Potential of allelopathic weeds for weed control in soybean

Several weed species were evaluated for their allelopathic potential, including bulbous buttercup (Ranunculus bulbosus L.), cutleaf eveningprimrose (Oenothera laciniata Hill), horseweed (Conyza canadensis (L.) Cronq.), and entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray). Extracts of bulbous buttercup inhibited germination of several weed species common to the Mississippi delta region. Bulbous buttercup collected from Pine Tree, AR were more inhibitory than those from Fayetteville. Greenhouse studies were conducted to evaluate method of residue placement (incorporated or soil surface) and residue rate (0, 0.5, 1, and 2 times the natural biomass production, denoted as X). In general, increasing residue rates resulted in fewer numbers of emerged plants. Method of residue placement also had an effect but was mainly dependent on species. Although the other weeds evaluated exhibited some activity, bulbous buttercup showed the greatest potential to be used as a natural cover crop.; Field studies were conducted at Pine Tree in 1996 and 1997 to simulate actual producer conditions. Factors evaluated consisted of residue rate (0, 1 and 2X) and method of residue management in tilled (incorporated) and no-tillage systems (either mowed or shredded onto the soil surface and standing residue). Little effects were noted in 1996. In 1997, increasing residue rates increased control of several weed species. This was also reflected by reductions in both plant counts and biomass production. Although some early soybean injury was evident, the crop soon recovered. Soybean yield increased with increasing residue rates of bulbous buttercup.; Laboratory experiments were conducted with plant extracts to identify chemicals that may be responsible for the observed activity. Extract fractions containing compounds with polar functional groups caused reduced germination of prickly sida and barnyardgrass. Analysis of these fractions showed the presence of several chemicals, some or all of which may be involved in the observed activity.; Greenhouse studies were conducted to evaluate commercially available allelochemicals for efficacy. The compounds were applied preemergence and postemergence at 5.6 and 11.2 kg ha{dollar}sp{lcub}-1{rcub}.{dollar} None of chemicals provided acceptable weed control at either rate or application timing.