Wild oat, a troublesome weed in small grain cereals, infests about 11 million ha of cropland in the United States. Diversifying cereal production with alternative crops, such as yellow mustard and canola, provides flexible cropping systems, decreases production risks, and may allow more effective weed suppression. A greenhouse study was conducted to assess the competitive ability of yellow mustard and canola with wild oat in 1999 and 2000, using replacement series interference experiments to relate the results to plant development stages. Yellow mustard, regardless of its proportion in mixture, reduced aboveground biomass of wild oat 33 to 66%, leaf and tiller number 34 to 36%, and panicle production 58% compared with wild oat in monoculture. Canola did not affect wild oat biomass in mixtures. Yellow mustard per plant biomass in 2000 and inflorescence production in 1999 decreased 30 and 20% with increased density of yellow mustard in mixtures. Yellow mustard biomass was not affected by the addition of wild oat to the mixture, indicating the greater importance of intraspecific competition between yellow mustard relative to interspecific competition with wild oat. Canola per plant biomass was affected more by interspecific competition with wild oat than by intraspecific competition. A second greenhouse experiment was conducted to compare plant height and biomass accumulation by the three species over 7 wk. Yellow mustard had the greatest biomass accumulation and plant elongation rate, followed by canola and wild oat. The greater competitive ability of yellow mustard with wild oat, compared with canola, is likely associated with the rapid growth and canopy elevation of yellow mustard.

Nomenclature: Wild oat, Avena fatua L. AVEFA; canola, Brassica napus L. ‘Sunrise’; yellow mustard, Sinapis alba L. ‘Idagold’.