Updated December 2, 2016:
Final Progress Report is downloaded in the File (optional) below

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WEED MANAGEMENT IN SOYBEAN CONTRIBUTES TO A FIELD-BASED
WEED CONTROL STRATEGY

Thomas J. Peters1, Andrew Lueck1 and Richard Zollinger2
1Extension Sugarbeet Agronomist and Weed Control Specialist and 1Research Specialist
1North Dakota State University and the University of Minnesota, Fargo, ND and 2Weed Control Specialist, North Dakota State University

Executive Summary

Weed management is a renewed production challenge for soybean growers in Minnesota and eastern North Dakota with the advent of resistant weeds. Emphasis on weed management must change from a series of tactical sprays to a planned strategy that incorporates regular scouting, positive identification of weeds, and mapping with emphasizes on prioritization control of target weeds based on population/density, difficulty to control, and longevity of seed viability in soil. The physical features of the field should also be a component of the plan including topography, soil type, and proximity to ditches and field boundaries. Weed management strategy places emphasis on the ‘field’ and targets weed control across crops that comprise the crop sequence in the field. Weed control should be two or three herbicides applied preemergence and/or postemergence alone or in mixtures that are efficacious against target weeds, are safe to rotational crops and represent herbicide diversity including herbicide tolerance trait(s) and herbicide site of action (SOA) families.

Experiments were conducted on natural populations of waterhemp and kochia in soybean in 2014, 2015 and 2016 (this report focuses on 2015). The experimental area also included corn and sugarbeet experiments to simulate waterhemp or kochia control in rotational crops in the field. Experiment was a randomized complete block design and four replications. Plots were 6-rows at 22-inch row spacing by 35 feet. Two or three effective soybean herbicides were applied over LibertyLink soybean. Soybean, corn, or sugarbeet were planted at commercial seeding rates using a JD Maxemerge planter. Herbicide treatments were applied with a bicycle wheel plot sprayer at 17 gallons per acre through 8002 XR flat fan nozzles pressurized with CO2 at 30 pounds per square inch to the center four rows of six row plots 35 feet in length. Data collection was a visual assessment of soybean safety and weed control at multiple time points during the growing season. Data were analyzed with the ANOVA procedure of ARM, version 2015.6 software package. In soybean, herbicide treatments that combined a chloroacetamide herbicide (SOA 15, a PPO inhibitor herbicide (SOA 14 and Liberty (SOA 10) grass and broadleaf weed control greater than other herbicide treatments. Soybean yield was not measured in these experiments.

Seventeen herbicides (19 active ingredients) representing 11 SOA families were applied across soybean, sugarbeet, and corn experiments. Across the crop sequence, twelve herbicides were applied in one of three crops, two herbicides were applied in two crops and three herbicides were applied in all three crops in the sequence. Multiple herbicides were from the same SOA family and were used within and across crops. That is, twelve herbicides applied in and across crops represented four SOA families.
There currently is no objective way to quantify herbicide diversity in and across crops in the sequence. Diversity indices that have been successfully used in ecological experiments to document species richness and evenness might be applicable to measure herbicide diversity. Diversity indices, evenness, and models that aid farmers in making herbicide selection decisions based on SOA families, weed efficacy, rotational crop safety, and economics choices are ideas for making informed decisions, for establishing a diversity ‘baseline’, and for quantifying diversity ‘improvement’ across the crop sequence. These ideas represent the next phase of this project.