This study was conducted to evaluate the chemical composition and fatty acids profile of six predominant date seed cultivars grown in Iran including Barhi, Estamaran, Mazafati, Khasi, Kharak and Zahedi and the biological effects of their tannins based on in vitro gas production. Organic matter digestibility (OMD), metabolizable energy (ME) and concentration of short chain fatty acids (SCFA) were estimated after 24 h of incubation without or with inclusion of polyethylene glycol (PEG). Results showed that date seeds of all cultivars contained high amount of neutral detergent fiber (NDF), ether extract (EE) and total phenolics (TP), which ranged from 689 to 782, 82 to 118 and 41 to 110 g/kg DM, respectively. However, they had low levels of crude protein (CP) (50 to 69 g/kg DM) and ash (10 to 26 g/kg DM). Gas chromatography re-vealed that the major unsaturated fatty acid was oleic acid (40.13 to 46.35 g/100 g fatty acids), while the main saturated fatty acid was lauric acid (20.96 to 26.25 g/100 g fatty acids). Except for Estamaran, all cul-tivars had low OMD (<334 g/kg DM) and ME (<4.1 MJ/kg DM). Inclusion of PEG increased gas volume (GV), OMD, ME and SCFA (P<0.05) suggesting the inhibitory effect of date seed tannins on microbial fermentation. Total tannins were negatively correlated with nutritive value (OMD and ME) of date seeds. It can be concluded that despite low digestibility and ME, date seed may be considered as an alternative feed because of high amount of structural carbohydrates and EE.

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