Cover crops and surface crop residues can be used to control or inhibit weeds in subsequent cash crops in three basic ways:
• By smothering and shading them so they don’t receive adequate air and light.
• By outcompeting them for nutrients.
• By producing an effect known as allelopathy, the toxic effect on weed seed germination and seedling growth that occurs as residues of some cover crops decompose.
The primary way to suppress weed seed germination and growth is to have a vigorous cover crop stand. Such a stand will simply out-compete weed seeds for light and nutrients (Teasdale and Daughtry, 1993). When the cover crop is killed, its thick residues remain on the surface and hinder weed growth by physically modifying the amount of natural light, soil temperature, and soil moisture that is necessary for weed seed germination.
It’s important to note that suppressing weeds by smothering them becomes less effective as cover crop residues decompose. How fast residues decompose depends on several variables. For instance, warm temperatures, rainfall, and field tillage can speed up the decomposition rate. Another important factor is the C:N ratio, the carbon-to-nitrogen ratio of different kinds of crop residues. Residues with a high C:N ratio, such as mature small grain cover crops like rye (which has a C:N ratio of around 50), have a much slower decom-position rate than legumes like hairy vetch (which has a C:N ratio of around 12). Mix-tures of legumes and small grains have an intermediate rate of decomposition (a C:N ratio of around 25).
Cover crop residues also interfere with weed emergence through the allelopathic effect (Creamer at al., 1996a). Scientists are still researching the many (and sometimes mysterious) allelopathic effects that one plant has on another through its allelo-chemicals, the chemicals a plant releases into the environment that can be toxic to other plants. Some scientists believe that the specific allelopathic effects of certain plants are enhanced by chemicals produced by actinomycetes, algae, fungi, or other microbes associated with particular plant root systems in the upper soil layers (Putnam, 1988). Where and how these allelochemicals originate is often hard to discern. Each chemical’s biological activity may be reduced or enhanced by other factors, such as microbe action in the soil and oxidation. Other factors, such as environmental conditions, insects, or disease pressure, can speed up the detrimental effects of allelochemicals on weeds.
In one study, researchers found that cereal rye residues on the soil surface suppressed most common annual broadleaf and grassy weeds for four to eight weeks (Smeda and Weller, 1996). Thus, using a rye cover crop could eliminate the need for a soil-applied herbicide at transplanting without depressing yield. The authors indicated, however, that post-emergence weed control of escaped weeds might be necessary in some years.
Researchers have reported that the cover crops listed in Table 1 have shown allelopathic effects on certain weeds. We should note that the allelopathic effects of crimson clover and hairy vetch are more apparent if the cover crop is incorporated rather than left on the surface in no-till management (Teasdale and Daughtry, 1993).
(Source – http://www.cefs.ncsu.edu/resources/organicproductionguide/covercropsfinaljan2009.pdf)