Use of phytotoxic plant residues for selective weed control

Abstract

Both fall- and spring-planted grasses were evaluated for weed-suppressing ability after desiccation by freezing, glyphosate or paraquat. Populations of Portulaca oleracea L. and Digitaria ischaemum (Schreb.) Muhl. were reduced by 70% and 98%, respectively, by residues of sorghum ( Sorghum bicolor L.). Total weed biomass and weight of several indicator species were also consistently reduced with residues of barley ( Hordeum vulgare L.), oats ( Avena sativa L.), wheat ( Triticum aestivum L.) and rye ( Secale cereale L.), as well as the sorghums. In general, the larger-seeded vegetables, particularly legumes, grew normally or were sometimes stimulated by the cover-crop residues, whereas several species of smaller-seeded vegetables were severely injured. The use of Populus wood shavings as a control mulch allowed separation of physical and chemical aspects of the residues. Glasshouse experiments with two soils confirmed both weed-suppressing and crop-stimulating effects of sorghum residues. Water extracts of sorghum herbage were toxic to indicator species in sterile bioassays: this suggests that phytotoxins are directly released by the plant residues.