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 premixes in corn that contain some combination of group 15 herbicides (metolachlor, acetochlor, pyroxasulfone), atrazine, mesotrione (Callisto), clopyralid (Stinger), and bicyclopyrone. 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 velvetleaf, burcucumber, morningglories, waterhemp, cocklebur and others. 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 mentioned 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. Warm weather is helping corn 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 HPPD (group 27) inhibitors and contact products. See table 8 on page 79 in the weed control guide for the height and growth stage restrictions of postemergence corn herbicides.
Big Bad Broadleaves in Corn. Be on the lookout for giant ragweed, burcucumber, and waterhemp. Every year is a good one for giant ragweed, and frequent rains are good for burcucumber and waterhemp. 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. If corn is larger than 12 inches, add a group 27 (mesotrione, tembotrione, topramezone) for foliar and residual activity on burcucumber and waterhemp. A dicamba product can be used for foliar activity on these weeds. If residual activity on waterhemp is needed, add a group 15 herbicide.
