Planting soybean green proves effective for waterhemp and Palmer amaranth suppression

Recently published research by Nunes et al. in the journal Weed Science shows planting soybean into a green, living cover crop provides effective control for some of the nation’s most troublesome weeds when integrated with pre-emergence (PRE) herbicides. More specifically, planting soybean over a cereal rye cover crop demonstrates an effective practice to both produce soybean and to suppress two problematic Amaranthus weed species: waterhemp and Palmer amaranth.  

″Our findings support the premise that planting soybean green is an effective recommendation to optimize cereal rye biomass production for effective waterhemp and Palmer amaranth suppression,″ says Rodrigo Werle, Ph.D., Associate Professor & Extension Cropping Systems Weed Scientist, University of Wisconsin-Madison, WSSA member scientist, and a contributing author to the journal research article. ″Integrating an effective pre-emergence herbicide program at soybean planting was also an important component of effective Amaranthus species control.″ 

Research results from across 10 U.S. states found that planting soybean green is a feasible management practice to optimize cereal rye biomass production and provide effective Amaranthus species management when combined with preemergence herbicide use.

Planting green provides more time for cereal rye to grow and produce biomass to suppress weeds, without interfering with timely planting, compared to terminating the cover crop one to two weeks prior to soybean planting. In addition, besides suppressing weeds, cover crops such as cereal rye contribute to reduced soil erosion, increased soil organic matter, and improved biological soil health. 

The newly reported research findings come from 10 U.S. states (Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, North Dakota, Pennsylvania, and Wisconsin) during the 2021 and 2022 growing seasons. ″It is evident from our findings that the planting green system can effectively optimize cereal rye biomass production and improve weed suppression,″ says Werle. ″Yet, soybean response to planting green can vary, with some studies reporting no effect on yield, and some studies reporting some yield reduction.″ 

Attention to detail when planting green is essential to optimize soybean stand establishment and thus yield in this system. ″Soybean stand was a key factor in maintaining soybean yields compared to no-till when planting green,″ emphasizes Werle. ″Farmers should follow best management recommendations for proper planter and equipment set-up to ensure effective soybean establishment under high levels of cereal rye biomass when planting green. Enough soil moisture is also critical for planting green success.″ 

With the rapid increase in herbicide-resistant waterhemp and Palmer amaranth populations in multiple U.S. states, non-chemical weed control methods, such as cover cropping, are gaining interest and popularity among soybean producers as an alternative weed management strategy. This study shows pairing cover crops with pre-emergence herbicides provides an even greater benefit than either practice done alone. 

″Planting soybean green provided a 44% reduction in Amaranthus species density compared to no-till,″ notes Werle. ″The use of PRE herbicides resulted in a 68% reduction in Amaranthus species density compared to no PRE. So overall, we found that planting soybean green is a feasible management practice to optimize cereal rye biomass production, which, combined with preemergence herbicides, provided effective Amaranthus species management.″ 

More information about planting soybean green and Amaranthus species weed control can be found in the article, ″Planting soybean green: how cereal rye biomass and preemergence herbicides impact Amaranthus spp. management and soybean yield″ in Volume 72, Issue 5 of Weed Science, a Weed Science Society of America journal, published online by Cambridge University Press. This multi-state collaborative study, sponsored by the United Soybean Board, was led by Jose Junior Nunes, Ph.D., a former doctoral student in Dr. Werle’s Cropping Systems Weed Science Research & Extension lab at the University of Wisconsin-Madison.