This project will address farmer/applicator questions regarding best practices to manage large pigweeds (Palmer amaranth and waterhemp) in Enlist soybeans. Specifically, we will 1) compare sequential applications of Liberty and Enlist One in small plots, 2) evaluate Liberty efficacy in a range of temperature and humidity conditions in the field, 3) compare herbicide treatments in on-farm field trial, and 4) collect weed seed samples to screen for herbicide resistance.

1. Identify the herbicide combinations and sequences that provide the best control of large Palmer amaranth and waterhemp
2. Develop a greater understanding of the influence of humidity and temperature on the effectiveness of Liberty (glufosinate)
3. Develop an understanding of glufosinate sensitivity/tolerance in Palmer amaranth and waterhemp across Kansas
4. Identify the herbicide treatments and environmental conditions that have the greatest economic benefit.

The information generated in this project will help prolong the usefulness of an important herbicide (Liberty) by using it in the most effective way possible. Even when a research project directly addresses farmer questions and concerns, the acceptance of results increases when research is done locally at commercial scale. This project will also help slow the development of glufosinate-resistant Palmer amaranth and waterhemp populations by detecting them as soon as possible. We will communicate the results via KSRE field days, newsletters and other electronic communications, Extension fact sheets, and a peer-reviewed publication.

Updated May 23, 2025:
Two small-plot field trials were established at the Ashland Bottoms Experiment Field near Manhattan, KS in May and June of 2024 to evaluate control of large (12" to 15") Palmer amaranth by
2,4-D and Enlist applied in various combinations, sequences, application intervals, and environmental conditions. The results of the first experiment to evaluate herbicide combinations, sequences, and application intervals were summarized and presented as a poster at the North Central Weed Science Society meeting in December 2024.

In the first experiment, initial results suggest that acceptable Palmer amaranth control can be achieved when two applications are made, regardless of herbicide combination, sequence, or application interval. However, single applications provided less control of Palmer amaranth. Soybean yield was similar for all treatments. In the second experiment, results suggest that applications made using 20 GPA provided more consistent control of large Palmer amaranth, although the differences were not statistically significant. This trial also suggests that Delta T is an suitable predictor of weed control, with delta T greater than 5 resulting in less than 90% weed control, and delta T less greater than 8 resulting in less than 75% weed control. These experiments will be repeated in 2025.

Pigweed samples were collected from 17 soybean fields during October 2024. Seeds were threshed and planted in flats in the greenhouse. Glufosinate (Liberty), 2,4-D, dicamba, and flumioxazin (Valor) were sprayed when pigweeds were 4" to 6" tall and monitored for 4 weeks. Due to greenhouse maintenance, planting was delayed, final assessments for some populations have not been completed.

Samples that have been completed at this time suggest populations with potential resistance to Liberty, Enlist, and Liberty + Enlist + Xtendimax.

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View uploaded report 4

This project directly answers questions posed by farmers and agronomist regarding best practices for managing large pigweeds in Enlist systems. The data generated will inform recommendations for the best interval for a re-spray, and more field-based data to determine the environmental conditions in which Liberty will be effective.