How we believe cereal rye cover crop suppresses waterhemp

cover crop

Nunes, Arneson, & Werle


February 24, 2023

Over the past few years, the WiscWeeds program has been preaching that a properly managed cereal rye (Secale cereale) cover crop can effectively suppress waterhemp (Amaranthus tuberculatus) emergence and development thus should be considered as part of integrated weed management programs (Figure 1).

Figure 1. Effective waterhemp suppression with cereal rye cover crop biomass (right; >4,000 lbs of cereal rye biomass per acre).

Despite showing you data that cereal rye cover crop can help with waterhemp management, we haven’t demonstrated how it actually works. Let’s put it this way, every herbicide has its own mode of action (how it kills a waterhemp plant), and we believe that cereal rye cover crop has its own sort of ‘mode of action’ that can help suppress waterhemp emergence and development.

When asked, most people think that the waterhemp suppression comes from the release of chemical compounds by the cereal rye cover crop (allelopathy). Although this is probably part of the mode of action, the weed science literature suggests that there is more to that. To address this, we conducted a study near Brooklyn (O’Brien Family Farms) and Janesville (Rock County Farm), WI, in the spring and summer of 2022 where we simulated a range of cereal rye cover crop biomass covering the soil surface going from 0 up to 10,000 lbs per acre. To simulate the various biomass levels, we harvested cereal rye plants from a field near Arlington, WI (Arlington Agricultural Research Station) in the spring, dried them in a big oven, and after weighing the samples, we spread the desired amount of biomass residue over the plots (Figure 2). Once everything was in place, we started collecting data:

Figure 2. 2022 research plots with different levels of cereal rye cover crop biomass (O’Brien Family Farms near Brooklyn, WI). Picture by Jose Junior Nunes (WiscWeeds PhD Student).

In summary, here is what we learned from the 2022 studies:

Our hypothesis is that all these factors combined comprise the cereal rye cover crop mode of action to suppress waterhemp emergence and development. The changes that the cover crop biomass causes in the weed seed bank environment reduce the signals that trigger waterhemp seed germination. Even when the germination is triggered, the mulch makes it harder for seedlings to emerge and establish. A way to think about this hypothesis is to see things from a waterhemp seed perspective. Each waterhemp seed has only one chance to serve its purpose, which is to germinate and become a plant capable of reproducing to replenish the weed seedbank (it is a cycle). If the germination is triggered but conditions are not favorable (like when we have the cereal rye cover crop mulch) and the seedling dies in the process, that is game over. But if the seed germination is never triggered, the seed will remain in the soil until conditions are favorable, waiting for its next chance. Now if it waits too long, it may decay in the soil and not serve its purpose. Thus, that is probably why we see fewer waterhemp plants where cereal rye cover crop is adopted and properly managed for effective weed suppression.

We know that there is a lot of “we believe/we think” in this article, but this is just the nature of research. Please do not just take our word for all of this. We encourage you to give it some thought, and if you feel like sharing your hypothesis(es) and research questions with us, please feel free to shoot us an email or drop a call so we can talk more about it. And don’t worry, we intend to replicate this research in 2023 to further validate our findings.

Waterhemp not a problem in your field? How about giant ragweed? A complimentary blog post is being developed by Guilherme Chudzik (WiscWeeds MS student) documenting his preliminary research findings on the impact of cereal rye cover crop on giant ragweed. Stay tuned!

The research reported herein is being led by Jose Junior Nunes (WiscWeeds PhD student). Click here to see Nunes’ poster presented during the 2022 North Central Weed Science Society Meetings (December 2022) in St. Louis, MO. Nunes wont first place presentation with this poster (congrats Jose!).

Additional resources:

This article was written by Jose Junior Nunes (PhD Student), Nick Arneson (Outreach Program Manager), and Rodrigo Werle (Assistant Professor). All authors are affiliated with UW-Madison.