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
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).