Tannins can inhibit enzymatic and microbial activity therefore affecting gas production. There is very limited information about effects of urea treatment on gas parameters in dietary of tannin. The objective of this study was to evaluate effects of processing of different levels of pistachio hull by urea on gas production in vitro. Experimental treatments were included: Control with 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). The treatments were incubated with 0.5 and 1 percentage of urea. Gas production was measured (Theodorou et al., 1994; McDonald, 1979). The gas pressure and volume was recorded before incubation (0) and 2, 4, 6, 8, 12, 24, 36, 48, 72, 96 and 120 h after incubation. Gas production kinetics were estimated using the equation described by Ørskov and McDonald (1979), y = a + b (1-exp-ct). Statistical analysis was performed using the 3 × 2 factorial procedure of SAS (P < 0.05). Our data exhibited that extent of gas production in treatments incubated with 0.5% urea in T2 was significantly higher vs. T1 and control. Extent of gas production in treatments incubated with 1% urea was significantly affected i.e., the value of gas production in T1 + 1% urea was lower than T2 + 1% urea and control + 1% urea. Indeed, T2 + 1% urea was significantly higher than control + 1% urea. Rate of gas production was not affected when co-incubation was done by 0.5% urea in different treatments while by incubation with 1% urea, the rate of gas production was significantly higher in T1 than control and T2. The treatments were incubated with 0.5 vs. 1% of urea led to increase of extent and reduction of rate of gas production with the exception of T2 which extent and rate of gas production were stable. It seems that urea supplementation can eliminate the negative effects of tannins on gas production.

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