Studies of the effects of "Mimosa bark extract" containing condensed tannins on milk production by grazing dairy cows and on ruminal protein metabolism in sheep. M.Agr.Sc. Thesis, Massey University, Palmerston North, New Zealand. Tannins, particularly condensed tannins (CT), either added to the diet or occuring naturally in the forage are advantageous because they protect dietary protein from degradation in the rumen. The aim of this study was to measure the effect of Mimosa bark extract which contained approximately 70% CT on grazing dairy cow performance and on ruminal protein metabolism in sheep. Two experiments were carried out over the spring season (September and October 1994). In experiment I, effects of CT in Mimosa bark extract upon blood urea concentration, milk yield and milk composition, liveweight and condition score of grazing dairy cows were evaluated. Thirty Friesian cows were allocated at random to 3 treatments :(1) Control : no Mimosa bark extract (no CT); (2) Low CT : 50 g/cow daily of Mimosa bark extract (2.4 g CT/ kg DM eaten); (3) high CT : 100 g/cow daily of Mimosa bark extract (4.8 g CT/kg DM eaten). Mimosa bark extract was given twice daily as a suspension by oral drenching during each milking. In experiment II, effects of CT in Mimosa bark extract on ruminal protein metabolism in sheep were evaluated. Six mature Romney sheep fitted with permanent ruminal cannulae were randomly assigned into 2 treatments in a cross-over design. The two treatments were (1) Control : no Mimosa bark extract (no CT); (2) High CT : 6.66 g/sheep daily of Mimosa bark extract (4.8 g CT/kg DM eaten). Mimosa bark extract was given twice daily as a suspension by oral drenching just after feeding. Dry matter intake, rates of DM disappearance by the in sacco method, rumen ammonia and blood urea concentration and apparent digestibility of dry matter and and nitrogen were measured. In experiment I, liveweight and condition score as well as milk yield and composition, were not influenced by CT. Lactose concentrations were higher in the low CT group than in the high CT group in all weeks of the experiment. Cows drenched with high CT had a lower (P<0.05) blood urea concentration than cows in the control group, and, in week I they were lower (P<0.05) than cows in the low CT group. In experiment II, rumen metabolism parameters, including dry matter intake, in sacco DM disappearance parameters (A, B, C and A+B) and apparent digestibility of DM and N were not influenced by Mimosa bark extract. However sheep drenched with high CT had lower rumen ammonia and blood urea concentrations (P<0.05) than the control in the whole period. These results indicate that Mimosa bark extract had no significant effect on milk production. However it did consistently and significantly reduce blood urea concentration in both cows (high CT group) and sheep and it reduced rumen ammonia concentration in sheep. This indicates that the CT did have some biological effect in the rumen namely, a reduced protein degradation in the rumen.