Extension Education: A Common Problem in Alfalfa Stand Establishment
There are a number of problems associated with alfalfa stand establishment.
One of the common problems is autotoxicity. This describes the production of some chemical compounds secreted by the plants that inhibit the growth or development of nearby plants of the same species. This is often found in many cases when alfalfa is reseeded in fields where alfalfa was a previous crop.
There are many negative effects from this autotoxicity problem. The initial negative impacts will be observed in the roots of young seedlings. Root growth will be inhibited or stopped and plant growth will be stunted. As a result, alfalfa yield will be reduced significantly.
The reasons or the chemical compounds responsible for causing autotoxicity are not well known. It is reported that toxins are more concentrated in roots than in shoots. The toxic compounds are water soluble and precipitation can easily wash them out to soils. Plowed down plants or plants killed by herbicides can further enhance this process by releasing more toxic compounds into the soils.
The effect and persistence of the toxic compounds on new seedlings vary with soil type and precipitation.
Autotoxicity may be a greater problem in heavier soils and in locations with low precipitation than lighter sandy soils. In heavier soils, the duration of the effect will be longer, but the intensity may be lessened by absorption of compounds by the soil particles.
On the other hand, the autotoxicity effect will be shorter in sandy soils because precipitation will carry out compounds quickly from the root zones by leaching. However, the toxic intensity may be higher during this short period.
Other factors enhancing autotoxicity include density and age of preceding crops and how much time has passed since the previous stand was killed.
The effect of toxins may disappear over time, but its impact stays on plants throughout the life-cycle of the affected stand. Therefore, affected alfalfa plants remain stunted and produce low yield after the autotoxicity effect has disappeared.
The question is how to protect alfalfa stand from this problem.
Please remember there is no single recommendation to follow for controlling autotoxicity in alfalfa. The following options may help in reducing the incidence of autotoxicity and increasing alfalfa yield.
First, avoid planting alfalfa after alfalfa. Instead, plant other crops for one or two years to avoid most yield decreases.
Kill old alfalfa stands in fall, and wait to plant new alfalfa seeds in the following spring. Alternatively, kill the old stand in spring and plant an annual crop, for example oats. Then, plant new alfalfa seeds in late summer.
Finally, in the case of reseeding, delay planting at least three weeks after killing the previous alfalfa stand.
Producers should expect that yields of new alfalfa stands will probably be reduced to some extent in all options except the first. In the long-run, however, yield will be increased from healthier alfalfa stands and offset the yield lost during the initial stand establishment phases.