Research experience by Rachel Teller, undergraduate student at University of Manitoba (supervised by Dr. Martin Entz)

To learn more about Rachel’s research project, listen to her interview on the MAKE Podcast (Manitoba Agriculture & Food Knowledge Exchange) on Spotify!

Weeds are a persistent challenge for farmers, competing with crops for sunlight, water, and nutrients. Every decision a farmer makes–whether it’s choosing a crop rotation, applying fertilizer, or tilling the soil–affects not only the crops but also the types and abundance of weeds that thrive in their fields. This relationship means that when farmers adopt practices to improve soil health, they must also consider how these changes will influence weed population. My research set out to explore this dynamic and uncover how practices for better soil health impact weed management.

Under the guidance of Dr. Martin Entz, I conducted my research at the Glenlea long-term rotation site, just south of Winnipeg. My study focused on two key questions:

1. How does including a perennial species in a conventional crop rotation influence the weed population?
2. How does adding nutrients to an organic crop rotation affect the weed population?

To investigate these questions, I used three different methods to evaluate weed populations:

1. Field Counts: In early June, when crops were just emerging, I counted and identified weed species in the field.
2. Biomass Assessment: Before harvest in the fall, weed biomass samples were collected to gauge the total weed presence.
3. Seedbank Analysis: I collected soil samples from each plot and grew them out in a greenhouse to examine the weed species and their abundance in the seedbank.

What Did I Find?

In conventional systems, incorporating a perennial species into the rotation resulted in fewer weeds across all three metrics. Additionally, the perennial rotations altered weed species composition, with fewer annual weeds like wild oat and green foxtail—species of particular concern due to their propensity for developing herbicide resistance. This highlights the potential of perennials to suppress problematic weeds in these systems.

In organic systems, an intriguing pattern emerged. Early-season measurements revealed more weeds in plots receiving added nutrients. However, by the end of the growing season, weed biomass was comparable across fertilized and unfertilized plots. Even more interesting, the soil seedbank data showed fewer weeds in fertilized plots, reversing the initial trend. The explanation lies in crop competition: while the weeds initially took advantage of the fertilizer, the added nutrients allowed crops to grow more robustly and outcompete the weeds over time. This ultimately reduced the number of weeds that reached reproductive maturity, thereby limiting their contribution to the seedbank for future seasons.

Why Does This Matter?

My findings demonstrate that adopting soil health practices can also deliver weed management benefits. Incorporating perennials in conventional rotations not only reduces overall weed numbers but also suppresses troublesome annual species. Similarly, adding nutrients in organic systems, while initially favoring weed germination, can strengthen crop competition and reduce the weed seedbank over the long term.

These results underscore the importance of considering weed dynamics when implementing practices to improve soil health. By understanding how management decisions impact weed populations, farmers can make informed choices that benefit both their crops and the environment.