Integrating cover crops and herbicides for horseweed and Palmer amaranth management in no-till soybean

Abstract

Palmer amaranth and horseweed are problematic weeds in no-till soybeans in Kansas. Integrating cover crops and herbicide programs could suppress weed populations. To determine the emergence pattern and survival of horseweed, a study was conducted across six locations in eastern KS in 2014-2015 and 2015-2016. Horseweed seedlings and leaf number per seedling were recorded at two-week intervals. Cumulative GDDs required to reach 50% horseweed emergence increased from north to south. Horseweed survival ranged from 4 to 90%, and majority of horseweed emerged in the fall. Field studies were conducted to determine effects of cover crops and herbicide programs on Palmer amaranth near Manhattan, KS in 2014-2015 and 2015-2016. Five cover crop treatments included no cover, fall-sown winter wheat, spring-sown oat, pea, and mixture of oat and pea. Cover crops were terminated in May with glyphosate and 2,4-D alone or with residual herbicides of flumioxazin and pyroxasulfone. By 10 weeks after termination in 2014-2015, Palmer amaranth biomass and density, averaged across cover crops. was 95 and 69% less with residual herbicides than without, respectively, and Palmer amaranth biomass was 98% less in winter wheat and 91% less in spring oat, averaged across termination methods, compared to no cover. Time to 50% Palmer amaranth emergence was delayed with winter wheat, spring oat, and spring oat/pea mix without residual herbicide. Soybean yields were greater with residual herbicide and greater with winter wheat or spring oat cover crop in 2014-2015. A field study was conducted to determine suppression effects of cover crop and herbicide programs on horseweed and Palmer amaranth near Manhattan, KS in 2015-2016. Three fall treatments included fall-sown rye, a residual herbicide tank mix of glyphosate, dicamba, chlorimuron-ethyl, tribenuron-methyl, and AMS, and no fall application. Four spring treatments included no spring application or three herbicide tank mixes: glyphosate, dicamba, and AMS alone or with flumioxazin and pyroxasulfone as early preplant, or as split applied with 2/3 preplant and 1/3 at soybean planting. Similar levels of horseweed suppression were observed when some control measure was used in fall or spring. Fall rye completely suppressed horseweed while the fall herbicide suppressed biomass by 93% and density by 86% compared to no fall application. Palmer amaranth suppression was observed when a spring herbicide application was used. In rye, total weed biomass was reduced by 97% or more across all spring treatments. Total weed biomass was reduced with a spring herbicide was used. Soybean yields were least when no herbicide treatment was used in the spring. An integrated program of fall cover crops or herbicide applications together with spring herbicide applications maintained soybean yields.