EFFECT OF SWARD SURFACE HEIGHT ON THE PERFORMANCE OF EWES AND LAMBS CONTINUOUSLY GRAZED ON GRASS CLOVER AND NITROGEN-FERTILIZED GRASS SWARDS

In three successive years, sward height was maintained at 3, 5, 7 or 9 cm on grass swards receiving a total of 300 kg N ha-1 in six equal monthly dressings from April, and on grass/clover swards receiving 50 kg N ha-1 as a single dressing in early spring. From turnout in April until weaning in July, 64 ewes and their lambs (mean litter size 1.5) were continuously grazed at the four sward surface heights on the two sward types. White clover content of grass/clover swards remained low throughout the experiment ranging from 0.2 to 7.4% of the herbage mass. During the first two years, lamb gains averaged over sward types were 204, 260, 285 and 308 g d-1 up to weaning, while in the third year gains were 238, 296, 296 and 260 g d-1 on 3, 5, 7 and 9 cm swards respectively. Ewes lost live weight on 3 cm swards but apart from this sward height had little effect on performance. During the autumn, weaned lambs gained -27, 87, 147 and 167 g d-1 on 3, 5, 7 and 9 cm swards respectively. Sward type had only a small effect on the performance of lambs up to weaning but in the autumn, mean gains of weaned lambs were lower on grass/N swards (73 g d-1) than on grass/clover swards (115 g d-1). Relative to 3 cm swards, carrying capacities of 5, 7 and 9 cm swards were 0.76, 0.57 and 0.52 respectively from turnout to weaning and 0.66, 0.52 and 0.44 respectively during autumn. Grass/clover swards carried 0.67 of the ewes carried by grass/N swards from turnout to weaning and 0.51 of the live weight carried by grass/N swards during autumn. The reaction of the two sward types to sward height did not appear to differ but in the third year there was evidence of a reduction in white clover content when swards were grazed at 9 cm. The data suggest that lamb growth rates will increase as sward height increases up to 9 cm and the evidence for this was stronger with weaned lambs in autumn than with suckling lambs in spring.