EFFECT OF CONDENSED TANNINS UPON THE PERFORMANCE OF LAMBS GRAZING LOTUS-CORNICULATUS AND LUCERNE (MEDICAGO-SATIVA)

A grazing experiment, conducted for 22 weeks in 1992/93 at Aorangi Res earch Station, AgResearch Grasslands, Manawatu, New Zealand, compared the productivity of weaned lambs grazing Lotus corniculatus (birdsfoot trefoil) and lucerne (Medicago sativa). Effects of condensed tannins (CT) in lotus were evaluated by studying the responses of lambs to twi ce daily oral supplementation with polyethylene glycol (PEG). A rotati onal grazing system with restricted feed allowance was used. Measureme nts were made of pre- and post-grazing herbage mass, the composition o f the feed on offer and diet selected, voluntary feed intake (VFI), li veweight gain (LWG), carcass growth, wool growth and the concentration of metabolites in rumen fluid. For both lotus and lucerne swards, the diet selected was mainly leaf. Lotus contained 34 g total CT/kg dry m atter in the diet selected, whilst there were essentially no CT in luc erne. Compared to lambs grazing lucerne, lambs grazing lotus had sligh tly lower VFI, and higher LWG, carcass weight gain, carcass dressing-o ut percentage and wool growth. PEG supplementation had no effect on th ese measurements or upon the composition of rumen fluid in lambs grazi ng lucerne. However, in lambs grazing lotus, PEG supplementation reduc ed wool growth (10.9 v. 12.1 g/day), slightly reduced LWG(188 v. 203 g /day), increased rumen ammonia concentration, and increased the molar proportions of iso-butyric, isovaleric and n-valeric acids and protozo a numbers in rumen fluid. PEG supplementation did not affect carcass g ain, carcass fatness or the molar proportion of rumen acetic, propioni c or n-butyric acids in lambs grazing lotus. It was concluded that the principal effect of CT in growing lambs grazing lotus was to increase wool growth without affecting VFI, thereby increasing the efficiency of wool production, that the greater rate of carcass gain of lambs gra zing lotus than those grazing lucerne was mainly caused by factors oth er than CT and that CT did not affect the rumen fermentation of carboh ydrate to major volatile fatty acids.