The practical application of field or natural leaching to rice straw was evaluated with the goal of improving biomass fuel value. Observations on three rice farms in the Sacramento Valley, California indicated that potassium, chlorine and total ash are leached from rice straw by rainfall regardless of rice variety, grain harvest method, straw arrangement, or stubble length. Leaching of sulfur by natural precipitation was not clearly established. In selected field plots leached straw was successfully collected in spring, even though biomass yields were variable (2.2-3.4 Mg ha(-1)) and equipment had to operate in difficult conditions. Total costs for collecting leached straw on an area basis ($77.07 ha(-1)) are 31% higher compared to collecting crude straw in the fall ($58.67 ha(-1)), due to reduced performance of machinery and addition of field curing operations. Analysis of historical rainfall data for the Sacramento Valley revealed that there is an 85% probability of receiving sufficient rainfall (250 mm, or more) for substantial natural leaching of straw during the winter period. The available period for mechanized collection of rice straw after the winter period ranges from 0 to 45 days, depending on drying time needed to accomplish favorable field conditions, and planting date of the next crop. The feasibility of spring collection of rice straw could be improved if straw collection equipment were better equipped to operate under wet field conditions. The commercial implementation of natural leaching of rice straw as a strategy to improve fuel quality depends on a combination of factors that include grain harvest and straw collection practices, rainfall intensity and distribution, and field-specific factors. (C) 2003 Elsevier Ltd. All rights reserved.