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.