Reducing autotoxicity in alfalfa

What you need to know

Allow at least one year between terminating an old alfalfa stand and re-seeding alfalfa.
Don’t delay spring reseeding. This can result in significant yield losses during the reseeding year.
Don’t thicken thinning stands of alfalfa with alfalfa. Use red clover or terminate the stand and seed something else.

Autotoxicity is when a plant chemically inhibits the growth or germination of another plant of the same species.

Although several factors influence autotoxicity -- such as soil texture, rainfall and termination timing -- research indicates the need for a break in the production and/or crop rotation to avoid potential stand establishment problems.

Typically, autotoxicity in alfalfa isn’t an issue, given the rotational schemes (e.g., corn following alfalfa). However, there are growers that wish to renovate their current alfalfa fields, and commonly ask about the impacts of doing so.

Research: Alfalfa autotoxicity

Study overview

Before sharing recommendations concerning alfalfa autotoxicity, we’ll show what happens in the most extreme settings. The images on this webpage are from the Institute for Agricultural Professional Field School plots on the St. Paul campus.

To demonstrate the extremes (i.e., no planting delay), two-year-old alfalfa was flail-mowed and the residue incorporated. After incorporation, alfalfa was Brillion-seeded into a well-prepared seedbed at 12 pounds per acre (Figure 1).

Because the chemicals associated with autotoxicity are thought to concentrate in the leaf material, a second treatment assesses the impact of roots-only on alfalfa autotoxicity (Figure 2).

Finally, alfalfa was seeded where there was no previous alfalfa crop, called the no-alfalfa check treatment (Figure 3). This enabled researchers to compare potential alfalfa autotoxicity from full plants or roots only, and ultimately assess how autotoxicity affects seedling establishment.

Study results

The most striking difference was between the autotoxicity treatments (e.g., full plant and roots only) and the non-autotoxicity alfalfa check plot. Autotoxicity drastically reduced alfalfa populations:

Check plots: More than 35 plants per square foot (Figure 3).
Full-plant treatment: 0.3 plants per square foot (Figure 1)
Roots-only treatment: 1.7 plants per square foot (Figure 2).

For both autotoxicity treatments, the entire plot (200 square feet) was counted. To maximize yields, growers need at least 25 plants per square foot at the onset of the first production year. Because all treatments were direct-seeded (i.e., Brillion) into well-prepared soil at 12 pounds per acre, we would expect stands greater than 25 plants per square foot.

What this means for producers

In most years, it's doubtful alfalfa growers will face with such extreme scenarios. There may be confounding factors such as difficulty preparing a firm seedbed when incorporating as much crop residue as this demonstration, which may have also contributed to poor alfalfa stands.

We created an extreme example to force the issue to demonstrate the potential risks. The severity of stand loss and probability of encountering autotoxicity when seeding into existing alfalfa likely would be less than shown here, but the potential for problems is real with significant risk of stand loss and incurred costs.

In years with significant winterkill, consider autotoxicity in alfalfa and be ready to rotate to a non-alfalfa crop. Emergency crops following alfalfa winterkill.

alfalfa test plot with almost no new regrowth, mostly bare ground — Figure 1: Full plant incorporation of alfalfa (i.e., roots and shoots) and subsequent impacts on re-seeded alfalfa establishment.

alfalfa test plot with a little new regrowth, mostly bare ground — Figure 2: Incorporated alfalfa roots and subsequent alfalfa autotoxicity on re-seeded alfalfa establishment.

alfalfa test plot with full regrowth — Figure 3: Non-autotoxicity alfalfa check plots. Alfalfa seeded following fall fallow.

Recommendations

We recommend the following management guidelines for reducing alfalfa autotoxicity.

Allow at least one year between terminating an old alfalfa stand and reseeding alfalfa, when possible. Though we were able to effectively establish alfalfa within a shorter window, evidence of auto-conditioning suggests a longer rotation interval may be better. Seed fall-, winter- or spring-killed alfalfa stands to something else in spring, and safely reseed to alfalfa possibly in late summer but, preferably, the following spring.
Remember that delaying spring seeding won't help. Delaying spring reseeding two to four weeks after tillage of a killed stand is not an effective strategy to alleviate autotoxicity. Delayed seeding can still result in significant yield losses during the reseeding year.
Don’t thicken alfalfa with alfalfa. When considering overall risk associated with alfalfa autotoxicity, it’s less risky to not attempt to thicken thinning stands of alfalfa with alfalfa, if possible. Use red clover or terminate the stand and seed something else. However, depending on soil type, alfalfa stand age, precipitation and termination timing, there are opportunities. This is especially true if you’re reseeding during the seeding year. Alfalfa autotoxicity risk calculator.

M. Scott Wells, Extension agronomist; David Nicolai, Extension educator and Roger Becker, Extension weed scientist

Reviewed in 2018