Seeding Alfalfa into Alfalfa

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With some alfalfa stands thinned or winterkilled this year, there may be the temptation to "thicken" stands by interseeding alfalfa into alfalfa. Recently, a feature article appeared in a popular farm magazine regarding the successful renovation of old alfalfa stands by seeding alfalfa into alfalfa using high rates of fungicide seed treatment. The Kansas crop consultant who recommended this practice dismissed the idea of autotoxicity and, based on his success with the practice, attributed failures with interseeding to disease problems that could be overcome with the higher fungicide rates.

High fungicide rates won't help

Based on research results and farmer experience, interseeding alfalfa into alfalfa has long been discouraged by agronomists because of autotoxicity, competition, and disease factors. The above mentioned magazine article brought about an influx of questions throughout the Midwest regarding the prospect of using high rates of fungicide to interseed alfalfa into alfalfa and prolong the life of the stand. In a recent edition of Agronomy Abstracts, the practice was put to the test by researchers at Kansas State University. High rates of fungicide were used on alfalfa seed interseeded into old alfalfa stands. Fungicide rate had no impact on seedling emergence and establishment and stands were not successfully interseeded.

The "zone of influence"

Recent research from Missouri has helped to quantify associated production loss from seeding new alfalfa in close proximity to old plants and established a critical plant density needed for successful interseeding. To do this, old plants from a 5-year old alfalfa stand were selected and all other plants within a 6-foot radius of the selected plants were killed with herbicides. Alfalfa seed was planted in four rows radiating 40 -inches out from each old plant in a "wagon wheel" fashion. Seed treatment fungicide and granular insecticide were used to control pests. Each row was then thinned and divided into five 8-inch sections for yield data collection. From the experiment, researches established a "zone of influence" radiating around the old alfalfa plants. This zone is described as follows:

Within 8 inches of the old plant: New seedlings rarely survive and yield only 30% of maximum.

From 8 to 16 inches away from old plant: New plants establish but yield 75% of maximum.

From 16 to 24 inches away from the old plant: New plants achieve maximum growth and yield.

Plant density impacts interseeding success

Using the zone of influence as a baseline, interseeding into an old alfalfa stand with a plant density of 0.2 plants per square foot or less would result in success. However, this density is far below what we would consider an optimum stand. At 0.4 plants per square foot, plants will establish but be low yielding. Any stand with greater than 1.3 plants per square foot will most likely result in an interseeding failure due to excessive zone overlap. Current recommendation stands

The practice of interseeding alfalfa into alfalfa for the purpose of thickening stands has rarely been successful unless old alfalfa plants are close to non-existent (for example, a winterkill situation). Although diseases may play a factor in the establishment of alfalfa into an old alfalfa sod, they certainly are not the only factor and autotoxic effects cannot be discounted or overcome without sufficient time between killing the old stand and establishing the new. Seeding alfalfa after a new seeding of alfalfa that has winterkilled can be done with little impact on germination and establishment.