Aims: To evaluate the effect of aflatoxin B1 (AFB1) on in vitro dry matter disappearance (IVDMD), gas production and ammonia-N formation of an alfalfa hay based diet using batch culture system. Place and Duration of Study: Department of Animal Science, between July 2011 and August 2012. Methodology: In an anaerobic batch culture system, 50 ml of buffered rumen fluid was dispensed into a 125-ml serum bottle containing 0.5 g dry matter (DM) of the experimental diet. Experimental treatments included four dose levels of AFB1 (0, 300, 600 and 900 ng/ml). All bottles were purged with anaerobic CO2, sealed and placed in a shaking water bath for 72 h at 38.6ºC. Gas production of each bottle was recorded at 2, 4, 8, 12, 16, 24, 48 and 72 h of the incubation and then gas released. The batch cultures were repeated in three incubation runs. After 72 h incubation, bottles were opened and 2-ml sample of each bottle were taken for ammonia-N analysis. The biomass residues were centrifuged and the pellet was dried at 65 °C for the determination of the residual DM and IVDMD. Results: Addition of AFB1 affected the rate and cumulative gas production (P<0.05), so, by increasing the level of AFB1 from 0 to 900 ng/ml, the gas production rate decreased from 0.071 to 0.051 and cumulative gas production decreased from 196.4 to 166.0 ml/g DM, respectively. In addition, IVDMD decreased significantly with inclusion of AFB1 in culture medium, so that the lowest and the highest IVDMD values were observed in treatments with 900 and 0 ng/ml AFB1, respectively (0.54 vs. 0.68). The results indicated that addition of AFB1 significantly (P<0.05) decreased ammonia-N concentrations, so the lowest value was observed at 900 ng/ml AFB1. Conclusion: The addition of different levels of AFB1 affected in vitro fermentation characteristic, as represented in reduced gas production, dry matter digestibility and ammonia-N concentrations. Therefore it is necessary to control and manage aflatoxin contaminations in ruminants.

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