Wishing away autotoxicity

By Mike Rankin

If ever there was a year that prompted the strong desire to disregard alfalfa autotoxicity and write it off as a bad dream, this is it.

The problem: It’s not a bad dream.

This spring has provided a double whammy for some farmers and at least a single punch to the gut for many others in terms of failed alfalfa stands.

First, there was widespread winterkill. Some lost a little, others I’ve spoken to had to write off as much as 80 percent of their stands. Of course, there were many portions of stands that were lost.

To mitigate that situation, additional alfalfa stands were seeded, and some of those were lost (or partially lost) to the relentless number of rain events.

In both cases, the desire to reseed alfalfa back into these fields is strong. But is that a prudent decision?

The answer, as is often the case, is maybe. Let’s break it down by situations, but first refresh our knowledge on that curious phenomenon known as autotoxicity.

It’s a real thing

Plants produce a wide range of chemicals aimed at warding off attacks by insects and diseases. Among these chemicals are some that inhibit the growth of other plant species. When compounds produced by one plant species are toxic to another, it is known as allelopathy.

The production of chemical compounds by a plant that are toxic to members of the same species is known as autotoxicity. Plants produce these compounds to help gain space for themselves and reduce competition from other plants for water, nutrients, and light. Alfalfa is a plant species that exhibits autotoxicity.

Medicarpin is thought to be one of the primary compounds produced by alfalfa that makes it autotoxic, but there are others. This compound is more concentrated in top growth than roots and is water-soluble, leaching readily into the soil from both decomposing plant material and growing plants.

Many factors involved

Once a stand of alfalfa is killed, whether by tillage, spraying, or winterkill, the autotoxic compounds are released into the soil environment. How long they remain and what affect they have on a new alfalfa seeding is a function of soil type, temperature, tillage, rainfall amount, and the amount of time between the old and new stand.

On sandy soils, where the toxins are more available and more easily taken up, the autotoxic effects are more acute but are less persistent because the toxin is thought to leach more quickly through the root zone.

On heavier textured soils such as clay, the toxins are more tightly adsorbed to soil particles. Consequently, the effects are less pronounced but are longer lasting. The autotoxic compounds are microbially degraded over time, so conditions that favor microbial growth such as warm, moist soils reduce the persistence of the toxins.

Tillage affects the level of toxin in the soil. More aggressive tillage will better mix and dilute the toxins. Wisconsin research showed the effects of autotoxicity to be greater in no-till fields than those that were moldboard plowed.

The age of the existing alfalfa stand will also affect autotoxicity. Younger plants (those one year old or less) contain fewer toxins than older plants.

The time interval between eliminating an old stand and planting a new one has an important influence on the effects of autotoxicity. The longer between killing the old stand and seeding the new one will reduce the effects.

The density of the previous alfalfa stand seems to have very little practical effect. University of Wisconsin studies have shown that stand densities as low as 30 stems per square foot exhibited autotoxicity similar to stands with 70 plants per square foot.

A total do-over

If you’ve simply lost an entire established alfalfa stand to winterkill that was seeded prior to last year, don’t try to reseed that field until next year. As a general rule of thumb, it’s best to have one year between the termination of an existing stand and the seeding of a new one.

Spring seedings of alfalfa following a fall-killed alfalfa stand from the previous year generally yield only 60 to 70 percent of a stand with a full year in between termination and seeding.

If the lost alfalfa stand was seeded last spring or fall, these fields can usually be reseeded with alfalfa without autotoxicity being a major concern. The same is true for new seedings from this spring that you may want to reseed during late summer or perhaps interseed into the thin or dead areas.

Bare patches

A second scenario occurs where there are large dead patches in an otherwise productive stand. These dead or very thin areas are often low spots where water and ice served as the executer.

In this case, most experts suggest filling these spots in with a species other than alfalfa. Red clover, ryegrass, or a perennial cool-season grass work well for longer-term production; a small grain can also work but is only a one-cut option.

Thickening a stand

When alfalfa stands become thin, either by natural attrition or from winterkill, there is often the inclination to simply no-till alfalfa into the existing stand. In this case, both plant competition and autotoxicity work against the development of productive new seedlings.

A classic University of Missouri study involved planting alfalfa seeds in a wagon wheel fashion around an existing alfalfa plant. Within 8 inches of the plant, new seedlings rarely survived. From 8 to 16 inches away from the existing plant, new seedlings established, but productivity was greatly diminished (see image).

Zone of influence of an old alfalfa plant on new alfalfa seedlings (University of Missouri)

If thickening the stand is the endgame, use a species other than alfalfa. Some producers routinely seed orchardgrass into existing, thinning alfalfa stands just to get another few years of production.