Milling Quality of California Medium-grain Rice

California’s medium-grain rice industry experiences a wide range of head rice yield (HRY). Average moisture of a representative paddy rice sample is the commercially used predictor of optimum harvest date to achieve high HRY. A two-year field study demonstrated that average rice moisture alone is not an adequate predictor of HRY. The history of rice moisture caused by varying meteorological conditions was needed to predict rice quality. Under typical calm conditions in California, daytime relative humidity is low and at night humidity increases, exposing rice to dew. During this meteorological pattern, HRY could be predicted by assuming that all kernels that dried below 15% moisture during the day would rehydrate at night and fissure, resulting in lost HRY. Harvest weather is also characterized by occasional episodes of dry north wind, lasting several days. These periods have insufficiently long rehydration periods to completely fissure kernels that dropped below 15% moisture, and actual HRY was much above predicted HRY. HRY dropped significantly during periods of dry north winds; however, rice value (government loan value minus drying costs) did not drop significantly during the windy period because the lower loan value was offset by lower drying costs. After the windy period ended, rice was again subject to nighttime dew and regained moisture, resulting in a large reduction in HRY and value. A combination of the range of individual kernel moisture at harvest and history of rice moisture influenced by weather conditions explained a great deal of the total HRY variation experienced by the California rice industry.