Competitive ability of rotational crops with weeds in dryland organic wheat production systems

While demand continues to grow and prices for organic grains have remained almost double those of conventional grains, 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. In organic systems, inherent competitive ability of crops is very important in managing and preventing weed infestations. The objective of this study was to identify crop species that could reduce weed pressure and compete with perennial weeds in a wheat (Triticum aestivum) rotation. To assess weed suppressive ability of alternative rotational crops, relative total weed biomass and relative biomass of two perennial and three annual weed species were examined over 4 years in three organic cropping systems where winter wheat was in rotation with alternative crops, as part of a long-term study examining dryland organic wheat production. Three years of continuous alfalfa (Medicago sativa) production reduced Canada thistle (Cirsium arvense) biomass during the alfalfa production phase, and reduced Canada thistle biomass in winter wheat following alfalfa, compared with wheat following winter pea (Pisum sativum). Alfalfa was competitive with wild oat (Avena fatua), though it competed poorly with winter annual grass weeds. Spring barley (Hordeum vulgare), when established successfully, suppressed jointed goatgrass (Aegilops cylindrica) more than winter triticale (x Triticosecale), winter wheat, and spring wheat, and demonstrated some competitiveness toward field bindweed (Convolvulus arvensis). Winter triticale competed poorly with field bindweed, but suppressed Canada thistle and downy brome (Bromus tectorum). All alternative rotational crops contained lower weed biomass than winter pea, the crop typically rotated with wheat in the region. Organic and conventional growers could gain benefits in perennial and winter annual weed suppression by incorporating alternative rotational crops into wheat rotations.