Weed Suppression By Forage Radish Winter Cover Crops
Lawley, Yvonne Elizabeth
Weil, Ray R
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Forage radish (Raphanus sativus L. var. longipinnatus) is a new winter cover crop in the Mid-Atlantic region. This study had three objectives: 1) to characterize the repeatability, amount, and duration of weed suppression during and after a fall-planted forage radish cover crop 2) to quantify its subsequent effect on direct seeded corn, and 3) to identify the mechanisms of this weed suppression. Forage radish cover crops were grown in ten site-years and followed by a corn crop in seven site-years in the coastal plain of Maryland. Forage radish was compared to rye (Secale cereale L.), oat (Avena sativa L.), and no cover crop treatments. Early and typical corn planting dates along with contrasting herbicide management strategies were compared over four site-years. Forage radish did not reduce population or yield in subsequent corn crops. Forage radish provided complete suppression of winter annual weeds in the fall and early spring but the suppression did not persist into the following cropping season. When forage radish cover crops were used in place of pre-plant burn down herbicide treatments to control weeds in early planted corn, some weeds were present at the time of corn emergence but corn yields were not reduced if emerged weeds were controlled with a postemergence herbicide. Controlled environment bioassays involving cover crop amended soil, aqueous plant extracts, and aqueous soil extracts along with a field experiment involving planted weed seeds did not provide evidence of allelopathy. In a residue moving experiment, no difference in spring weed suppression was observed if forage radish residues were removed prior to killing frost in November or left in place to decompose in three of four site-years. These results were supported by planting date experiments where fall ground cover and spring weed suppression was greatest for earlier planting dates of forage radish cover crops. Thus, rapid and competitive fall growth, rather than allelopathy, is the most likely mechanism of weed suppression by forage radish winter cover crop. Strategies to utilize the weed suppression of forage radish cover crops should focus on fall weed suppression and the early spring pre-plant window of weed control.