Over three growing seasons (1989 to 1991) perennial ryegrass swards were cut at regular 3-week intervals or changed to a 2, 4, 5, 6, 7, 8 or 9-week regrowth interval in the early (April to mid-June), mid (late June to mid-August) or late season (late August to mid-October) at the Agricultural Research Institute of Northern Ireland. Fertilizer was applied according to the length of the regrowth period to supply the equivalent of 2·2 kg N/ha per day with an annual rate of 416 kg N/ha.

Lengthening the regrowth interval by a single 6-week interval increased the annual yield of herbage DM and digestible organic matter (DOM) from 10·13 to 11·15 and from 6·73 to 7·47 t/ha respectively. The effect increased with increasing length of regrowth period and was greatest in the early part of the season. In swards cut on a regular 3-week regime there was no significant difference in organic matter digestibility (OMD) throughout the season (mean 69·4%), whereas N concentration increased steadily throughout the season, ranging from 25·6 g/kg DM in April to 41·8 g/kg DM in October.

During the variable regrowth periods, the greatest response in DM and DOM yield to increasing length of regrowth interval was obtained in the early part of the season in spite of a rapid fall in OMD. There was little to be gained by lengthening the regrowth interval beyond 6 weeks in mid-season, due to low and declining herbage OMD. However, in the late season period, the rate of decline in herbage OMD tended to decrease after week 5 and so the rate of decline in DOM accumulation was reduced, making length of regrowth period in the late season less critical.

The treatments imposed had little effect on yield of herbage N (range 293–354 kg N/ha).