Postemergence herbicide options are limited for grain sorghum. In this context, managing tough weeds for Kansas grain sorghum growers requires a strong residual herbicide program upfront, applied either before or at planting (preemergence). A strong residual program has several benefits, like reducing the selection pressure associated with overreliance on the already limited postemergence herbicide options, reducing later flushes of weeds with extended emergence windows like Palmer amaranth, and allowing the emergence and growth of a competitive grain sorghum crop for an early canopy formation. Once a dense grain sorghum canopy has formed, emergence of most weeds will typically be reduced because of temperature, light quality and quantity changes in the zone of seed germination.
It is important to keep in mind that the efficacy of residual herbicides is affected by several factors like environmental conditions (precipitation timing and amount after application, temperature), herbicide properties (water solubility), soil properties (pH, organic matter, texture), and the abundance and species composition of the soil seedbank. In general, effective early-season weed control can be achieved when the residual herbicides applied are appropriate for the soil and weed population, and an activating rainfall occurs within the first few days after application. In contrast, reduced weed control commonly occurs with dry weather conditions due to the lack of water for the herbicide to be dissolved in the soil solution for weed uptake.
Residual herbicide options for grain sorghum
Seven active ingredients can be used for preemergence weed control in grain sorghum in Kansas:
- atrazine (Aatrex, others)
- S-metolachlor (Dual Magnum, others) or metolachlor (Parallel, others)
- acetochlor (Harness, others)
- dimethenamid (Outlook)
- mesotrione (Callisto),
- saflufenacil (Sharpen)
- imazamox (Imiflex, if an igrowth grain sorghum hybrid is planted)
Atrazine has some activity on some grasses (foxtails) but is considered a broadleaf herbicide and will control susceptible populations of kochia, pigweeds, morningglories, mustards, and ragweeds. However, the two major constraints with atrazine are the large number of atrazine-resistant weed populations and the potential reduction in residual control length (enhanced atrazine degradation) associated with repeated application of the herbicide to the same field.
Group 15 herbicides such as acetochlor, metolachlor/S-metolachor and dimethenamid control most annual grasses, and certain small-seeded broadleaf weeds. When selecting metolachlor/S-metolachlor products, it is important to note that products that contain S-metolachlor have a greater concentration of the active form of the herbicide, so they will have lower use rates and are more effective than products that contain metolachlor. Treating grain sorghum seeds with Concep III safener protects the crop from group 15 herbicide injury by increasing how fast these herbicides are broken down in the crop.
Mesotrione (Group 27) controls broadleaf weeds (Palmer amaranth, cocklebur, velvetleaf, common lambsquarters, giant ragweed) and some grass weeds.
Saflufenacil provides control of many broadleaf weeds (common lambsquarters, marestail, mustards, nightshade, Palmer amaranth, redroot pigweed, wild sunflower, and velvetleaf).
Imiflex (imazamox is) is labeled only for use in igrowth® (imazamox-tolerant) sorghum. Imazamox provides broad-spectrum control of broadleaf and grass weeds, including redroot pigweed, common cocklebur, common lambsquarters, mustards, nightshade, sunflower, velvetleaf, foxtails, and fall panicum. Imazamox can control Palmer amaranth quite well, provided the population is not resistant to imazamox.
Herbicide combinations are essential to improve control
Weed control is improved with the use of residual herbicides from multiple effective sites-of-action (SOA) groups. The benefits of using two or more active ingredients include: expanded spectrum of weed control, slowing the selection of herbicide-resistant weeds, and a greater likelihood of successful weed control when weather conditions are not favorable. With the variable and extreme weather conditions in dryland production systems of western Kansas, preemergence herbicide combinations are crucial for early-season control of the toughest weeds like Palmer amaranth, which has an extended emergence window and a widespread occurrence of herbicide resistance.
Some examples of premix products that can be applied in grain sorghum are listed below.
- Group 15 and 5 premixes (S-metolachlor and atrazine) are available (Bicep II Magnum, Bicep Lite II Magnum, others) with different concentrations of each active ingredient, allowing growers to choose depending on whether they want to use more atrazine or more S-metolachlor. Group 15 and 5 premixes of acetochlor and atrazine, available with the same concentrations of both active ingredients (Degree Xtra, FulTime NXT), can also be used.
- A premix of Group 14 and 15 (Sharpen + Outlook: Verdict), as well as premixes of Group 15 and 27 (S-metolachlor and mesotrione) (Zemax, Coyote, Calibra), allow farmers to use residual herbicides that do not contain atrazine. These alternative options can be particularly helpful for fields where atrazine-resistant weed populations are present or where a reduced length of residual control was documented due to repeated use of atrazine.
- Three-way premix options (atrazine + S-metolachlor + mesotrione) commonly used as residual herbicide programs in grain sorghum include Lumax EZ and Lexar EZ. These products differ in the concentrations of the three active ingredients. One advantage of a mix of atrazine and mesotrione is the reported improved activity for weed control. Previous research reported an improved control of atrazine-resistant and HPPD-resistant Palmer amaranth populations when these two herbicides were mixed for application.
Our research at the Agricultural Research Center in Hays (ARCH) in 2024 showed that atrazine alone provided the least control of Palmer amaranth compared to FulTime NXT, Calibra, Atrazine + Calibra, Verdict + Outlook, or Dual II Magnum + Callisto . Also, Palmer amaranth control with herbicide programs with two SOAs (Outlook + Aatrex 4L and Parallel + Aatrex 4L) was lower (88%) relative to other pre-emergence herbicide treatments that included three SOAs (98 to 100%) (Figure 1).
Figure 1: Palmer amaranth control, 43 days after preemergence herbicide treatment (applied the day of planting) in 2024 at the ARCH. The abbreviation fb means “followed by”. Degree Xtra and Outlook + Aatrex were applied 21 days after the preemergence application of Lumax EZ. Graph by Jeremie Kouame, K-State Research and Extension.
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 and follow all label instructions before use.
For more detailed information, see the online version of the 2025 Chemical Weed Control for Field Crops, Pastures, and Noncropland Guide or check with your local K-State Research and Extension office for a paper copy.
Jeremie Kouame, Weed Scientist – Agricultural Research Center, Hays
jkouame@ksu.edu
Patrick Geier, Weed Scientist – Southwest Research & Extension Center, Garden City
pgeier@ksu.edu
Sarah Lancaster, Extension Weed Management Specialist, Manhattan
slancaster@ksu.edu
Tags: sorghum grain sorghum herbicides pre-emergence residual herbicides