Herbicide Choices for Palmer Amaranth

“What is the best herbicide for use in fields infested with Palmer amaranth” has been a common question this spring. There are a number of products that could be used, the key is maximizing their effectiveness. Most of our Palmer amaranth plants are resistant to glyphosate and many are resistant to Group 2 herbicides, so we cannot rely on these products to provide control.

1. Use the appropriate rate (full rate for the soil type)
For pre-packaged mixtures, examine what products are included and what rate of each product is being applied. There are a number of soybean herbicides that contain more than one active ingredient, but one or more of those active ingredients may be at a low rate. Sources for this information are the Corn or Soybean Weed Management Guide http://extension.udel.edu/ag/weed-science/weed-management-guides/. See table 3 page 8 in the Corn Guide or table 3 page 9 in the Soybean Guide as well as table 7 for typical use rates when used alone).

Soybean Herbicide Active Ingredients with Good Preemergence Control for Palmer Amaranth

*Trade names given are only an example and others trade names may be available

Note many of the Palmer amaranth are Group 2 resistant so this group of herbicides was not included

Corn Herbicide Active Ingredients with Good Preemergence Control for Palmer Amaranth

*Trade names given are only an example and others trade names may be available

Note many of the Palmer amaranth are Group 2 resistant so this group of herbicides was not included

2. Apply the herbicides in a timely fashion.
Herbicides applied more than 7 days before planting means the postemergence herbicides must be applied earlier than normal. Be sure to use your residual herbicides close to planting so when the postemergence sprays are made about 4 weeks later, the crop is approaching canopy closure.

3. Postemergence sprays will need to include a product that is highly effective on Palmer amaranth since most of our plants are glyphosate resistant. Applications need to be made to Palmer amaranth plants before they reach 4 inches; in most situations this is three to four weeks after the preemergence herbicide application has been made.

UD Weed Science has had consistent postemergence control in soybeans with PPO herbicides (Reflex, Blazer Ultra, or Goal); of these products Reflex will provide good residual control as well. Liberty (used with Liberty Link soybeans) has been very effective for Palmer amaranth, but does not provide any residual control.

In corn, HPPD herbicides such as Callisto, Impact/Armezon, or Laudis (all combined with 1 pt to 1 qt/A of atrazine) have provided very good control of Palmer amaranth. While all of these products will provide residual control, Callisto provides the longest residual control. If an HPPD herbicide (Group 27) will be used postemergence, refer to the herbicide labels to determine if there are limits on use of an HPPD herbicide at planting. In most situations, if a HPPD-inhibiting herbicide will be use postemergence, there is seldom benefit for using one at planting. Dicamba is also effective for postemergnece control of Palmer amaranth, but it does not provide effective residual control.

4. Finally, we have not seen any triazine-resistant Palmer amaranth in the region. But as we use more triazines (atrazine, metribuzin, and simazine) for Palmer amaranth control, we need to be sure we also incorporate other effective herbicide groups for our soil-applied treatments.