Autotoxicity in Alfalfa - What it is & How it Affects Alfalfa

Warren C. Thompson National Forage Specialist: America’s Alfalfa

As reported by Dr. John Jennings and associates at the University of Missouri in 2000, autotoxicity in alfalfa is an allelopathy. The allelopathy was first described and reported by E.L. Rice in the 2nd edition of the Academic Press Inc. in 1984. The malady is best described as ‘the direct or indirect harmful (or beneficial) effect of one plant on another through the production of chemical compounds that escape into the environment’. In alfalfa production, this means that old (living) alfalfa plants or residual litter from old plants can (and usually do) destroy or greatly reduce the vigor of new alfalfa plants.  
 

The infection system kicks in as mature leaves drop and new seedlings attempt to emerge through this ‘infected shield’. It is in this situation that new emerging seedings are killed or suppressed severely when growers attempt to re-seed to thicken failing older alfalfa stands. This is also the situation that causes stand failure and lower yielding less persistent stands when following older alfalfa stands immediately with new alfalfa plantings without intervening row crops. It is simply a matter of the residue from the older plants contaminating (infecting) new plants. When you plant corn for instance, you are diluting the polluting affects that carry over from the old alfalfa. What is the old saying, “the best solution to pollution is dilution”?

The first time that I heard about this problem of following alfalfa with alfalfa was in the fall of 1939! My major professor Dr. E. N. Fergus announced one cold November day in a forage crops course he was teaching that included a study of alfalfa production and management; “Gentlemen, you cannot follow alfalfa immediately with alfalfa and produce successful and enduring alfalfa stands”. Immediately, hands shot up and asked, “why and what’s the problem”? I will never forget the ‘gist’ of his reply; “we really don’t know what the problem is. Just take my word for it and someday, someone will identify the problem and give us some usable answers”.

He was right. We have discovered why alfalfa does not grow well when following and old alfalfa stand. But to date, as far as I know, no one has identified the chemical other than it is an allelopathy and some of the more important factors that tell the whole story. Our current professionals know that the major problem comes mainly from deteriorating leaf (and stem) mass that accumulates in the soil surface and ultimately works its way into parts of the soil profile.

How to deal with Autotoxicity:

The best system we know of about to establish a great new, productive alfalfa stand following alfalfa is to completely destroy the old alfalfa stand (best to plow if possible) and plant the land to corn. Two years in corn is better than one year. In most alfalfa producing areas, (especially in the Midwest, Southeast, and Eastern states), erosion can be a problem. I suggest you seed the land after row crop harvest to winter small grains. This cover crop will help reduce erosion, provide excellent soil cover and in many years may produce another grain or forage crop.

With the introduction of specialized chemicals and equipment to replace plowing, no-till crop production is increasing in much of the country. When growers are into this system, eliminating all of the old alfalfa becomes quite a chore and unless all of the old plants are killed/ removed, autotoxicity becomes a continuing factor.

Since Roundup (Glyphosate) came on the scene, farmers in many areas have used it with varying degrees of success as a substitute for plowing. With herbicides, their best successes have continued to be when growing corn before making the new seeding. Folks who have been most successful with this system have sprayed with Roundup and possibly Gramoxone found it best to spray before planting the second year to be sure they kill the more hardy surviving plants and persisting weeds.

A Summary: I have attempted to un-complicate very complex malady that occurs in alfalfa. Frankly, I feel that this malady is more of a threat to the expansion and use of alfalfa than any unsolved mystery associated with alfalfa. Researchers have been studying autotoxicity for over 30 years and studies will continue until better answers are available. Basically, I like to think of this malady as a ‘disease of association’ meaning “the closer a young alfalfa plant is to an older living alfalfa plant or is growing in undisturbed yet decaying aftermath of older alfalfa stands the better it’s chance of infection”. Should infected plants survive, they will not live to a ripe old age and forage yields will be less than if the stand had been established in rotated soil conditions. To get around this problem, remove the infection potential before seeding and I advise that you avoid quick fixes you may have heard about as they usually fizzle.

Even though autoxicity is with us and will likely be a malady that we will be dealing with for several years to come, all is not lost and there is an upside to it. Alfalfa is a great nitrogen-fixing legume. When the land is rotated regularly, growers are able to ‘harvest’ the accumulated nitrogen to elevate grain yields. This accumulation range can vary from 100 and 150 pounds per acre when the alfalfa stand is over two years old and this organic form of nitrogen can be figured into the base recommendations for attaining best grain or grain silage yields.

Note: I have refrained from giving exacting chemical recommendations for weed control. They vary widely from one state to another. My advice is to call your County Agent or your State Extension Weeds Specialist for specific recommendations as to rates, timing and perhaps alternative herbicides for your farm.

What about resistant varieties? There are none at present. But when resistant varieties are developed we will likely be the outfit that will develop and release them and you will be the first to know.