of ALS-sensitive giant ragweed populations.
Post-emergence
Enlist 3 soybeans: Option to spray 2,4-D choline (Enlist One™) or glufosinate (Willowood Glufosinate 280SL) or both.
LibertyLink or LLGT27 soybeans: Glufosinate (Willowood Glufosinate 280SL)
XtendFlex beans: Glufosinate (Willowood Glufosinate 280SL)
Burndown
A2,4-D ester (2,4-D LV6 Value Pick) or dicamba (Dicamba DMA 4 Value Pick) in combination with glyphosate (Willowood Glypho 5) or paraquat (Willowood Paraquat 3SL). Note seven days needed between 2,4-D ester application and soybean planting.
These rates are general recommendations. It's essential to read and follow the specific instructions on the product labels for accurate application rates and guidelines.
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