Correlation of Degree-Days with Annual Herbage Yields and Livestock Gains

On California's winter annual rangelands precipitation controls the beginning and end of the growing season while temperature largely controls seasonal growth rates 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 annual type range in northern California. Degree day values were determined by summing daily degree-days from the beginning of the growing season after germinating rainfall until the clipping or weigh dates. Accumulated degree-days accounted for 74 to 91% of the variation in seasonal herbage yield while accumulated days (AD) accounted for 64 to 86% of the variation. Together, ADD and AD accounted for 94 and 86%, 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 seasonal herbage yield profile based on key growth curve inflection points and using simple field observations with 3 clipping dates and ADD is proposed.