There is increasing interest in the development of later lambing systems for crossbred ewes which more closely match ewe nutritional requirements with grass growth (Stone, 1988). Lambing a proportion of the flock during summer potentially ensures a better continuity of lamb supply but the superior growth rate of summer born lambs dictates that finishing is not delayed to as great an extent as would be predicted from lambing date. The objective of the present study was to determine whether it is possible to alter lamb growth by controlling herbage availability without detriment to ewe body condition.

The liveweight change of sixty four Mule ewes and their summer (24th May) born twin lambs was evaluated when grazing unfertilised clover rich swards maintained at a sward surface height (SSH) of 4-6cm (Treatment A) or rising from 4-6 to 6-8cm (Treatment B). Grazing treatments were duplicated with equal numbers of animals per treatment and commenced on 19/6/90 (day 0). Throughout the grazing period SSH was monitored twice weekly using a sward stick and was maintained within the target ranges by movable electric fencing. Samples for determination of herbage mass and botanical composition were obtained at monthly intervals. Lambs were weighed fortnightly, ewes were weighed and condition scored monthly until weaning on 28/8/90 (day 70).