Herbicide Recommendations for Noxious Pigweeds

General Guidelines

It is important to learn about these two invasive pigweeds (Palmer amaranth and Waterhemp).

• Know where they are prevalent across the country.
• Know where they have been found in Pennsylvania.
• Know the weed biology (aggressive germination, competitiveness, seed production, and herbicide resistance).
• Know how they spread (equipment, feed, grain, hay, manure, mulch and seed).
• Know how to reduce their impact including preventing their movement and spread.

Develop strategies to address Palmer amaranth and Waterhemp in Commerce.

• Identify and address pathways for the movement and spread of these weeds.
• Identify and address effective, consistent and complementary containment measures.
• Collaborate with academia, government and industry to take effective preventative actions.

Herbicide recommendations

Plant into a clean seedbed.

• use tillage or an effective burndown herbicide for no-till

Always consider herbicide sites of action (SOA) or Group numbers.

• Use tank mixes with different effect SOAs
• Use multiple and effective SOAs and rotate with crops
• Avoid using the same SOA each year

Use effective soil applied herbicides at planting.

• Use Group 14 (PPO) in soybean followed by Group 27 (HPPD) in corn. Include Group 15 herbicides in the both corn and soybean.
• Use full labeled rates

For postemergence treatments, make timely applications and apply to small pigweed (<4 inches tall).

• Never apply glyphosate alone to emerged pigweed
• Avoid the use of ALS type (Group 2) herbicides for pigweed control

Scout fields within 10 to 14 days after application.

These recommendations assume Group 9 (glyphosate) and Group 2 resistance (ALS-inhibitors).

Soybeans

Consider planting Liberty Link soybean.

Plant into a clean seedbed.

• Use tillage or an effective burndown herbicide for no-till

Use effective soil applied residual herbicides.

Effective preemergence herbicides are essential at or before planting.

• sulfentrazone (Authority) or flumioxazin (Valor)-based herbicides
• Use full labeled rates

Scout fields 10 to 14 days after application.

Apply timely early postemergence herbicide to small pigweed (<4 inches tall).

• Reflex (1 pt) + glyphosate (or Liberty in Liberty Link soybeans)
• Can add residual (group 15) to reduce need for another post herbicide (Dual, Warrant, Zidua/Anthem, Outlook)

Corn

Best option: plant corn - many more choices for management.

Apply effective residual herbicide at or before planting.

• Atrazine premix - good
• Lexar, Corvus + atrazine - better

Scout field 10 to 14 days after application.

Apply post herbicide when pigweed is small (<4 inches tall).

• Better to include herbicide with residual activity in addition to dicamba, glufosinate, or 2,4-D (Atrazine, Callisto (or other mesotrione product), metolachlor, acetochlor or Zidua/Anthem)
• Avoid glyphosate or ALS inhibitors

Forages (alfalfa, grass hay/pastures)

Routine mowing will help reduce pigweed seed production.

• However, pigweed stumps can regrow if not cut low enough

Alfalfa could be treated with Chateau or Prowl H2O plus Gramoxone between cuttings.

In grass hay or pastures, apply 2,4-D and/or dicamba or triclopyr-containing herbicides