The effect of sward height and its direction of change on the herbage intake, diet selection and performance of weaned lambs grazing ryegrass swards

Four sward height treatments were imposed by continuous variable stocking using at least ten Suffolk x Greyface lambs per plot from late July to late August: constant 3.5 cm, constant 6.0 cm, 3.5 cm increasing to 6.0 cm and 6.0 cm decreasing to 3.5 cm. The treatments were established on two swards given fertilizer N applications over the season of 97 and 160 kg N ha(-1) respectively. Animal density was greater on the high fertilizer treatment, at the lower sward height and especially on the decreasing height treatments. Liveweight change of lambs was higher (P < 0.001) on the 6-cm than on the 3.5-cm treatments (+ 59 vs-13 g d(-1) and was also higher (P < 0.001) on the increasing than on the decreasing sward height treatments (+ 92 vs-26 g d(-1)). Herbage organic matter intake (OMI), measured on two occasions in the experiment, was greater (P < 0.001) on the 6.0 cm than on the 3.5-cm sward heights whereas values for the increasing sward height treatments were much greater than those for the decreasing sward height treatments. There was little difference in the organic matter digestibility of the diet between treatments. Diets were composed largely of lamina, although there was more pseudostem and dead herbage in the diets of lambs grazing the decreasing than the increasing sward height treatments at the end of the experiment. Bite mass was closely related to OMI but the treatment and period differences were relatively greater than for OMI. Bite mass was more closely related to the depth of the lamina layer (award height-pseudostem height) than it was to sward height. There was evidence that pseudostem acted as a barrier to defoliation on these short swards and also that the proportion of youngest leaf in the diet was positively related to sward height and to increases in sward height. Sward height and especially the direction of change in sward height, together with associated stock density, were potent influences on lamb growth rate. This was a consequence of differences in herbage intake, which was strongly influenced by bite mass.