On Caiilomia's winter a~ud r8ngdanda precipitation controls the beginning and end of the growing season while temperature Lirgely controls se8sonai growth tmtes within the growing season. Post-germination accumulated degree-days (ADD) account for the length of the growing season and variation of daily temperature. Simple correlations of ADD and herbage yield or resultant livestock gains were determined at 5 locations in ammal type range in northern California. Degree day values were determined

more »

... y summing daily degree-days from the beginning of the growing season after germinating rainfail until the ciipphrg or weigh dates. Accumulated degree-days accounted for 74 to 91% of the variation in seasonal herbage yield while accumuiated days (AD) accounted for 64 to 86% of the variation. Together, ADD and AD accounted for 94 and 8696, respectively, of the variation in stocker cattle weights. Regression coefficients relating ADD to herbage yield appear to predict maximum site productivity. A procedure for estimating a seasonai herbage yield profile based on key growth curve intlection points and using shnple field observations with 3 clipping dates and ADD is proposed. Year-to-year variation in range herbage yield has frequently been attributed to variations in precipitation (Sneva and Hyder 1962). However, Duncan and Woodmansee (1978) were unable to show a relationship between herbage yield and precipitation on California annual rangeland. Pitt and Heady (1978) identified 5 annual range weather variables that explained 73% of the variation in March standing crop. Three of these variables were temperature related. Another set of 5 variables explained 90% of the variation in June standing crop. Two of these were temperature variables. The annual range growing season can be partitioned into fail, winter, and spring periods, Fall precipitation and cooling winter temperatures determine the length of the fail growing season. The duration of slow winter growth is variable depending on the beginning and ending dates of the cold season. The length of the rapid spring growth period is also variable depending on the date that warm spring temperatures begin and the date spring soil moisture becomes depleted. Thus, precipitation controls the beginning and end of the whole growing season while temperature controls the end of the fail and beginning of the spring growing season. The winter annual growth habit of California's annual rangeland appears to be an ideal system for explaining forage productivity based on accumulated degreedays (ADD), which integrates season length and temperature. Equations derived from degreedays to estimate or predict phenological stage or growth rate have been criticized (Wang 1960) but Bauer et al. (1984 concluded that Senior, sccahd, and third authors arc extensions 'alist professor, and extension Tc ialiat, Agronomy and Range Sci. Dep., Univ. o p". '. Cabforma, Davis 95616. Fourth, Ifth, and sixth authors arc farm advisors, Yuba, Butte, and Madcra Counties, Univ. of Calif. Coo . Ext., Berkeley 94720. Seventh author is director, San Joaqum Exp. Range, Call omia State Univ. Fresno 93710. Eighth author is supcnntendent ? (retired), Univ. of Caliiomia Hopland Field Sta., Hopland 95449. The authors wish to thank Drs. W. Williams and J. Menke, professors, Agronomy and Range Sci. De fp- Univ. of California, Davis, for reviewmg this manuscript and providing thought ul suggestions. Acknowledgement also goes to the many research assistants and other employees who dependably collected data at various locations for many years.