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This past two weeks have produced environmental conditions which have led to problems in our soybean fields.

The use of PPO herbicides for control of waterhemp, Palmer amaranth and other pigweed species have had an impact on soybean emergence we rarely see in this district.  With the combination of wet and cool conditions these PPO herbicides have caused some conditions that have required replanting in some fields.  What is happening in this situation is the herbicides are being applied pre-emergence over the top of newly planted fields. 

Soil-residual herbicides are important components of integrated weed management programs and provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.  One class of residual herbicides are the PPO or protoporphyrinogen herbicides.  Soil-applied PPO-inhibiting herbicides, including saflufenacil, flumioxazin, and sulfentrazone chemistries, are very effective for control of pigweed species.  These herbicides can cause soybean injury under certain weather conditions.  Applications of residual PPO herbicides made immediately before or after soybean planting result in a highly concentrated layer of herbicide near the emerging soybean plants.  In contrast, a herbicide is often more widely distributed within the soil profile by the time of soybean emergence when applications are made several days or weeks prior to planting and particularly after a good rain.

So what has been the problem?  When it rains the herbicides have not had a chance to go into soil solution and are very concentrated on the soil surface.  The cold conditions then slow soybean emergence growing through this concentrated “herbicide” layer, allowing the seedling to take in a very concentrated dosage of the herbicides.  It then burns the hypocotyls as they emerge thru soil.  The cold conditions also slow the plant’s ability to metabolize the herbicide, stressing the plant even more.

Even with this damage a possibility, growers need to be reminded that this family of chemistry is an important mainstay in control of this family of weeds today.  No longer can you rely on glyphosate to provide control.

Steps that can help reduce this situation are:

• Spray before planting, allowing herbicides to move into the soil after a rainfall
• If you cannot wait to plant, switch to a very early postemergence treatment of Anthem, Zidua or Flexstar chemistry. These herbicides have to be activated by rain before weed emergence to be effective.
• Use a combination of these two by layering them and making a second application 21 – 28 days after planting.

Remember, if waterhemp emerges you have very little left in the arsenal of herbicides to take it down postemergence.

Randy Stephens
District 7