The aim of this study was to evaluate the effects of tannic acid (TA) and pistachio by-product tannin extract (PBE) on N utilization and efficiency of microbial nitrogen synthesis (EMNS) in Saanen dairy goats. The experimental design was a change over design with three treatments and three periods using six mid lactation multiparous dairy goats. Tannic acid solution and PBE were added to fresh alfalfa at ensiling to get the final concentration of about 1% tannin as dry matter (DM) basis. Dietary treatments were as follows: 1) untreated alfalfa silage (AS), 2) AS treated with TA (AS+TA) and 3) AS treated with PBE (AS+PBE). Intake of DM, organic matter (OM) and N and also milk yield and composition (as percent or as g/d) were not affected by tannin additions. A considerable shift occurred in the pattern of N excretion in goats fed PBE diet compared to control silage diet. Urinary N losses had a tendency (P<0.10) to lower (17.4 vs. 19.2 g/d) and fecal N losses was higher for goats fed AS + PBE compared with those fed the AS silage diet (14.6 vs. 12.6 g/d). As a proportion of N intake or as g N loss per d, retained N was higher (P<0.05) in goats fed AS + PBE compared to those fed the control diet. Microbial nitrogen synthesis (MN) was not affected by tannin additions. However, a trend (P<0.10) for less efficiency of microbial nitrogen synthesis (EMN) was observed in tannin fed goats compared to those fed the control diet. Although, tannins added to AS may exert some beneficial effects on N utilization and also environmental N outputs, they may interfere with rumen fermentation lead to decrease in EMN in goats especially those fed with PBE.

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