Evaluating cover crops and herbicides for horseweed and Palmer amaranth management

Abstract

Horseweed and Palmer amaranth are common weeds in Kansas that compete against many row crops. Horseweed can emerge in different seasons depending on the year. Palmer amaranth emerges from spring throughout the summer months and has a rapid growth rate with higher temperatures. Three separate studies were conducted near Manhattan, KS from 2016 to 2018 to determine (1) horseweed control in no-till soybean with cover crops and herbicide programs with and without residual activity (2) emergence timing in KS of eight horseweed populations collected from MO, IL, KS, and KY, and (3) Palmer amaranth control in response to three Protoporphyrinogen Oxidase (PPO) inhibitors applied every three days once Palmer amaranth plants reached 2.5 cm tall. Cereal rye reduced weeds biomass by 78% and weed density by 75% by 8 weeks after cover crop seeding in the fall. At cover crop termination two weeks prior to soybean drilling. Cereal rye reduced horseweed biomass more than herbicide treatments, but after termination weed control was similar across treatments. Soybean yields were greater with herbicide treatments in year one, but there were no differences in soybean yields among cover crop and herbicide treatments in the second year. Emergence of all eight horseweed populations occurred at the same time. Most horseweed emergence occurred in the spring in the first year, while all horseweed populations emerged in the fall in the second year. Environmental conditions were driving factors for horseweed emergence, but horseweed seed source did not influence emergence timing. All PPO-inhibitor herbicides controlled Palmer amaranth at similar levels within an application timing. PPO-inhibitor herbicides need to be applied within three days after Palmer amaranth plants reach 2.5 cm tall to achieve greater than 90% control.