I’ve heard that cover crops can be an effective means of suppressing weeds. Are all cover crops equally effective, and are they best sown as monocultures or mixtures?
Alwyn, this is a complicated questions. In general, we have found that winter cover crops have the largest effect on winter annual weeds like marestail. Planting the cover as early in the fall provides the best results. Cover crops that emerge quickly and cover the ground are important. Mixtures may be best for achieving this effect. For example, mixing cereal rye with oats or radish can provide fall ground cover very quickly. We observed almost 100% marestail suppression in the spring when we had 100% ground cover in the late fall around Thanksgiving. Suppressing summer annuals is more difficult and requires high biomass cover crops. Cereal rye is often a dominant cover crop for achieving good summer annual weed suppression, but mixtures can be helpful here as well. I’m sure others have opinions on this so hopefully we will hear more…..
Choosing a cover crop for weed suppression depends on the cropping system and the weed species in question. First, what is the phenology (growth period) of the main crop? Summer annual crops, like corn and soybeans, require winter annual covers. Winter annual crops, like winter wheat, can work with a quick summer annual cover, but can also be frost-seeded in late winter with a forage legume such as red clover to provide summer cover that extends into the following spring, providing both weed suppression and an organic N source to the following summer annual crop. Second, the seedling recruitment process and physiology of the weed will determine what types of covers work well to suppress it. Common waterhemp has a very small seed, emerges from shallow soil layers, and is highly responsive to soil N. A thick cereal rye mulch can strongly inhibit waterhemp emergence by temporarily tying up soil N, releasing allelochemicals into the soil, and physically blocking its growth. A rye cover crop won’t do as much against a perennial weed, such as Canada thistle, emerging from an energy-rich rhizome. In a case like this, taking repeated cuttings from a short cycle warm season cover, like sorghum sudangrass, can make it costly for the thistle to send up new shoots, and deplete its energy reserves.
A very good resource for deciding which cover crop species/mixtures are right for your cropping system and weed community is:
‘Managing cover crops profitably’, by the Sustainable Agriculture Network (1998)
The free download of this book is available here: http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition