Weed Suppressive Potential of Sudangrass is Driven by Interactions of Root Exudates and Decomposing Shoot Residue

Sudangrass [Sorghum bicolor (L.) Moench var. sudanense] is an increasingly popular forage and cover crop, but farmers have questions about how to manage sudangrass residues for maximum weed suppression. The objective of this greenhouse study was to quantify the relative contribution of sudangrass root exudates (e.g., sorgoleone) and decomposing shoot residue to green foxtail [Setaria viridis (L.) P. Beauv.] suppression across a range of soil mixtures. Green foxtail emergence was reduced by up to 76% by shoot residues and by up to 55% by root exudates. Synergism between decomposing shoot residues and root exudates was observed as the combination of treatments delayed time to >50% green foxtail emergence by 4 to 12 days. Results suggest that the weed suppressive potential of sudangrass is driven by independent and synergistic effects of decomposing shoot residue and root exudates in the soil. Thus, farmers removing sudangrass shoot residue by grazing or haying could experience reduced weed suppressive benefits from this crop. S is often planted as a summer cover crop or forage crop and can be useful for increasing soil organic matter and suppressing weeds when planted at high densities (Forney and Foy, 1985; Weston, 1996). Many studies have demonstrated the capacity for sorghum [Sorghum bicolor (L.) Moench] and sudangrass to suppress germination, emergence, and growth of weeds and other plant species (Hoffman et al., 1996; Nimbal et al., 1996b; Roth et al., 2000; Weston et al., 1989). The mechanism of suppression may be related to a combination of factors, including interplant competition during sudangrass growth (Weston, 1996), physical interference of mulched residues with weed seed germination and growth (Teasdale and Mohler, 1993), delayed nutrient availability to weeds as sudangrass residues are decomposed (Liebman and Davis, 2000), negative soil microbial feedback effects (Klironomos, 2002), or chemical allelopathy (Einhellig and Souza, 1992). Published in Crop Management DOI 10.2134/CM-2013-0037-RS © 2014 American Society of Agronomy and Crop Science Society of America 5585 Guilford Rd., Madison, WI 53711 All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. S.E. Wortman, Assistant Professor, Dep. of Crop Sciences, Univ. of Illinois at Urbana-Champaign, 1201 S. Dorner Dr., Urbana, IL 61801; J.J. Schmidt and J.L. Lindquist, Graduate Student and Professor, Dep. of Agronomy and Horticulture, Univ. of Nebraska-Lincoln, 279 Plant Science Hall, Lincoln, NE 68583. Received 30 Oct. 2013. *Corresponding author (swortman@illinois.edu).