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Late Season Weed Control in Corn and Soybean

Posted: August 21, 2012

Hopefully you’re still not needing to control weeds in corn and soybean, but if you are....

As we move into silage harvest season, producers may notice some problem weed situations. Also, as soybean leaves start to turn yellow, problem weeds may become more noticeable. At this point in the season, it is difficult to do much with the exception of a harvest aid prior to corn or soybean grain production. In corn, this type of application requires high clearance equipment or aerial application, so hopefully these fields are the exception and not the norm. Below is a brief list of products available for late season application or harvest aid for corn and soybeans.

Corn

  • Basagran — No restrictions
  • Impact/Armezon — Can be applied up to 45 days prior to harvest.
  • Glyphosate — Up to 0.75 lb ae/acre (32 fl oz of a 3 lb ae/gal formulation) to corn a week or more prior to harvest. Must be applied to grain when moisture is 35% or less and after maximum kernel fill.
  • Gramoxone SL — Apply 1.2 to 2 pt/acre after black layer and at least 7 days before harvest. The higher rate can be used to desiccate mature broadleaves and grasses over 18 inches tall. Be sure to include a nonionic surfactant.
  • 2,4–D LV4 — Apply 1 to 2 pt/acre after the hard dough or dent stage. Do not forage or feed corn fodder for 7 days after application.

Soybean

  • Clarity — Apply 8 fl oz to 2 quarts after soybean pods have reached a mature brown color and at least 75% leaf drop has occurred. Wait at least 14 days before harvest.
  • Glyphosate —In Roundup Ready soybeans, glyphosate may be applied through flowering (R2 stage) for late season control. For pre-harvest, glyphosate may be applied to Roundup Ready and conventional soybeans after 80% leaf drop (loss of all green color). Apply up to 0.75 lb ae/acre (32 fl oz of a 3 lb ae/gal formulation) in 10-20 gallons of water/acre to control weeds that may interfere with harvest or to control perennials such as quackgrass or Canada thistle. Allow a minimum of 7 days between application and harvest. Do not graze or harvest for hay within 25 days of application.
  • Gramoxone SL — Apply 8 to 16 fl. oz/acre plus nonionic surfactant (1 qt/100 gallons of spray) to soybean plants that are mature (65% or more of the seed pods have reached mature brown color or seed moisture is 30% or less. Do not apply within 15 days of harvest and do not graze or harvest for forage or hay.


A special note on burcucumber: Burcucumber can germinate and emerge late into the season and it’s not until it gets above the crop canopy that it is often noticed. This late in corn, there are few options for managing this troublesome weed. Aside from the corn herbicides mentioned above, Beacon (applied before tassel emergence) and Buctril (tassel) can also be sprayed to slow burcucumber. As we get near harvest, the pre-harvest applications are targeting grain production prior to a frost and really are intended just to allow combine harvest of the crop. Gramoxone is probably the most effective of these three options at this time.

In soybean, as noted above, only glyphosate, Gramoxone and Clarity may be used for late season control. Again, Gramoxone is probably the most effective of these treatments for burcucumber control.

Finally, if burcucumber can be harvested for silage and the seeds are immature or still green, the ensiling process will kill the seeds. If the corn or soybean is harvested for grain, the mature seeds will simply end up back in the field (or in the grain) making the problem worse for future years.

Contact Information

Dwight Lingenfelter
  • Program Development Specialist
Email:
Phone: 814-865-2242
William S. Curran
  • Professor of Weed Science
Phone: 814-863-1014