Palmer amaranth is a troublesome weed species displaying the ability to adapt and evolve resistance to multiple herbicide modes of action, and additional weed suppression tactics are needed. Growing interest in the use of cover crops (CCs) has led to questions regarding the most appropriate forms of CC management prior to cash crop planting in order to maximize weed suppression benefits. Experiments were conducted between 2021 to 2023 to test 1) cover crop termination timing (i.e., green or brown); 2) CC biomass amount; and 3) CC termination method (i.e., rolled or left standing) on Palmer amaranth suppression. Treatments included “planting brown” (cereal rye terminated 2 wk before soybean planting), “planting green” (cereal rye terminated at soybean planting), and a no-CC (winter fallow) check. Palmer amaranth emergence was evaluated at 4 and 6 wk after soybean planting, and yield was calculated at harvest. Palmer amaranth emergence was reduced when a CC was planted compared with the no-CC check, and more suppression was observed as CC biomass increased. This decrease in emergence is potentially due to a decrease in light reaching the soil surface and physical suppression as CC biomass increased. Yield, however, was unaffected by any CC management practice, indicating that growers can tailor CC termination practices for weed suppression. This information will provide better recommendations for farmers interested in using CCs for weed suppression. Overall, the importance of CC biomass accumulation to achieve weed suppression is highlighted in our findings. Additionally, we add to the growing body of documentation that soybean yield may be variable from year to year as a result of CC presence.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Watson; cereal rye, Secale cereale L.; soybean, Glycine max (L.) Merr.