Post-harvest Weed Control by Jason Bond

The reality of weed control in the Midsouth today is that the weed control season is never over.  Because of glyphosate-resistant weeds, we are either in the process of killing weeds or we should be thinking about how to kill them.  Late summer is the time to evaluate weed control programs.  Where are the problem spots?  Does a field need to be rotated to a different crop to better manage the weed spectrum?  Which herbicide program worked?  Which did not work?  If it did not work, why?  Make thorough notes and refer to these when designing weed control programs for 2012.

Judging by the turnrows and ditchbanks that have been treated with all types of herbicide concoctions, awareness of the repercussions of allowing weeds to produce seed is increasing.  Harvest in the Delta will be in full swing long before the official end of summer.  Harvest season is generally so busy that no one is thinking about weed control; however, weed control after harvest with the intent of preventing seed production can pay long-term dividends.  Weeds are relentless.  Just because the crop is gone does not mean the weeds stop growing.  Many fields that look clean from the turnrow right now contain emerged weeds that are patiently waiting for the crop canopy to start drying down.  When these weeds get some sunlight, they will start growing and eventually produce seed.  Also, some summer annual weeds like Palmer amaranth (pigweed) and barnyardgrass are not picky about when they emerge, and they can produce seed rather quickly.  If a field is harvested in August or early-September, Palmer amaranth and barnyardgrass can easily produce one and sometimes two crops of seed before the first frost.  Weed seed production after harvest can nullify even the best control efforts. 

Because of when the crop matures, wheat, corn and early soybean fields are most susceptible to weed problems after harvest (see photos).  The rice field pictured below was planted following two consecutive years of corn.  The barnyardgrass that emerged and produced seed after corn harvest in 2008 and 2009 created a nightmare scenario when the field was rotated to rice in 2010.  Rotating a field to corn is a good technique to manage severe infestations of glyphosate-resistant Palmer amaranth because several effective pigweed herbicides are labeled for corn.  However, all the benefits of using corn to reduce the Palmer amaranth population can be negated if the weeds emerging after harvest are not managed. 

How do fields need to be managed after harvest to prevent weed seed production?  A vague weed scientist response to many questions is, “It depends on the year”.  For postharvest weed control, it really does depend on the year.  Consider 2009 and 2010 as examples.  In 2009, over 21 inches of rainfall were recorded by the weather station at Stoneville between August 15 and November 1.  Conversely, only 4 inches of rainfall was recorded over the same interval in 2010.  In 2010, nearly all tillage was complete in the Delta by November, but the previous year most fields could not be disked until spring because of the wet weather in late 2009.  We were unable to do any postharvest weed control research in 2009 because the fields were too wet.  The same studies were initiated in 2010, but only required two passes with a disk to prevent weed seed production. 

For postharvest weed control, our best control recommendation at this time is, “Just do something”.  The primary goal of weed control in the fall is preventing seed production.  Disk it.  Spray it.  Disk it then spray it with a residual herbicide.  Or, spray it with a nonselective herbicide then disk it.  As long as the weeds do not produce seed, then the system was successful. 

A second consideration both during and after harvest is sanitation.  Harvest and tillage equipment are excellent tools for spreading weed seed.  A land plane or a combine’s straw spreader can turn an isolated patch of glyphosate-resistant weeds into a widespread problem.  Combines, disks, subsoilers, etc. should be cleaned to remove weed seed before moving to the next field, especially if some fields have glyphosate-resistant weeds and some do not. 

Few farming operations have enough labor and equipment and almost no farms only grow a single commodity.  When corn harvest leads to soybean harvest, and soybean harvest leads to cotton harvest, there is not much time left to think about weed control.  Also, after a field is harvested, there is no return on an additional weed control investment in the current year.  But, the big picture must be considered when fighting glyphosate-resistant weeds.  Decisions made this year carry over into next year. Controlling weeds after harvest is a good way to get a jump start on next year.