Effect of heading date of perennial ryegrass cultivars on tillering and tiller development in spring and summer

Three cultivars (two diploid and one tetraploid) in each of three maturity groups (early, intermediate and late) of perennial ryegrass were sown in 10 m(2) plots, replicated four times, in Northern Ireland in June 1997 in a study of the effect of heading date on tiller development (including initiation to flower) and turnover of tillers produced at specific times in spring in 1998 and 1999. The plots were harvested seven times in each year. Annual dry-matter production was similar for all groups in each year. In spring and early summer of both years, tiller density of the diploid cultivars was 1.5 times greater than that of the tetraploid cultivars and the mean tiller density over all swards in June was about 0.40 times greater than that in April. Maximum proportions of reproductive tillers in the early, intermediate and late maturity groups, determined from apical dissections, were found in early April, mid-May and early June, respectively. Although a high proportion of tillers, which were present when annual observations commenced in spring, was decapitated at the first harvest in the early group, the previous population density was maintained by rapid production of new tillers during May, including those from suppressed tiller buds during reproduction. It is concluded that the relationship between heading date and rate of tiller turnover (including flowering) at specified times in spring is important in sward management throughout the early part of the growing season and should be taken into account in tiller-based grass growth models.