Agronomic and Sociological Aspects of Organic Cropping Systems in the Dryland Pacific Northwest
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Of the 4.8 million acres devoted to organic winter wheat (Triticum aestivum L.) production in the U.S., less than 1% of organic wheat acreage is located in Washington State, despite the highly productive soils and record wheat yields of eastern Washington. Growers reported inadequate organic weed and pest control methods and low yields as being the main barriers to producing organic grains. To improve our understanding of organic management practices in the wheat production region of the inland Pacific Northwest, a survey was conducted of certified organic small grains and forage producers in five northwestern states. A field study investigated weed control in organic wheat rotations via crop competitiveness and rotation design, as well as the ability of poultry versus green manure to maintain soil nitrogen. The resultant organic wheat grain yields and quality were compared to conventional yields to assess the viability and potential of certified organic wheat production. Crops in rotation with wheat impacted relative weed biomass and weed species composition. Perennial crops competed most strongly with the perennial weeds Canada thistle (Cirsium arvense [L.] Scop.) and field bindweed (Convolvulus arvensis L.), two of the most troublesome weeds reported by growers in the survey. Barley (Hordeum vulgare L.) and triticale (x Triticosecale), two alternative grain crops in wheat rotations, competed more with all weed species than wheat. Rotation crop choices could be made based on competitiveness with weeds. The use of poultry manure as fertilizer resulted in wheat yields and quality similar to conventional wheat. Winter pea (Pisum sativum L.) green manure did not accumulate sufficient levels of biomass and nitrogen, and subsequent wheat yields were low due to nitrogen deficiency. Levels of soil inorganic nitrogen following alfalfa (Medicago sativa L.) were sufficient for high-yielding wheat the following year, but volunteer alfalfa created weed pressure in the wheat crop. Alternatively, intercropping spring pea with spring wheat as a green manure strategy in organic wheat systems increased subsequent winter wheat yields by 1000 kg ha-1. This study demonstrated the potential to produce competitive winter wheat yields and identified successful weed and soil nitrogen management strategies under certified organic production.