Prepare, Or Suffer The Scourge Of Giant Ragweed

5 things to know to keep this herbicide-resistant weed out of your fields

Because it has adapted to repeated applications of various herbicides, giant ragweed’s effect on soybean crops cannot be understated.

“Managing for herbicide-resistant giant ragweed is truly a matter of planning and preparation,” says Bill Johnson, Ph.D., professor of weed science at Purdue University. “Failure to do so can lead to crop yield losses up to 90 percent.”

In a recent Focus on Soybean webcast on the Plant Management Network, Johnson recommended strategies for combating giant ragweed.

“Putting the post-emergence treatments on in a timely manner is really important. We cannot let these plants get big or we increase the risk of failure to control them,” he says. “We want to scout our fields late in the year to see how well our control tactics are working. This is going to be the best way for us to determine whether or not we have a resistance problem that is developing and if our current strategies are effective.”

Johnson says farmers should remember these five key takeaways to manage giant ragweed:

1. Giant ragweed germinates early.

It can outgrow and outcompete your soybeans for sunlight, moisture and nutrients. Crop yield losses of 30-90 percent have been documented.

2. Giant ragweed is becoming resistant to more herbicides over time.

It is important to determine if the ragweed you are treating is resistant to Group 2 (ALS inhibitors like FirstRate, Classic®, or Scepter), Group 9 (glyphosate) or both types of herbicides. When using the appropriate herbicide, make sure to apply the product as labeled to reduce the chance of creating more herbicide-resistant plants.

3. Giant ragweed can also be a host for stem-boring insects.

The double trouble here is that not only do you have pests that can affect crops, but these stem-boring insects help giant ragweed resist the impact of herbicide applications. When large plants are sprayed with post herbicides, these stem boring insects can reduce the efficacy of these herbicides.

4. Field preparation is the key to successful giant ragweed control.

• No-Till – A good burndown treatment coupled with a pre-emergence herbicide provided the best results by reducing the size and frequency of surviving ragweed, allowing the soybean crop to establish and establish a dense crop canopy. This also creates a scenario in which any need to treat with a post-emergence herbicide will most likely only be to address small ragweed plants that may have survived or recently germinated from the seed bank.
• Conventional-Till – Thorough tillage is a must. Giant ragweed have large, resilient seeds that can germinate from a depth of 5 inches, making it tough for soil-applied herbicides to effectively reach emerging all seeds in the seedbank.

5. Timing of post-emergence application is critical.

Herbicide-resistant giant ragweed can have different levels of resistance to Group 2 and Group 9 post-emergence herbicides. It is important to know the level of resistance in the plants you’re treating.