AbstractThe results or estimations ofin vitrodigestibility of three standard herbage samples, analysed weekly over about 48 weeks per year, have been examined using time‐series methods of Box and Jenkins to separate seasonal and non‐seasonal trends. The study includes data from March 1971 to February 1981. The dominant feature of variability is the seasonal component with a cycle of 52 weeks; no other significant cycles of longer periods were detected. The model of Box and Jenkins (1976) was found to be the most appropriate in describing the three series. A decrease in thein vitrodigestibility of the low standard, which is 10 years old, occurred over the last year or so, although the reasons for this apparent storage effect are unkn