Alfalfa Autotoxicity

• 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 breakdown in the soil. Rain can also wash the toxins below the root zone to reduce the affect of autotoxicity. Initially autotoxicity is more severe in light soils but the toxins persist in heavier soil.

• 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 winterkilled can be reseeded with out an autotoxicity affect. 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.