Eric Harwood1, Richard Koenig1, and Aaron Esser2. (1) Washington State University, PO Box 646420, Pullman, WA 99164-6420, (2) WSU Coop Extension, Ritzville, WA 99169
Dryland wheat producers in the low rainfall zone (<450 mm annual precipitation) of eastern Washington commonly utilize winter wheat-tillage fallow rotations. Annual cropping or chemical fallow reduces wind erosion in this susceptible area. However, late seeding is required in these situations due to a lack of seed zone moisture at normal planting times. The objective of this study was to determine if phosphorus (P) and/or seeding rates can be altered to improve late seeded recrop or chemical fallow winter wheat yield. Winter wheat was grown at three locations in eastern Washington in 2004-05 and two locations in 2005-06. One site had chemical fallow and the others standing winter wheat stubble. Seeding rates were 45 and 78 kg/ha. Phosphorus rates were 0, 22, 45, 67, and 90 kg P2O5/ha in 2004-05, and 0, 11, 22, 45, and 67 kg P2O5/ha in 2005-06. The chemical fallow site had both early and late seeding dates. Measurements included stand density, dry matter accumulation and partitioning, grain yield, yield components, and total P uptake. Averaged across seeding rates, grain yield at recrop sites increased linearly with P rate (0.7 and 1.6 kg grain/kg P2O5 applied in 2004-05; 3.9 kg grain/kg P2O5 applied in 2005-06). The yield response to P at the chemical fallow site was quadratic in 2004-05, compared to a linear response with the 45 kg/ha seed rate and no response to P with the 78 kg/ha seed rate in 2005-06. At the chemical fallow site, yield for the early seeding date was 289 kg/ha (9.8%) higher than the late seeding date in 2004-05, and 865 kg/ha (30.6%) lower than late seeding in 2005-06 due to unfavorable seed zone moisture at early planting. Phosphorus increased yields of recrop and chemical fallow winter wheat; however, seeding date and rate showed variable responses depending on year.
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