Theses and Dissertations Collection

Farmers in the Inland Pacific Northwest (IPNW) typically cultivate dryland wheat-based crop rotations, which often include spring crops and, in lower precipitation areas, fallow. As part of the Landscapes in Transition (LIT) project, the CropSyst model was employed to evaluate traditional (business-as-usual, BAU) and novel crop rotations at two locations: St. John, Washington, an annual crop-fallow transition site, and Genesee, Idaho, an annual crop site. Incremental (INC) and aspirational (ASP) crop rotations, which included winter peas and cover crops, respectively, allow farmers to intensify and diversify operations through the inclusion of winter crops and reduction in fallow. Data collected during LIT field studies, carried out during the 2018-2021 growing seasons, was used to parameterize and calibrate the CropSyst model and evaluate its ability to simulate field conditions. Subsequently, scenario testing explored the viability of the alternate crop rotations under long-term, historic climate conditions.

The developed CropSyst models appeared powerful and robust, especially given the variability and range of the observed data. CropSyst achieved good agreement between predicted yield and biomass and observed values on an overall, rotational, and yearly basis (R2 greater than 0.5 and root mean square error, RMSE, less than the standard deviation, σ, of observations). Additionally, soil moisture simulated by Cropsyst exhibited greater accuracy than the precision available in the observed datasets. Crop nitrogen was also predicted well (R2>0.5 and RMSE<σ). However, more work is necessary to improve simulated soil inorganic nitrogen.

Long-term simulations of the models, from 1980-2010, suggest opportunities exist for adoption of diversified and intensified rotations in both the annual crop and annual crop-fallow transition regions of the IPNW. In both Genesee and St. John, INC produced the greatest biomass and yield and displayed the greatest stability, based on its coefficient of variation (cv). Baling a cover crop for forage markedly increased ASP’s yield and presents an opportunity for producers to diversify. Overall, winter crops better utilized available water and reduced water loss. However, in St. John, the inclusion of fallow in the BAU rotation reduced its susceptibility to drought. Scenario testing suggested that altering management practices strategically can improve outcomes. For example, in St. John, terminating the previous cover crop earlier in the year prevented any detrimental effects on subsequent ASP winter wheat yields. Additionally, the inclusion of legume crops (winter pea, chickpea, or a nitrogen-fixing cover crop) reduced fertilization requirements.

CropSyst simulations emphasize relationships and trade-offs existing between cropping choices and production outcomes and contribute to a better overall understanding of these traditional and novel crop rotations. However, consideration of factors not captured by CropSyst, such as weeds, disease, and pests would prove valuable. Additional model simulations could include future climate to explore longer-term implications of changing environmental conditions.