Demonstrating The Extreme: Autotoxicity In Alfalfa

By M. Scott Wells, David Nichol, and Roger Becker

Autotoxicity is the chemical inhibition by a plant on the germination or growth of a plant of the same species, and has been suggested as one of the mechanisms for poor alfalfa emergence and growth. Although several factors influence autotoxicity such as soil texture, rainfall, and termination timing, research indicates a break in the production and/or crop rotation is needed avoid potential stand establishment problems. Typically, autotoxicity in alfalfa is not 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 what are the impacts of doing so. Before we present the UMN Forage Teams recommendations concerning alfalfa autotoxicity, lets take this opportunity to show what happens in the most extreme settings.

The below images are from the Institute for Agricultural Professional Field School plots located on the St. Paul campus. To demonstrate the extremes (i.e. no planting delay), 2-year old alfalfa was flail mowed and the residue incorporated. After incorporation, alfalfa was Brillion seeded into a well-prepared seedbed at 12 lb/ac (Figure 1). Since the chemicals associated with autotoxicity are thought to be 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, the ‘no-alfalfa check’ treatment, to compare to the potential for alfalfa autotoxicity from incorporation of full plant (i.e. roots + shoots) or roots only on establishment of alfalfa seedlings (Figure 3).

Figure 1. Full plant incorporation of alfalfa (i.e. roots + shoots) and subsequent impacts on re-seeded alfalfa establishment.

Figure 2. Incorporated alfalfa roots and subsequent alfalfa autotoxicity on re-seeded alfalfa establishment.

Figure 3. Non-autotoxicity alfalfa check plots. Alfalfa seeded following fall fallow.

 The most striking difference was between the autotoxicity treatments (e.g. full plant, and roots only) and the non-autotoxicity alfalfa check plot. Since all treatments were direct seeded (i.e. Brillion) into well-prepared soil at 12 lb/ac, we would expect stands greater than 25 plants/ft2. In the check plots stands were greater than 35 plants/ft2 (Figure 3). Alfalfa populations where drastically reduced by autotoxicity with 0.3 plants/ft2 for the full plant treatment, and 1.7 plants/ft2 for the roots only treatments (Figure 1 and 2). For both the autotoxicity treatments, the entire plot (200 ft2) was counted. In order to maximize yields, growers need at least 25 plants/ft2 at the onset of the first production year. It is doubtful in most years; alfalfa growers will be faced with such extreme scenarios. There may be confounding factors such as the difficulty of preparing a firm seedbed when incorporating as much crop residue as we did in 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. Therefore, in years with significant winterkill, alfalfa growers should give consideration to autotoxicity in alfalfa and be ready to rotate to a non-alfalfa crop. Visit http://www.extension.umn.edu/agriculture/forages/presentations/docs/warm-season-grasses-as-emergency-forages-wells.pdf for information on emergency crops following alfalfa winterkill.

In summary, the UMN Forage Team recommending the following management guidelines for reducing alfalfa autotoxicity:

1. When possible, allow at least one year between terminating an old alfalfa stand and re- seeding alfalfa. Though we were able to establish alfalfa effectively within a shorter window, evidence of autoconditioning suggests that a longer rotation interval may be better. Thus, fall-, winter-, or spring-killed alfalfa stands should be seeded to something else in spring, and safely re-seeded to alfalfa possibly late summer but preferably the following spring.

2. Delaying spring re-seeding 2 to 4 weeks after tillage of a killed stand is NOT an effective strategy to alleviate autotoxicity because delayed seeding can still result in significant re-seeding year yield losses.

3. When considering overall risk associated with alfalfa autotoxicity, if possible, it is less risky to NOT attempt to thicken thinning stands of alfalfa with alfalfa. Use red clover or terminate the stand and seed something else. However, giving soil type, alfalfa stand age, precipitation, and termination timing there are opportunities especially in the seeding year to re-seed alfalfa. Please visit http://www.extension.umn.edu/agriculture/ag-professionals/docs/alfalfa-autotoxicity-risk-spreadsheet.xls for alfalfa autotoxicity risk calculator.

Source:umn.edu