A major problem with grass is that during ensilage the proteins are degraded and nitrogen lost as non protein nitrogen. In the animal a disadvantage of grass silage is that the proteinaceous nitrogen is readily degraded in the rumen to ammonia. There is considerable evidence in the literature indicating that ruminal undegradable protein is a desirable component of some feeds (AFRC, 1993). This is valuable since proteins that by-pass the rumen may be digested in the intestine and the resultant amino acids absorbed. Tannins have been identified by other workers as suitable for the protection of protein in the rumen. However, there is some dubiety regarding the subsequent efficient release of protein in the intestine (McNabb et al., 1993). The effect of quebracho or mimosa tannins or formaldehyde or a combination of tannin/formate or formaldehyde/formate on silage total tract disappearance of dry matter (DM), nitrogen and total amino acids (TAA) was examined in this study.