The optimal control theory is applied to the mathematical model of grass growth developed by Johnson & Thornley (1983) to work out an optimal cutting frequency and cutting intervals which maximize total amount of harvested yield over the whole growing season in the year. It reveals that, under the typical weather conditions at Hurley with water and nutrient supply non-limiting, four cuts, carried out on 25 May, 10 July, 23 August and 23 October, respectively, give maximum total harvested dry matter of ryegrass. The physical meaning of the optimal cutting schedules is explained.