Aims: To determine the chemical composition and estimation of nutritive value of different by-product feedstuffs (BPF) using in vitro gas production technique.Place and Duration of Study: Department of Animal Science, between February 2013 and June 2013. Methodology: In an anaerobic batch culture system, 50 ml of buffered rumen fluid was dispensed into a 125-ml serum bottle containing 0.2 g dry matter (DM) of the experimental treatments. Experimental treatments included five by-products (pomegranate peel and seed, apple pomace, walnut hull, almond hull). All bottles were purged with anaerobic CO2, sealed and placed in a shaking water bath for 96 h at 38.6ºC. Gas production of each bottle was recorded at 3, 6, 9, 12, 24, 36, 48, 72 and 96h of the incubation and then gas released. The batch cultures were repeated in three incubation runs. The biomass residues were centrifuged and the pellet was dried at 65ºC for the determination of the residual DM and in vitro DM disappearance (IVDMD). Results: The total tannins and phenol content were higher (P<0.01) in almond hull and pomegranate peel than in the other BPF. The total tannins content ranged from 0.34% in apple pomace to 9.78% in almond hull. The total phenol of pomegranate peel, pomegranate seed, apple pomace, walnut hull and almond hull were 10.9, 1.20, 0.76, 3.80, and 10.6%, respectively. The rate (c) and cumulative gas volume (b) was significantly higher (P<0.01) for apple pomace than the other feedstuffs. There were significant differences (P<0.01) among feedstuffs about lag time. Apple pomace showed higher (P<0.01) organic matter digestibility (OMD), in vitro dry matter digestibility (IVDMD), metabolizable energy (ME), short chain fatty acid (SCFA) and lower (P<0.01) pH than the other feedstuffs. Conclusion: The higher values obtained for the potential gas production in apple pomace will indicate a better nutrient availability for rumen microorganisms (P<0.01).

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