2018
Late Herbicide Applications for a Palmer Amaranth: Can We Prevent a Train Wreck?
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Contributor/Checkoff:
Delaware Soybean Board
Category:
Sustainable Production
Keywords:
AgricultureCrop protectionHerbicide
Parent Project:
This is the first year of this project.
Lead Principal Investigator:
Mark VanGessel, University of Delaware
Co-Principal Investigators:
Kurt Vollmer, University of Maryland
Project Code:
Contributing Organization (Checkoff):
Delaware Soybean Board
$13,919
Institution Funded:
University of Delaware
$13,919
Brief Project Summary:

Palmer amaranth has become one of the “driver weeds,” a species that needs control strategies targeted specifically for it. Fields infested with Palmer amaranth are found throughout the region. Most postemergence herbicides are not effective on larger weeds. Research has demonstrated that herbicide applications made to 6 to 8 inch tall Palmer amaranth has not provided consistent control. Plus, the weed has developed widespread resistance to glyphosate and ALS-inhibiting herbicides. Research objectives are to evaluate the effectiveness of various approaches for control of Palmer amaranth and other problem weed species, and determine interaction of rye cover crop with soil-applied herbicide applications.

Key Benefactors:
farmers, agronomists, Extension agents, weed scientists, herbicide companies

Information And Results
Project Deliverables

1. Report on various options for management of large Palmer amaranth plants

Final Project Results

Updated November 6, 2019:
Palmer amaranth control with a single POST application did not provide acceptable control, with 83% control from Enlist Duo being the highest. Liberty plus Enlist One provided only 75% control at 5 weeks after treatment, yet no Palmer amaranth plants were present at harvest. In this trial, Reflex plus glyphosate provided better control than Flexstar GT.

Sequential applications of Engenia plus glyphosate, Enlist Duo, and Liberty provided 97% control or better when rated 4 weeks after treatment. Sequential applications of PPO herbicides Reflex, Cobra, or Flexstar provided similar level of Palmer amaranth control as a single application. Adding Cobra with sequential applications of Engenia, Enlist Duo or Liberty did not improve control; tank mixtures with Enlist Duo resulted in less control than Enlist Duo by itself.

Weed seeds per plant were quite variable and ranged from 4,748 to 38,584. The nontreated plants had the highest number of seeds, yet it was lower than what is often reported. The soybeans were drilled in 7-inch rows presumably increasing soybean competitiveness and helping to limit Palmer amaranth seed production.

There were no yield differences among treatments with the same soybean variety. Herbicide treatments that caused soybean leaf burn (Cobra) did not impact soybean yield, nor did they delay soybean maturity.

Based on preliminary results from one year, sequential applications of dicamba, 2,4-D choline or Liberty are needed to control larger Palmer amaranth plants. Provided Palmer amaranth plants are not taller than 17 inches, excellent control can be achieved with sequential applications of dicamba, 2,4-D choline or Liberty. Sequential applications of PPO herbicides, or including Cobra with sequential applications did not improve control.

View uploaded report PDF file

Based on preliminary results from one year, sequential applications of dicamba, 2,4-D choline or Liberty are needed to control larger Palmer amaranth plants. Provided Palmer amaranth plants are not taller than 17 inches, excellent control can be achieved with sequential applications of dicamba, 2,4-D choline or Liberty. Sequential applications of PPO herbicides, or including Cobra with sequential applications did not improve control. Nontreated plants had the highest number of seeds, but narrow row spacing (7") likely contributed to lower overall seed production than reported in the literature due to soybean competition with weeds that limited Palmer amaranth seed production. Future work will examine using cover crop systems to reduce weed emergence and early-season weed growth, in addition to the use of residual herbicides. Such work will provide recommendations on the use of residual herbicides in fields with cover crops.

The United Soybean Research Retention policy will display final reports with the project once completed but working files will be purged after three years. And financial information after seven years. All pertinent information is in the final report or if you want more information, please contact the project lead at your state soybean organization or principal investigator listed on the project.