Post on May 6, 2021

To successfully grow sorghum, a preemergence weed control program is essential. Recent research completed by both Texas A&M University and Kansas State University has shown that an increasingly high number of fields across the sorghum belt have populations of weeds that have either increased tolerance or, in some cases, developed 100% resistance to popular herbicides. While resistance is easy to spot, tolerance is harder to recognize. Tolerance is when the weeds require a higher dose of herbicide for control or when activity is inconsistent in less-than-ideal conditions. This increase in tolerance is one reason why consistent weed control from field to field or from year to year has become more difficult to achieve.

A great example of increased weed tolerance is how Palmer amaranth reacts to atrazine. When I first started my career as a weed scientist/agronomist and was conducting multiple weed control trials annually, I could almost guarantee a grower nearly 100% control of Palmer amaranth with a 1X rate of atrazine. Truth be told, I could usually achieve this control at a 0.5X rate! Sometime around 2005, this outcome began to change, and weed control with atrazine alone became more inconsistent. Atrazine was still a great product, but it just needed a little help.

Growers will get the best results when they use two or more active ingredients with different modes of action for preemergence weed control. Below are the most common and effective premixes for weed control in sorghum:

  1. Bicep II Magnum, Bicep Lite II Magnum, Cinch ATZ, other generics
    Contain atrazine and S-metolachlor (some generics use metolachlor)
  2. Degree Xtra, Fultime NXT
    Contain atrazine and acetochlor
  3. Lumax EZ, Lexar EZ
    Contain a three-way mix of atrazine, S-metolachlor and mesotrione
    Each of these mixes requires sorghum seed treated with the safener Concep III.

If Palmer amaranth is the primary weed of concern, the two-way mixes of atrazine and S-metolachlor or atrazine and acetochlor normally provide good control. Growers should use the maximum rate for the specific soil type and crop rotation. Adding mesotrione to make a three-way mix is appropriate in heavier-textured soils, especially in fields where weed resistance to atrazine exists. Soil-type restrictions apply to both atrazine and mesotrione and should not be ignored because significant crop injury can occur.

If growers cannot use atrazine because of soil type (only use in medium- and fine-textured soils) or other considerations, S-metolachlor (Dual), acetochlor (Warrant) or dimethenamid-P (Outlook) should be used as a preemergence treatment. Then, in most cases, growers can apply atrazine at early postemergence to provide additional residual control.

Another non-atrazine preemergence treatment that has gained in popularity over the last few years isVerdict + Outlook. Verdict contains saflufenacil (Sharpen), which provides good burndown activity and some soil residual. The product can be a good one to use if small weeds are present at planting. In addition to saflufenacil, Verdict contains dimethenamid-P. However, Verdict does not have enough dimethenamid-P to provide adequate residual control. For this reason, it is recommended that growers apply 10 ounces of Verdict plus an additional 10 ounces of Outlook and, as always, should check labels for soil-type restrictions.

The main reason for preemergence weed control failure with any of the mentioned treatments is heavy rainfall after application, which leaches the herbicide away from the weed seed in the soil. Most weed seed germinates from the top 1 inch of soil. If growers suspect that leaching has occurred, they should consider applying S-metolachlor, acetochlor or dimethenamid-P to the emerged crop as soon as possible. While these herbicides will not provide any control of emerged weeds, they will control weeds that have not emerged and lengthen the amount of residual control that can be expected later in the season.

S-metolachlor vs metolachlor

Growers sometimes ask about the difference between S-metolachlor and metolachlor. These are slightly different forms (isomers) of the same molecule; however, S-metolachlor is more effective than metolachlor when applied at the same rate. When a label lists the active ingredient as metolachlor, it actually contains four different metolachlor isomers. In contrast, a label that lists the active ingredient as S-metolachlor will contain approximately 88 percent of the more active S-metolachlor isomer.

Regarding formulations, growers also want to know the meaning of ‘II’ in Dual II Magnum and Bicep II Magnum. The ‘II’ indicates that the product includes the safener benoxacor. This safener has nothing to do with sorghum, but it increases the ability of corn to metabolize metolachlor and reduces the potential for plant injury. As mentioned earlier, sorghum seed must be treated with Concep III safener when using metolachlor, acetochlor or dimethenamid-P.

In 2017 and 2018 the United Sorghum Checkoff Program sponsored preplant and PRE weed control trials in the High Plains of Texas and Kansas and a PRE trial in Georgia. Attached is a PowerPoint presentation summarizing these trials. Most of the results presented are from trials conducted by Wayne Keeling, Ph.D., at Texas A&M University research facility in Lubbock, Texas, and Randall Currie, Ph.D., at Kansas State University in Garden City, Kansas.

Key takeaways from the trials:

  1. Premixes of Bicep formulations and Degree Xtra or Fultime NXT gave better weed control than atrazine alone.
  2. Degree Xtra and Fultime NXT are premixes of atrazine + acetochlor (Warrant) and had a tendency to give better control of pigweed and other broadleaf weeds than Bicep formulations.
  3. Lumax EZ (Callisto + atrazine + S-metolachlor) consistently gave the best season-long control of broadleaf weeds. Callisto applied alone did not give season-long control. Callisto + atrazine gave good control, but not as good as the three way mix. Keep in mind Lumax EZ cannot be used on sandy soils.
  4. Preplant applications of any of the above mentioned premixes applied 14 days prior to planting resulted in season-long control as good as the same treatments applied PRE.
  5. Verdict + Outlook was somewhat inconsistent in controlling pigweed in these trials. However, it was one of the better treatments for kochia and Russian thistle control in Kansas.
  6. Dual Magnum and Warrant only provided short-term control and did not provide season-long control of pigweed or other broadleaf weeds in Texas or Kansas.
  7. S-metolachlor provided as much as 30 percent better control than metolachlor. This could likely be offset by increasing the metolachlor rate.
  8. Valor applied 14 days preplant provided good initial control but began to fail after 30 days. Some minor crop injury was observed, especially with the higher 2 oz rate. Valor label requires treatment be applied 30 days prior to planting sorghum.
  9. In Georgia in sandy soils, crop injury was high with any Callisto containing treatments. Dual Magnum provided season-long control of weeds and annual grasses.

All the different environments and circumstances that determine if and how any given herbicide can be used cannot be discussed here. Growers should consult the product label for the specific use of each herbicide.