By Mark Bernards and Lowell Sandell, University of Nebraska Extension weed specialists.

No-tillers should weigh the benefits and risks of post-emergence herbicide applications to corn. Many corn fields have not received post-emergence herbicide applications because of frequent rains and many windy days.

Corn in many fields is nearing canopy closure and in the V8 to V12 growth stage and 20 to 36 inches tall. Because the weeds are small, control should be excellent and result in clean fields at harvest if post-emergence herbicides can be applied before the canopy closes.

But If weather conditions prevent herbicide application until the canopy closes and the herbicides cannot be applied with drop nozzles, weed control may be less than expected . Much of the herbicide will be intercepted by the crop canopy and can't reach the targeted weeds.

When corn reaches this stage, it’s good to weigh whether herbicide applications will be profitable or merely make the crop look better. If corn is nearing canopy and weeds are just starting to emerge, the yield loss caused by these late emerging weeds is likely slight. Use the University of Nebraska’s YieldLoss Calculator to estimate yield losses based on weed density and crop stage in fields.

Allowing late-emerging weeds to grow will have limited effect on crop yield, but this may slightly increase weed seeds in the soil and the field may look messy than 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.

However, we have been contacted regarding fields where glyphosate was broadcast applied after the corn reached 48 inches tall. We believe these off-label glyphosate applications likely contributed to the poor pollination and significant yield loss in these fields.

  • 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.
  • HPPD-inhibiting herbicides (Callisto, Laudis, Impact) are labeled for use 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. The value of applying them this late is minimal unless weeds are very small or rescue treatment is needed for velvetleaf escapes.

Hindsight Is Always 20/20

This was an excellent year to invest in a pre-emergence 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 post-emergence application of glyphosate or glufosinate) have provided outstanding weed control on the species they are effective at controlling. Weed pressure in fields that received a pre-emergence herbicide is typically light and yields have been protected from weed competition.

(Editor’s Note: This article first appeared in the University of Nebraska’s June 18, 2010, issue of CropWatch. It has been edited for length and clarity.)