Recent wet, rainy weather has created some weed management challenges for Indiana growers.  In this article we will hit on a few key points to consider based on current challenges.

Delayed weed control in corn.  Indiana corn growers rely heavily on atrazine premixes in corn.  Rain will not have completely washed all of the herbicide away, but may have compromised overall activity.  Scout fields as soon as possible to determine if weeds are escaping.  Obviously giant ragweed is a big concern, but wet conditions and dilution of atrazine can result in failures to control cocklebur, sunflower, velvetleaf, burcucumber, morningglories, and waterhemp.  If corn is less than 12 inches tall and you haven’t used all of the atrazine allowed by the label, it would be wise to add atrazine to the other postemergence herbicides being applied to corn to control the above mention weeds and provide some soil residual activity.  If corn is more than 12 inches tall, you cannot use any additional atrazine. 

Weed escapes in large corn in all of Indiana.  When the warm weather hits us late this week and through the weekend, corn will grow rapidly.  Postemergence corn herbicide options become limited when corn is 12 inches tall, and really limited on corn at the V6 or later growth stage.  Also keep in mind that large weeds are much more difficult to control.  To avoid crop injury on corn under other stresses, try to keep spray out of whorls, especially with ALS inhibitors and contact products.  See table 6 in the weed control guide for the height restrictions of postemergence corn herbicides.

Big Bad Broadleaves.  Be on the lookout for giant ragweed and burcucumber.  Every year is a good one for giant ragweed, and frequent rains are good for burcucumber.  Both weeds have relative long emergence patterns and herbicides with both foliar and residual activity are needed.  If corn is not 12 inches tall, consider adding some atrazine to the postemergence product to extend the residual window of activity on these weeds.

How do I pick a postemergence herbicide for corn? Going on the assumption that most of the corn grown in Indiana is Roundup Ready or Liberty Link, we assume growers will use glyphosate or Ignite as the base herbicide. Many different herbicides can be tankmixed with glyphosate or Ignite to broaden the spectrum of weeds controlled and provide residual activity in the soil. The best advice we can give you is to consult table 4 on page 28 of the weed guide to help in selecting tankmix partners for weeds you are trying to control.