Allelopathy can be regarded as a component of biological control in which plants are used to reduce the vigour and development of other plants. Allelopathy refers to the direct or indirect chemical effects of one plant on the germination, growth, or development of neighbouring plants. This can be through the release of allelochemicals while the plant is growing or from plant residues as it rots down. These chemicals can be released from around the germinating seed, in exudates from plant roots, from leachates in the aerial part of the plant and in volatile emissions from the growing plant. Both crops and weeds are capable of producing these compounds and in this case the desired effect is the impaired germination, reduced growth and poor development of weeds.
Potentially allelopathy could be used in various ways:
- to manipulate the crop-weed balance by increasing the toxicity of the crop plants to weeds thereby reducing weed germination in the direct area of the crop, which is the most difficult area to control physically
- as cover crops to suppress weed germination and development over a whole field in part of a rotation
- as mulched residues or incorporated residues which could prevent weed germination and allow transplanted crops to be grown, producing a residual weed control effect
Many crops have been reported as showing allelopathic properties at one time or another and farmers report that some crops such as oats seem to clean fields of weeds better than others. The current list includes: wheat, barley, oats, cereal rye, brassicas, red clover, yellow sweet clover, trefoil, vetch, buckwheat, lucerne, rice, sorghum.
However, several weed species have also been reported to show allelopathic properties. They include couch grass, creeping thistle and chickweed. Where they occur together they may have a synergistic negative effect on crops.
Allelopathic effects might also depend on a number of other factors that might be important in any given situation:
- Varieties: there can be a great deal of difference in the strength of allelopathic effects between different crop varieties
- Specificity: there is a significant degree of specificity in allelopathic effects. Thus, a crop which is strongly allelopathic against one weed may show little or no effect against another
- Autotoxicity: allelopathic chemicals may not only suppress the growth of other plant species, they can also suppress the germination or growth of seeds and plants of the same species. Lucerne is particularly well known for this and has been well researched. The toxic effect of wheat straw on following wheat crops is also well known
- Crop on crop effects: residues from allelopathic crops can hinder germination and growth of following crops as well as weeds. A sufficient gap must be left before the following crop is sown. Larger seeded crops are effected less and transplants are not affected
- Environmental factors : several factors impact on the strength of the allelopathic effect. These include pests and disease and especially soil fertility. Low fertility increases the production of allelochemicals. After incorporation the alleopathic effect declines fastest in warm wet conditions and slowest in cold wet conditions