Pre Emergence Weed Control

Weed control is critical for the success of any crop, and grain sorghum is not an exception. Unlike many other crops, the nonselective herbicides glyphosate and glufosinate cannot be used post-emergence to control weed escapes. In addition, research repeatedly has shown that maintaining weed control during the first 30 days after crop emergence is critical to protecting the yield potential of sorghum.

To successfully grow sorghum, a pre-emergence weed control program is essential. Recent research completed by both Texas A&M and Kansas State Universities have 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 100 percent resistance to popular herbicides. While resistance is easy to spot, tolerance -- which is when the weeds require a higher dose for control or activity is inconsistent when conditions are less than ideal, is harder to recognize. This increase in tolerance to popular herbicides is one reason why consistent weed control from field to field or from year to year has become more difficult to achieve.

Growers will get the best results when they use two or more active ingredients with different mode-of actions for pre-emergence weed control. Below are the most common and effective pre-mixes for weed control in sorghum:

  • Bicep II Magnum, Bicep II Lite Magnum, Cinch ATZ, other generics
    Contain atrazine and S-metolachlor (some generics use metolachlor)
  • Degree Xtra, Fultime NXT
    Contain atrazine and acetochlor
  • Lumax EZ, Luxar EZ
    Contain a three-way mix of atrazine, S-metolachlor and mesotrione

All of these mixes require sorghum seed that is treated with the safener Concep III. 

If Palmer amaranth is the primary weed of concern, the two-way mixes of atrazine and either S-metolachlor or acetochlor normally will provide good control. Use the maximum rate for the soil type and crop rotation. Adding mesotrione to make a three-way mix is appropriate in heavier textured soils, especially in those fields where weed resistance to atrazine is known to exist. Soil type restrictions do apply to both atrazine and mesotrione and should not be ignored or significant crop injury can occur. If atrazine cannot be used because of soil type (only use in medium- and fine-textured soils) or other considerations, metolachlor (Dual), acetochlor (Warrant) or dimethenamid-P (Outlook) should be used pre-emergence. Then, in most cases, atrazine can be applied early post-emergence to provide additional residual control. 

Another non-atrazine pre-emergence treatment that has gained in popularity over the last few years isVerdict + Outlook. Verdict contains saflufenacil (Sharpen), which has good burndown activity as well as 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 to apply Verdict at 10 ounces plus an additional 10 ounces of Outlook. Check labels for soil-type restrictions.

The main reason for pre-emergence weed control failure with any of the mentioned treatments is heavy rainfall after application that leaches the herbicide away from the weed seed in the soil. Most weed seed germinate from the top one inch of soil. If this is suspected to have happened, 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 those weeds that have not emerged and lengthen the amount of residual control that can be expected later in the season.

S-metolachlor vs metolachor

S-metolachlor and metolachlor 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.  Differences in weed control between S-metolachlor and metolachlor can be observed, particularly in fields with high weed populations and when environmental conditions are less than ideal. If metolachlor is used, increase rate at least 25% to account for its lower weed control activity.

Dual Magnum vs Dual II Magnum

Growers sometimes asked what is 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.

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