Aims: An in vitro gas production technique was conducted to investigate the effect of oil content of nine ruminant protein feedstuffs including whole soybean, soybean meal, flaked soybean, linseed, fish meal, canola meal, cottonseed meal, sunflower meal and corn gluten, on protein fermentation. Study Design: Complete randomized block design Methodology: Nine protein feedstuffs were chosen and 15 mg N of both intact or oil free dried and milled samples were inserted into 125 mL serum bottles. Buffered rumen fluid was added 10 g of rapidly fermentable carbohydrates and incubated for 4 h. Then, 60 mL of the incubated medium was poured in each serum bottle to start the gas production procedure. Results: Gas production constant rate of the feed samples diminished after oil extraction (0.026 vs. 0.022 respectively). Whole and flaked soybean displayed the greatest magnitude of gas production constant rate among the other feedstuffs (P=.05). Also, the extent of gas production for fish meal, flaked and whole soybean was numerically lower than the other feed samples. A fast gas production was observed for flaxseed among the oil free samples. A decline in gas production constant rate occurred after the oil extraction might be due to the suppression in protein degradability of feedstuffs evaluated. Conclusion: The oil free samples generally produced, or tended to produce, less in vitro gas production constant rate than the intact samples. In addition, results of this study provided evidences that rumen fermentation patterns, as evaluated by gas production technique, is affected when different protein feed sources were incubated.

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