Post-Harvest Weed Control Options

Harvesting has begun and we are getting numerous calls about post-harvest weed control options for Palmer amaranth.  While there is no single plan that will work on every farm, the ultimate goal should be to limit Palmer amaranth seed production.

Tillage (disking, chisel plowing, etc.) is a common means of post-harvest weed control and a way to encourage rapid degradation of crop residues.  However, this process can also bring new weed seeds to the surface, allowing them to germinate.  It is still a long time until November, which is historically when the weather begins to become a factor in limiting germination of Palmer amaranth.  Considering the earliness of this year’s harvest, it is our opinion that the soil seed bank should be disturbed as little as possible to prevent new emergence.  Since the soil seed bank and that the effective germination zone for Palmer amaranth is at or near the soil surface, it is imperative that no additional seed is brought to the surface via tillage.  If Palmer amaranth was effectively controlled in-season, there is a good chance that the number of seed currently in the germinating zone has been somewhat depleted from the residual and postemergence herbicides.  Knowing this, most of our current recommendations, especially following corn and grain sorghum, are to shred the remaining vegetation (preferably with a flail mower) and try to create a mulching effect with the crop residue to prevent Palmer amaranth emergence.  Palmer amaranth will not germinate or grow easily where there is heavy canopy or cover.  Once this “mulch” is in place, the few Palmer amaranth plants that emerge should be controlled with an application of paraquat.  This system should remain in place until it becomes imperative to get back in the field and conduct normal tillage in preparation for next year’s crop.

There are also some farmers taking advantage of the early harvest and great commodity prices by following corn harvest with soybean planters (see  http://www.msagconsultants.com/wp-content/uploads/2012/08/COrn-fb-beans.jpg).  While this goes right along with the above concept about keeping some kind of ground cover on the field, there are a couple of things to keep in mind.  First is to make sure this is not “off label” by planting soybean following application of such herbicides as atrazine, Halex GT, Laudis, Realm Q, etc.  In some situations, soybean injury could be severe, and the soybean crop may also be in violation of the label.  Second, if soybean is planted into known populations of glyphosate-resistant Palmer amaranth, we encourage using a LibertyLink variety as the likelihood of receiving rainfall for incorporation of a residual herbicide in a Roundup Ready system is low this time of year.  Additionally, if Roundup Ready soybean is planted and a residual herbicide is applied (which it should), the rotational intervals associated with the residual herbicide should be considered, especially if the field will be planted to wheat this fall or corn/grain sorghum next spring.