Poor management can damage crops
Weed suppression is one of the many benefits of cover crops. Weed suppression by cover crops is achieved either indirectly, such as competition for light or nutrients, or directly by affecting seed viability, seed germination and seedling establishment. Chemicals released from cover crop root exudates or residue decomposition are generally responsible for toxicity known as allelopathy.
Unfortunately, if a cover crop residue can kill weeds, it can also kill crops. Most cases of cover crop injury to cash crops are due to poor cover crop management and can be avoided.
Allelopathic cover crops
Many cover crops are called “weed fighters” because of their superior ability to suppress other plants. Many of those cover crops produce toxic compounds from their roots or during residue decomposition. Rye is the most common cover crop known for its toxic residue. This cover crop is widely used and adapted to most growing conditions. Severe injury from rye has been reported on both direct-seeded and transplanted vegetables.
Other cover crops reported to produce toxic compounds are hairy vetch, cowpea, sorghum sudangrass, buckwheat and members of the Brassicaceae (cabbage) family like oilseed radish, yellow mustard, oriental mustard, rapeseed and turnip. The strength of the allelopathic effect varies with both the cover crop species and cultivar. Within specific cover crops, the extent of injury varies with the amount of cover crop biomass produced, the cash crop that follows, type and quality of planting material and environmental condition.
Generally, direct-seeded crops are more susceptible than transplants, especially for small-seeded species. High cover crop biomass will sustain high concentrations of the allelochemicals in the soil for a long period, unless rainfall or irrigation leaches allelochemicals from cover crop residue.
The notion of a plant-back period is well known for herbicide application and soil fumigation. This is the minimum period of time that should follow herbicide or soil fumigant application before crop planting. The plant-back period for most cover crops has not been established. However, for “hot” cover crops like rye, oilseed radish and other Brassica species, our program recommends a plant-back period of at least two weeks when adequate cover crop biomass are present during cover crop incorporation.
Studies have shown that those cover crops can cause total stand loss in many vegetables if the crop was seeded less than 10 days following cover crop incorporation. It is also important to know that the plant-back period should be extended in the absence of adequate rainfall or irrigation.
photo by Brian Prectel USDA Agricultural Research Service