Herbicide resistant weeds in soybean have become an important consideration when designing a weed control program. One of the pillars of herbicide resistance management is timely detection of resistant populations. The long-term goal of this project is to establish an herbicide resistance monitoring program to provide timely detection of resistant weeds for soybean farmers in Pennsylvania. Once this project is completed, growers would be able to submit leaf samples for herbicide resistance diagnostics with short turnaround time (i.e., days, as opposed to several months with current techniques). Our program could be used to detect herbicide resistance before herbicide application to assist with in-season decision making. As the first step towards this goal, we are assessing the state of resistance distribution in the Commonwealth. In the 2022 growing season, we sampled annual ryegrass, marestail, Palmer amaranth, and waterhemp, the most commonly observed species in fields. After the end of sampling in November 2022, we began greenhouse experiments to test for 17 commonly used herbicides.

The long-term goal of this project is to establish an herbicide resistance monitoring program to provide timely detection of resistant weeds for the Pennsylvania soybean farming community. The monitoring program will be based at Penn State University and, in the long-term, will provide quick tests for soybean farmers in Pennsylvania to submit their weed samples for herbicide resistance diagnosis. The first step towards this goal, and the objective of the year 1 of this project (FY 2022-23), is to:

1 - Conduct a statewide survey to characterize the predominance of herbicide resistance in Pennsylvania. Populations of Palmer amaranth (Amaranthus palmeri), waterhemp (Amaranthus tuberculatus), common ragweed (Ambrosia artemisiifolia), marestail (Conyza canadensis), and Italian ryegrass (Lolium multiflorum) will be sampled.

2- Screen for resistance to chlorimuron, imazethapyr, cloransulam, dicamba, 2,4-D, metribuzin, glyphosate, fomesafen, acifluorfen, mesotrione, clethodim and fluazifop.

We observed that resistance to glyphosate, ALS inhibitors (e.g., Pursuit), and atrazine in waterhemp is widespread, but not in all populations. A single population displayed resistance to 2,4-D and dicamba. We also observed that products containing flumioxazin (e.g., Valor), fomesafen (e.g., Reflex), glufosinate (e.g., Liberty), and tiafenacil (Reviton) are still efficient. Populations of Italian ryegrass displayed resistance to fluazifop (e.g., Fusilade) and glyphosate. All marestail populations were resistant to glyphosate and chlorimuron (e.g., Classic), but were controlled by atrazine, 2,4-D, glufosinate, and dicamba.

Updated August 23, 2022:
The goal of this project is to establish an herbicide resistance monitoring program to provide timely detection of resistant weeds for the Pennsylvania soybean farming community. As the first step towards this goal, we are currently conducting a survey to determine a “baseline” herbicide resistance presence in the Commonwealth. To date, we have focused our efforts on sampling annual ryegrass from eight soybean fields that survived herbicide applications. The sampling locations are available in the map below (Figure 1). We continue to sample fields with annual ryegrass escapes, and expect this number to increase in the next weeks. Several summer weeds are now close to completing their life cycles (Palmer amaranth, waterhemp, common ragweed, foxtails), and we will initiate sampling in the next weeks as well. Once the season is over, we will move our work to a control environment facility where collected populations will be tested to 12 herbicides (Classic, Pursuit, FirstRate, XtendiMax, EnlistOne, Metribuzin, Roundup PowerMAX, Reflex, Ultra Blazer, Callisto, Select MAX, and Fusilade).

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Updated December 31, 2023:
Establishing an herbicide resistance monitoring program for Pennsylvania soybean growers

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We observed that resistance to glyphosate, ALS inhibitors (e.g., Pursuit), and atrazine in waterhemp is widespread in Pennsylvania, but not in all populations. A single population displayed resistance to 2,4-D and dicamba. We also observed that products containing flumioxazin (e.g., Valor), fomesafen (e.g., Reflex), glufosinate (e.g., Liberty), and tiafenacil (Reviton) are still efficient. Populations of Italian ryegrass displayed resistance to fluazifop (e.g., Fusilade) and glyphosate. All marestail populations were resistant to glyphosate and chlorimuron (e.g., Classic), but were controlled by atrazine, 2,4-D, glufosinate, and dicamba. Given weed populations displayed distinct resistance patterns, growers could benefit from a system for quick herbicide resistance diagnosis. In 2023, we considerably expanded our sample size to obtain a better representation of the state of herbicide resistance in PA. Future work will test those populations against key soybean herbicides.

This project has several practical implications: (i) awareness of herbicide resistance patterns and predominance may assist with a more robust design of weed control programs; (ii) herbicide resistance characterization may allow the identification of management practices that select for resistance (and susceptibility) beyond herbicide selection pressure (e.g. cover crop use, tillage frequency, crop rotation); (iii) data on herbicide resistance distribution may assist funding agencies and chemical companies to direct the research and development to better address herbicide resistance. Given weed populations displayed distinct resistance patterns, growers could benefit from a system for quick herbicide resistance diagnosis. In 2023, we considerably expanded our sample size to obtain a better representation of the state of herbicide resistance in PA. Future work will test those populations against key soybean herbicides.