LOVELOCK - Many non-farmers may wonder why alfalfa growers tear up their alfalfa fields and plant wheat or some other small grain. Alfalfa can remain productive and profitable from four, to sometimes 10 years or more, but as plant population declines renovation eventually becomes necessary.
So why don't they just replant new alfalfa to renovate the old stand? The answer is, reseeding alfalfa immediately following alfalfa is not recommended in most states due to the negative effects of autotoxicity, seedling disease and insect pests, which can build up in old stands.
So what is auto-toxicity? Autotoxicity in alfalfa is described as a process in which established alfalfa plants produce a chemical or chemicals that escape into the soil and reduce establishment and growth of new alfalfa if seeded too soon following the old stand. The autotoxic nature of alfalfa has been hypothesized to be a result of environmental selection. Perennial alfalfa originated around the northern coast of the Mediterranean. During part of the time when perennial alfalfas were evolving, the Mediterranean basin was a hot, dry desert. Development of the autotoxic trait may have reduced competition from nearby new seedlings for scarce soil moisture. The chemical or chemicals responsible for autotoxic effects has not been conclusively identified although several have been implicated. The most conclusive evidence to date suggests that the chemical medicarpin may be a primary cause, but some of the characteristics observed for auto-toxicity have not been reported from medicarpin.
There is general agreement that autotoxicity does exist in old alfalfa fields and can cause poor establishment of new alfalfa seedlings, however the rotation interval recommended for successful establishment varies widely among states. In a 1996 survey of agronomists and forage specialists in 40 states, autotoxicity was ranked as the second most important problem for seeding alfalfa after alfalfa, when compared to soil-borne disease, soil-borne insects, soil-moisture depletion by the old stand, and other factors. Soil-borne disease was ranked as the most important problem.
The same survey respondents also reported the minimum rotation interval recommended for their area. Recommended intervals of six or 12 months were most frequently reported, but the range was two weeks to 24 months. Many respondents commented that their most common recommendation was to reseed alfalfa after rotation with a non-legume crop grown for one or more seasons.
The reasons why rotation intervals should be so different from one area to another are not completely known. An assessment of research may help explain the variation in reseeding recommendations from state to state.
The role that soil texture plays in the dissipation of the autotoxic chemical has only recently been studied. Recent research, indicate that autotoxic chemicals passed more rapidly through leaching columns of sandy soil than through columns containing silty clay loam. The autotoxic effect on root growth was stronger (reduced root growth more) in the sandy soil, but persisted longer for the silty clay loam.
This suggests that in the short term autotoxicity may be more severe in sandy soils, however, with irrigation the autotoxic factor may be leached out of the root zone more easily in sandy soils than in soils of heavier texture. Further research is needed to determine the amount of rainfall or irrigation needed for different soil textures to allow shortened rotation intervals.
Variable responses to autotoxicity have been observed in alfalfa planted after alfalfa. Many times the primary response considered is whether or not a stand is established. Autotoxicity primarily affects seed germination and early root growth with root growth being affected more than germination. In severe cases, autotoxicity may cause stand failure, but sub-lethal exposure can alter root growth and development, thus causing an auto-conditioning effect of reduced stand and yield. This auto-conditioning effect can cause modest reductions in plant stands or yield that would be difficult to see in production fields.
The autotoxic zone of influence appears to prevent the thickening of old stands, but further work in variety development may produce alfalfa varieties resistant to autotoxicity so interseeding could be possible. Currently, rotation intervals of at least 12-months have been effective in avoiding autotoxic effects. Irrigation on light-textured soils may help dilute the autotoxic chemical making it possible to shorten the rotation interval. Continued research into production management and identification of the autotoxic compound may make continuous alfalfa production possible.
Source: "Understanding Autotoxicity in Alfalfa", John Jennings, Extension Forage Specialist University of Arkansas Cooperative Extension Service.[[In-content Ad]]
