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Extension Update from Jenny Rees

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Crop Update:  So grateful for some rain for much of the area last week!  Updated soil moisture status at http://jenreesources.com. The crops are rapidly growing now as are the weeds.  Some were seeing Palmer shooting heads at soil level already…last year we

palmer headed at soil

Palmer shooting a head at the soil surface. Photo courtesy Matt Kirchhoff.

didn’t see that till late July.  Many have been in the process of postemergence herbicide applications.  We revisited a CropWatch article regarding best management practice considerations for postemergence dicamba-based applications to corn based on the research that is available.  Please see the full articlewith more explanation at http://cropwatch.unl.edu.  Briefly, those practices include:

  • Consider a quick irrigation (rainfast/irrigation timing based on label requirements for the product you’re applying) of only 1000 gal/ac to help reduce any potential volatility.
  • Don’t use dicamba products in both corn and soybean to reduce selection pressure and resistance.
  • Check for temperature inversions and wind speed. Temperature inversions can be tested by using Innoquest SpotOn® inversion tester and testing the temperature at 1 meter and 3 meters.  If the temp is cooler at 1 meter than 3 meters, a temperature inversion is occurring and spraying is not recommended.
  • Consider using the more restrictive RUP dicamba requirements regarding wind speed, boom height, etc. Also consider not using AMS with any dicamba product even though it is labeled for use in some of the corn dicamba products.  This may result in you needing to increase the glyphosate rate to the highest labeled rate to increase efficacy.  Amit Jhala will showcase research on efficacy of dicamba products with and without AMS at the South Central Ag Lab Weed Science Field Day on June 27.

Volunteer corn is also a major issue in many corn and soybean fields in the area and there’s two articles in CropWatch at http://cropwatch.unl.edu addressing this topic.  A number of grass herbicides are available for control in soybean.  The challenge is in the continuous corn fields.  If you had glyphosate resistant corn last year and used a different technology such as Liberty or Enlist, you have some other options this year.  For Liberty Link corn this year, two applications, each of 32 to 43 fl oz/acre, could be made.  Remember that Liberty will NOT be effective if Roundup Ready + Liberty Link hybrid corn was planted last year.  Regarding Enlist corn, Assure® II is the only grass herbicide labeled to control volunteer corn with this technology.  It can be applied at 5 to 12 fl oz/acre in Enlist Corn for selective control of volunteer corn.  Please be sure to read and follow all label requirements.  A few farmers have also discussed their past experiences with cultivation, using either one or two passes and their concern about the soil moisture situation this year.

So how much yield loss can be anticipated from volunteer corn?  Perhaps more than one would think with more loss occurring in soybean!  Studies were conducted in several mid-western states at various densities including 3500, 5000, 7000 and greater volunteer corn plants per acre.  To envision this, imagine 3.5, 5, and 7 volunteer corn plants respectively in 1/1000 of an acre (17’5” in 30” rows).  Some fields this year have much higher densities than this!  Clumps of corn impact yield more than individual plants.

UNL research found a volunteer corn density of 3500 plants/acre led to 10% yield reduction in soybean. Doubling the density to 7000 plants/acre led to a 27% yield reduction.  South Dakota State University data revealed similar trends. A volunteer corn density of 5000 plants/acre resulted in a 20% yield reduction (12 bu/acre yield loss in 60 bu/ac soybean).

Clumps of volunteer corn in soybean led to greater yield loss as they were more competitive than individual plants. In the UNL study a density of 3500 clumps of corn/acre resulted in a 40% yield reduction. Researchers in Minnesota and Illinois also found increased competition with clumps of volunteer corn versus individual plants. Clumps of corn (7-10 plants/clump) were established at different densities. Depending on the location and year, soybean yield was reduced 1% for every 75-115 clumps/acre.

A recent UNL research study found highest yield reduction occurred when volunteer corn was left uncontrolled or when it was controlled too late at the R2 (full flower) soybean growth stage.  The combined density at this greatest yield reduction was at 24,710 volunteer corn plants per acre plus 1,235 volunteer corn clumps per acre.

In corn, UNL research found a volunteer corn population of 3500 plants/acre resulted in a 2% yield reduction in corn. Doubling the density to 7000 plants/acre caused a 5% yield reduction.  Clumps of volunteer corn led to greater yield loss as they were more competitive than individual plants. A density of 7000 clumps of corn/acre resulted in a 14% yield loss compared to a 5% yield loss with individual plants.  So volunteer corn in general can be fairly competitive especially to our legume crops.  It can also be a problem regarding harboring corn rootworm in soybean fields, reducing the advantage of the rotation from that perspective.

Also, an update on my soybean planting date demo at the fairgrounds: I wasn’t counting on rabbits!  All the soybeans were reduced to stems below the point of recovery.  So there is no demo but the groundskeeper preferred they took out my soybeans instead of his petunias 🙂Will try again next year.

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