Pennsylvania Soybeans: Invasive Pigweeds Rising Above Canopies

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This is the time of year that that you will start to notice infestations of the new invasive pigweeds, palmer amaranth and waterhemp, as they climb above the soybean canopy after surviving the POST herbicide program.

Herbicide Resistance Info


The first reaction for most is to look for an herbicide solution, but this can be challenging. These populations are glyphosate resistant and maybe ALS or group 2 herbicide resistant (Pursuit, Classic, etc.).

Potential active soybean products are usually members of the Group 14 or PPO inhibitors like Cobra, Flexstar/Reflex, Cadet/Marvel, and Ultra Blazer, which can be effective on small seedling pigweeds. However, these herbicides will not kill these large, more mature plants.

In addition, with some exceptions, most products should be applied no later than R2 (flowering) – Of the Group 14 herbicides, Cobra (lactofen) allows application up to 45 days before harvest or R6 (full seed), Cadet or Marvel, 60 days, Flexstar/Flexstar GT/Reflex 45 days, and Ultra Blazer 50 days before harvest.

Even Liberty, which can be applied up to 45 days before harvest to Liberty Link soybeans, or the new dicamba products which can be applied up to R1 for Xtend soybeans (Xtendimax, Fexapan, and Engenia) will not control these large escaped weeds. With smaller infestations, consider walking the fields and removing the weeds by hand.

The Pennsylvania Soybean Board along with PA soybean growers provided some financial support for some new educational tools last year. One is a 40 gal recycled paper bag that is to be used for bagging and burning or burying large pigweed plants (BAG IT!) to prevent seed production and spread. Keep a few of these in your pickup. With small severe infestations, you may consider destroying the crop and the weeds by mowing and/or herbicide application.

On dairy farms, perhaps the soybeans (and corn) could be harvested for silage, which may occur prior to Palmer amaranth seed production. Harvesting all plant material and ensiling should also kill some of the weed seeds that could be present as we move into the fall.

It is very important NOT to allow Palmer amaranth seed to spread beyond the current infestation and to try to reduce the potential for new seed production. The potential to spread this problem at harvest via the combine is great, so anything that can be done to control the pigweeds prior to crop harvest is imperative.

If you are attending Ag Progress Days next week, pick some up bags and other educational materials from the Penn State Field and Forage (Agronomy) Team in the J.D. Harrington Building near the corn maze. Also, visit our Palmer Amaranth webpage for the latest invasive pigweed management options.