Palmer Amaranth Continues To Spread In Pennsylvania

Jul 22, 2015
By William Curran
In the last few days, we have heard of three more farms identifying fields with Palmer amaranth (Southeast, South central, and the western part of the state). All three are full-season soybeans and some of the pigweeds were above the canopy and beginning to flower (see image). In these situations, glyphosate was applied postemergence sometime earlier this season and it obviously did not control the problem. The first reaction for most is to look for an herbicide solution, but this can be challenging. These populations are glyphosate resistant and many may also be 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, Ultra Blazer, etc., however they will generally not kill these large more mature plants, they will burn the soybean foliage and set the beans back, and potentially impact soybean yield. With some exceptions, these products should be applied no later than R2 (flowering) - See last week’s FCN article about lateness of soybean herbicide application. 
Interestingly, the Cobra (lactofen) label allows application up to 45 days before harvest or R6 (full seed). Additionally, products that contain fomesafen (Flexstar/Reflex) have a 10-month recrop restriction to corn, so applications beyond early July have a greater risk for herbicide carryover. With smaller infestations, consider walking the fields and removing the weeds by hand. If possible, plants should be pulled and removed from the field to ensure they do not reestablish. Plants should be placed on field edges and allowed to dry. It is generally too early in late July for seed production, but do not transport plants away from the infested field and bury or burn plants that may contain viable seed. With small severe infestations, you may consider destroying the crop and the weeds by mowing and/or herbicide application. Dicamba plus or minus 2,4-D are probably the preferred products. We are not sure what the crop insurance ramifications are with this option, but it’s probably not good. 
On dairy farms, perhaps the soybeans 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. Here is a summary of the options presented today:
  • Apply rescue PPO herbicide to control smaller weeds and reduce weed seed production.
  • Hand rogue lighter infestations and dispose of plants on field edge.
  • With small severe infestations, destroy the crop and weeds before seed set.
  • On dairy farms, harvest soybeans for silage.
  • Do not transport Palmer amaranth seeds into uninfested fields via combines or other machinery.