A rising plate meter (RPM), which enables rapid, non-destructive estimation of herbage mass of pastures, is useful in grazing management. Herbage mass is estimated from the RPM readings using a linear regression model, which has the parameters a (constant) and beta (slope). In this study, we analyzed the seasonal changes in the two parameter values for total and green herbage (TH and GH, respectively) masses in Zoysia-dominated pastures, and examined the potential for incorporating a periodic function (PF) of the parameters into the equation for estimating herbage mass. The values of both a and beta showed a weak seasonal periodicity for TH. No periodicity and a clear seasonal periodicity were detected in the a and beta values for GH, respectively. Compared with the linear regression models developed separately for individual dates, the equation incorporating a PF for beta improved the estimation accuracy for the GH mass (from 0.64 to 0.77 as R2), though it did not affect the predictability for the TH mass. Incorporation of a PF into the model relating herbage mass to the RPM reading increases the sample number and broadens the data range by allowing the use of all seasonal data, and produces a single equation useful for estimating herbage mass across seasons.