As spring approaches, farmers may be deciding to replace an old stand of alfalfa with a new stand. The question of how soon new stands can be seeded into an old stand have been debated for years.

The problem with planting too quickly is a condition called autotoxicity. Autotoxicity is where the old alfalfa plants that have been killed or allowed to decompose in the ground produce chemicals that inhibit germination of new plants resulting in a poor stand.

The question has gotten to be a hot topic, said Monroe Rasnake, a University of Kentucky Extension agronomist.

You can follow alfalfa with alfalfa, people are doing it, he said. But there are several factors that must be considered in doing so.

Research has been done in Kentucky where the old stand was killed either by tillage or herbicide and then a new stand planted after a three-week wait. A good stand was established, Rasnake said. Over three years, they were also able to get good yields. But the research did show a rapid decline in stands.

Work has been done in other states as well that showed good stands being established after waiting a few weeks after killing the old stand. But these studies had difficulty in getting good yields from the fields.

Alfalfa fields that followed corn had about twice the yield as fields where alfalfa followed alfalfa, Rasnake said. Part of the problem was attributed to injury to the tap root of the new seedlings.

The compounds involved in the process have not been identified making it difficult to control but the compounds have been found to be in higher concentration in the leaf than stem. Reducing the amount of leaf by a fall cutting or a first cutting in May, will help reduce the leaves and perhaps lower the autotoxicity in stands killed by tillage. It will not work when herbicides are involved because the leaves are necessary for the chemical to be effective.

Breeding efforts are under way to try to find alfalfa varieties that are tolerant to autotoxicity, but none are currently available.

Until a better method is found, Kentucky growers would be best advised to continue providing at least one growing season between alfalfa stands. The minimum is a fall kill with a spring seeding or a spring kill with a fall planting. One could be spring plowing with a corn silage crop and alfalfa seeding in the fall. If corn is harvested for grain, alfalfa will need to be seeded the next spring.

Using rotation has more advantages than reducing autotoxicity problems, Rasnake said.

Other benefits include weed, insect and disease suppression. The rotational crops can also utilize the nitrogen that has been fixed by the alfalfa.