The objective of this study was to investigate the effects of pistachio by-products (PBP) in replacement of alfalfa hay (AH) on ruminal fermentation, blood metabolites, and milk fatty acid composition in Saanen dairy goats fed a diet containing fish oil. For this purpose, 15 multiparous lactating Saanen goats (on day 25 postpartum, 38.159 1.2 kg) were randomly assigned to three treatment diets in a completely randomized design with five goats per treatment. The treatment consisted of feeding

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... following diets: (1) control (AH), (2) 30% PBP, and (3) 30% PBP 'polyethylene glycol (PEG, 1 g/Kg of dry matter [DM]). The experiment lasted 21 days, including 16 days of adaptation to the experimental diets followed by a five-day sampling period for determining milk yield and composition, dry matter intake (DMI), and blood metabolites. The results showed that DMI was affected (PB 0.05) by using PBP in the diets. The goats fed PBP had a lower (PB0.05) DMI than those in other treatments. No differences were observed in milk yield and composition among the dietary treatments. The goats fed PBP (with and without PEG) had higher (P B0.05) amounts of C16:0 and trans-C18:1 isomer in their milk fat than those fed AH. C24:0 was detected at higher (P B0.05) concentrations in goats fed AH than those in other treatments. Overall, different treatments had no significant effects on the concentrations of saturated, monounsaturated, and polyunsaturated fatty acids (PUFA) in milk fat. Trans fatty acids (TFA) in milk fat exhibited higher (PB0.05) concentrations in PBP and PBP-PEG treatments than in AH. However, no differences were detected in the concentrations of short, medium, and long chain fatty acids among the treatments. No differences were observed in rumen pH among the treatments, while rumen ammonia-N concentrations were lower (P B0.01) in goats fed PBP than those in the other treatments. Treatments showed no differences with regard to blood metabolites (i.e., cholesterol, triglyceride, blood urea nitrogen, total protein, albumin, and glucose). These findings indicate that the inclusion of PBP in replacement of AH in the diet of dairy Saanen goats have no adverse effects on ruminal fermentation and blood metabolites. Moreover, PBP is capable of modifying the fatty acid profile of milk in dairy goat.