Next in our series of difficult-to-control driver weeds is a much-too-common foe for growers (and allergy sufferers) — giant ragweed. From farmland to roadside ditches, giant ragweed can be found in most areas of the United States. Its size and adaptive nature make it extremely problematic for corn and soybean growers, especially now that it’s developed resistance to glyphosate and ALS-inhibitor herbicides. Learn more about the strengths and weaknesses of giant ragweed and control tactics for this driver weed.
The Bad News (Strengths)
- Giant ragweed is extremely prolific. It’s not only capable of producing thousands of seeds per plant, but it also produces pollen at an alarming rate, making it a nasty allergen. According to university weed experts, a single plant can produce 10 million pollen grains per day and one billion during its life cycle. When compared to corn, which can produce around 4.5 million during its life cycle, you can clearly see the prolific nature of pollen production in giant ragweed.
- High levels of pollen mean increased chances of cross pollination in giant ragweed. When cross pollination transpires, more herbicide resistance can occur when those herbicide-resistant traits are carried over from plant to plant.
- New populations of giant ragweed are able to germinate and emerge in production fields from March through July.
- Giant ragweed is extremely competitive and can grow up to 16 feet tall, shading out neighboring crops and competing for the sunlight and nutrients necessary for plant vitality and yield.
- It is estimated that one plant for every 110 square feet can reduce yield by up to 50 percent in soybeans.
- Stem-boring insects can be detrimental to glyphosate control of glyphosate-susceptible populations of giant ragweed when applications are made after weeds are 6-inches tall.
The Good News (Weaknesses)
- A monoecious weed, giant ragweed grows separate male and female flowers on the same plant. This structure keeps genetic diversity limited to that plant so that it does not pass on that genetic diversity in the form of a mutation to surrounding giant ragweed plants. This slows the development of resistance and cross resistance to multiple herbicide modes of action, allowing for more time to eliminate that genetic mutation before it becomes something we can’t control.
- No-till systems may have a slight advantage in seed bank persistence because of predation and natural weathering of giant ragweed seeds.
- Controlling stem-boring insects can have a positive effect on giant ragweed control.
- Long-term no-till systems combined with adequate herbicide control programs can lead to improved control of giant ragweed populations and significant reduction of giant ragweed seed bank in the soil.
*Note: Current known herbicide resistance includes Groups 2 (ALS) and 9 (glyphosate). There is some known cross resistance between the two groups.
- Know your resistance profile. Have your population tested to know what herbicides will be effective.
- Start clean. Consider tillage and/or an effective burndown to control and eliminate actively growing weeds. Strategies should include multiple effective modes of action for chemical control in burndown. Consult your local agronomist or university extension office for specific burndown recommendations.
- Utilize an effective soil residual herbicide. Using these products in multiple sequential applications is encouraged, where possible, to delay and diminish the germination and emergence of giant ragweed to allow soybeans a competitive advantage in row closure, canopy and plant density.
- Utilize an effective post-herbicide program with layered residuals and multiple modes of action. Apply post-emerge applications to weeds less than six inches tall using multiple herbicides for adequate coverage. Follow early-post applications with a sequential application within 14–21 days to control late-emerging plants.
- Narrow rows. In some cases, narrow-row soybeans may be warranted to increase photosynthetic competitiveness and decrease giant ragweed growth habits.
- Cover crops. Cover crops have shown an increased ability to lessen seed bank populations and delay emergence.
- Manual eradication. In severe infestations, manual eradication may be necessary for several years to get the population under control.
- Conventional herbicide system. Using a conventional herbicide system that relies on multiple, timed passes with soil-applied residuals and effective post-applied herbicides with current broad spectrum herbicides can be an effective strategy. This system is generally costly; however, the increase in yield from reducing weed competition generally outweighs the cost of treatment.
Trait System Usage
Use trait systems that provide the ability to apply multiple effective modes of action simultaneously to combat herbicide resistance. For instance, the Enlist E3® system allows Enlist One® with 2,4-D choline to be applied with both glyphosate and glufosinate and additional tank mix partners to eliminate and delay further giant ragweed emergence. This system combines many of the strategies discussed into a flexible platform.
To learn more about giant ragweed and how to manage this hard-to-control driver weed on your farm, contact your local Stine agronomist or university extension specialist.
Resources and Citations:
Biology and Management of Giant Ragweed
Management of Herbicide-Resistant Giant Ragweed
Arkansas Field Crop Weed Control Resources
Postemergence Control of Giant Ragweed in Soybeans
Palmer amaranth: ID, biology and management