By Curtis Thompson, Extension Weed Specialist

Severe grass and broadleaf weed pressure will reduce grain sorghum yields and can make harvest very difficult. Good crop rotation and herbicide selection are essential components of managing weeds in grain sorghum.

Options Before Planting

In a wheat-sorghum-fallow rotation, it is essential that broadleaf and grassy weeds do not produce seed during the fallow period ahead of grain sorghum planting.

Always control those summer annual weeds after wheat harvest soon enough to prevent seed production. It is equally important that winter annual grasses and broadleaf weeds are not allowed to head/flower in spring, producing seed before the sorghum is planted. Most winter annuals produce seed in April and early May.

If you are anticipating problems with glyphosate-resistant pigweeds, it may be very important to include in the April burndown treatment a soil residual product. This can help minimize pigweed (Palmer amaranth and waterhemp) emergence in late April and May, prior to planting sorghum. A pound of atrazine may provide the needed protection unless the pigweed population is atrazine resistant. Atrazine + chloracetamide herbicides can be used effectively, however. 

The Valor label allows the use of 2 oz product/acre applied 30 days or more prior to sorghum planting. It is essential that at least one inch of precipitation fall during the window between Valor application and sorghum planting. Valor will control glyphosate-resistant and triazine-resistant pigweeds as it has a different mode of action than glyphosate and atrazine.

An effective burndown prior to planting is essential if any weeds have emerged. Sorghum should always be planted into a weed-free seedbed. The addition of a dicamba product or 2,4-D with glyphosate generally will control broadleaf and grass weeds effectively, provided an earlier burndown treatment has been applied in March or April.

There is a waiting period of 15 days between application and sorghum planting when using 8 fl oz of Clarity. Current 2,4-D labels do not address a waiting period ahead of planting sorghum; however, for corn or soybeans a 7-day waiting period is required for 1 pint or less of 2,4-D ester when used in the burndown.

In sorghum, the best choice of herbicides will depend on the weed species present. Broadleaf weeds generally can be controlled with a combination of preemergence and postemergence applied herbicides. With the development of herbicide-resistant weeds, however, complete weed control is becoming increasingly difficult.

Figure 1. Atrazine + 2,4-D ester applied too late to provide adequate Palmer amaranth control. The application was made to 20” Palmer amaranth and 20” sorghum on July 6.

Control of pigweeds in sorghum is an increasing concern across the state. Using a soil-applied chloracetamide herbicide with atrazine (such as Bicep II Magnum, Bicep Lite II Magnum, Outlook + atrazine, Degree Xtra, Fultime NXT, or generic equivalents of these products) will greatly enhance controlling pigweeds. Some of the broadleaf escapes producers can expect when using the chloracetamide/atrazine mixtures are devilsclaw, puncturevine, velvetleaf, morningglory, and atrazine-resistant kochia.

The addition of 10 oz of Verdict, which is a mix of 2 oz of Sharpen and 8.3 oz of Outlook, with a chloracetamide/atrazine herbicide can help control triazine-resistant pigweeds and kochia, and control large-seeded broadleaf weeds such as velvetleaf, morningglory, sunflower, and others. The chloracetamide/ atrazine herbicides will do a very good job of controlling most annual grassy weeds.

Using a product such as Lumax EZ or Lexar EZ, which contains mesotrione (Callisto), preemergence will help control triazine-resistant pigweeds and kochia. With the lower price of generic herbicides, these treatments are becoming very economical.

A weakness of all soil-applied programs is that precipitation is required for activation. Without activation, poor broadleaf and grass control can be expected. Once precipitation is received, the herbicides are activated and weed control measures are in place. Weed escapes prior to this activation will need to be controlled with postemergence applied herbicides.

Grass Control

Grass control in sorghum can be a difficult task in some cases. If a field has severe shattercane or longspine sandbur pressure, planting grain sorghum is not recommended. For other annual grassy weeds, it will be important to apply one of the chloracetamide herbicides. Grasses that emerge before the soil-applied herbicides are activated will not be controlled.

There are no herbicides currently labeled for postemergence grass control in conventional grain sorghum. Although atrazine and Facet L have grass activity and can control tiny grass seedlings, it’s generally not a good practice to depend on these herbicides for grass control. Facet L is the new liquid formulation of quinclorac (previously Paramount 75 DF) and has excellent activity on field bindweed.

A new technology for grass management is Inzen sorghum, a non-GMO type of sorghum. Growing Inzen sorghum will allow the use of nicosulfuron (an ALS grass herbicide) applied postemergence to control labeled annual grasses when they are small. This technology will likely not be commercially available until the 2019 growing season at the earliest, however. An article titled “Status of new ALS-resistant Inzen sorghum technology” in Agronomy eUpdate No. 621, March 24, 2017, provides a current update on this topic.

Post-Emergence Options

Postemergence broadleaf herbicides for sorghum are most effective when applied in a timely manner. Weeds that are 2-4 inches tall will be much easier to control than weeds that are 6-8 inches tall, or larger. Controlling weeds in a timely manner will result in less weed competition with the crop compared to waiting too long to control the weeds. Atrazine combinations with Huskie, Banvel, 2,4-D, Buctril, or Aim (or generic versions of these herbicides) can provide excellent broad-spectrum weed control.

Huskie should be applied at 12.8 to 16 fl oz/acre with 0.25 to 1.0 lb of atrazine, NIS 0.25% v/v or 0.5% v/v HSOC (high surfactant oil concentrate), and spray grade ammonium sulfate at the rate of 1 lb/acre to sorghum from 3-leaf to 12 inches tall. Huskie alone, without atrazine, can now be applied to sorghum up to 30 inches tall prior to flag leaf emergence, however it will be less effective.

Huskie is effective on kochia, pigweeds, and many other broadleaf weed species. Huskie is most effective on small weeds. The larger pigweed and kochia get, the more difficult they are to control. Temporary injury to sorghum is often observed with Huskie.

The presence of certain weed species will affect which postemergence herbicide programs will be most effective. See the grain sorghum section in the K-State 2017 Chemical Weed Control Guide (SRP 1132) to help make the selection.

The crop stage at the time of post-emergence herbicide applications can be critical to minimize crop injury. Delayed applications to large sorghum increase the risk of injury to the reproductive phase of grain sorghum, thus increasing crop injury and yield loss from the herbicide application. Timely applications not only benefit weed control, but can increase crop safety. Always read and follow label guidelines.