Abstract: Samples of unfractionated forage, isolated NDF and residue insoluble in 90% Ethanol were fermented in vitro and gas production was monitored. The gas volume associated with the ethanol soluble (A fraction) was determined as the difference between the gas from the whole forage and from the ethanol residue.The gas yield associated with the fraction insoluble in 90% ethanol but soluble in neutral detergent soluble (B1 fraction) was determined by subtracting the isolated NDF gas curve from the corresponding ethanol residue curve. This experiment included untreated Corn Silage (CS) and chemically treated corn silage. The rate of gas formation from the A fraction was approximately rather than (p<0.05) the B1 fraction. The gas production of A fraction was less than (p<0.05) the B1 fraction. The CS was treated with urea (10g kg^-1) or formaldehyde (4g kg^-1). Cumulative gas production was recorded at 2, 4, 8, 12, 16, 24, 36, 48, 72 and 96 h of incubation and McDonald equation was used to describe the kinetics of gas production. Treatment with urea decreased (p<0.05) gas production at 96 h by 217.6 vs 236.7, 240.0, 232.56, 254.0 (mL g ¹) for CS, Formaldehyde treated (CSF), urea and formaldehyde treated (CSFU), residue insoluble in 90% ethanol (EIR) and isolated NDF, respectively. The maximum rate of gas production decreased (p<0.05) in CS from 0.028 to 0.023, 0.025, 0.027, 0.0235 and 0.0268 for CSU, CSF, CSFU, EIR and isolated NDF, respectively. The gas production of soluble and insoluble fractions (a+b) decreased (p<0.05) from 261.8 in EIR to 241.8, 240, 225.0 238.7 and 239.3 mL g ¹ for CS, CSU, CSF, CSFU and isolated NDF, respectively. The gas production at 96 h in EIR was (p<0.05) rather than the other treatments. Associative effects were calculated as the difference between the observed gas production for mixture of urea and formaldehyde and the individual inclusion (urea or formaldehyde). Associative effects generally observed as decreasing of gas production with duration of incubation. The strong correlation between extent of gas production in incubation times and on dry matter in situ disappearance was obtained. The poor correlation in initial times between gas production and in vitro dry matter and in situ dry matter disappearance observed resulting the improved production of in vitro and in situ dry matter disappearance from gas production in later times of incubation due to all potential components were fermented and produced gas. It is concluded that the associative effects cause decreasing of gas production specially in CSU and also resulted the difference between of the gas formation and the rate of gas production of the A fraction and the B fraction. There is strong positive correlation between gas production with in vitro and in situ dry matter disappearances.