Alfalfa Autotoxicity: Why you shouldn’t overseed alfalfa into an existing stand.

by Glenn E. Shewmaker
Extension Forage Specialist, University of Idaho

Alfalfa can be overseeded into a new seeding within about one year. After one year of a stand, overseeding is usually not successful. The factors that cause the overseeding failure are 1) competition for light, water, and nutrients; 2) autotoxicity; and 3) disease. New alfalfa seedlings are slow to establish and thus have difficulty competing with larger plants and weeds. Moreover, new alfalfa plants are stunted in growth. This is because alfalfa creates autotoxicity. Autotoxicity is a process in which established alfalfa plants produce chemicals that move into the soil and reduce establishment and growth of adjacent new alfalfa. A similar term, allelopathy, is the direct or indirect harmful effects of one plant on another through the production of chemical compounds that escape into the environment.

How important is alfalfa autotoxicity? Agronomists in 41 states ranked autotoxicity just after soil-borne diseases in order of importance when alfalfa is planted too soon after alfalfa (Jennings & Nelson, 1991). A well-known effect of the autotoxic chemicals is that seed germination is delayed but not entirely inhibited. A little-known effect of alfalfa autotoxicity is reduced development of the seedling taproot. Seedlings affected by autotoxicity or allelopathy can be more susceptible to other stress factors including seedling disease, insects, and environmental conditions. Plants that survive initial effects at establishment may exhibit chronically depressed growth compared to unaffected plants.

Characteristics of the Autotoxic Chemical(s)

Autotoxins are released from leaf, stem, and crown tissue as well as from seeds, hay, and dried roots. Autotoxicity is likely caused by an interaction of phytotoxic chemicals which are phenolic-like compounds (Hegde and Miller 1992) and medicarpin (Dornbos et al., 1990). These compounds are water soluble and extractable from fresh alfalfa herbage, and are not a product of microbial action.

How many years should you wait before reseeding alfalfa?

In three of six experiments, yields of the first harvest in the year following reseeding averaged 15% lower following alfalfa than a control crop. Thus autotoxic response may be delayed. Yield reductions were not associated with stand density, yield in the reseeding year, or stand characteristics of the killed stands (Seguin et al. 2002). They concluded that autotoxic response in alfalfa is subtle and hard to detect in the seeding year, but more apparent in the following year. This delayed response could be the result of changes in root morphology (Nelson et al. 1997). Autotoxic effects extend for several years. Yield was reduced by 0 and 6-month rotation intervals compared to 12 and 18-month rotation intervals in the first, second, and third years of production (Jennings, Nelson, Coutts; Univ. Arkansas & Missouri). Plant density increased slightly, from 9 to 11 plants/ft2 after a 6-month rotation interval.

Lateral Zone of Autotoxic Influence

A recent study in Missouri attempted to thicken two old alfalfa stands, with densities of 2.1 and 2.4 plants per square foot. No-till interseeding in April resulted in weak, stunted seedlings that died during the first summer (Jennings & Nelson, 2002). The lateral autotoxic zone was 8 to 10 inches from established plants. Density of established alfalfa stands would have to be less than 0.7 plants per square foot before new seedlings could be productive. Seedling density within the 8-inch zone would average 70% of the control, but would yield 44% of the control. The negative effect on yield continues on through the second and subsequent years (Jennings & Nelson, 1991).

Remedies

Anecdotal evidence suggests that deep tillage, coarse textured soils (sand), and plentiful irrigation can reduce the effects of the autotoxic chemicals. The theory is that mixing soil with tillage and over irrigating to leach the autotoxic chemicals below most of the roots should reduce the effects. Some entrepreneurs claim they have a product that helps bacteria break down the autotoxins. I suggest that producers ask for independent verification of these claims such as from a university study.

The Threshold for Overseeding

Consider overseeding a cereal grain or orchardgrass into a weak alfalfa stand when alfalfa drops below a minimum of 4-5 plants per square foot or 40-55 stems per square foot. Overseeding grasses into alfalfa helps maintain hay yields, provides excellent hay for beef and horses, and reduces the chance for bloat when used for pasture. In Wisconsin Cosgrove (1996) determined that 55 or more stems per square foot was necessary for maximum yields. This level is the critical point when irrigated alfalfa fields begin to lose profitability. I strongly recommend if that critical point is reached, the stand should be rotated out of alfalfa or overseeded with a productive grass which could add one or more additional production years.

Summary

Rotation out of alfalfa is the best practice before reseeding to alfalfa. The recommended rotation interval for reseeding alfalfa after alfalfa is at least one year. To minimize pest and disease pressure, about four years should lapse between alfalfa crops. Crop rotation is the preferred practice for reseeding stands. Autotoxicity was ranked second after soil-borne diseases for importance in reseeding old alfalfa. Some studies suggest it may be a primary factor that increases the effects of disease and other stresses on the crop.

Alfalfa autotoxicity does not always cause stand failure. Surviving stands may be affected by “autosuppression” which may change plant morphology or may suppress yield and plant density compared to normal stands. Chronic autosuppression caused by autotoxicity can cause long-term reductions in alfalfa plant density and yields lasting for at least 3 years after seeding.

There are other reasons to rotate. Nitrogen credits should be used to reduce fertilizer costs of other crops and to limit environmental problems. Rotation will reduce pest and disease pressures and improve profitability. If a producer wants to extend the stand longevity, the recommendation is to overseed with a grass or cereal grain to temporarily improve yield.

References

Cosgrove, D. 1996. Effect of phytophthora resistance levels and time of planting on alfalfa autotoxicity. p. 73–75. In Proc. 1996 Am. Forage and Grassl. Counc. Conf., Vancouver, BC, Canada. 13–15 June 1996. Am. Forage and Grassl. Counc., Georgetown, TX

Dornbos, D.L., Jr., G.F. Spencer, and R.W. Miller. 1990. Medicarpin delays alfalfa seed germination and seedling growth. Crop Sci. 30:162-166.

Hegde, R.S., and D.A. Miller. 1992. Scanning electron microscopy for studying root morphology and anatomy in alfalfa autotoxicity. Agron. J. 84:618–620.

Jennings, J.A., and C.J. Nelson. 1991. Reseeding old alfalfa stands. p. 162–165. In Proc. 1991 Am. Forage and Grassl. Counc. Conf., Columbia, MO. 1–4 Apr. 1991 Am. Forage and Grassl. Counc., Georgetown, TX.

Jennings, J.A., C.J. Nelson, and J.H. Coutts. 1996. Assessment of research and recommendations for alfalfa autotoxicity in the U.S. p. 126. In 1996 agronomy abstracts. ASA, Madison, WI.

Jennings, J. A., and C. J. Nelson. 2002. Zone of autotoxic influence around established alfalfa plants. Agron. J. 94:1104–1111 (2002).

Nelson, C.J., J. Jennings, S.U. Chon, and J. Coutts. 1997. Dealing with alfalfa autotoxicity. p. 13-25. In The Central Alfalfa Improvement Conf., 25th, La Crosse, WI. 16-18 July 1997.

Seguin, P., C.C. Sheaffer, M.A. Schmitt, M.P. Russelle, G.W. Randall, P.R. Peterson, R.R. Hoverstad, S.R. Quiring, and D.R. Swanson. 2002. Alfalfa autotoxicity: Effects of reseeding delay, original stand age, and cultivar. Agron. J. 94:775-781.