THE EFFECT OF SWARD HEIGHT AND ITS DIRECTION OF CHANGE ON THE HERBAGEINTAKE, DIET SELECTION AND PERFORMANCE OF WEANED LAMBS GRAZING RYEGRASS SWARDS

Four sward height treatments were imposed by continuous variable stock ing using at least ten Suffolk x Greyface lambs per plot from late Jul y 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 establi shed 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 o n the decreasing height treatments. Liveweight change of lambs was hig her (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 organ ic matter intake (OMI), measured on two occasions in the experiment, w as 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 gr eater than those for the decreasing sward height treatments. There was little difference in the organic matter digestibility of the diet bet ween treatments. Diets were composed largely of lamina, although there was more pseudostem and dead herbage in the diets of lambs grazing th e decreasing than the increasing sward height treatments at the end of the experiment. Bite mass was closely related to OMI but the treatmen t and period differences were relatively greater than for OMI. Bite ma ss was more closely related to the depth of the lamina layer (award he ight-pseudostem height) than it was to sward height. There was evidenc e that pseudostem acted as a barrier to defoliation on these short swa rds and also that the proportion of youngest leaf in the diet was posi tively related to sward height and to increases in sward height. Sward height and especially the direction of change in sward height, togeth er with associated stock density, were potent influences on lamb growt h rate. This was a consequence of differences in herbage intake, which was strongly influenced by bite mass.