Dietary oil content is potent to alter the nitrogen utilization by ruminal microbiota which are major source of protein for the ruminant host animal. The aim of the present study was to investigate the in vitro effect of oil content of fish meal and flaxseed (containing 151 and 376 g oil/ kg OM, respectively) on protein fermentation using gas production technique. Feed samples were milled to pass a 1 mm screen then the oil content of a part of each sample was extracted. All gas production procedures were run (n=4) for intact and the oil-free samples .Two rams (42:1:4kg, body weight) were used as rumen fluid donors. To avoid a too high input of nitrogen from the rumen fluid, the rumen fluid was diluted 1:19 with a buffer/mineral solution. To be certain that nitrogen was the limiting factor to fermentation, 10 g/I rapidly fermentable carbohydrates (glucose, 3.33 g/I; xylose, 3.33 g/I and wheat starch, 3.33 g/I) were added to the buffered rumen fluid and incubated at 39 °C.This preincubation was performed in a 4 I bottle. During this pre-incubation, all available nitrogen from the rumen fluid was incorporated into bacterial nitrogen components, mainly protein, in order to make nitrogen limiting to microbial growth. After only 4 h of incubation, gas production had ceased and 60 ml of the free nitrogen buffered rumen fluid, with the rapidly fermentable carbohydrates, was moved into a 125 ml bottle, to follow the gas production for 72 h, already containing a feed sample with exactly 15 mg nitrogen. Gas production parameters were estimated using a non-linear equation of p=b (1_e-ct),where p is the gas production at time t, b is the gas production from fermentable fraction (ml), c is the gas production constant rate (ml/h) and t is the incubation time (h). Values of b (mean:l:SE) for intact fish meal and flaxseed were 216:1:7and 244:1:6and for oil-free fish meal and flaxseed were 181:1:4and 242:1:6, respectively. Values of c for intact fish meal and flaxseed were 0.033:1:0.003 and 0.027:1:0.001 and for oil-free fish meal and flaxseed were 0.031:1:0.001 and 0.030:1:0.001,respectively. Results showed that the extraction of oil from fish meal caused a significant decline of parameter b by 16 percent (P<0.05). However, it did not alter the parameter c. On the other hand, oil-free flaxseed had a significant (P< 0.05) higher constant rate compared with that of the intact flaxseed (0.030 vs. 0.027). It seems that oil-free flaxseed may liberate more nitrogen under in vitro condition used in the present study, so gas is produced faster than that of the intact sample.

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