Cover crops for weed suppression
Considering the whole system is required when incorporating soil health practices on-farm. Often times, there are concerns about potential issues that including something like cover crops may cause; for example, disease and pest transfer, potential to contaminate grain, soil water and temperature conditions. However, looking for additional benefits to the entire system should be considered, too. In this Soil Health Minute, I'd like to talk about the benefit of using cover crops in rotation for weed suppression.
Weed suppression using cover crops can come in the form of competition for resources. For example, using cover crops after wheat provides a mode of action for weed control via competition for remaining nutrients in the soil after the crop is harvested. It's the idea of "pick your green or Mother Nature will do it for you," where you seed a cover crop you know you can control before Mother Nature puts a weed (sometimes resistant) in the field. The key to this concept is to pick a cover crop you know you can control — keep it simple.
The "pick your green" principal is the same when using cover crops on saline areas. If barley is seeded as a cover crop, it may outcompete some of the herbicide resistant weeds that may grow in saline areas when left bare. Plus the benefits of managing water and building residue to reduce evaporation are the primary goals of using a cover crop in this scenario.
In many systems, cereal rye is being used as a tool to control early season weeds prior to soybean. Think about it: Cereal rye establishes in the fall, is cold tolerant and starts growing early in the field. The competition for resources and the allelopathic effect from the roots are a win for weed suppression. In North Dakota State University studies, we are seeing 10 times less weed biomass in areas where cereal rye is growing prior to planting soybean than in areas where there is no cereal rye.
When conditions are so dry, there are certainly some concerns that water use by cover crops outweighs the weed suppression benefits. In this case, we need to think about the entire system and figure out what we can adjust to make it work. There needs to be a balance between the biomass of the cover crops grown (i.e. water use) and the residue produced by the cover crops to reduce evaporation (i.e. water storage). I'm not sure that we have all the answers at NDSU to make recommendations, but seeding rates and what is included in the mix can always be adjusted. This is where experience and gut feeling comes into play.
Discussions about seeding rates and mixes will come up at field days, so be sure to visit the NDSU Soil Health webpage to see a schedule of field tours (ndsu.edu/soilhealth). You can also follow me on Twitter (@NDSUsoilhealth).