UNL CropWatch June 18, 2010: Weighing the Risks of Late Herbicide Applications to Corn

UNL CropWatch June 18, 2010: Weighing the Risks of Late Herbicide Applications to Corn

June 18, 2010

Due to the frequent rains and numerous windy days this spring, many corn fields have not received postemergence herbicide applications. The purpose of this article is to discuss 1) when postemergence herbicide applications are most beneficial, and 2) crop injury risk from late postemergence herbicide applications.

The 2010 cropping season was an excellent year to invest in a preemergence herbicide application. Because of good precipitation and herbicide activation, even "set-up" rates (one-half to two-thirds rates of herbicides, intended to be followed by a postemergence application of glyphosate or glufosinate) have provided outstanding weed control on the species they are effective at controlling. As a result, weed pressure in fields that received a preemergence herbicide is typically light and yields have been protected from weed competition.

Corn in many fields is nearing canopy closure and in the V8 to V12 growth stage and 20-36 inches tall. Because the weeds are small, if postemergence herbicides can be applied before canopy closure, control should be excellent and result in clean fields at harvest. However, if weather conditions prevent herbicide application until the canopy does close and the herbicides cannot be applied using drop nozzles, weed control may be less than expected because much of the herbicide will be intercepted by the crop canopy and not reach the targeted weeds.

When corn reaches this stage, producers should determine if they are making economically profitable or cosmetic applications. If corn is nearing canopy closure, and weeds are just starting to emerge, the yield loss caused by these late emerging weeds is likely slight. The YieldLoss Calculator may be used to estimate yield loss based on weed density and crop stage for your field. Although allowing late-emerging weeds to grow will have limited effect on crop yield, it may result in a slight increase in the amount of weed seed in the soil, and the field may look "messier" than desired at harvest.

The other risk to consider is whether the herbicide application will have a negative effect on pollination and grain yield. Pesticide labels are usually specific about the maximum growth stage for application. Adhering to the label is important for crop safety. Corn in the V6 to V10 growth stages is forming the ear and determining how many rows and kernels it will contain. Herbicide application and other stresses at this growth stage can have a negative effect on ear formation and yield potential.

Herbicide Application Timing

  • Glyphosate is labeled for broadcast applications up to 24 inches. From 24 inches to 30 inches, drop nozzles are recommended. Drop nozzles are required by the label for applications from 30 inches to 48 inches.  Research has shown that applying glyphosate broadcast to tall corn (V8 and V10) caused pollen deformation and reduced pollen viability, but did not reduce seed set or grain yield because corn produces much more pollen than it needs (Thomas, et al. 2004.Weed Science 52:725-734). However, we have been contacted regarding fields where glyphosate was broadcast applied after the corn reached 48 inches tall. We believe that these off-label glyphosate applications likely contributed to the poor pollination and significant yield loss observed in these fields.
  •  Photo - Corn herbicide injury

    Figure 1. Corn injury from a late application of nicosulfuron.

  • ALS herbicides (Resolve, Steadfast, Permit, Accent/Nic-It, Beacon, Option, Capreno) should be applied prior to V7. When applied after V7 they can cause ear deformation (Figure 1).
  • HPPD-inhibiting herbicides (Callisto, Laudis, Impact) are labeled through V7 (Laudis), V8 (Callisto), or taller corn (Impact).
  • Glufosinate (Ignite) is labeled up to V7 Liberty Link corn.
  • PPO inhibitors (Cadet, Resource) are labeled up to V10 (Resource) or 48 inches (Cadet). Both are most effective if weeds are less than 2 inches tall so the value of applying them this late is minimal, unless the weeds are very small or if it is for a rescue treatment where velvetleaf has escaped.

Mark Bernards
Extension Weeds Specialist, Lincoln
Lowell Sandell
Extension Educator, Weed Science, Lincoln



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