When the aggressive nature of Palmer amaranth is combined with the limited post-emergence herbicide options in grain sorghum, problems often arise – even when an adequate preemergence herbicide program is used. This article covers post-emergence herbicide options for Palmer amaranth control in grain sorghum. Combinations of the herbicides listed here will generally improve control, and all of the options are most effective when applied to small (under 4 inches tall) weeds.
Figure 1. Palmer amaranth in a grain sorghum field that escaped treatment earlier in the growing season. Photo by Sarah Lancaster, K-State Research and Extension.
Atrazine can control sensitive populations of Palmer amaranth and can be combined with other herbicides to enhance effectiveness. Recommended rates range from 0.25 to 2.0 pounds of atrazine (0.5 to 4 pints). Atrazine should be applied with crop oil or surfactant to control emerged weeds. Atrazine can be applied to grain sorghum between 3-leaf and 12 inches or between 6 and 12 inches in western Kansas. Be sure to observe rate limits for your area.
Aim (carfentrazone) is a Group 14 herbicide that can be applied to grain sorghum between 4 inches and boot stage. It is less effective than some of the other herbicides in this article and requires good coverage for maximum effectiveness. Aim can be tank-mixed with atrazine, 2,4-D, dicamba, bromoxynil, and Huskie. Aim is likely to burn grain sorghum leaves, especially if applied in hot, humid weather or with crop oil. Leaf burn will also be greater if Aim is applied with bromoxynil.
2,4-D is an effective herbicide option to control Palmer amaranth. However, crop response should be expected, especially if applied in hot, humid conditions. Crop responses can include rolled leaves, lodging, and brittle stems. Grain sorghum is most tolerant of 2,4-D applications when it is 5 to 10 inches tall. Drop nozzles should be used when applying 2,4-D to grain sorghum greater than 8 inches. To reduce crop response, apply lower rates (2/3 pint) with atrazine, Aim, bromoxynil, or Huskie. Using crop oil in tank mixes with 2,4-D, will increase crop injury.
Dicamba, at the rates used in grain sorghum (0.5 pint), may be less effective on Palmer amaranth than 2,4-D. It can be applied to grain sorghum between 2 and 15 inches. Drop nozzles should be used if grain sorghum is 8 inches or taller to avoid damaging seed heads. Crop response, including rolled leaves and lodging, should be expected, especially if applied in hot, humid conditions. Dicamba can be tank-mixed with Aim, atrazine, and bromoxynil.
Bromoxynil can be applied from the 3-leaf stage through boot stage. Crop response will be less with bromoxynil than other herbicides, but bromoxynil alone will not control Palmer amaranth larger than 4-leaf. Adequate spray coverage is needed for maximum effectiveness.
Huskie (pyrasulfutole+bromoxynil) is most effective when mixed with atrazine (up to 1 pound). When used alone, it can be applied between 3-leaf and 30 inches and should be applied with HSOC (high surfactant oil concentrate) or AMS + NIS. Huskie will cause leaf burn (Figure 2), which can be greater in fields where mesotrione was applied pre-emergence. As needed, Huskie plus atrazine may be tank-mixed with phenoxy broadleaf herbicides such as 2,4-D or dicamba.
Figure 2. An example of leaf burn caused by a post-emergence application of Huskie. Note that the large Palmer amaranth plants were not controlled by this tank-mix of Huskie and atrazine. Photo by Sarah Lancaster, K-State Research and Extension.
Additional information can be found in the 2023 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland, K-State publication SRP-1176 - https://bookstore.ksre.ksu.edu/pubs/SRP1176.pdf
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Weed Science Extension Specialist
slancaster@ksu.edu
Jeanne Falk Jones, Multi-County Agronomist
jfalkjones@ksu.edu