Delayed Seeding of Alfalfa Avoids Autotoxicity after Plowing or Glyphosate Treatment of Established Stands

Re-establishment of alfalfa (Medicago sativa L) has often been unsuccessful because of autotoxic effects of the crop on seedlings. This study was conducted at East Lansing, MI on an Udallic Ochraqualfs, fine loamy, mixed, mesic soil. The first objective was to determine the days required to eliminate autotoxic effects for 1-, 4-, and 6-yr old stands of alfalfa with 1.4 Mg ha⁻¹ dry weight of topgrowth after treatments of plowing or glyphosate [N-phosphonomethyl)glycine]. The second objective was to determine autotoxic effects of 1.4, 2.7, and 4.0 Mg ha⁻¹ dry weight of topgrowth from year-old alfalfa plowed prior to seeding the same day. Check treatments were seedings after plowed Kentucky bluegrass (Poa pratensis L.), corn (Zea mays L.), or fallow. Alfalfa was seeded for Objective 1 on 23 June 1982, 12 d after plowing alfalfa and 19 d after glyphosate application on 6-yr-old alfalfa; on 5 June 1984 and 29 May 1985, 0, 7, 14, 21, 28, and 195 d (29 May 1985 only) after plowing alfalfa or glyphosate application on year-old alfalfa; and on 11 June 1986, 7, 14, 21, and 28 d after plowing alfalfa or glyphosate application on 4-yr-old alfalfa. Seedings for Objective 2 were made on 5 June 1984. Densities and yields of seedings equaled checks when made at least 14 d after plowing alfalfa or 21 d after glyphosate application on established alfalfa. Seedling densities were similar, but lower than the check, after three levels of alfalfa topgrowth were plowed and seeded the same day. Results indicate alfalfa can be re-established without significant autotoxicity if seedings are made at least 2 wk after plowing or 3 wk after glyphosate application on established alfalfa or after seeding failure. For maximum killing of old alfalfa and to avoid autotoxicity, plowing of alfalfa and seeding at least 2 wk after plowing, or early-fall or spring glyphosate application on alfalfa followed by no-till seeding at least 3 wk after glyphosate application, are recommended. Contribution of the Michigan Agric. Exp. Stn.; partially supported by the Michigan Agric. Exp. Stn.