Comparative studies of herbage intake and performance of lambs grazing Yorkshire fog (Holcus lanatus) and perennial ryegrass (Lolium perenne) pastures in winter

Abstract

A comparative trial was carried out to assess the grazing behaviour, and performance of lambs grazing Yorkshire fog (Holcus lanatus cv. Massey Basyn)/white clover (Trifolium repens cv. Grasslands Tahora) and perennial ryegrass (Lolium perenne cv. Grasslands Nui)/white clover pastures and to evaluate the effects of low concentrations of condensed tannins (CT) in the grasses on body and wool growth under continuous stocking management in winter. Thirty‐six spring‐born female lambs of mean initial live weight 29 ± 2.3 kg were allocated to balanced groups in sets of six, with three replicates of the two pasture treatments. Individual paddocks were grazed continuously for 7 weeks from 30 May to 15 July. Half of the lambs were drenched with 10 g polyethylene glycol (PEG: molecular weight 4000) twice daily at 0830 and 1700 hrs and the remaining lambs were drenched with water as a control. Yorkshire fog swards had greater herbage mass and bulk density, lower proportions of live leaf and white clover, and a higher proportion of dead material than perennial ryegrass swards. Ryegrass had a consistently superior organic matter digestibility (80 vs 77 ± 0.3%, P < 0.05) and higher total CT content than Yorkshire fog (2.1 vs 1.8 + 0.07 g/kg DM, P < 0.05). Lambs had higher herbage digestible organic matter intake (888 vs 716 ± 24.8 g/day, P < 0.05) and carcass weight gain (89 vs 70 ±2.7 g/day, P < 0.05) on ryegrass than on Yorkshire fog. PEG administration did not significantly influence liveweight or carcass gain, but improved the rate of wool growth in lambs on ryegrass and decreased it in lambs on Yorkshire fog (Species x PEG interaction, P < 0.01). It is concluded that the better performance of lambs grazing the perennial ryegrass pastures reflected greater herbage intake and diet digestibility. Concentrations of CT in both pastures were too low to materially affect animal performance.