The liveweight gain of Limousin X Friesian heifers grazing perennial ryegrass / white clover swards of different clover content and the effects of their grazing on sward botanical composition

An experiment was carried out in 1992 and 1993 to examine the effect of white clover content of perennial ryegrass/white clover swards on the performance of Limousin x Friesian heifers. Swards with low (L), medium (M) and high (H) white clover contents were established and managed by continuous variable stocking. A compressed sward height of 5.5 cm was maintained using a buffer fence to vary plot areas, with herbage surplus to grazing requirements cut, removed and yields measured. The mean white clover proportions for treatments L, M and H were 0.02, 0.19 and 0.18 in 1992 and 0.13, 0.16 and 0.31 in 1993 respectively. White clover contents of the swards reached a maximum in August and September, and differences between treatments diminished. There was no significant difference between treatments in the content of white clover in the swards in autumn 1993. Liveweight gains of heifers increased asymptotically with increasing white clover content of the sward. Below a white clover herbage mass of 300 kg DM ha(-1), there was little effect on liveweight gain, which was 0.70 kg day(-1) over the grazing season. Between 400 and 450 kg DM ha(-1) white clover, liveweight gains were 0.85-0.90 kg day(-1). While clover content of the sward did not significantly affect utilized metabolizable energy output, the mean output over the grazing season in the two years front liveweight gain and herbage yield was 78 GJ ha(-1). It is suggested that, using this grazing system, white clover reached an equilibrium with a mean herbage mass of about 400 kg DM ha(-1) over the grazing season.