It has been found that the rate of herbage production from perennial grass swards, cut at a stage suitable for grazing, is lower in midsummer than in late spring. This occurs even with an abundant supply of water and fertilizer. The growth rate of green material was measured in swards differing in leaf area and light interception, Results did not suggest that higher growth rates could be obtained by attention to this relationship. Examination of the tiller population of grass swards showed that, in summer, herbage yields from cocksfoot and timothy were correlated with the average tiller size. This was not so in either of 2 varieties of perennial ryegrass. During May, June and July there were changes in the erectness of the swards as measured by the proportion of the crop which could be harvested with a mowing machine. This proportion was maximal when the highest rates of production of herbage were recorded. The importance is discussed of the change from a phase when mature tillers are forming inflorescences to one when herbage is derived from young, vegetative tillers formed in the same summer.