EFFECT OF ALTERING THE LENGTH OF THE REGROWTH INTERVAL IN EARLY, MID AND LATE-SEASON ON THE PRODUCTIVITY OF GRASS SWARDS

Over three growing seasons (1989 to 1991) perennial ryegrass swards we re 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 a pplied according to the length of the regrowth period to supply the eq uivalent of 2 2 kg N/ha per day with an annual rate of 416 kg N/ha. Le ngthening the regrowth interval by a single 6-week interval increased the annual yield of herbage DM and digestible organic matter (DOM) fro m 10.13 to 11.15 and from 6.73 to 7.47 t/ha respectively. The effect i ncreased with increasing length of regrowth period and was greatest in the early part of the season. In swards cut on a regular 3-week regim e there was no significant difference in organic matter digestibility (OMD) throughout the season (mean 69.4%), whereas N concentration incr eased steadily throughout the season, ranging from 25.6 g/kg DM in Apr il to 41.8 g/kg DM in October. During the variable regrowth periods, t he greatest response in DM and DOM yield to increasing length of regro wth 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 declini ng herbage OMD. However, in the late season period, the rate of declin e in herbage OMD tended to decrease after week 5 and so the rate of de cline in DOM accumulation was reduced, making length of regrowth perio d in the late season less critical. The treatments imposed had little effect on yield of herbage N (range 293-354 kg N/ha).