I’m about to till in sorghum-sudangrass cover crop. Will they have allelopathic effects on subsequent crops?
Answer: Sorghum-sudangrass is often grown as a cover crop to reduce erosion, increase soil organic matter, and suppress weeds, but you’re right to be concerned about its allelopathic effects on subsequent crops. These effects can include death or stunting due to the presence of a number of inhibitory compounds including sorgoleone, phenolic acids, and dhurrin, which converts to cyanide. However, the presence of these chemicals is not permanent (or else nobody would use this cover crop), and according to the University of California Davis, waiting at least six to eight weeks for these compounds to leach and degrade before transplanting into the residue is usually sufficient to avoid the allelopathic effects. Appropriate irrigation or rainfall is necessary to facilitate leaching. The 2009 UC study states:”We studied the effects of sudex, a sorghum hybrid used as a cover crop, on subsequent crops of tomato, broccoli and lettuce started from transplants. Within 3 to 5 days of being transplanted into recently killed sudex, all three crops showed symptoms of phytotoxicity including leaf necrosis, stunting and color changes. There was 50% to 75% transplant mortality in all three species. Plant growth and development, as determined by biomass measurements, were also significantly affected. Yields of mature green tomato fruit and marketable broccoli and lettuce heads were reduced significantly. Tomato, broccoli and lettuce should not be transplanted into sudex residue for at least 6 to 8 weeks, or until the residue has been thoroughly leached.”For more information on the issue of allelopathy in sorghum-sudangrass, read the full UC study. For more information on cover crops, see the ATTRA publication Overview of Cover Crops and Green Manures.