Factors listed to consider when deciding to reseed alfalfa


As county-based Extension Educators, this time the year we get in-person visits and telephone calls from producers with concerns related to their row crops, pasture, and backyard garden operations.

One such recent question from a producer was related to alfalfa autotoxicity. Pipes were laid through the producer’s alfalfa field, and he wanted to know if he can reseed the area disturbed when the pipes were laid, with alfalfa. His concern was that the toxins from the established alfalfa may affect the germination and establishment of reseeded alfalfa. 

According to an article from Michigan State University, alfalfa autotoxicity is a trait which causes alfalfa to be toxic to its own seedlings. 

•The cause of alfalfa autotoxicity has never been fully explained                                                   

•Alfalfa autotoxins do not affect any other crop.

 Research shows that autotoxicity is affected by several factors. When alfalfa is killed, be it by spraying or tillage, toxic chemicals are released into the soil. How long the toxins remain and the effect they have on the new seedlings depends on soil type, temperature, tillage, rainfall amount and time since termination.

For example, on sandy soils toxins are readily available and easily taken up but are less persistent because they leach quickly through the root zone Irrigation and rainfall help leach toxins out of the sandy soil profile after termination of old stand but before reseeding. Under dry weather conditions, delaying reseeding may be necessary because toxins from the terminated old stand may persist in the soil profile. On heavier soil, however, because the toxins are tightly adsorbed to soil particles, their effect is less but longer lasting. Hence, delaying reseeding, after termination, on soils containing clay is advised.

The level of toxins in the soil is also affected by tillage. Research in Wisconsin has shown the effects of autotoxicity to be greater in no-till fields than those that were moldboard plowed. Soil disturbance by tillage helps to better mix and dilute the toxins.

Moreover, the time interval between terminating an old stand and planting a new one also has a significant impact on the effects of autotoxicity. The longer the lapse between killing the old stand and planting the new one, the less the effect of toxins.

In our particular case, because the area to be reseeded is a strip within an existing alfalfa field, distance between the old and new alfalfa is another factor to consider. Research indicates that for seedlings to survive and grow normally, they have to be at least 16 inches from an existing alfalfa plant. The decision to reseed or not to reseed, therefore, depends on your situation.