The rapid pace of planting in late April followed by rain has resulted in many fields where the corn is now emerged and the preemergence herbicides and nitrogen applications haven't been made yet. Of particular concern are no-till fields where corn planting was completed prior to killing the emerged weeds.
"These fields should be priority for action since weeds that have a head start on the crop can begin to impact crop yields very early in the season," observes Bob Hartzler, an Iowa State University Extension weed management specialist.
Researchers evaluating how long weeds can compete with corn before impacting yields (the critical period) often report that corn can tolerate weeds until the V3 to V4 growth stage of corn plants. However, these findings are based on corn being planted into a clean seedbed. "When corn is planted into a weedy seedbed, yields can be impacted much sooner," says Hartzler.
Need quick action now to apply herbicides before emerged weeds get bigger
Another reason farmers need to take quick action in these fields is that several of the early-emerging weed species are some of the most difficult to control once they get a little size to them, says Hartzler. These tough to control weeds include horseweed or marestail, giant ragweed and lambsquarters.
Most preemergence herbicides also are registered for application after corn has emerged. However, their activity on emerged weeds varies, he points out. If weeds are present, you need to determine whether the postemergence activity of the residual herbicide is sufficient to control the weeds present in the field or if an additional herbicide with better postemergence activity is needed.
Another issue many Iowa farmers are now facing is the need to apply nitrogen and the desire to minimize trips across the field, notes John Sawyer, ISU Extension soil fertility specialist. Urea-ammonium nitrate or UAN alone can be applied to emerged corn, and the risk of injury to the corn plants depends upon the UAN application rate per acre, the corn stage of growth and on weather conditions.
Can you apply N either alone or mixed with herbicide over top of corn plants?
Conservative suggestions are to limit postemergence applications of UAN to 90 lb. of N per acre when corn is at the V3 to V4 growth stage and to 60 lb. of N per acre at the V7 stage. Applications beyond the V7 stage of corn growth are not recommended, says Sawyer. Also, keep in mind that the risk of injury to the corn plants increases during hot, dry conditions--if nitrogen is broadcast over the top.
What about mixing liquid N with the herbicide and applying both at once? "While many preemergence herbicides are applied using UAN as a carrier, this practice is only recommended prior to crop emergence," says Sawyer.
The combination of herbicides with UAN greatly enhances the foliar activity of these products and poses a real threat of killing all emerged tissue contacted by the spray, he warns. Almost all herbicides prohibit application in nitrogen solutions after the corn crop has emerged, because of the risk of severe crop injury.
Some farmers and fertilizer/chemical dealers might try to rationalize this combination if the corn is at the VE to V1 growth stage since the growing point is still underground. "While corn often can recover quickly from loss of the shoot at this growth stage, the herbicide may influence the plant's ability to recover and therefore result in yield loss," warns Sawyer. You must be very careful if you try this. He doesn't recommend applying herbicide/N tankmixes on corn that has already reached this growth stage.