Can we solve the mystery of alfalfa autotoxicity?
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Alfalfa is an important forage crop for dairy cattle, but it has an unusual trait that can challenge hayfield management. It suppresses germination and growth of its own seedlings. This is known as autotoxicity, and is a form of alleleopathy, which refers to a more general phenomenon in plants that suppress growth of other plant species. Alfalfa does not interfere with establishment of other species like forage grasses, clover, or rotation crops.
About alfalfa
Alfalfa is the nation’s third most valuable field crop at roughly $8.7 billion per year. While alfalfa has many uses, it’s most often grown for animal agriculture forage due to its nutrient-rich profile. It is especially valuable as a perennial forage crop for its high crude protein and energy content and ability to fix nitrogen. Like any other crop, alfalfa faces insect and diseases challenges, but current varieties are exceptionally hardy and able to withstand a variety of environmental pressures.
Autotoxicity and Autosupression
There are two significant management consequence to autotoxicity. The first one is obvious—direct failure of germination and seedling establishment when planting new alfalfa into soil that previously grew alfalfa. This is the reason that producers cannot effectively thicken aging alfalfa stands by overseeding with more alfalfa seed. The second consequence of autotoxicity, autosuppression, is both more common and harder to recognize. With autosuppression, new seedlings emerge and grow. Unbeknownst to aboveground observers,, their taproots are permanently damaged even when the new seedlings look superficially normal. Once the seedling taproot is damaged, it may compensate to some degree by increasing side branching, but the overall effect is reduced persistence and lifetime yield for that plant. It will never be the plant it could have been. Autosuppression is the reason for lengthy rotation interval recommendations before reseeding alfalfa into a field where it was previously grown.
Why does this happen?
The specific cause of alfalfa autotoxicity has never been identified. Growth-inhibiting chemical compounds released from alfalfa tissues into soil have been the leading suspects, including a large range of phenolics and saponins, but no single compound or combination of compounds have been proven. Three conditions are required to prove chemical allelopathy:
- A plant must produce compounds capable of suppressing germination and growth
- Those compounds must be present at high concentrations in soil,
- There must be no other plausible environmental or biological factors that could explain the problem.
It is challenging to set up field experiments that can control all possible environmental, genetic, and management factors that might contribute to autotoxicity. Therefore, much of our existing knowledge has been obtained from laboratory bioassays using extracts of plant material. While useful, this does not always play out as expected in a field situation. There is some laboratory evidence that alfalfa varieties differ in genetic potential to produce or tolerate autotoxins.
How does it work?
Autotoxins are more concentrated in leaves than in stems or root tissue. Toxins are water-soluble and leach out of soil in proportion to rainfall or irrigation. Autotoxicity 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 speeds the rate of dissipation. A classic experiment in Missouri showed that mature alfalfa plants have reduced seedling survival extending in an 8-inch diameter around the crown, with autosuppression extending out to 16 inches. Stands with only 0.8 plants/ft2 can exhibit autotoxicity.
This means that an alfalfa stand that is well below the economic replacement threshold of 5 plants/ft2 may still have too much autotoxicity to allow reliable reseeding of a new stand. Current MSU Extension recommendation is to include tillage in alfalfa termination practices and then wait at least six months to one year before attempting to replant alfalfa, possibly longer if soil is heavy.
We are unable to be more specific because we simply have no way to know exactly when the autotoxicity is gone. More precise predictions regarding autotoxic potential of the soil would assist alfalfa producers to make appropriate planting decisions. If no-till is used or drought conditions exist, wait up to two years between alfalfa rotations. A year or two of corn silage between alfalfa rotations is a good strategy.
Bioassay challenges
My research group began working on autotoxicity in 2018 using funds from the alfalfa checkoff program and Project GREEEN. Our initial efforts focused on developing a fast three-day bioassay that might be used in the MSU Plant Diagnostic Lab to flag soils where alfalfa establishment was likely to fail. We hoped this could provide a decision tool for producers.
Unfortunately, while we were able to detect differences among soils with this short bioassay, validation tests showed it is not reliable at detecting potential for long-term autosuppression. We have not given up the idea of a diagnostic test but it will not be a fast turnover test and may take several weeks to provide an answer.
A new hypothesis
Meanwhile, PhD student Paige Baisley was doing field plot research to look at the effect of replanting alfalfa into stands where existing alfalfa was terminated 2 versus 20 weeks earlier. An interesting observation emerged.
Soils expected to be greater in autotoxicity from alfalfa stands fertilized according to MSU Extension recommendations were unexpectedly often also deficient in soil nutrients essential to seedling establishment, specifically potassium and phosphorus. This led us to a new hypothesis that nutrient deficiency may play a role in autotoxicity. Plants of all species actively exude a rich mixture of chemicals into the rhizosphere, which is the soil layer immediately around the roots. These chemicals, many of which are phenols, help plants obtain soil nutrients and therefore plants may produce more of them when nutrient stressed. This could explain why some failing alfalfa stands are more autotoxic than others.
Research Objectives
- Identify the compounds responsible for autotoxicity.
- Determine how root function and soil microbiology interact with factors such as soil fertility to influence chemical development and release.
- Begin the process of breeding alfalfa varieties that do not cause this problem.
- Communicate with growers via MSU Extension education.
The end goal of the research is to pave the way for developing new varieties that either produce less toxins or are able to tolerate them more effectively. This research may also serve as a framework for other crops that exhibit autotoxicity.
Continuing research
In 2023, my lab received a $980k grant from the USDA-NIFA Alfalfa Seed and Forage Systems program to continue our work towards identifying causative compounds in autotoxicity while adding new efforts to study how root exudates, the alfalfa root microbiome, nutrient deficiency, and alfalfa genetics may be related to the problem. This project also includes Dr. Sarah Lebeis from the MSU Plant Resilience Institute and Dr. Virginia Moore, an alfalfa breeder at Cornell University. Currently, we are working on categorizing possible autotoxins in root exudates, planning how to best evaluate possible interactions with potassium fertility, and analyzing microbial populations from alfalfa root microbiome. Our Cornell colleagues have begun the process of screening populations for tolerance to autotoxicity. It is early in the project, but we are very excited about the results to date and look forward to having more to share with the dairy industry in the future.
Want to learn more?
Dr. Cassida spoke on this topic at an MSU Extension Virtual Breakfast in 2023. You can view her talk, and learn more about the virtual breakfast series here.
She also runs the MSU Forage Connection website, which contains information on current research projects, events, and variety trial results related to forage crops in Michigan.
