Effect of continuous stocking by sheep at four 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 quality, 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 increased 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 1·58 and 1·51 t OM ha^?1 (s.e.m. 0·051, NS) respectively. Mean N content of 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 0·002 to 0·074 of herbage mass. However, from tissue turnover rates it was estimated that its contribution to herbage production was in the range of 0·049–0·219 of net herbage growth. Total growth increased with increasing SSH in both sward types, with maximum growth rates in GN swards 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) respectively. Senescence rates ranged between 13·3 and 50·1 kg DM ha^?1 d^?1 and tended to be higher in Period 2 than in Period 1. Net production increased with increasing SSH in Period 1, while in Period 2 net production declined at SSH above 6·5 cm. The increased net herbage production in taller swards was not associated with greater utilized metabolizable energy production at sward heights above 5 cm.