Severe grass and broadleaf weed pressure reduces 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. 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 have been applied prior to winter annuals going into reproductive, flowering/heading, stages.
An effective burndown prior to planting is essential, as 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 has been used. There is a waiting period of 15 days for planting sorghum after a burndown application of 8 fl oz of Clarity. Most 2,4-D labels don’t address a waiting period ahead of planting sorghum, however for corn or soybeans a 7-day waiting period is required if 1 pint or less of 2,4-D ester is 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 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, Bullet, Lariat, Guardsman Max, G-Max Lite, Degree Xtra, 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, morningglory, atrazine-resistant kochia, and atrazine-resistant pigweeds. The chloracetamide/atrazine herbicides will also do a very good job of controlling most annual grassy weeds.
Using a product such as Lumax or Lexar preemergence, which contains mesotrione (Callisto), will help control the triazine-resistant pigweeds and kochia. The addition of 10 oz of Verdict can help control triazine-resistant pigweeds, as well as the large-seeded broadleaf 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 herbicides.
Grass control in sorghum can be a difficult task in some cases. If a field has severe shattercane 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 grain sorghum. Although atrazine and Paramount have grass activity and can control tiny grass seedlings, it’s generally not a good practice to depend on these herbicides for grass control.
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, is effective on kochia, pigweeds, and many other broadleaf weed species. It should be applied at 12.8 to 16 fl oz/a with 0.25 to 1.0 lbs of atrazine and spray-grade ammonium sulfate to sorghum from 3-leaf to 12 inches tall. Bayer CropScience has applied to EPA for a label change suggesting that a nonionic surfactant at 0.25% v/v be added to the spray mixture.
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 2013 Chemical Weed Control Guide (SRP 1081) to help make the selection:
The crop stage at the time of postemergence herbicide applications can be critical to minimize crop injury. Delayed applications risk 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.
Curtis Thompson, Weed Management Specialist