To avoid autotoxicity in new alfalfa stands, growers should keep fields alfalfa-free for at least 12 months prior to reseeding.

That’s according to results of a three-year University of Missouri study funded by Pioneer Hi-Bred International.

“This research has given us a better understanding of how autotoxicity impacts the alfalfa plant,” says Missouri agronomist Jerry Nelson. “It indicates the alfalfa-free interval should be at least 12 months in order to establish a stand and minimize or avoid losses.”

Plant and soil bioassays were used to evaluate the impact of autotoxicity on germination, root growth and shoot growth. They showed that reduced germination appears to be due to a slowing and killing of the primary root within the germinating seed. Roots that do survive are then smaller and more branched, reducing the plant’s capacity to tolerate drought and mak-ing the plant less productive, especially in drier conditions.

According to Nelson, one of the most significant findings is that alfalfa does not outgrow the initial effects of autotoxicity.

“Alfalfa appears to have this ‘memory’ of response to the autotoxins that is termed autocon-ditioning,” he says. “Even if a producer reseeds and appears to have successfully established a stand following a three- to six-month alfalfa-free interval, autoconditioning will lower the productivity of the stand over an extended period because of the initial damage. These losses can’t be visually assessed, and the potential economic losses go well beyond seeding failure. Over the long-term, yields can be 8-29% lower where the stand is impacted by autotoxicity.”

Interseeding alfalfa to thicken thin stands also isn’t advised. Nelson’s research shows that the presence of just one plant per 2.8 square feet will decrease yields following interseeding by nearly 30%.

“It’s better to thicken the stand by reseeding with a legume such as red clover, since it is tolerant of alfalfa autotoxins,” says Nelson.

Researchers also have discovered that the severity of autotoxicity varies among soil types. While sandier soils tend to be more toxic in the short-term, autotoxins are leached much more quickly from those soils. In clay soils, autotoxins bind to soil particles and leach more slowly.

“Reseeding intervals may be shorter for sandy soils compared to clayey soils,” says Nelson.

Alfalfa varieties do not vary in their production of autotoxic compounds. All of the varieties tested showed equal damage when the stands were killed right before seeding. Likewise, es-tablishment after all varieties was equally successful when the reseeding interval was one year.

But bioassays of seedlings showed that germplasms differed in their tolerance to auto-toxins, suggesting that breeders may be able to make progress in developing varieties toler-ant to autotoxins.