EFFECT OF WEATHER AND RICE MOISTURE AT HARVEST ON MILLING QUALITY OF CALIFORNIA MEDIUM-GRAIN RICE

Abstract

Californias medium-grain rice industry experiences a wide range of head rice yield (HRY). Average moisture ofa representative paddy rice sample is the commercially used predictor of optimum harvest date to achieve high HRY. Atwo-year field study demonstrated that average rice moisture alone is not an adequate predictor of HRY. The history of ricemoisture caused by varying meteorological conditions was needed to predict rice quality. Under typical calm conditions inCalifornia, daytime relative humidity is low and at night humidity increases, exposing rice to dew. During this meteorologicalpattern, HRY could be predicted by assuming that all kernels that dried below 15% moisture during the day would rehydrateat 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 droppedbelow 15% moisture, and actual HRY was much above predicted HRY. HRY dropped significantly during periods of dry northwinds; however, rice value (government loan value minus drying costs) did not drop significantly during the windy periodbecause the lower loan value was offset by lower drying costs. After the windy period ended, rice was again subject tonighttime dew and regained moisture, resulting in a large reduction in HRY and value. A combination of the range of individualkernel moisture at harvest and history of rice moisture influenced by weather conditions explained a great deal of the totalHRY variation experienced by the California rice industry.