Allelopathy, in general, signifies a plant’s ability to discourage other plant growth around it. The aversion can be caused from “phtyotoxic” compounds (allelochemicals) in the soil environment, resulting in a harmful effect on neighboring plants, according to a research paper published by Dalhousie University in Nova Scotia, Canada. Allelopathic conditions can also be caused by a “resource competition” by one plant, effectively stealing the nutrients needed for a competing plant’s health. Finally, allelopathic conditions can also occur when one crop, like Annual Ryegrass, competes by shading out competing plants from sunlight. Or, in the case of a cover crop with more vegetative matter, the residue of that crop can suppress the growth of other plants.
Annual ryegrass, when used as a cover crop, can be planted in the spring, interseeded when corn is about mid-calf to knee high. It germinates, then lies nearly dormant until fall (because of the shade created by the corn foliage, when the corn is harvested and the ryegrass resumes growing. If planted after harvest in the fall, ryegrass can still effectively suppress annual weeds from propagation if it gets adequate rainfall and establishes before freeze up.
With something green on the field all winter, the chances for competing weeds to grow is diminished considerably. So, in addition to protecting fields from erosion in the winter and spring, annual ryegrass keeps other unwanted weeds from getting established.
In terms of annual ryegrass’ effect on corn, it has been shown to have no allelopathic impact on corn or soybeans. Moreover, the cover crop sequesters, or takes up, excess nitrogen in the soil while growing, then gives it back to the corn crop and soil once the cover crop is terminated in the spring.