Improved fallow systems are needed to reduce the negative effects of increased soil erosion and reduced soil quality. Field experiments were conducted to determine weed suppression attained with yellow sweetclover grown as a green manure fallow replacement crop. Yellow sweetclover was undersown in field pea, flax, or Indian mustard and then killed in June of the following fallow year. Living yellow sweetclover competed strongly with weeds during the first fall and spring of fallow. Weed biomass accounted for <1 to 12% of the total plant biomass when yellow sweetclover was terminated in June. Yellow sweetclover residues remaining after termination of growth continued to provide excellent weed suppression. Weed densities in April before planting the succeeding wheat crop were 75 to 97% lower in yellow sweetclover than in untreated fallow treatments. Yellow sweetclover controlled the perennial weeds dandelion and perennial sowthistle, as well as the annuals kochia, flixweed, Russian thistle, and downy brome. Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that a portion of the weed suppression effect may be due to allelopathic compounds being released from decomposing yellow sweetclover. Results will be used to develop more sustainable agronomic practices in regions where fallow is still widely employed.
Nomenclature:Dandelion, Taraxacum officinale Weber in Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; field pea, Pisum sativum L. ‘Trapper’; field pennycress, Thlaspi arvense L. THLAR; flax, Linum usitatissimum L. ‘McGregor’; flixweed, Descurainia sophia (L.) Webb. ex Prantl DESSO; indian mustard = oriental mustard, Brassica juncea (L.) Czern. and Coss. ‘Lethbridge 221’ BRSJU; kochia, Kochia scoparia (L.) Schrad. KCHSC; perennial sowthistle, Sonchus arvensis L. SONAR; Russian thistle, Salsola iberica Sennen and Pau SASKR; spring wheat, Triticum aestivum L. ‘Katepwa’; yellow sweetclover, Melilotus officinalis (L.) Lam ‘Norgold’ MEUOF.