Pre-emergence weed control—key to successful grain sorghum production

Although growers have already planted grain sorghum in parts of Texas, growers in much of the Plains and other regions of the country will be planting sorghum over the next few weeks. To successfully grow sorghum, a pre-emergence 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 and agronomist conducting multiple weed control trials annually, I could almost guarantee a grower nearly 100% control of Palmer amaranth with a one-time rate of atrazine. Truth be told, I could usually achieve this control at a 0.5-time 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 pre-emergence weed control. Below are the most common and effective premixes for weed control in sorghum:

• Bicep II Magnum, Bicep Lite II Magnum, Cinch ATZ, other generics:
  Contain atrazine and S-metolachlor (some generics use metolachlor);
• Degree Xtra, Fultime NXT: Contain atrazine and acetochlor; and
• 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, metolachlor (Dual), acetochlor (Warrant) or dimethenamid-P (Outlook) should be used as a pre-emergence treatment. Then, in most cases, growers can apply atrazine at 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 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 pre-emergence 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.

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.

For a discussion on post-emergence herbicide options, growers can visit the Sorghum Checkoff website here.

Editor’s note: Brent Bean, Ph.D., Sorghum Checkoff Director of Agronomy, Lubbock, Texas. For more information visit www.sorghumcheckoff.com.