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- As alfalfa fields mature, the plants begin to produce toxins in the soil that result in autotoxicity, which stops alfalfa seedlings from establishing. Alfalfa autotoxicity can occur if alfalfa follows alfalfa in a rotation or if a current stand of alfalfa is inter-seeded to try to thicken it.
- The specific toxin that is produced is called medicarpin.
- Autotoxins reduce seed germination, decrease plant density, reduce establishment success, and reduce growth and vigour of the alfalfa seedlings. New roots are swollen, curled, discoloured and do not develop hairs, significantly impacting yields. Stands affected by autotoxicity appear to regrow slower after each cutting.
- The water-soluble autotoxins are more concentrated in the leaves and stems than in the roots. Removal of top growth will help reduce the concentration of medicarpin in the soil. The higher the temperature, the faster the toxins will break down in the soil. Rain can also wash the toxins below the root zone to reduce their autotoxic effect. Initially autotoxicity is more severe in light soils, but the toxins persist longer in heavier soils.
- There is no clear consensus on the minimum period of time required between removing the old stand and reseeding a new stand of alfalfa. Time is required to break down the toxins or to move them below the root zone. If the alfalfa stand is 2 or more years old, an intervening year of an alternate crop is required before rotating back to alfalfa.
- Toxins are not produced in the first year in new seedlings. Therefore, seeding failures or new seedlings that were winter-killed can be reseeded with out an autotoxicity effect. Thickening old stands by inter-seeding more alfalfa is rarely, if ever, successful. The new seedlings will initially germinate, look good but then die out over the summer months.
- Research has shown that the 'zone of influence' or the radius around the old alfalfa plant that will contain toxins is 16 inches.
Allelopathy in Alfalfa - available in PDF format only
Autotoxicity During Re-Establishment of Alfalfa is a Serious Concern - available in PDF format only
Autotoxicity in Alfalfa: Causes, Effects and Rotation - available in PDF format only
Managing Alfalfa Autotoxicity - available in PDF format only
Seeding Alfalfa Fields Back Into Alfalfa - available in PDF format only
Understanding Autotoxicity in Alfalfa - available in PDF format only
Alfalfa Autotoxicity: Effects of Reseeding Delay, Original Stand Age and Cultivar - available in PDF format only
Autotoxic Compounds from Fresh Alfalfa Leaf Extracts: Identification and Biological Activity - available in PDF format only
Zone of Autotoxic Influence around Established Alfalfa Plants - available in PDF format only