Attempts to reseed alfalfa immediately after alfalfa or to thicken existing stands of thinning alfalfa with alfalfa sometimes fail. This phenomenon has often been attributed to a condition known as autotoxicity in alfalfa. Autotoxicity is the negative effect of well-established alfalfa plants, either living or recently killed or plowed under, on the germination, emergence and vigor of new alfalfa seedings.

Controlled experiments and producer testimonials have reported a range of severity for this response, from no response at all to a complete lack of seedling emergence. Understandably, this has lead to some confusion and a broad range of recommendations regarding alfalfa re-seeding. The fact that the recommended range between alfalfa plantings ranges from 2 weeks to 2 years across the country demonstrates just how little we understand this phenomenon.

One of challenges to predicting when autotoxicity in alfalfa will occur is that the substance(s) responsible for the autotoxic response is unknown, and thus the exact mechanism by which autotoxicity occurs is unknown. However, it is known that, of all plant parts, alfalfa leaves contain the highest concentrations of the autotoxins. There is also good evidence that the autotoxins are water-soluble, in other words, they are ultimately leachable out of the rooting zone.

Recent work in Missouri has indicated that soil texture influences the potential severity of the autotoxic response. The potential severity is greatest in sandy soils, but the autotoxins appear to be flushed below the rooting zone rapidly, making the period of danger in sandy soils relative brief. In contrast, the amount of rainfall (or irrigation) required to leach the autotoxins from a sandy soil will only dilute them in a clay soil. Thus, the period of concern in a clay soil is longer.

The Missouri work has also demonstrated that even when no observable reduction in seedling emergence occurs, the growth of the seedling taproots can be affected. The growth of these taproots is stunted and the resulting plants are "autoconditioned" to be lower yielding and more susceptible to other stresses. This is difficult to observe in the field, because stands can appear to be unaffected in terms of stand density, and yet be lower yielding than they should be. This could help explain why producers who seed alfalfa immediately after alfalfa sometimes claim to see no negative response in terms of establishment. The Missouri work suggests that they may be suffering a yield suppression that is not visible to the naked eye.

While there are a number of laboratory and greenhouse experiments documenting the existence and characteristics of the autotoxic response in alfalfa, there are very few well-designed field experiments, especially in northern environments. Thus, in the early 90's, we undertook a field experiment to study the potential occurrence and severity of autotoxicity in alfalfa in Minnesota.

The objective of our trial was to determine the affect of stand age and interval between plowing and reseeding on alfalfa autotoxicity. The trial was seeded at three locations in Minnesota: Rosemount, Lamberton, and Waseca where the soil types were a silt loam, a clay loam, and a loam, respectively, and repeated twice at each location. Thus, 3 locations and repetition of the experiment at each location sum to 6 total environments in which autotoxicity was studied.

The influence of stand age was investigated by seeding '5262' alfalfa every year at each location beginning in 1991 and 1992 with the goal of producing stands that were 3-, 2-, and 1-year old in 1994 and 1995. A non-alfalfa control was included at each location; the control was oats re-seeded every year at Lamberton, tall fescue at Rosemount, and corn seeded every year at Waseca.

In October of the year before reseeding, all alfalfa stands were sprayed with 1 qt/ac of 2,4-D ester to kill the existing alfalfa stands. In May the following year, killed stands were chisel plowed, disked and harrowed. Then, alfalfa was reseeded at 13 lb/ac either immediately, 2 weeks after tillage, or 4 weeks after tillage, resulting in seeding dates of about May 1 (no delay), May 15 (2 week delay), or June 1 (4 week delay). This treatment was evaluated because previous research in Michigan suggested that a 2-3 week delay after tillage was sufficient delay before reseeding to avoid autotoxicity.

One of the most commonly reported effects of autotoxicity is reduced stand densities of re-seeded alfalfa. Interestingly in our experiment, alfalfa seedling density, counted 30 days after seeding, was similar when re-seeding followed alfalfa or another crop, regardless of the alfalfa stand age (Table 2; one year of data shown). This suggests that there was no evidence of a severe autotoxic response. Where stand densities differed (in 2 of 6 environments), the differences were not consistent with an autotoxic response.

A delay before re-seeding affected initial alfalfa stands in 4 out of 6 environments, indicating variable environmental conditions at seeding, i.e., optimal conditions for gemination and emergence occurred at different times depending on the environment.

Some previous reports suggest that while initial stand densities may not be affected, ultimately, seedlings weakened by autotoxicity die and thus stand densities of the re-seeded alfalfa are reduced. However, this was not borne out in our trial. Stand counts in the fall of the re-seeding years showed reduced stands following alfalfa compared to corn, oats, or tall fescue in only 1 of 6 environments (Rosemount 1995; data not shown).

While the previous crop affected re-seeding year alfalfa yields in 3 of 6 environments, the significant responses were not consistent with an autotoxic response. In fact, re-seeding year alfalfa yields following alfalfa sometimes exceeded those following non-alfalfa controls.

Delaying seeding following spring tillage had a consistent negative effect on re-seeding year yields (Figure 1; one year of data shown). Total yield reductions compared to re-seeding directly after tillage on May 1 averaged 32% and were as high as 75% at Lamberton in 1994. These significant yield reductions were due simply to the shorter season available for growth and the more favorable moisture and temperatures that usually occur for alfalfa growth in the spring.

In 3 of 6 environments, first cutting alfalfa yields in the year following re-seeding were 19% lower following alfalfa than the non-alfalfa controls. This suggests the presence of the autoconditioning response documented in the Missouri research, however we did not dig up alfalfa plants to determine if their root morphology was abnormal.

When alfalfa was killed with 2,4-D ester the previous fall, stands of alfalfa re-seeded after tillage the following spring were consistently good. However, there was some evidence of autoconditioning, the more subtle manifestation of autotoxicity. Also, in our scenario, delaying re-seeding after spring tillage did not affect stand density and was, in fact, detrimental to re-seeding year yields.

In reality, alfalfa that winterkills does so sometime between October and May; thus, our results may not be representative of attempting to re-seed winterkilled stands. Furthermore, if existing alfalfa stands had been sprayed (killed) in spring rather than the previous fall, responses may have been different. (However, in addition to autotoxicity concerns, there would be serious residual herbicide concerns if the goal was to re-seed alfalfa in spring following a spring kill with 2,4-D ester.)

Thus, Minnesota research has shed some light on alfalfa autotoxicity, but the phenomenon still remains somewhat cloudy. In light of our work and previous research, we offer the following recommendations regarding re-seeding alfalfa into alfalfa.

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.

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 results in significant re-seeding year yield loss.

Do NOT attempt to thicken thinning stands of alfalfa with alfalfa. Use red clover or terminate the stand and seed something else