COVER CROPS AFFECT SORGHUM SEEDLING GROWTH

Recently killed cover crops often interfere with crop seedling growth. Controlled-environment and field studies were conducted to characteri ze the nature and persistence of cover crop interference with sorghum [Sorghum bicolor (L.) Moench] seedling growth and to test several seed -zone management practices that might alleviate detrimental effects. G ermination, root and shoot length, and disease incidence of sorghum ge rminated at 25 degrees C for 5 d in soil collected 2, 4, 7, 14, 23, an d 32 d after killing cover crops indicated legume cover crops were mor e detrimental to seedling growth than were nonlegumes. Surface residue s, subsurface residues, and residue leachates contributed to the delet erious effects. Seedling shoot disease incidence of 50% persisted thro ugh 32 d when legume residues were mixed into soil or placed on top of soil at planting, but disappeared by 7 to 14 d if residues were remov ed. Pathogenic organisms isolated from lesions on seedlings indicated legume cover crops increased damage due to Rhizoctonia solani Kuhn. In a no-till field study, stand density was reduced 15% and aboveground seedling dry weight was reduced 45%, from 85 to 45 mg plant(-1) 28 d a fter planting, when sorghum was planted 1 d after killing crimson clov er (Trifolium incarnatum L.) compared with planting 21 d or longer aft er killing. Insecticide, activated charcoal, or CaO2 seed coating impr oved sorghum stand density 15%, but did not affect seedling size. In-f urrow fungicide drench had no effect on stand density, but phytotoxic effects of the fungicide reduced shoot and root growth rates in both f ield and controlled-environment studies. Residue removal combined with selected in-furrow treatments may allow the interval between cover-cr op killing and successful no-till planting to be reduced to less than 7 d.