Effects of Herbicide Management Practices on the Weed Density and Richness in 2,4-D- Resistant Cropping Systems in Indiana

Abstract

Commercialization of 2,4-D-resistant soybean varieties allows for postemergence (POST) applications of 2,4-D in soybean. With the increase in POST applications of 2,4-D in soybean, shifts in weed populations may occur. A long-term field trial was conducted over seven years in a corn-soybean rotation. Weed populations were subjected to four herbicide strategies with variable levels of 2,4-D reliance. The strategies used included: 1) diversified glyphosate strategy with six herbicide sites of action (SOA); 2) 2,4-D reliant strategy with three SOA; 3) diversified 2,4-D reliant strategy with seven SOA; and 4) fully diversified strategy with eight SOA. Soil residual herbicides were utilized for both corn and soybean years, except for the 2,4-D reliant strategy which only utilized a residual herbicide during the corn years. A 52% or greater reduction in weed densities for all herbicide strategies, except the 2,4-D reliant strategy, was observed by the end of the study. However, the density of weeds tolerant to 2,4-D, such as monocots, increased after three years of selection pressure, and more than doubled after five years of selection pressure in the 2,4-D reliant strategy. Additionally, in the 2,4-D reliant strategy with three SOA, species richness was 30% higher in the soil seedbank compared to herbicides strategies with six or more SOA. In order to delay weed shifts, diversified herbicide strategies with more than three SOA that include residual herbicides should be used in corn:soybean rotational systems that utilize 2,4-D-resistant soybean.