Pre-emergence herbicide programs for corn were discussed in a recent eUpdate. The role of pre-emergence herbicides is similar in both corn and grain sorghum, and some herbicides are similar. But, fewer post-emergence herbicide options, particularly for Palmer amaranth and grass control, make an effective pre-emergence herbicide even more important for successful sorghum production. A table summarizing weed species response to various grain sorghum herbicides can be found on page 48 of 2021 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1162) at: https://bookstore.ksre.ksu.edu/pubs/SRP1162.pdf
Herbicide groups of soil-applied residual herbicides for grain sorghum
Photosystem II inhibitors (Group 5). Atrazine is a common component of many pre-plant and pre-emergence herbicide premixes for sorghum. It controls a wide variety of broadleaf weeds, including pigweeds, ragweeds, morningglories, and mustards, as well as some grasses. However, atrazine resistance has been reported for many weed species. Atrazine use rate is influenced by soil type, soil pH, and organic matter, and use is prohibited in instances where water contamination is likely. Unless your situation prohibits atrazine use, it is recommended to include atrazine when you apply Group 15 and Group 27 herbicides.
Fatty acid inhibitors (Group 15). Dimethamid-P (Outlook), S-metolachlor (Dual II Magnum), metolachlor, and acetochlor, are also a common component of many pre-plant and pre-emergence herbicide premixes for sorghum. In general, these products are very effective in controlling most annual grasses and small-seeded broadleaf weeds, except kochia. Though resistance to Group 15 herbicides have been reported in other states, resistance has not been confirmed in Kansas to date. Group 15 products are most effective when applied with atrazine, unless atrazine is not allowed.
HPPD-inhibitors (Group 27). Mesotrione (Callisto, others) controls kochia, pigweeds, velvetleaf, and many other broadleaf weeds, as well as grasses. Mesotrione should be applied with atrazine, which is often included in premixes (Lexar EZ, Lumax EZ, others). Some mesotrione-resistant weed populations have been identified in Kansas.
PPO-inhibitors (Group 14). Saflufenacil (Sharpen) controls pigweeds well; however, kochia control is marginal. Verdict (saflufenacil + dimethenamid-P) has excellent activity on pigweeds, kochia, and large-seeded broadleaf weeds. However, the length of residual activity can be shorter than other pre-emergence products.
Another pre-emergence option that is new for grain sorghum in 2021 is imazamox (IMIFLEX). Imazamox is an ALS-inhibitor (Group 2) that will control grasses such as foxtails, crabgrass, fall panicum, and barnyardgrass. It will also control cocklebur, sunflower, velvetleaf, and pigweeds – if the populations have not developed resistance. Imiflex can only be used in igrowth grain sorghum varieties. Additional comments about igrowth grain sorghum can be found in eUpdate Issue 833 in an article titled “IMIFLEX herbicide receives EPA approval for igrowth grain sorghum”.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Extension Weed Science Specialist