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Be Cautious of Autotoxicity When Developing Old Alfalfa Stands

07 June 2013

CANADA - Although many plants produce chemicals to defend against pests, some give off chemicals that hinder other plant species, which is why farmers should be wary when developing old alfalfa fields.

Alfalfa autotoxicity is one factor to be carefully considered before developing rejuvenation plans, according to Stephanie Kosinski, forage specialist at Alberta Ag-info Centre. 

“Certain plant species give off chemicals that affect the growth and development of other plants. This is called allelopathy. Alfalfa has an allelopathic chemical that inhibits the growth of other alfalfa plants. It is said to be autotoxic, or toxic to itself,” states Mrs Kosinski.

The autotoxic chemicals produced by alfalfa are water soluble and can leach into the soil from decomposing plant material and growing plants. The chemical causing autotoxicity in alfalfa has not been positively identified, but is thought to be ethylene and/or possibly medicarpin. The autotoxic chemical is found in higher concentrations in the leaves and flowers than the stems and roots of alfalfa plants.

“As soon as a stand is killed, the autotoxic chemicals are released into the environment from the decomposing alfalfa plant material,” says Mrs Kosinski. “Once they enter the soil, they will remain there until they break down or are moved by water. The length of time these toxic chemicals remain in the soil depends on soil type, temperature and rainfall.

“On sandy soils, you will see more acute effects of the toxic chemicals, but they will last for less time than on heavier textured soils. This is because they will be quickly leached out by rain," she adds. "On soils with more clay, the toxic chemicals are more strongly attached to soil particles, resulting in a lower level of damage over a longer period of time.”

The age of the alfalfa stand affects autoxicity. Stands that are two or more years old will contain more toxins than stands that are one year old or less. In addition, alfalfa plants have a higher level of toxins when flowering compared to alfalfa that is vegetative.

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