Competitiveness of Rotational Crops with Weeds in Dryland Organic Wheat Rotations.

While demand for organic foods continues to grow, few growers in the dryland wheat production region of eastern Washington produce organic grain. Growers have cited weed control constraints as a top factor preventing adoption of organic production practices, and low precipitation in the summer limits rotational crop choices. The objective was to identify crop species that could be rotated with wheat to reduce weed pressure and compete with perennial weeds. To assess weed suppressive ability of rotational crops, weed pressure and species composition was monitored in four organic cropping systems utilizing different rotational crops with winter wheat, as part of a long-term study examining dryland organic wheat production. Three years of alfalfa production reduced Canada thistle biomass by up to 100% during the alfalfa production phase, and Canada thistle biomass in winter wheat was lowest following alfalfa. Alfalfa also suppressed wild oat and field bindweed, though it did not compete well with other winter annual grasses. Field bindweed biomass was highest in winter triticale and lowest in spring barley, which was the most suppressive of all weed species compared to all other crops in this study. Winter triticale did not compete with field bindweed but competed well with Canada thistle and winter annual weed spp., producing greater yields than barley. Community analysis indicated that, compared with cereal-intensive systems, three years of alfalfa shifted the composition of the weed community generally away from perennial weeds, and towards annual weeds. Rotational crops can compete strongly with certain weed species over others, and specific crop rotations could be prescribed to prevent and suppress specific weed species in organic cereal rotations in eastern Washington.