With the many windy days that kept sprayers out of the fields this spring, corn has emerged in some fields before the planned nitrogen and/or herbicide applications were made. So what do you do now? "Liquid UAN by itself can be applied over small emerged corn up to around 60 to 90 lb. of N per acre without causing yield loss, although some burning will occur," says Brian Lang, an Iowa State University Extension field agronomist at Decorah in northeast Iowa.
Most herbicide labels do not allow herbicides to be applied using UAN as a carrier after the corn has emerged because of the risk of crop injury. Corvus herbicide is labeled to be applied on corn up to the 2-leaf stage of corn growth. The label does not forbid applying Corvus postemergence with UAN as a carrier, but it does state that this is not recommended, notes Lang.
Some preemergence herbicides such as Dual II Magnum can be applied after corn emergence, but they will not control emerged weeds. Other herbicides can cause severe injury or death to the corn plants if applied when the corn is spiking, such as Roundup on non-GMO corn. Be sure to check labels. Some farmers may choose to change their planned herbicide or nitrogen applications. The following article gives more details on applying UAN and herbicides after corn emergence at www.ipm.iastate.edu/ipm/icm/2007/4-23/uan.html.
It's been a very good spring for weed development - not good news
We're seeing some green come up in fields in Iowa, and it's not all corn and soybeans: weed pressure is starting to build. "It's been a very good spring for weed development and that's not good news," says ISU Extension weed management specialist Mike Owen. "It's also been windy. The application of herbicides has been difficult. It's been difficult to get the spraying done on a timely basis in many field situations."
Given the early planting of corn and soybeans that has occurred this spring, "we are a little worried that weeds are going to get a jump on some farmers' intentions to apply preemergence herbicides," says Owen. "As we now move in to serious soybean planting, we're concerned that the corn weed management plans are going to take second fiddle to getting a soybean crop planted."
Protect your corn yields from weeds - don't wait too long to spray
Owen adds, "I'm encouraging everyone to make sure weeds are managed prior to the emergence of corn and soybeans. If that's not possible, you need to get on the stick pretty quick to make sure the weeds don't co-exist with the corn or the soybean crop very long so that you protect crop yield."
Here is a link to a May 2, 2010 article by Bob Hartzler, another ISU Extension weed specialist, regarding timely weed control to avoid potential yield losses from waiting too long before you apply your postemergence herbicide program. Go to