Producers concerned with old alfalfa stands dying after several years of production should avoid reseeding directly back to alfalfa.
Mature alfalfa plants produce toxins that result in toxicity in the soil, which prevents establishment of new seedlings.
“The specific toxin produced is called medicarpin,” said Trevor Lennox, a forage specialist with Saskatchewan Agriculture.
“It is present mostly in the leaves and stems, rather than in the root system itself.”
He said the toxin leaches into the ground surrounding old alfalfa plants up to a radius of 40 centimetres.
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Lennox said this toxicity will affect seed germination and decrease plant density over time.
“The most effective way to deal with autotoxicity is to rotate to another crop for a year or two.”
“It is recommended that producers seed an annual cereal crop for a minimum of one year to give the soil a chance to cleanse itself and to take advantage of the nitrogen-rich soil nutrients the broad-leafed legume has synthesized over the years.”
While there is no consensus on how long it takes for the toxicity to disappear, variables can affect the return of conditions more conducive to seedling vigour.
“The toxin is water soluble, which means that heavier soils with reduced drainage ability will exhibit the problem longer than lighter soils which are more permeable,” he said.
“Tillage can help reduce the autotoxicity effect. In fact, producers are advised to remove top growth before tillage to help reduce leaching risks.”
Producers should also be aware that higher temperatures are known to break down the toxic compound more rapidly and that heavier rain can move them away from the root zone faster.
Toxins are not produced in the first year of new seedlings, which means seeding failures or new seedlings that were winterkilled can be reseeded without any toxicity affect.
