Alfalfa is a popular commercial legume forage plant which is dried into alfalfa hay for horses, cattle and other livestock. It is well documented that the mature (1 year old +) plant produces a toxin known as ‘medicarpin’ which damages newly sown seed and those plants which do manage to germinate from it. This is known as ‘autotoxicity’ because the plant is toxic to itself.
Farmers are advised to cut and remove the top growth before ploughing to destroy the roots of the alfalfa but the chemicals released remain in the soil and rain will wash surface chemicals into the root zone. The recommendation for farmers is to grow cereals or other crops in the soil before trying to seed again with alfalfa.
Source http://www.omafra.gov.on.ca/english/crops/field/autotox.htm
