Dry conditions continue to persist in Southern WI: Poor Early Weed Control is Evident

Dry conditions continue to persist in Southern WI: Poor Early Weed Control is Evident

Vince M. Davis, Extension Weed Scientist

What happened to this poor soybean stand?

picture of soybean field with dandelion damage

Soybean field damaged by dandelion competition in dry conditions.

Poor stand establishment…Disease…Insects…?   Answer: Dandelion competition in dry conditions!

A drive around much of Southern WI right now, particularly in the afternoon, would result in a very common scene in many crop fields…..that is, crop stress to various degrees from slight to severe.  What is most evident (at least to me), is how much of the variability in the visual crop stress symptoms are related to poor early-season weed control.  This is NOT the year to be losing moisture in the soil profile to weeds.  In the above picture, a grower thought there was a problem with disease.  A scouting trip in this no-till field very quickly revealed that patchy dandelion pressure was enough under these droughty conditions to cause plant stress to the point of plant death.

This year we are currently experiencing the same conditions in our research trials at Arlington Research Farm where severe weed pressures, especially dandelion, are enough to stress corn and soybean plants to the point of no return in several cases.  I don’t write many articles that don’t emphasize the importance of scouting, and this one will too.  I’d also like to point out that I’ve more than once heard growers complain they’re not sure what to do with their yield monitor maps.  Well, here’s a suggestion for this year, make weed scouting maps including weed density estimates as detailed as possible at the POST herbicide application timing, and overlay them with yield monitor maps this fall. It might prove very interesting and insightful.

But, what to do now?      

There is still the question that is the bigger one, and that is what to do now?  We know the weeds are stressed with thick cuticles making POST herbicides not work well, but on the other hand, the crops (and weeds) are getting large.  I feared, and warned, of these difficult conditions that may occur for controlling weeds at the POST timing in the WCM two weeks ago if dry conditions persisted: http://ipcm.wisc.edu/download/wcm-pdf/WCM19_13.pdf .  Now, we are even further in difficult times.  In many cases, corn has gotten beyond the height (30”) and development (V8) restrictions on the glyphosate label, meaning it can no longer be applied.  For broadleaf weeds, Status® (BASF Corporation) herbicide can still be used at 5 fl oz/acre on corn up to 36” in height, or V10.  As a special precaution, make certain the label is followed regarding procedures to clean the sprayer following Status applications to avoid sprayer contamination and damage to soybean fields in subsequent applications.

In soybean, I would not delay POST applications of glyphosate because large weeds could be causing tremendous yield loss this year.  If rain is likely within a day or two of your desired application timing, it might be worth waiting for a rain that will make weeds resume more normal growth.  However, in many cases it may not be worth waiting.  Just because a rain will make the weeds grow quickly again, the structure of the aged cuticles won’t change that quickly and so only part of the problem that makes weeds difficult to control is solved with rain. Moreover, rapid weed growth may cause weeds to surpass recommended sizes on the label quickly, also working against you.  If it’s not worth waiting, which not to mention also depends on your pending work load, there are a few things that can be done to help improve control.

  • Scout weed heights by species carefully, and use an appropriate rate of herbicide.  In many cases, that is going to mean using the maximum amount of glyphosate that can be used in a single application on soybean: (1.5 lb ae per acre).
  • Use a tank-mix herbicide partner if needed to improve certain species.
  • Increase adjuvant concentration.  Non-ionic surfactant (NIS) can be increased to 0.5% v/v, and that could improve performance of generic glyphosate formulations.  For some other herbicides, rates of COC and UAN may need to increase, or even considerations of using MSO.  However, for contact herbicides, increasing volume may be more effective than increasing adjuvant rates.  Make sure you read the labels closely regarding these possible changes, and remember, as a general rule of thumb, if increasing adjuvant concentration increases weed control then it also increases the likelihood of negative crop response.
  • Apply early in the morning.  Weeds are likely to be less stressed at this time.  They will have the most active growth in the morning and likely present the most surface leaf area to intercept herbicide droplets.

It’s no consolation that our neighboring states are suffering, but parts of Illinois and Indiana are also experiencing similar problems with dry conditions.  Here are suggestions from my colleagues in those states addressing similar issues this season:

Herbicide Applications in Dry Conditions by Legleiter and Johnson 6/20/12: http://www3.ag.purdue.edu/btny/weedscience/documents/Dry_Conditions.pdf

Considerations With Postemergence Herbicides Applied During Wet or Dry Conditions by Aaron Hager 6/22/2012: http://bulletin.ipm.illinois.edu/article.php?id=1666

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