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ALFALFA

Zone of Autotoxic Influence around Established Alfalfa Plants

^a Coop. Ext. Serv., Univ. of Arkansas, P.O. Box 391, Little Rock, AR 72203
^b Dep. of Agron., 210 Waters Hall, Univ. of Missouri, Columbia, MO 65211

* Corresponding author (jjennings{at}uaex.edu)

Received for publication December 11, 2000. Interseeding alfalfa (Medicago sativa L.) to thicken declining alfalfa stands is seldom successful due to autotoxicity. Our objective was to characterize the lateral zone of autotoxic influence around established plants. Experiments were conducted in South Missouri in alfalfa fields of ‘Apollo’ [Location 1 (L1)] and ‘Cody’ (L2) in 1992 and ‘Cimarron’ (L3) in 1993. Soils were Poynor cherty silt loam (loamy-skeletal over clayey, siliceous, mesic Typic Paleudult) at L1, Ashton silt loam (fine-silty, mixed, mesic Mollic Hapludalf) at L2, and Waben very cherty silt loam (loamy-skeletal, siliceous, mesic Ultic Hapludalf) at L3. Established alfalfa was killed in May with herbicide within 2 m of selected alfalfa test plants, and then 1 yr later, Cody alfalfa was seeded within 1 m around the test plants. Granular chlorpyrifos and seed treatment metalaxyl at planting did not improve alfalfa seedling establishment or yield. Seedlings were smaller (p < 0.05) near both live and dead test plants at L1, indicating mainly autotoxicity, but at L2, yield per plant was reduced more within 25 cm from live than from dead plants, indicating competition was involved. Clipping test plants more frequently at L3 did not improve establishment and reduced seedling yield for six of eight harvests. Seedling density and dry matter yield averaged 70 and 44% (p < 0.05) of the control, respectively, within 20 to 25 cm of test plants, an area equivalent to a stand density of 8 plants m^-2. The zone of influence around established alfalfa plants involves both competition and autotoxicity and needs to be considered in replant decisions.

Abbreviations: +I+F, insecticide and fungicide added • -I-F, no insecticide or fungicide added • L1, L2, and L3 are Locations 1, 2, and 3, respectively

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