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Autotoxicity in alfalfa: Causes, effects and solutions

Greg E. Blaser and Kristi M. Larsen Published on 15 September 2011

0711fg_blaser_fg_1The average stand of alfalfa lasts between five to six years.

Once it becomes evident through stand and stem evaluation, or through increased pest population, that the alfalfa stand needs to be replaced, the grower should consider some potential problems with replanting a new crop too quickly.

University studies have shown that there should be a minimum of one year before re-establishing alfalfa because of autotoxicity in existing alfalfa.

Autotoxicity is a form of allelopathy that affects alfalfa plants. Allelopathy is defined as the effect a plant has on another through the production of chemical compounds released to the environment.

Medicarpins and phenolics are the possible compounds, though the true identity remains a mystery.

The water-soluble chemicals are most readily found in leaves and flowers, though the compound can be found in the stem and roots, according to a study done by J Volenec and K Johnson of Purdue University Extension.

Stand evaluation
Some growers in the Pacific Northwest automatically replace their alfalfa stand every four years; however, if the grower wants to continue with the existing stand, they need to assess its condition first.

When a grower determines to replace the alfalfa stand, the decision is based on stem and stand count.

Initially, plants per square foot was the standard used to determine when to replace alfalfa; however, in the last several years alfalfa replacement has been assessed by stem count.

As a general guide, the number of plants per square foot should be greater than 25 in the establishing year, greater than 12 in the spring of the first production year, greater than eight in the spring of the second production year and greater than five in the spring of the third production year, according to N. Glazier, UCCE Farm Adviser.

However, the preferred method of stand evaluation is a stem count per square foot. This method has been proven to be a good indicator of potential yield. Stems should be four inches or taller to be counted. Other basic guidelines in stem evaluation include the following:
• If stem count is 55 or more, expect no change in yield.
• If stem count is between 40 and 50, expect some reduction as the stand declines
• If there are less than 39 stems per square foot, there will be a decline or reduction in yield.

In a one-year study done at BYU-Idaho in five-year-old alfalfa, the results, though not statistically proven, indicated that a poor stem count not only affected the yield but also increased weed and pest problems.

With an increased weed problem, there would not only be a reduction in yield, but also in feed quality.

Once the grower has determined to replace the existing stand, there are some questions that need to be addressed.

How much time should be allowed before replanting?
Research in Michigan shows that a three-week waiting period was all that was needed before replanting; however, in Missouri there was a yield loss of 8 percent if alfalfa was planted within three weeks.

In Wisconsin, yield was reduced 70 percent when seeded two weeks after plowing existing alfalfa, and 30 percent after four weeks.

Figure 1 shows the zone of influence of an old alfalfa plant and the survival rate of new-seeded alfalfa. This also shows the distance interval between the existing plant and the new seedling.

Guidelines given from Purdue and other university studies show that at least one year would be the best option. However, some growers in the Pacific Northwest are doing a minimum two-year rotation.

What rotation crops could be included to utilize the one-year or two-year rotation?
In many Western states, a typical rotation would be a cereal crop grown for either grain or hay. In the Pacific Northwest, the rotation crops include corn, barley, wheat and potatoes.

The primary purpose of rotation following alfalfa is to utilize the fixed nitrogen. Some growers have indicated some reluctance to growing alfalfa long term.

Due to lack of equipment and other issues, some growers find that the rotation from alfalfa cannot extend for one or two years.

They have had some success in planting within two to three months. The first year’s production is acceptable; however, yield reduction is noticeable in the following years.

These growers may want to consider finding other rotation crops that use the equipment they already have, such as timothy grass, oats and other grasses.

Using a rotation crop provides the time to reduce the water-soluble, autotoxic chemicals found in the soil following alfalfa.

Plants younger than a year have fewer toxins than older plants. This means that with a less-than-acceptable stand of new seedlings, the opportunity exists to reseed within one year’s time and the new seedlings will have a strong chance of survival.

Much research has been done to find an explanation for failures in seeding into an existing stand. Some factors for failure in overseeding are competition for light, water, and nutrients; autotoxicity and disease; according to Glen Shewmaker, University of Idaho Extension forage specialist.

To reduce the risk of toxicity, the following guidelines may be useful.

How to deal with autotoxicity
The majority of the studies show that the best way to deal with autotoxicity is to allow a minimum of one year between the existing stand and the new planting.

Irrigation or rainfall may help dissolve or reduce the water-soluble chemicals, causing them to leach out of the soil profile.

Autotoxicity also affects the germination of seedlings. The recommendation for waiting at least one year would be the least risk, although some growers have planted just weeks following the elimination of the previous stand.

Jack Kuln, a Utah grower, has developed his own technique for planting alfalfa back into alfalfa. He says to experiment on 10 to 15 acres, “to get the fertility up the way it should be, scratch the ground up to get rid of the compaction; the germination has not been as high as it could be.”

However, he makes a gamble, and hopes it was the correct one. Fortunately, this method has worked for him; some growers have seen a yield increase with this practice, while others have not.

Evidence suggests deep tillage of alfalfa fields will mix the soil and reduce the autotoxic chemicals. Soil texture will determine the amount of toxins in the soil.

Sand, for example, is one type of soil that distributes the chemicals and makes it easier to leach. If the soil is clay, delays in planting should be increased.

Some growers suggest, after harvesting the last cutting, to remove it as soon as possible, and when everything has been removed, spray with a herbicide and till.

The idea is to remove most of the allelopathic chemicals before they have time to settle into the soil.

Most studies have determined that a rotation out of alfalfa is best before reseeding back into alfalfa. Not only will a rotation interval between alfalfa crops reduce the toxicity, rotation will also reduce diseases, insects, weeds and other pest pressures.

As an added bonus, rotation out of alfalfa will also provide utilization of fixed nitrogen to other crops.  FG

References omitted due to space but are available upon request.