Frosted corn —
The majority of the crops are recovering from the frost that hit May 9, but producers need to keep a few things in mind when it comes time for their first herbicide application.
Bob Hartzler, professor of agronomy and Iowa State University Extension weed specialist, said that spraying corn without allowing it to fully recover from the frost could end up further damaging the plant.
“Any herbicide is going to stress the crop and especially if the corn plant is in the process of recovering from the frost or any other environmental damage it might have,” said Hartzler.
Fortunately there have been relatively good growing conditions since the frost hit and Hartzler said if the corn is showing two new leaves it is most likely out of danger and healthy enough to handle a herbicide application.
“Consider the seriousness of the weed problem. If it’s not bad, maybe you can wait, but if there is a lot of weed pressure, that could be worse than any frost damage done to the corn,” he said.
Although many of the corn plants appear to be recovering from the frost damage, another issue will be staging the correct growth of that frost-damaged corn.
The leaf loss, Hartzler said, from the frost will make it very easy to underestimate the actual growth of the corn plant.
According to Hartzler, assume that a field is staged on May 29 and has four visible leaf collars. Normally, you would call this a V4 plant.
“However, if this field was planted in mid-April and on May 9 had two-emerged leaves that were killed by frost; the actual development stage would be V6 rather than V4.
“You need to keep in mind the two leaves that are gone from frost,” said Hartzler. “There are some restrictions on timing for herbicide applications based on crop injury potential and those restrictions are based on leaf numbers.”
With some careful assessment, those dead leaves might be present to help you count the stage, but in many cases will not be. Hartzler also said if staging the corn by height, the same problem of underestimating the stage of development still exists.
The main concern will be herbicides for which application restrictions are based on crop tolerance concerns.
According to Hartzler, many sulfonylurea herbicides such as Accent Q, NIC-IT, Resolve and Steadfast prohibit broadcast applications beyond the V6 stage. Applications made later than the V6 stage may damage the ear since it is initiated at this time.
Growth regulator herbicides also have application timing restrictions based on crop injury risk.
Hartzler recommends documenting which fields have been damaged by frost and determine the number of leaves lost to freezing to help avoid problems.
Contact Kriss Nelson at firstname.lastname@example.org.
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