Weed Management in Continuous Corn

Chris Boerboom, Ext. Weed Scientist

Recent discussions on increasing corn acreage have raised concerns about how continuous corn production will affect various management practices, including pest management.  While continuous corn may affect insect and disease management, I believe the effect on weed management will be relatively minor.  Because corn is not a “host” of weeds like corn is a host of disease or insects, corn will not cause specific weed species to increase just because it is planted.  Rather, weeds may increase because other practices used in corn production allow weed shifts or increased weed populations.

Let’s look at how continuous corn would compare to a corn-soybean rotation and see where such differences might exist. 

1. Time of planting.  Certain weed species may be more successful in early or late planted crops.  However, the time difference between corn and soybean planting is minimal and both are currently planted in early spring.  The increased time to plant additional corn acres will basically extend into the time that was used to plant soybeans.  The net effect seems to be negligible. 

2. Canopy development.  Narrow row soybeans can create a more complete canopy (i.e. greater shade) than corn.  This could affect the survival of late emerging weeds.  For instance, more dandelions might establish in no-till corn than in no-till soybeans even if early season control was the same. The canopy of soybeans in 30-inch rows might create shade that is more similar to a full corn canopy.  In either case, I don’t envision that any canopy differences would lead to a weed shift.  Any differences in weed density should be manageable if they occur.

3. Herbicide use.  Certain advantages might exist with continuous corn as compared to a corn soybean rotation.  Specifically, corn herbicide carryover should never be a concern when corn is the following crop.  Increased corn acres could also lessen the risk for corn herbicide drift onto soybeans if these fields are now planted to corn.  On the negative side, more acres may need to be sprayed in the same short time period.  If corn acres are being sprayed postemergence, this time period is short and it’s critical to remove weed competition.  Of course, preemergence herbicides in the corn program would lessen this concern. 

4. No-till systems.  No-tilling corn in to soybean residue is relatively easy. However, no-tilling corn into corn stalks may require more row cleaning to move residue.  This would be a concern if a residual herbicide was applied preplant.  If aggressive row cleaners are used, they may move the preplant herbicide out of the corn row and lead to poor control.  A solution to this situation would be to apply the same residual herbicide as a preemergence application after planting.

5. Herbicide resistant weeds.  If we look back in history, continuous corn fields led to herbicide resistant weeds in some cases because the same herbicides were used year after year.  Could that happen again?  It certainly could.  However, it could also happen with a corn-soybean rotation if both crops are glyphosate resistant and glyphosate is the primary herbicide being used in both crops.  We still recommend that herbicide diversity should be maintained whether continuous corn or a corn-soybean rotation.

Overall, I don’t see where continuous corn will present any special weed management problems or where continuous corn differs greatly from a corn-soybean rotation.  Both crops are summer annuals and weeds really don’t care if they are growing in one crop versus the other.