Research results support using PRE herbicides to control glyphosate-resistant Palmer amaranth in soybeans
The occurrence/continued development of glyphosate-resistant (GR) Palmer amaranth (PA) in soybean fields is a threat to continued profitable soybean production in the US. The most up-to-date information about managing this troublesome weed in soybeans can be found in a publication titled “Palmer Amaranth Management in Soybeans” from the United Soybean Board.
An integral component for managing GR PA is the application of effective soil-applied or preemergence (PRE) herbicides at or soon after soybean planting. In fact, according to the information in the above-linked article, this management component is essential. Group 14 herbicides, mixtures of Groups 14 and 15 herbicides, and mixtures of Groups 5, 14, and 15 herbicides have been the most effective and consistent PRE herbicide control options.
Recent research conducted in Nebraska provides additional support for the use of PRE herbicides when targeting GR PA. Details of that research and results from it are published in Agron. J. 113:1920-1933 (2021) (https://doi.org/10.1002/agj2.20615) . A summary of the research conduct and its findings follows.
• Field experiments were conducted in 2018 (dry year–below average rainfall in May-July) and 2019 (wet year–above average rainfall entire growing season) in a nonirrigated field with silt loam soil and confirmed GR PA that was the predominant weed present. Winter weeds were killed with an application of glyphosate plus paraquat before planting a dicamba/GR soybean variety in early- to mid-May at the site each year.
• PRE herbicide treatments of 1) no PRE herbicide, 2) flumioxazin (Valor SX–Group 14), and 3) a premix of flumioxazin/metribuzin/pyroxasulfone (Fierce MTZ–Groups 14, 5, and 15) were the main plot treatments. PRE herbicides were applied on the same day as soybean planting.
• Palmer amaranth removal timings at V1, V3, V6, R2, and R5 soybean stages were the subplot treatments. Plots were sprayed with dicamba (XtendiMax with VaporGrip) at each respective removal time, and were kept weed-free following PA removal with the dicamba herbicide.
• As expected, both treatments with PRE herbicides had greatly reduced PA density throughout the study. Densities in 2018 and 2019, respectively, were 85, 11, and 17 and 1122, 15, and 268 plants/m2 for the three respective treatments at the V1 soybean growth stage. The large difference between the two years was attributed to the abundant rainfall in 2019.
• Season-long, uncontrolled PA populations past planting reduced soybean yield 20% to 50%. Yield reduction was generally greater in the treatment that did not receive a PRE herbicide.
• In this study, the critical time for Palmer amaranth removal (CTPAR) was based on a 5% loss of soybean yield. In 2018 (dry year), the CTPAR for the three treatments were V1 [18 days after emergence (DAE)], V3 (33 DAE), and V2 (24 DAE) for the three treatments, respectively. In 2019, the CTPAR was V6 (40 DAE), V6 (42 DAE), and R1 (45 DAE), respectively. The authors attributed the variability in CTPAR between 2018 and 2019 to the diverse rainfall patterns of the two years.
• The authors concluded from their results that: 1) when no PRE herbicide is applied, PA should not be allowed to compete with soybean past the V1 stage in a dry year, and beyond R6 in a wet year; 2) the PRE herbicide can delay CTPAR depending on the PRE herbicide used and seasonal growing conditions; 3) despite some differences in CTPAR between the two PRE herbicides, the best management practice is to use a PRE herbicide with components that have multiple modes of action; and 4) by reducing early-season weed competition, the PRE herbicide can delay the time for a POST herbicide application.
Composed by Larry G. Heatherly, June 2021, larryh91746@gmail.com