There are a lot of ways to keep a weed seed down with cover crops.
Above ground, you can rob them of the light, temperatures, moisture and energy they need to germinate. Or, if you’re a cereal rye cover crop, you can take the low road, head below ground, and hit them where it hurts — in the roots!
This phenomenon is known as “allelopathy,” and it is a growing area of interest for plant breeders, who want to help farmers gain access to strains of weed-repelling cereal rye.
Taking the High Road: Above Ground Weed Suppression
The weed suppression provided by cover crops is mainly due to the thick plant material barrier they create on the soil surface, making conditions harsher for weeds to emerge and compete with crops.
Cover crop residues reduce weed emergence in four ways:
- Minimizing soil temperature fluctuations, which are critical for many weed seeds to break dormancy and initiate germination
- Preventing light exposure for weed seeds, which decreases germination
- Cooling down the soil, decreasing weed seed germination rate and seedling growth
- Forcing seedlings to invest more energy to emerge — many seedlings that do not have enough energy will fail to establish and die
All these benefits depend on having enough cover crop residues on the ground. Unfortunately, we do not always reach the desired amount of biomass. What can we do in those cases? With the right cover crop species and cultivar, we can still suppress weeds below ground.
Taking the Low Road: Below Ground Weed Suppression
Cereal rye produces allelochemicals that are released through the roots to the soil. These allelochemicals damage and frequently kill new root tissue of germinating weed seeds and emerging seedlings. The release of allelopathic substances to the soil complements the physical barrier effect of the cover crop, but more importantly, it can favor weed suppression in cases where biomass production is not ideal.
Recently, the Cover Crop Breeding Network incorporated allelopathy activity as one of the key traits in the cereal rye breeding program. The breeding program takes advantage of the large genetic diversity of the crop and a pipeline of laboratory, greenhouse, and field screenings to identify and select the most promising lines.
Using this approach, the network has identified genetic lines that have both high allelopathy and biomass production. An important component of the network’s breeding pipeline is to cross allelopathic lines to locally adapted varieties to ensure weed suppression, vigorous growth, and resilience. Currently, testing of cereal rye allelopathic lines is being conducted in North Carolina, Maryland, New York, Wisconsin, and Minnesota, but the goal is to produce varieties that are adapted to more states and climatic regions.
For more information, visit The Cover Crop Breeding Network website.
Text and photos by Ramon Leon, North Carolina State University; header photo by Claudio Rubione, GROW.