AbstractA study was made to characterize the effect of the duration of regrowth on the pattern of changes in the major physiological processes involved in the net accumulation of herbage, and so to provide a rational basis for optimizing production under rotational grazing.During regrowth following a severe defoliation, rates of canopy photosynthesis, and so the rate of production of new leaves, increased rapidly but there was a delay before there was a corresponding increase in the rate of leaf death. Although the amount available for harvest (crop live dry weight,W) continued to increase as the duration of regrowth was extended from ‘short’ (12–13 days), through ‘medium’ (19–23 days) to ‘long’ (30‐34 days), there was not a continued increase in the average growth rate – the increase in the weight of the crop, (W – Wo), divided by the duration of regrowth,t, In this study, the average growth rate (based on changes in the weight of lamina alone) increased as the duration of regrowth was extended from 12‐13 to 19‐23 days but changed little as the duration of regrowth was extended from 19‐23 days to 30‐34 days. In spring and summer, elongating stems increased the average growth rate (of lamina plus stem) up to 30‐34 days but the accumulated stem material could not reliably be harvested by sheep.A mathematical model was used to explain why the average growth rate is characteristically insensitive to the duration of regrowth beyond a given minimum duration. For practical purposes, we suggest from the results of this study that regrowths of at least 14 days but less than 28 days will be effective in achieving not only close to the maximum average growth rate of highly digestible material, but also in sustaining a densely tillered, leafy sward which regrows rapidly from severe defoliation and is more reliabl