Managing alfalfa autotoxicity

Alfalfa has a trait that is unusual among forage crops – it contains natural chemicals that are toxic to its own seedlings.

Key Points:

  • Alfalfa autotoxicity is a trait which causes alfalfa to be toxic to its own seedlings.
  • The cause of alfalfa autotoxicity has never been fully explained and can cause direct failure of germination and seedling establishment.
  • Alfalfa autotoxins do not affect any other crop

Since alfalfa originated in a dry climate, this characteristic could have been useful in ensuring plants were spaced far enough apart for best survival. This is known as autotoxicity, a form of alleleopathy, which refers to the more general phenomenon of plants that contain chemicals that are toxic to other plants. Plants use allelochemicals to discourage other plants from growing near them, reducing competition for soil nutrients, water and light resources. 

The cause of alfalfa autotoxicity has never been fully explained. Proposed chemical culprits include medicarpin, coumarin and chlorogenic acid, but none of these have been confirmed. We suspect the problem may be caused by a combination of these compounds rather than a single one. The practical result of autotoxicity is direct failure of germination and seedling establishment in some cases. The most damaging effect is harder to see, because autotoxicity permanently damages root systems on seedlings that may appear to have established successfully from above ground. Once the alfalfa taproot is damaged, it may compensate to some degree by increasing branching, but the overall effect is reduced persistence and lifetime yield for the stand. This phenomenon is called autosuppression.

Autotoxicity is the factor that prevents farmers from being able to thicken declining alfalfa stands by overseeding with more alfalfa. Stands more than one year old are almost certain to have built up enough toxin so that overseeding alfalfa into alfalfa will fail. Alfalfa is also not effectively able to reseed itself on the same site as the parent plant, and therefore alfalfa stand density inevitably declines over time until reaching the point where productivity is no longer economical. Autotoxicity also affects decisions about how soon a field can be replanted to alfalfa after termination of a stand.

A complex mix of environmental, genetic and management factors influence the degree and duration of autotoxicity and autosuppression. This makes it difficult to predict exactly how long the effects will last in a given field. It is challenging to set up field experiments that can account for all the possible combinations of factors. Therefore, much of our knowledge on autotoxicity is obtained from laboratory bioassays using extracts of plant material and do not always play out as expected in a field situation. There is some laboratory evidence that alfalfa varieties differ in toxin production, tolerance to toxins, or both.

The toxic factor is more concentrated in leaves than in stems or roots. It is water-soluble and leaches out of soil in proportion to rainfall. It dissipates faster in light-textured than heavier soils, but the initial toxic effect right after termination is often greater in the light-textured soils. Tillage helps speed the rate of dissipation. A classic experiment showed that mature alfalfa plants have a ring of autotoxicity extending in an 8-inch diameter around the crown. That means an alfalfa stand with less than one plant per square foot may still have 100 percent field coverage for autotoxicity if plants are evenly spaced. This is much lower than the alfalfa density threshold of four to five plants per square foot for profitable hay or haylage production. This means most farmers will be thinking about replacing alfalfa stands long before autotoxicity has been diluted enough by natural plant death.

MSU Extension recommends several options for managing alfalfa autotoxicity and improving the odds of a successful alfalfa re-establishment. The best option is fall termination as soon as there is enough regrowth to take up the herbicide after the last harvest, followed by tillage to speed up toxin degradation over the winter. The field should then be rotated to another crop the following season.

It is important to note that alfalfa autotoxins do not affect any other crop, so there are no restrictions on what the rotation crop can be. Corn is a good option because it can take full advantage of the nitrogen credit provided by the terminated alfalfa and is often already grown on the same farms that grow alfalfa. Another good option is to take a first cutting of alfalfa in spring, followed by termination and immediate rotation to a late-planted forage crop such as a late silage corn, forage sorghum, or oats with peas.

In no-till systems, two years of rotation into another crop may be needed for autotoxin breakdown with either fall or spring termination. Some farmers report success with fall alfalfa termination by plowing and then replanting alfalfa the following spring. The probability of success with this short-turnaround method will be increased when there has been prior success on a particular farm, therefore indicating that local soil conditions and climate are favorable for fast toxin breakdown.

For more information on managing autotoxicity, contact Kim Cassida or Phil Kaatz with MSU Extension.

This article was originally ran in Michigan Farmer Magazine.

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