Autotoxicity is the negative effect of well-established alfalfa plants on the germination, vigor and emergence of alfalfa seedlings. Pioneer Hi-Bred International, Inc., funded the studies through the company’s Crop Management Research Awards program.

“Our survey of 20 university Extension agronomists showed recommendations on when to reseed alfalfa after killing old alfalfa stands vary from state to state,” says Jerry Nelson, professor of agronomy at the University of Missouri, who conducted the research. “The period for keeping fields alfalfa-free also varied from zero to six months, while some recommended more than 12 months.

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

Long-term impact

Plant and soil bioassays were used to evaluate the impact of autotoxicity on germination, root growth and shoot growth. These tests show reduced germination appears to be due to a slowing and killing of the primary root within the germinating seed.

Those roots that do survive are then smaller and more branched, reducing the plant’s capacity to tolerate drought and making the plant less productive, especially in drier conditions. According to Nelson, one of the most significant findings of this evaluation 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 autoconditioning,” says Nelson. “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 of time 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 percent to 29 percent 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 percent.

“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.

Soil type affects

Researchers also have discovered that the severity of autotoxicity also varies depending on soil type. While lighter, sandier soils tend to be more toxic in the short-term, the autotoxins are leached much more quickly from sandy soils. In clay soils, the autotoxins bind to soil particles and leach more slowly. “Reseeding intervals may be shorter for sandy soils compared to clayey soils,” says Nelson.

In addition, 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, establishment after all varieties was equally successful when the reseeding interval was one year.

Bioassays of the seedlings showed germplasms differed in their tolerance to the autotoxins, suggesting that breeders may be able to make progress in developing varieties tolerant to autotoxins.

“Dr. Nelson’s research evaluated aspects of autotoxicity that hadn’t been researched before, and the findings provide sound new management solutions,” says Tom Doerge, agronomy research manager for Pioneer. “Pioneer funded the project because the information from it will help our customers improve their production and optimize the performance of our products.”

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