By Curtis Thompson, Extension weed management 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.
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 are not allowed to head in spring, before the sorghum is planted. Thus, an effective burndown should be applied before winter annuals go into the flowering/heading stages. 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 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/acre applied 30 days or more prior to sorghum planting. It is essential that 1 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, this is becoming increasingly difficult.
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, 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, atrazine-resistant kochia.
Using a product such as Lumax EZ or Lexar EZ preemergence, which contains mesotrione (Callisto), will help control the triazine-resistant pigweeds and kochia. The addition of 10 oz of Verdict, which is a mix of 2 oz of Sharpen and 8.3 oz of Outlook, can help control triazine-resistant pigweeds as well as the large-seeded broadleaf weeds. The chloracetamide/atrazine herbicides will do a very good job of controlling most annual grassy weeds.
A weakness of all soil-applied programs is that rainfall is required for activation. Without activation, poor broadleaf and grass control can be expected. Once rain 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 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 for control of labeled annual grasses when they are small. DuPont has developed a liquid formulation of nicosulfuron that has an anticipated registration in the next few months. Currently a 75% a.i. nicosulfuron dry formulation product has received a registration for Inzen sorghum. Hybrid availability is extremely limited in 2016. Advanta and Pioneer will be marketing Inzen hybrids. Hopefully availability will improve over time.
Postemergence broadleaf weed control herbicides are available for grain sorghum. These products will be 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, the newest herbicide registered in sorghum should be applied at 12.8 to 16 fl oz/acre with 0.25 to 1.0 lbs 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 2016 Chemical Weed Control Guide (SRP 1126) to help make the selection:
The crop stage at the time of postemergence 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.