Effective Post-Emergence Weed Control

A successful preemergence weed control program is important for sorghum growers. However, if for some reason a preemergence treatment was not applied growers should still consider applying one of these soil active products soon after emergence of the crop. If weeds have already emerged a herbicide with post activity will still be needed, but these herbicides with soil activity will hopefully prevent future weeds from emerging.  

Growers can choose from several products for post-emergence broadleaf weed control in sorghum. Atrazine combined with crop oil is one of the most effective herbicides when weeds are small, especially for control of Palmer amaranth. Soil restrictions often prevent the use of atrazine as a pre-emergence treatment, but these restrictions do not apply once the crop has emerged. However, growers should check for restrictions that apply to any rotational crop thatmayfollow grain sorghum.

Clarity (dicamba) applied at 8 ounces per acre or 2,4-D applied at 1.0-1.5 pints per acre also can be used safely in grain sorghum. But beware, these products can cause injury if not used properly.The addition of surfactants and other adjuvants with Clarity or 2,4-D tend to increase crop injury and, for this reason, are not recommended. Clarity or 2,4-D should be applied after all sorghum has emerged but before the height exceeds 15 inches. Drop nozzles are recommended once sorghum exceeds 8 inches in height. Occasionally, some plant leaning or leaf rolling may occur, but these effects are usually outgrown within two weeks. Weeds less than 4 inches in height are much easier to control than larger weeds. If dicamba drift is a concern, the new low-volatile products Engenia, FeXapan and Xtendimax are labeled for use in sorghum.

Growers can use Starane Ultra in place of dicamba or 2,4-D if kochia or morning glory species are present. Starane Ultra should be applied with a nonionic surfactant and has very good activity on these weeds, though it is weak against pigweed. Starane Ultra also is safer on sorghum plants and can be broadcast-applied up to the seven-leaf stage.

One of the most effective products used in sorghum for broadleaf weed control is Huskie, an HPPD herbicide plus bromoxynil (Buctril). Although most effective on small weeds, growers can use Huskie as a rescue treatment on larger weeds if necessary. Huskie can be applied on up to 30-inch-tall sorghum plants prior to flag leaf emergence. Best control is achieved with the addition of a low rate of atrazine plus adjuvants. Temporary sorghum leaf spotting and yellowing likely will occur, but sorghum typically rebounds from these injury symptoms within a few days. 

Another good treatment for larger weeds is the use of bromoxynil + atrazine + dicamba. The rates of each will be dependent on the size of the weeds. The suggested rates are 0.5-1.0 pt of bromoxynil (2E formulation), 0.75-1.0 qt atrazine, and 2-4 oz dicamba.  No adjuvant is required. The bromoxynil may cause some leaf burn, but injury to the sorghum is generally less than with Husky. 

For fields with nutsedge, Permit is the herbicide of choice. Growers should use Permit with crop oil or nonionic surfactant. Permit can be applied to sorghum up to 15 inches in height, but the productis not effective on many broadleaf weeds. If both nutsedge and broadleaf weeds are present,growers shouldconsider using Yukon, a premix product that contains both Permit and dicamba. 

A final product for consideration is Peak, which is also an ALS inhibitor.Best control is achieved when Peak is applied with atrazine or dicamba, and growers can use iton sorghum plants up to 30 inches in height. Crop rotation restriction to cotton or soybeans is 18 months and often prevents its use.

Unfortunately, no good postemergence grass control options exist for sorghum at this time. Atrazine or quinclorac, sold under various trade names, may provide some control if applied to very small grass. For now, sorghum growers must rely primarily on preemergence herbicides for annual grass control.