Timing Weed Control
Application timing is important for optimum herbicide performance and to avoid unacceptable crop injury. Small grass and broadleaf weeds that are 2 to 4 inches tall are easier to control compared to taller weeds. Weed control the first four weeks after grain sorghum emergence is reported to be the most critical period of weed control.1
Crop injury from in-crop herbicide applications can occur if using improper additives, tank-mix partners, or timing. It is important to read label directions as different brands and label updates could differ from one year to the next. Drop nozzles are recommended for several herbicide applications in tall grain sorghum and this equipment should be available in case delayed.
Yield losses from weed competition are greater in sorghum compared to most grain crops. As much as 30 to 50 percent yield loss can be associated with weed competition.1 Potentially one pigweed plant per 3 foot of row, left uncontrolled until sorghum reaches the threeleaf stage, can reduce yield potential by 10%.2 However, grass weeds can be more difficult to control due to the lack of selective herbicides. Heavy infestations of grassy weeds may cause up to a 20% yield reduction in the first two weeks after sorghum germination.2
POST Herbicide Options
Species such as Johnsongrass and shattercane may be a challenge to control as they are biologically similar to cultivated sorghum. Johnsongrass is a perennial and, unlike in corn, does not have a selective herbicide option in sorghum. In general, there are limited herbicide options for emerged grass weeds in sorghum. Some small grasses may be controlled with an overhead application of atrazine.
Atrazine has also been commonly tank-mixed for postemergence broadleaf weed control in sorghum. Maximum use rates should be recognized for atrazine if sensitive crops are planned for next season. Small pigweed and waterhemp weeds can be controlled with a tank-mix or pre-packaged herbicide. Herbicide products should be rotated and chosen based on past activity for the weed spectrum of a field. Over time, pigweeds and waterhemp can develop tolerance to herbicide families.
Sorghum seeds treated with a safener (such as Concep® III) can be safely grown in soil treated with Warrant® Herbicide and Degree Xtra® herbicides. Warrant Herbicide and Degree Xtra can also be POST-applied and are residual herbicides that have active ingredients from the chloroacetamide family. Safeners protect sorghum from potential injury from herbicides in the chloroacetamide family. Warrant Herbicide and Degree Xtra are important products that can control annual grasses and late flushes of small-seeded broadleaf weeds such as pigweeds.
Table 1. POST Herbicide Options for Broadleaf Control in Sorghum*
|Common Name (brand)||Sorghum Height Recommendations||Comments|
|Apply when sorghum is 6 to 12 inches tall (drop nozzles after crop is 12 inches)||Apply before pigweed species reach 6 inches
Carryover injury can occur if there is little seasonal rain
|Acetochlor + Atrazine
|Apply until sorghum is 11 inches tall||Avoid using liquid fertilizer carriers
Do not apply with a temperature forecast of 85° F within 24 of application
|Apply until sorghum is 11 inches tall||Avoid using liquid fertilizer carriers|
|2,4-D amine 4
|From 6 to 15 inches (drop nozzles when taller than 8 inches)||Do not apply during boot, flowering, or dough stages|
|From spike to prior to 15 inches (drop nozzles when taller than 8 inches)||Performance may be best when applied to sorghum between the 3 and 5 leaf stages and weeds are less than 3 inches|
|5 to 30 inches before head emergence||Crop oil concentrate (COC) is preferred over nonionic surfactant (NIS) for additives in drier climates|
|3 to 7-leaf (drop nozzles for 7-leaf up to boot stage)||Use NIS for additive
Good control for kochia and morningglory
|Through 6-leaf stage||NIS recommended at 0.25% v/v
Effective on small weeds
|Halosulfuron (Permit® Herbicide)||2-leaf through layby stage before head emergence||NIS at 0.25% v/v as and additive
Weak on pigweed control
|Metsulfuron (Ally® XP)||3 to 15 inches before boot stage (use drop nozzles over 10 inch height)||Do not use surfactant or crop oil additives
Apply when weed heights are less than 6 inches
|Bromoxynil (Buctril® 2E or Buctril® 4EC)||3-leaf to prior to preboot stage in grain sorghum (recommended rates differ)||Leaf burn may occur with spray additives
Often combined with atrazine
|Diuron (Direx®)||Directed sprays after 15-inch height||Lower rates for small broadleaves and higher rates for grasses and taller broadleaves|
|Pyrasulfotole + bromoxynil (Huskie®)||3-leaf to 30-inch (prior to flag leaf emergence)||Best performance on weeds 4 inches or less.
Negative crop response can occur
|Halosulfuron + dicamba (Yukon®)||2-leaf through 15 inches (drop nozzles in sorghum greater than 8 inches)||COC or NIS recommended additives|
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