EFFECT OF SWARD SURFACE HEIGHT ON HERBAGE INTAKE AND PERFORMANCE OF LACTATING EWES LAMBED IN WINTER AND CONTINUOUSLY STOCKED ON PASTURE

Thirty-six 4-year-old Border Leicester x Romney (BR), Poll Dorset x BR (PBR) and Suffolk x BR (SBR) ewes balanced for litter size were assigned, 6 days after the mid-point of lambing (29 June 1989), to one of three sward surface height (SSH) treatments replicated twice (six ewes/1 ha paddock). The mean SSH during the 54 days of the trial were 2.6, 4-4 and 7-8 cm. Herbage organic matter intake (OMI) was measured using intraruminal chromium controlled release capsules during weeks 3, 4, 7 and 8 of lactation. Ewe OMI was not influenced by SSH during week 3 of lactation. During weeks 4 and 7 of lactation, ewe OMI increased with SSH up to a SSH of 7-8 cm. In week 8 of lactation there was no significant difference in OMI between ewes grazing the 4.4 or 7.8 cm SSH treatments, suggesting maximum intake at this stage of lactation is reached at 4.4 cm SSH. A maximum OMI of 2.6 +/- 0-1 and 3.0 +/- 0.1 kg OM/ewe per day (P < 0.05) was reached in week 4 of lactation on the 4.4 and 7.8 cm SSH treatments, respectively. Ewes grazing the 2-6 cm swards had a relatively constant OMI throughout the experiment (range 1.8 +/- 0.1 to 2-0 +/- 0.1 kg OM/ewe per day). However, these ewes lost 10.5 kg of liveweight while those on the 4.4 and 7.8 cm SSH treatments maintained a constant liveweight. Sward surface height had no effect on ewe midside wool growth rate or mean fibre diameter (measured at day 60 of lactation) or on lamb liveweight gain. Ewe breed cross had no effect on ewe OMI or midside wool growth. Lambs reared by SBR ewes were heavier (20.4 +/- 0.5 kg) at day 60 of lactation than lambs reared by PBR (18.7 +/- 0.6 kg) or BR (18.4 +/- 0.6 kg) ewes (P < 0-05). These results suggest that for Winter(June)-lambing ewes in good condition a SSH of 4.0 cm under continuous stocking should ensure near to maximum ewe and lamb performance. However, ewes in good body condition at lambing stocked on a SSH down to 3 cm will rear lambs to acceptable weaning weights but at the expense of ewe liveweight loss and possible long-term carry-over effects on future production if compensatory weight gains are not achieved during the spring pasture growth period.