Even a vigorous and productive new alfalfa seeding will eventually thin to a less than optimum plant density over a period of time.  At this point, producers typically terminate the stand using some combination of herbicide and tillage.   Although it is sometimes desirable to use a no-till drill and interseed back into the old alfalfa stand, most researchers and producers agree that seeding alfalfa into a declining alfalfa stand is rarely successful.  Failed attempts have often been blamed on disease pathogens, insects, old plant competition, and autotoxicity.

    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:
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.