An experiment was carried out from August to early November 1994 to examine differences in diet selection, herbage intake, grazing behaviour and animal performance between weaned lambs rotationally grazing swards of annual ryegrass (Lolium multiflorum)\white clover (Trifolium repens) and Yorkshire fog (Holcus lanatus)\T. repens with or without Lotus corniculatus. There were four replicate groups of six lambs per treatment. The effects of condensed tannins (CT) on lamb production were assessed

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... twice-daily oral administration of 10 g polyethylene glycol (PEG ; molecular weight 4000) to half the lambs on each sward. The Lotus content of all swards was very low, and results are presented here for main sward comparisons meaned over lotus treatments. Overall mean estimates of pre-grazing herbage mass and sward surface height for the annual ryegrass and Yorkshire fog swards respectively, were 5820 v. 4360p190 kg DM\ha (P 0n001) and 29 v. 21p0n6 cm (P 0n001). The coefficient of organic matter digestibility (OMD) of the diet selected and herbage intake were higher on Yorkshire fog than on annual ryegrass (0n78 v. 0n74p0n080 g\kg ; P 0n05, and 1070 v. 860p57 g OM per lamb per day, P 0n05 respectively), reflecting the higher content in the diet of grass green leaf (980 v. 930 g\kgp14 g\kg, P 0n05) and the lower content of dead material (80 v. 110p15 g\kg, P 0n08). Lambs grazing on Yorkshire fog swards had higher clean wool growth rate (1470 v. 1280p30 mg\cm per day, P 0n01) and greater fibre diameter (31 v. 29p0n2 µ, P 0n001), greater liveweight gain (152 v. 108p5n5 g\day, P 0n001), final weight (42 v. 38p0n5 kg, P 0n001), carcass weight gain (89 v. 69p2n5 g\day, P 0n001), carcass weight (19 v. 17p0n3 kg, P 0n001) and soft tissue thickness (GR value 11 v. 8p0n5 mm, P 0n01), and lower faecal egg counts (FEC ; square root transferred values 9n2 v. 11n0p0n4 eggs\g fresh faeces, P 0n01) than lambs grazing annual ryegrass swards. Similar dietary concentrations of condensed tannins (CT) between Yorkshire fog and annual ryegrass swards (4n2 v. 3n7 DMp0n2 g\kg, P 0n08) increased clean wool growth (1440 v. 1310p32 mg\cm# per day, P 0n05), fibre diameter (30n7 v. 29n5p0n21 µ, P 0n01) and liveweight gain (141 v. 120p4n3 g per lamb per day, P 0n01), although differences in carcass weight (17n9 v. 18n2p0n3 kg) and FEC transformed values (9n6 v. 11n0p0n6 eggs\g fresh faeces) were not significant. The effects of CT on animal performance were greater in Yorkshire fog swards. CT had no significant effects on diet selection, herbage intake and grazing behaviour patterns.