This experiment was conducted to evaluate the effects of processing protein meals by tannin extracted from pistachio by-product on apparent total tract digestibility, ruminal fermentation and performance of dairy cows. For tannin extraction, one kilogram of sun-dried pistachio by-products was immersed in four liters of water for 48 h. Then, pistachio by-products were removed from the water and the extract obtained was sprayed on protein supplements (canola meal and soybean meal) with an equal ratio of 1:1 (L/kg) and was dried in the shade. Eight nulliparous Holstein dairy cows with 27 ± 49days in milk(DIM) and milk production of 38 ± 4 kg/d were used in the form of a replicated Latin square (4×4) design. The experiment was performed in four 21-day periods consisting of 14 days for adaptation and 7 days for recording. The experimental treatments were as follows: 1) ration based on soybean meal, 2) ration based on soybean meal processed with the extracted tannin by water-extracted tannin from pistachio by-product, 3) ration based on canola meal and 4) ration based on canola meal processed with water-extracted tannin from pistachio by-product. Spraying extracted tannin on protein supplement increase concentration by 370 and 246% in soybean meal and canola meal, respectively. Intake of dry matter (DM) and total tract apparent digestibility were not affected by treatments. Although daily milk production (kg/d) and milk protein percentage were not affected by treatments, cows fed tannins produced more milk and milk protein. Tannins decreased rumen ammonia concentration only in the canola diets. Soybean treatment had the highest and processed canola treatment with tannin had the lowest rates of ammonia nitrogen in the rumen. Treated protein supplements with tannins decreased ruminal pH, soybean treatments showed the highest and processed canola treatment had the lowest pH rates with tannin. Although blood urea nitrogen was not affected by the treatments, processing the rations with tannin reduced blood urea nitrogen linearly however; other blood metabolites were not affected by treatments. Results of this study show that under our experimental conditions, processing protein supplements with tannin had no negative effect on animals, Further experiments should be done in this regard and on the rations containing higher levels of tannin. تفاصيل المقالة