Corn and Soybean Weed Control in a Wet Year

Mike Hunter, CCE – North Country Regional Agriculture Team

small common lambsquarters
Small common lambsquarters that emerged before the soybean planted in this field. Photo taken in Jefferson County June 2019

The cool, wet month of May and start of June has created some challenging weed management situations for both corn and soybean.  Unfortunately, delayed planting seasons force growers to focus so much on getting the corn and soybean planted they may not have had the opportunity to make a timely planned preemergence (PRE) herbicide application.

Here is a common situation that we are already encountering this season.  We have a field with corn or soybeans planted and cool conditions have delayed crop emergence but the weeds have already emerged before the PRE herbicide treatment was made.  Do we stick to our original plan and apply a PRE herbicide to this field or do we need to make adjustments to the herbicide program?

If your planned PRE herbicide application has been delayed, it is very important to carefully consider your herbicide choices and make necessary adjustments if any weeds are emerged at the time of application.  With adequate rainfall, PRE herbicides can provide excellent weed control; however, once the weeds are emerged they will generally need some additional product to the tank mix.  The additional product could be another herbicide to add to the tank mix or just an adjuvant such as non-ionic surfactant (NIS), crop oil concentrate (COC) or methylated seed oil (MSO).  There will be many more options in corn than soybeans.

Corn fields not treated with an herbicide prior to crop emergence need to be looked at carefully.  If very small weeds are emerged at the time of the PRE application the answer may be as simple as adding adjuvant to the PRE herbicide.  Consult the herbicide label and follow the adjuvant recommendations based on the products in the tank mix.

If the corn has emerged and the annual grasses are over 1 inch tall and the broadleaf weeds are 2 to 3 inches tall, it may be necessary to add another herbicide to the PRE herbicide.  If the corn is glyphosate tolerant, you may only need to add glyphosate to the preemergence herbicide program.  Using this same scenario with conventional corn, you will likely need to include a postemergence (POST) herbicide to the PRE herbicide.  Examples of POST tank mix herbicides to consider for control of both emerged annual grasses and broadleaf weeds include: Revulin Q, Realm Q, Resolve Q, Capreno, Laudis, Armezon.  The effectiveness of these POST herbicides varies with the control of different annual grasses making proper weed identification critical.  Again, check the herbicide label prior to making any herbicide applications.

If you are using a PRE soybean herbicide it will likely be an Herbicide Group 2 (Pursuit, Python, Firstrate), 3 (Prowl, Treflan, Sonalan), 5 (TriCor, Dimetric, metribuzin), 7 (Lorox, Linex), 14 (Valor, Sharpen) or 15 (Dual, Warrant, Outlook).  Soon after soybeans are planted, there is a narrow window to make certain PRE herbicide applications.  Valor (flumioxazin), Sharpen (saflufenacil), metribuzin and any premixes containing these active ingredients must be applied prior to crop emergence.  Lorox (linuron) is another PRE soybean herbicide that must also be applied prior to crop emergence.  Prowl, Treflan and Sonalan are applied prior to planting soybeans.

Soybean fields not treated with a PRE herbicide after crop emergence where very small weeds have emerged can be more difficult to deal with, especially if a population herbicide resistant tall waterhemp is present.  Recently, Dr. Bryan Brown, NYS Integrated Pest Management Program, conducted tall waterhemp herbicide resistance screening trials at Cornell University.  Using tall waterhemp seeds collected from three different fields in New York, preliminary results indicate that two populations were resistant to glyphosate (i.e. Roundup, Group 9), three populations resistant to atrazine (i.e. Aatrex, Group 5) and two populations resistant to imazethapyr (i.e. Pursuit, Group 2).  Fortunately, none of the tall waterhemp screened were found to be resistant to lactofen (i.e. Cobra, Group 14).

If a population of multiple resistant tall waterhemp is present, our effective herbicide options are limited.  The PRE herbicides that will provide control of multiple resistant (Group 2, 5, 9) tall waterhemp include Dual, Warrant, Outlook (S-metolachlor, acetolchlor, dimethenamid-P), Prowl, Treflan, Sonalan (pendimethalin, trifluralin, ethafluranlin) Valor SX (flumioxazin) and Lorox, Linex (linuron).  If both the soybeans and multiple resistant tall waterhemp have emerged, our effective herbicide options are very limited.  Dual, Warrant and Outlook are the only PRE herbicides listed that can be applied POST; however, these products will not control emerged weeds.  In this situation it would be necessary to include either Reflex or Cobra (Group 14) to the tank mix to provide control of the emerged tall waterhemp.

Soybeans with the herbicide resistant technologies such as Liberty Link (glufosinate tolerant i.e. Liberty), Xtend (dicamba tolerant i.e.Xtendimax, Engenia, FeXapan) and Enlist E3 (2,4-D i.e. Enlist, glufosinate and glyphosate tolerant) provide additional options for POST control of resistant tall waterhemp.

This spring has provided very limited opportunities to plant corn and soybeans due to frequent rainfall and wet field conditions.  This challenging spring has also made it difficult to apply planned PRE herbicides in a timely manner.  It is important to carefully scout your fields before making any herbicide application to make sure the right products are included in the tank mix. And as always, check the herbicide label prior to making any herbicide applications.


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