Two experiments were conducted to evaluate the crude protein fractionation, in vitro gas production and the biological effects of tannins in alfalfa silage treated with different levels of pistachio by-products (PB) extract. PB was soaked in water with a ratio of 1:5 (w/v) for 12 h. After filtering and concentrating, the crude extract was added to fresh alfalfa to a final concentration of 1%, 2% and 3% tannin dry matter (DM) and then ensiled for 60 days. Four treatments were as follows: alfalfa silage without addition of PB extract as a control (T0), PB tannin added at 1% (T1), 2% (T2) and 3% DM (T3). Treating alfalfa silage decreased the formation of non-protein nitrogen (NPN) compounds and increased crude protein fraction B2 during ensilage. Cumulative in vitro gas production after 96 h incubation, organic matter digestibility (OMD) and metabolizable energy (ME) decreased in T2 and T3 compared to the control. The gas production and short chain fatty acids (SCFA) of treated silages increased in the presence of polyethylene glycol (PEG) which indicates that the presence of phenolic compounds and tannins depressed the gas production. It can be concluded that treating alfalfa silage with PB extract can reduce the nitrogen losses during ensilage and degradability of organic matter (OM) by microbial inhibition.

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