EFFECT OF CONTINUOUS STOCKING BY SHEEP AT 4 SWARD HEIGHTS ON HERBAGE MASS, HERBAGE QUALITY AND TISSUE TURNOVER ON GRASS CLOVER AND NITROGEN-FERTILIZED GRASS SWARDS

The effects of continuous stocking by sheep at sward surface heights ( SSH) of 3, 5, 7 and 9 cm in grass/clover (GC) and nitrogen-fertilized grass (GN) swards were examined in relation to herbage mass and qualit y, clover content, tiller density and rates of herbage production and senescence in two periods in each of three grazing seasons (1987-89). The GN swards received a total of 300 kg N ha-1 each year in six equal dressings from March; GC swards received a single dressing of 50 kg N ha-1 in March each year. Herbage mass measured from ground level incr eased linearly with SSH with overall mean herbage masses of 0.89, 1.38 , 1.78 and 2.12 t OM ha-1 (s.e.m. 0.024, P < 0.001) at SSH of 3, 5, 7 and 9 cm respectively. GN and GC swards had mean herbage masses of 1.5 8 and 1.51 t OM ha-1 (s.e.m. 0.051, NS) respectively. Mean N content o f herbage on GN swards was greater than that on GC swards and declined with increasing SSH. Crude fibre (CF) content of herbage was similar for both sward types and increased with increasing SSH. Clover content of GC swards remained low throughout the experiment, ranging from 0.0 02 to 0.074 of herbage mass. However, from tissue turnover rates it wa s estimated that its contribution to herbage production was in the ran ge of 0.049-0.219 of net herbage growth. Total growth increased with i ncreasing SSH in both sward types, with maximum growth rates in GN swa rds of 143 and 130 kg DM ha-1 d-1 and in GC swards of 88.2 and 85.4 kg DM ha-1 d-1 in Periods 1 (up to early July) and 2 (after July) respec tively. Senescence rates ranged between 13.3 and 50.1 kg DM ha-1 d-1 a nd tended to be higher in Period 2 than in Period 1. Net production in creased with increasing SSH in Period 1, while in Period 2 net product ion declined at SSH above 6.5 cm. The increased net herbage production in taller swards was not associated with greater utilized metabolizab le energy production at sward heights above 5 cm.