High amount of pistachio by-product are produced annually in Iran that most of them are considered as agricultural waste. Main problem regarding feeding PB to livestock is the presence of phenolics and tannins which can affect their utilization by ruminants. In vitro studies have shown that tannin reduced microbial protein synthesis due to its antimicrobial effects. The main objectives of the current study were to evaluate the effects gamma irradiation (10 kGy), sodium hydroxide (NaOH), urea and polyethylene glycol (PEG) on the tannins and phenolic compounds, gas production (GP) and microbial protein synthesis (MPS) of PB.
Pistachio by-product was obtained from a pistachio processing factory and was air dried before it was used in this study. Samples were treated with 4% NaOH, 4% urea solution and gamma radiation (10 kGy) and assigned in a completely randomized design. Phenolic compounds were analyzed according to standard methods. Gas production (GP) test was performed and produced gas were recorded at 2, 4, 6, 8, 12, 24, 24, 48, 72 and 96 h. PEG were used in line with treatments. Metabolisable energy (ME), digestible organic matter (DOM) and volatile fatty acids (SCFA) were estimated via available equations. In another run of gas test (using 10.0 atom % 15N-double labeled urea in buffer solution), at the 12 h of incubation gas volume was recorded and the contents of the syringe were transferred quantitatively to tubes for determination of apparently undegraded residue, 15N incorporation and efficiency of microbial mass production. To estimation gas production coefficients were done by Orskov and McDonald equation and data were analyzed using the GLM procedure of SAS.
NaOH and urea treatments decreased (P<0.01) total phenolic (TP), total tannin (T), condensed tannin of PB and GR decreased (P<0.01) TP, T of PB. Among treatments NaOH decreased TP, TT and CT more than others (%60.43, %80.25 and % 88.89, respectively). Cumulative GP at 96 h varied between 18.76 and 29.13 ml/200 mg DM. The lowest (P<0.01) GP at 96 h observed with NaOH and urea treated PB. Incorporation of 15N into the microbial mass was lowest (P<0.01) in NaOH (0.013 mg) and urea (0.014 mg) in comparison with the control (0.017 mg). Efficiency of microbial protein synthesis (EMPS) expressed as 15N enrichment in microbial mass (15N) per ml of produced gas (G) after 12 h of incubation was highest (P<0.01) for NaOH treated PB (0.049 mg 15N/ml G) followed by control, urea and GR treated PB. NaOH and urea treatments decreased (P<0.01) ME, DOM and SCFA production.
Findings of the present study suggest that although supplementation of PB with NaOH, urea and GR would decrease T, but they decreased gas production and 15N enrichment in microbial mass. Data of the present study showed that the elimination of tannin from PB does not necessarily mean improving its fermentation properties in in vitro.