Variability in straw quantity and quality can have critical impacts on biomass industries. To generate better information on this variability for rice residues, trials with eight common California rice cultivars were planted at multiple sites for the 1999 and 2000 seasons. Straw yields averaged 11-2 Mg ha(-1) in 1999 and 8.5 Mg ha(-1) in 2000 with a consistent range of 2-3 Mg ha(-1) between the highest and lowest yielding varieties at each site. Straw-to-grain ratios were also higher in 1999 averaging 1.50 kg kg(-1) with high variability while in 2000 they were a more typical 1.04 kg kg(-1) with little difference by site or variety. The length of the pre-heading period was the strongest indicator for straw yield. Each one day increment in the length of the time to 50% heading resulted in an additional 8.4 kWh m(-2) of solar energy and 0.2 Mg ha(-1) of straw production at an efficiency slightly over 1%. Average stem weight ranged from 1.3 to 2.6 g and increased stem weight corresponded to higher yield but lower stand density. Harvested straw yield is also strongly affected by cutting height with a non-linear distribution resulting in nearly half of the straw biomass occurring in the lower third of the plant. Forty percent of biomass was in the internode sections of the stem; 53% in leaf and sheath, 4% in nodes and 3% in the panicle excluding hull and seed). Stem (culm) fraction decreases and leaf fraction increases from the base of the plant to the panicle. Since many properties vary by botanical fraction, height of cut influences both the yield and composition of the straw. The ability to predict the amount and composition of the biomass material allows for greater control in the design and mobilization of the harvesting system.