The definitive version is available at: http://dx.doi.org/10.1017/S0021859699007261Previamente, ha sido publicado en el congreso: ITEA, Vol. Extra 20 nº 2: 496-498 (1999).https:///digital.csic.es/handle/10261/16352Previamente, ha sido publicado en el congreso: ITEA, vol. Extra 20 Nº 2: 499-501 (1999). https://digital.csic.es/handle/10261/16373A study on the use of quebracho tannins as chemical additives was carried out at the Spanish Council for Scientific Research, Leon, Spain during 1998 by conducting two experiments. In the first experiment, three ruminally cannulated ewes were used to determine in situ degradability of soya bean meals (SBM) treated with different doses of quebracho tannin. Samples were prepared by spraying 100 g SBM with 100 ml distilled water containing 0, 1, 5, 10, 15 or 25 g of commercial quebracho powder (S-0, S-Q1, S-Q2, S-Q3, and S-Q5, respectively). Intestinal digestibility of non-degradable protein was estimated in vitro. The rapidly degradable protein fraction of all quebracho treated soya bean meals was different (P < 0.05) from the non-treated SBM (S-0: 0.154), with values ranging from 0.032 to 0.133. S-Q4 and S-Q5 showed lower fractional rates of degradation of the protein than S-0 (0.042 and 0.046 v. 0.082, respectively). By contrast, in vitro digestibility of the non-degradable protein was not significantly affected by the treatments imposed, with the exception of treatment with the highest dose of quebracho tannin in which intestinal digestion was reduced (0.939 v. 0.826 for S-0 and S-Q5, respectively. P < 0.05). In the second experiment, ten ruminally cannulated ewes were used to examine the effect of a daily dosing of quebracho tannin into the rumen on the degradation of the quebracho-treated SBM. Long-term dosing of quebracho tannin did not increase the ability of the rumen microbes to degrade tannin treated SBM. It was therefore concluded that quebracho tannins could be used as chemical additives for improving the digestive utilization of protein-rich feeds in sheep.This work was supported by the Inter-ministerial Commission of Science and Technology (CICYT) of Spain (Project AGF98-0874).Peer reviewed