Project Details:

Title:
Late Herbicide Applications for a Palmer Amaranth: Can We Prevent a Train Wreck?

Parent Project: This is the first year of this project.
Checkoff Organization:Delaware Soybean Board
Categories:Weed control
Organization Project Code:
Project Year:2018
Lead Principal Investigator:Mark VanGessel (University of Delaware)
Co-Principal Investigators:
Kurt Vollmer ((not specified))
Keywords: herbicide, Palmer Amaranth

Contributing Organizations

Funding Institutions

Information and Results

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Project Summary

Palmer amaranth is a recent problem species in the region and has become one of the “driver weeds”, a species that needs control strategies targeted specifically for it. Palmer amaranth originated in the deserts of the southwestern United States and northern Mexico. As a result, it is well adapted to stressful conditions. Palmer amaranth tolerates drought and heat better than other weeds. Palmer amaranth is capable of continuing to photosynthesize at temperatures higher than soybeans or corn, allowing it to grow while these crops have “shut down”. This allows Palmer amaranth to grow rapidly throughout the summer and outcompete soybeans. Fields infested with Palmer amaranth are found throughout Delaware as well as eastern shore of Maryland, southeastern Pennsylvania, and New Jersey. This species is related to our common pigweed, but it is much more aggressive. It is common for this plant to grow as tall as 6 feet, and a single plant will produce a tremendous number of seeds. Palmer amaranth has a prolonged germination period that requires the use of postemergence herbicides.

Most postemergence herbicides are not effective on larger Palmer amaranth plants. University of Delaware research has demonstrated that herbicide applications made to 6 to 8 inch tall Palmer amaranth has not provided consistent control. This results in a short time period that Palmer amaranth can be effectively controlled with postemergence herbicides. If weather is not conducive to spraying during this time or if a sprayer is not available, farmers are looking for guidance on strategies to deal with larger plants.

Complicating Palmer amaranth control in Delaware is the widespread resistance to glyphosate and ALS-inhibiting herbicides (Group 2) such as Classic, Raptor, and Pursuit. Farmers are relying on PPO-inhibiting herbicides (Group 14) such as Reflex, Cobra, or Flexstar or Liberty (Group 10) for postemergence Palmer amaranth control. Previous research, some funded by DSB, has demonstrated that increasing the use of adjuvants has only minimal effect on improving Palmer amaranth control.

Limited research has been done with dicamba or 2,4-D to control larger Palmer amaranth plants. A preliminary trial in 2017 found dicamba, Liberty or Reflex applied to 14-inch tall Palmer amaranth plants provided less than 70% control. However, control improved if a second herbicide application was made 7 days later. In addition to improved control, Palmer amaranth seed production appeared to be reduced with the second herbicide application. Most farmers realize the challenges they face if Palmer amaranth plants are large, but it is difficult to make decisions on the best way to minimize Palmer amaranth’s impact on yield and contribution to the seedbank.

Herbicide resistance is increasing in DE as new infestations of Palmer amaranth are discovered and new weed-resistant biotypes are reported (i.e. common ragweed). With no new herbicide sites-of-action available for the near future, integrated weed management strategies need to be evaluated and implemented. Cover crops are one tactic that UD Weed Science has been investigating and has shown considerable amount of potential for both winter annual weeds such as horseweed as well as Palmer amaranth. One concern with integrating cover crops is its impact on soil-applied herbicides. Information is lacking on the potential effect of cover crop to intercept the herbicide spray. In addition, farmers need to know if herbicides are washing off the plant residue to reach the soil surface. While cover crop systems are reducing weed emergence and early-season weed growth, many situations still require the use of residual herbicides. Information is needed on how best to recommend the use of residual herbicides in fields with cover crops. The research objectives are to evaluate the effectiveness of various approaches for control of Palmer amaranth and other problem weed species.

We examined the effectiveness of strategies to control Palmer amaranth when larger than recommended sizes using soybeans with the following herbicide-tolerance traits: Liberty Link (Enlist E3); 2,4-D-resistant (Enlist E3); dicamba-resistant (Xtend) and Roundup Ready (Xtend). Large Palmer amaranth plants, 12 to 14-inches tall, were treated with Engenia (dicamba) plus glyphosate, Enlist Duo (2,4-D choline+glypohsate), Reflex plus glyphosate and Liberty as single as well as sequential applications. Engenia and Reflex were applied as a tankmixture with glyphosate. Flexstar GT is a prepackaged mixture of fomesafen and glyphosate with proprietary adjuvants. Sequential applications were made 7 days apart. In addition, the sequential treatments included Cobra (lactofen) to assess the potential benefit of including herbicides with additional modes-of-action. Fomesafen label does not allow more than one application so it was not used as part of a sequential application. We also examined if the order in which herbicides are applied influenced control.

Soybeans were drilled no-till on June 18. The research area was sprayed with glyphosate plus Liberty before planting to eliminate any emerged weeds. Outlook was applied after planting at 12 fl oz to provide limited control of annual weeds.
All herbicides were applied at 20 g/A; with appropriate spray tips (coarse droplets for dicamba and 2,4-D; medium droplets for Liberty and Reflex); at maximum allowable rates; and with appropriate adjuvants. The first application was made on July 20. Plots were evaluated for Palmer amaranth control. At harvest, five female Palmer amaranth plants were measured for height, clipped at the ground level for recording biomass, and threshed to determine seed production. Plots were harvested to determine impact of treated Palmer amaranth plants on yield. However, use of multiple varieties did not allow comparison of yields across the various herbicide programs.

Project Objectives

1. Evaluate various options for management of large Palmer amaranth plants
2. Determine interaction of rye cover crop with soil-applied herbicide applications

Project Deliverables

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

Progress of Work

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.

Benefit to Soybean Farmers

This research will benefit Delaware’s soybean growers and the soybean industry by knowing potential options when spraying large Palmer amaranth plants and expand the options of non-chemical strategies to help control plants with few to no herbicide options.

Performance Metrics

Determination of most effective applications of herbicides to control Palmer amaranth after plants reach 8" height.
Development of novel methods of pairing chemical and non-chemical control methods to effectively combat Palmer amaranth infestation.

Project Years