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Drought challenges PRE, POST weed treatments

Shelley E. Huguley swfp-shelley-huguley-sorghum-combine-harvest.jpg
Drought requires special weed management considerations.
Sorghum Agronomist Brent Bean says producers need to consider drought conditions and adjust accordingly prior to weed treatments.

Drought conditions complicate sorghum preemergence (PRE) and postemergence (POST) herbicide applications. As drought persists in the Southwest, Sorghum Checkoff Agronomist Brent Bean says there are important management tips producers need to consider before treating their fields.

"Drought conditions tend to make weed control a challenge with both PRE and POST-applied herbicides," Bean says. "In sorghum, growers especially rely on good preemergence weed control."

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For pre-applied herbicides to be effective, Bean suggests the following:

1)  The herbicide must be moved into the soil in close proximity to the weed seed. Although this can be accomplished by tillage, producers are mostly reliant on rainfall or irrigation to place the herbicide in the soil weed seed zone. How much rainfall is needed? Usually between 0.25 and 0.5 inches are required, depending on some of the factors listed in #2.

2)  Enough herbicide must remain in the soil water to kill the germinating weed. This is dependent on the following:

a. Solubility of herbicide in water.

b. Amount of soil water present. The wetter the soil, the more herbicide will be present to be taken up by the germinating weed.

c.  Affinity of herbicide for being absorbed into the soil. This will differ by herbicide. Weeds do not take up herbicide absorbed to the soil. In general, soils with higher pH, CEC, organic matter and clay content will absorb more herbicide, which is why increased herbicide rates are recommended in these soils.

To increase PRE control during a drought, Bean recommends the following:

  1. Use the highest labeled rate. The higher the rate, the better the opportunity for enough herbicide to be in the soil solution to be effective under drier conditions.
  2. Try to time planting and pre-application just prior to rainfall. " I know, easier said than done!"
  3. Consider light tillage to incorporate the herbicide if rainfall is not received within a few days after application. The longer the herbicide remains on the soil surface, the more it potentially degrades and at some point will not be effective, even after a rainfall event.
  4. Consider re-applying the pre herbicide as a post application if conditions remain dry. Almost all of the PRE herbicides used in sorghum are labeled and safe for POST application. Check label.
  5. Select a herbicide that is more active under drier conditions. Although S-metolachlor, sold under a host of trade names and in pre-mixes with atrazine, is a great PRE herbicide there is evidence that both acetochlor (Warrant, Fultime NXT, others) and dimethenamid (Outlook, Verdict, others) may be more effective under drier conditions.

For more information, review Absorption of Soil-Applied Herbicides by Iowa State Extension Weed Scientist, Bob Hartzer.

POST Weed Control

Weeds growing in drought conditions typically have a thicker waxy cuticle on the leaves that must be overcome by the herbicide, Bean says. Also, stressed weeds have a reduced growth rate and translocation slows, reducing the movement of herbicide to where it can cause the most damage (site-of-action) within the plant, he adds.

Bean recommends producers take the following actions to increase POST weed control success during a drought:

  • Use the highest labeled herbicide rate
  • Good coverage is essential
  • Treat small weeds. We always talk about the importance of weed size for optimum control, but it is even more important for the control of stressed weeds.
  • Use recommended good quality adjuvants. Consider adding fertilizer – AMS, UAN, etc, when appropriate (see label).
  • Take advantage of any rainfall event to apply herbicides to weeds that are actively growing.
  • Consider contact herbicides. Drought stress affects the performance of translocated herbicides more so than contact herbicides. Examples of contact herbicides that can be used in sorghum are Aim and Buctril. Maybe tank mix these with other POST herbicides.
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