Improved Weather Boosts Crops And Weeds

Midwest Extension specialists are concerned about surging weed growth.

Published on: Jul 16, 2013

Lots of sunshine and overall humid weather is having a major impact on crop and weed growth.

While farmers welcome the resurgence in crops, Extension scientists are concerned about weed control, particularly with Roundup resistance showing up some major Minnesota weeds.

"Although it is recommended to target post-emergence herbicide applications to weeds no more than 3 to 4 inches in height, this was not possible in many fields due to wet and/or windy conditions," said Liz Stahl, an Extension crops educator based in Worthington. "Timely weed control has been a challenge this year, particularly where a pre-emergence herbicide was not applied."

How to deal with weeds a foot high and taller is the hot topic across the Corn Belt, she noted.

Improved Weather Boosts Crops And Weeds
Improved Weather Boosts Crops And Weeds

"Resistance is a real issue," she said. "In Minnesota, we have confirmed populations of glyphosate-resistant giant ragweed, common ragweed and waterhemp. This doesn't mean that all plants of these species in Minnesota are resistant to glyphosate, but the potential is there for resistant populations to exist."

Resistance to other chemistries in a number of weed species including waterhemp also has been confirmed in the state.

"Resistance to multiple herbicide chemistries, or sites of action, can stack up," she added.

To help prevent and manage resistance, Stahl said it is important for growers to diversify weed management practices and identify weed issues early. If weeds were not controlled by a full-labeled rate of herbicide and there were no obvious problems with application of the product, suspect resistance. Use a different herbicide site of action next time (site of action numbers are on most herbicide labels these days). Use mechanical weed control to control escapes. Prevent seed production of suspected resistant weeds. Use full labeled rates and target applications to small weeds. Use a pre-emergence herbicide with residual activity in the future.


Weeds are very prolific producers of seed, she said. Waterhemp, for example, has the potential to produce more than 100,000 seeds per plant. One resistant plant or patch can quickly lead to an infested field once a combine runs through the field, spreading weed seeds behind it.

If you have large weeds in your fields, Bruce Potter, integrated pest management specialist at the U-M Southwest Research and Outreach Center, offered these key management points, gleaned from herbicide industry representatives and Jeff Gunsolus, U-M agronomist:

1) A second application of glyphosate should not be expected to control waterhemp where the first application has failed.

2) PPO herbicides (e.g. Flexstar, Cadet, Cobra, etc) are options for rescue treatment.

3) Rescue treatments have a lower success rate than timely planned herbicide applications.

4) Weed size and herbicide carryover concerns limit the herbicide options for waterhemp control in soybean this time of year. For example the corn re-crop restriction for Flexstar GT is 10 months and the maximum water hemp size is 4 inches. Lactofen (Cobra) does not have the long replant restriction, has 6" waterhemp on the label and is another option.

5) Try to maximize the success of the PPO herbicide for rescue treatments after a glyphosate failure.

a) These are contact herbicides and excellent coverage is critical to kill waterhemp growing points.

b) Use a minimum of 15 gallons/acre and 40 PSI; 20 gallons or more is better.

c) Flat fans will perform better much better than AI nozzles. Select adjuvants to optimize performance.

d) Adjuvant details vary slightly by product but in general, crop oil concentrate/methylated seed oil plus ammonium sulfate are good, high surfactant loads.

6) Note that mixing a contact herbicide like Cobra or Flexstar with glyphosate can result in reduced control as the contact herbicide may reduce the ability of glyphosate to move adequately throughout the weed. If weeds are larger than what is on the label for optimal control, applying a product alone versus in a tank mix may be the best option.