Tannins are a heterogeneous group of phenolic polymers of the plant origin that can be detrimental when consumed by herbivores. Pistachio hull (PH) is a by product that has a high tannin content. Tannins can also inhibit enzymatic and microbial activity and together with inhibition of gas production. The aim of this study was to evaluate effects of different levels of Pistachio Hull on in vitro extent and rate of gas production. The rumen fluid was obtained from 4 fistulated sheep which were fed twice daily with a diet containing alfalfa hay (60%) and concentrate (40%). Experimental treatments were included: Control consisted of 30% alfalfa, 20% straw and 50% concentration (0.42% tannin), T1) 15% alfalfa, 15% PH, 20% straw and 50% concentration (1.06% tannin) and T2) 30% PH, 20% straw and 50% concentration (1.83% tannin). Gas production was measured on procedure (Theodorou et al., 1994; McDonald, 1979). The pressure and volume of gas was recorded before incubation (0) and 2, 4, 6, 8, 12, 24, 36, 48, 72, 96 and 120 h after incubation. The linear regression equation between reading pressure and volume for determination of corrected volume of gas was estimated. Cumulative gas production data were fitted to the model of Ørskov and McDonald (1979), y = a + b (1-exp-ct). Statistical analysis was performed using the MIXED procedure of SAS (2000) using a completely randomized design. Extent of gas production decreased as the amount of tannin in the diet increased, but this decrease was significantly in treatment 2 (1.83% tannin) relative to T1 (1.06% tannin) and control (0.42% tannin; P < 0.01). Rate of gas production was not affected by the amount of tannin in experimental treatments. It seems that tannins could be inhibiting for microbial activity and thus reduced the amount of gas production.

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